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Talanta 51 (2000) 645665

Polarographic and voltammetric behavior of selenious acidand its use in analysis

P. Zuman a,*, G. Somer b

a Department of Chemistry, Clarkson Uniersity, Potsdam, NY13699-5810, USAb Department of Chemistry, Faculty of Science, Gazi Uniersity, Ankara 06500, Turkey

Received 24 August 1999; received in revised form 25 October 1999; accepted 26 October 1999

Abstract

Solution chemistry of Se(IV), in particular the acid base properties, salt and complex formation, chemical

reduction and reaction of Se(IV) with organic and inorganic sulfur compounds are briefly summarized. The

electrochemical reduction of Se(IV) on dropping and stationary mercury electrodes is dealt with in some detail. The

effects of antecedent acidbase equilibria and of consecutive reactions of the reduction product, Se2, adsorption of

their products, and effects of added metal ions are discussed. The principles and applications of stripping analyses for

determination of ultratraces of Se(IV) are summarized. The behavior on unreactive (Au, Pt, carbon) and reactive (Hg

Ag, Cu) electrodes are compared. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Acidbase; Sulfur; Metal ions

www.elsevier.com/locate/talanta

1. Introduction

Selenium is present in the earth crust as the

seventeenth most common element and occurs in

volcanic eruptions, some minerals, soil and waters

in variable quantities. In trace amounts it is a

biologically essential element for vertebrates, but

can be toxic when introduced in larger quantities.

It is usually present as Se(VI) or Se(IV) and in

organoselenium compounds.

In this review, attention will be paid to electro-chemical properties of selenious(IV) acid and its

anions as well as practical applications of electro-

analytical techniques. As the electrochemical be-

havior of Se(IV) is accompanied by chemica

reactions, such reactions will be briefly mentioned

first.

2. General properties of Se(IV)

2.1. Acidbase properties

Selenious acid can undergo in aqueous solu-

tions the following equilibria:H3SeO3

+K1

H2SeO3+H+ (1)

H2SeO3K2

HSeO3+H+ (2)

HSeO3K3

SeO32+H+ (3)

* Corresponding author. Fax.: +1-315-2686610.

0039-9140/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 3 2 3 - 9

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For the individual dissociation steps the follow-

ing pKa-values have been reported:

pK3pK1 pK28.33 [1]2.72 [1]3.3 [1]

8.32 [2]2.62 [2]

The pK1-value has been determined [1] spec-

trophotometrically using values of Hammett acid-

ity functions Ho as a first approximation for

logaH+. The existence of the protonated form

of Se(IV) in solutions of sulfuric acid has been

confirmed by cryoscopic and conductometric

measurements and by 77Se NMR data [3].

2.2. Salt and complex formation

Solutions of selenious acid react with numerous

metal cations to form slightly soluble, white pre-

cipitates [4]. In unbuffered solutions formation of

slightly soluble salt (K3=3.8109) o f C u2+

with selenite has been confirmed using polarogra-

phy [5]. On the other hand, Pb2+ forms under the

same conditions a 1:1 complex (Kf=1.4104),

which is relatively soluble [5]. Shifts of half-wave

potentials of the polarographic wave of cadmiu-

m(II) ions with increasing concentration of selen-

ite indicates formation of soluble complexes

between Cd2+ and SeO32 in ratio 1:1 with Kf=

2104 ([6]).

For electrochemical studies involving mercury

electrodes, the formation of salts of Se(IV) specieswith mercury ions is of particular interest. For the

slightly soluble [7] mercury(I) (Ks=2.31015)

and mercury(II) (Ks=1.41014) compounds

were proposed structures HgSeO3 ([8]) and Hg3(HSeO3)2(SeO3)2([9,10]). It has been suggested [4]

that in the presence of calomel occurs a dispro-

portionation of Se(IV) into Se(VI) and Se(0) to-

gether with an oxidation of Hg(I) to Hg(II).

2.3. Chemical reductions

Most common metals(0), with the exception of

Au, Pt and Pd, reduce Se(IV) in acidic media, as

the cation H3SeO3+ (similarly as for example

H3CrO4+) is a strong oxidizing agent. Reductions

with Mg, Al and Zn yield both Se(0) and Se(2),

whereas Raney nickel and Cu yield Se2 ([4])

The reaction with Fe2+ yields Se(0) ([11]).

Se(IV) can be reduced quantitatively to Se(0) by

iodide[4,12] following reaction (4):

H2SeO3(l)+4H++4I Se(0)+I2(s)+3H2O

(4)

For the equilibrium constant K= [H2SeO3][H+

]4[I]42 was reported [13] the value 1.4

1014. In order for the reaction between Se(IV)

and I to be quantitative, pH should be smaller

than 1.0 ([12]).

This reaction can be used as a basis of titrimet-

ric determination of Se(IV) using a thiosulfate

solution [12] or an amperometric titration with

two polarized electrodes [14] or potentiometric

titration at constant current [14] using a Pt anode

and C cathode. Iodide ions can be also generated

electrochemically on a carbon electrode and this

can be used for a coulometric titration with two

platinum electrodes (E=0.1 V) for amperomet-ric end point detection [15,16]. Similarly, the cou-

lometric titration can be carried out using

electrogenerated titanium(III) [17].

Both the hydrazinium ion and its aryl deriva-

tives [4,18,19] yield predominantly Se(0), but un-

der optimal conditions it is possible to reduce

Se(IV) to Se2 ([18]). Se(0) is also obtained in the

reduction of Se(IV) by protonated forms of hy-

droxylamine and semicarbazide [4,20], together

with a smaller amount [21] of H2Se. Reductions

with sodium borohydride [22] and a mixture ofH3PO2 and HI [20] yields H2Se.

Aldehydes reduce Se(IV) to Se(0) and do not

form addition compound as with sulfites [4]

Ascorbic acid reduces also Se(IV) to red form of

Se(0); the reaction can be utilized for titrimetric

determinations of Se(IV) [23].

The complex reactions of Se(IV) with sulfur

compounds will be discussed in Section 2.4.

2.4. Reactions with sulfur compounds

Among inorganic sulfur containing species con-

siderable attention has been paid to the reaction

of Se(IV) with sulfite. This reaction has been used

in some analytical procedures to separate Se from

other components of the analyzed sample. In this

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procedure elemental Se (obtained by reduction of

Se(IV) with iodide) is extracted from the sample

quantitatively at pH 89 by an excess of sodium

sulfite. The reaction is chemically reversible, as

upon acidification with hydrochloric acid the ele-

mental selenium is again formed [12].

There is a strong indication that the composi-

tion of the reaction product may vary with a

stoichiometric ratio of Se(IV) to S(IV). Unfortu-

nately, the reaction between Se(IV) and S(IV) has

been often carried out in unbuffered solutions.

When SeO2 was mixed with solutions of H2SO3,

NaHSO3 and Na2SO3 complex acid base and

hydration-dehydration equilibria are established

and the nature of the reacting species is difficult to

identify.

Under conditions used in the most recent study

[24] the predominant product has been proved to

be selenotrithionate, with selenium as central

atom to which two SO32 groups are linked, either

by SeO or by SeS bonds. Based on an analogyof the reaction of Se(IV) with thiolates, the follow-

ing reaction scheme (5)(8) has been proposed:

HSO3+H2SeO3HSeSO5

+H2O (5)

HSeSO5+HSO3

OSe(OSO2)2

2+H2O (6)

OSe(OSO2)22+HSO3

Se(OSO2)22+H++SO4

2 (7)

Se(OSO2)22slow

Se(SO3)22 (8)

Reactions (5) to (7) are supposed to be fast so that

reaction (8) is the rate determining step. Neverthe-

less, no evidence has been offered for the kinetics

or intermediates involved in these processes.

Similarly, the reaction of Se(IV) with thiosulfate

[25] has also been carried out in unbuffered solu-

tions. In a reaction mixture consisting of 0.5 M

SeO2, 2 M N a2S2O3 and 2 M HCl selenopen-

tathionate is formed (9):

H2SeO3+4S2O32+4H+

Se(S2O3)22+S4O92+3H2O (9)

As selenopentathionate is formed [26] in reaction

of selenotrithionate with thiosulfate, it is probable

that selenotrithionate is an intermediate in the

formation of selenopentathionate.

In selenopentathionate, the central Se can be

bound either to two sulfurs (O3S-S-Se-S-SO3)

to one sulfur and one oxygen atom (O3S-S-Se-

O-SSO2), or to two oxygens (O2S-S-O-Se-O-

SSO2). At higher pH-values the selenopentathion-

ate is cleaved and yields Se(0):

Se(S2O3)22S4O6

2+Se(0) (10)

Finally, the reaction of Se(IV) with thiocyanate

yields a selenium dithiocyanate [27] (11):

H2SeO3+4HSCN

Se(SCN)2+NCSSCN+H2O (11)

For the reaction of thiolates with Se(IV) a 4:1

stoichiometry was established and the following

reaction scheme proposed [2831]:

RSH+H2SeO3RSSeO2H+H2O (12)

RSH+RSSeO3H(RS)2SeO+H2O (13)

(RS)2SeORSH+RSSeOSR (14)

RSH+RSSeOSR(RS)2Se+RSOH (15)

RSH+RSOH(RSSR)+H2O (16)

In a consecutive reaction Se(0) is formed (17):

(RS)2Se RS

Se+RSSR (17)

This latter reaction is the basis of a spectropho-

tometric procedure [32] for determination of

Se(IV), in which elemental selenium is dissolved

by an excess of thiolate (18):

Se+nRS(RS)nSe (18)

Formation of species of the type (RS)2SeO

named selenotrisulfides or bis(thio)selenides, has

been reported as products of reactions of alkylthi-

ols [29], 2-mercaptoethanol [31], thiosalicylic

[33,34] and thioglycolic [35] acids, but particularly

(because of their physiological importance) for

cysteine [36,37] and glutathione [36,38]. The mix-

ture of Se(IV) and glutathione catalyzes genera-

tion of superoxide radicals [39] and reacts withmercury compounds [40,41]. Se(IV) reacts also

with methionine [42], but the product of this

reaction was not identified. The reaction of Se(IV)

with 1, 2-dimercaptoethane yielded in a pH-de-

pendent reaction [43] the compound:

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The electrochemical reduction of (RS)2Se and of

(RS)n

Se is assumed to involve a two-electron re-

duction (19) followed by a reduction of RSSR by

Se2 ions (20) [38]:

(RS)n

Sen+2enRS (19)

Se2+RSSR2RS+Se (20)

Bis(alkylthio)selenides RSSeSR undergo also

readily nucleophilic attacks, either on S or Se

depending on substituent R [44]. For glutathione,

formation of GSSeSG predominates at pH 2 and

pH4 at a ratio [GSH]:[Se(IV)]4:1 and this

species is reduced [45] by glutathione reductase

(gl.r.) (21):

GSSeSG+TPNH+H+gl.r.

GSH+GSSeH

+TPN+ (21)

3. Electrochemical reduction

3.1. Reductions on mercury electrodes

3.1.1. Reduction at the dropping mercury

electrode

In the first observation of polarographic reduc-tion of Se(IV) the indistinct waves in 1 M HCl

were attributed to consecutive reductions to +2,

0, and 2 oxidation states, the single wave in an

ammonia-ammonium buffer to the formation of

Se(0) [46]. All these attributions turned out to be

wrong. Lingane and Niedrach [47] in a communi-

cation well advanced for the time-period recog-

nized correctly that between pH 0 and 11 the

reduction of Se(IV) occurs in three different

waves, i1, i2 and i3. These authors concluded

correctly that in each of these waves anotherionized form of selenious acid is reduced. Never-

theless, due to the less developed state of theory

of polarographic processes that involve an-

tecedent acid base equilibria at that time, their

attribution of i1 to the reduction of H2SeO3, i2 to

that of HSeO3 and i3 to that of SeO3

2 was

incorrect. These authors [47] proved that electrode

processes in waves i3and i1 (the latter followed by

a reaction of Se2 formed with mercury ions)

correspond to a six-electron process, but their

attribution of wave i2 to a four-electron reduction

was incorrect. In the following group of studies

[48 53], the role of interaction of Se2 ions

formed by electrolysis with mercury ions was mis-

understood and a process involving four-electron

reduction of Se(IV) and a formation of elemental

Se was erroneously proposed. Further studies of

the reduction of Se(IV) in various buffers [5459]

did not offer substantial new mechanistic

information.

The sharp-edged portion of wave i, in acidic

media was correctly attributed to a formation of a

mercury selenide [60] formed in an anodic process

[61]. The selenide ions, formed in a six-electron

reduction of Se(IV), react with mercury ions

formed by anodic dissolution of mercury. Result-ing selenide is slightly soluble and adsorbed at the

mercury surface. Similarly as sulfide ions, selenide

ions are manifested on polarographic current

voltage curves by at least two anodic waves

[47,62]: the wave with the characteristic sharp

upper edge at about 0.6 V is formed at low

concentrations of selenide first. At concentrations

of selenide higher than about 12104 M an-

other anodic wave increases with increasing con-

centration of selenide at about 0.2 V (actual

potentials depend on pH). The two anodic pro-cesses result in different adsorbed layers, but it is

questionable, if these two adsorbates differ in the

oxidation state of mercury, in the stoichiometry of

the mercury selenide formed or just in the struc-

ture of the adsorbate (e.g. surface orientation)

The two adsorbates certainly occupy different ar-

eas at the electrode surface. Both these anodic

waves are superimposed on the cathodic wave of

the reduction Se(IV)Se(2). The description of

the process occurring at 0.7 V depends on the

direction of the voltage scan: When the potentialis scanned from negative (e.g. 1.0 V) to more

positive potentials (e.g. 0.0 V), the anodic current

corresponds to an oxidation of mercury and for-

mation of an adsorbate, as corresponds to Eqs

(22)(24) from left to right. When the potential is

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oppositely scanned from more positive to more

negative potentials, the increase in current at

about 0.7 V corresponds to a desorption of the

selenide and reduction of mercury ions, corre-

sponding to Eqs. (22)(24) from right to left:

nHgHgn2++2e (n=1 or 2) (22)

Hgn

2++2e2Hgn

(23)

HgnSe (HgnSe)ads (24)

In 3 M H2SO4 the limiting current of Se(IV)

decreased over period of hours when standing in

the presence of metallic mercury. The decrease

was larger when a dropping mercury was im-

mersed into the standing solution than in the

presence of an unstirred mercury pool. The pas-

sage of drops of mercury through the solution

was hence more effective than the contact with the

mercury pool surface [61]. The white precipitate

formed was attributed to a formation of a mer-

cury selenite.A formation of mercury selenide in the course

of reduction of Se(IV) in a perchloric acid solu-

tion has been confirmed [63] by controlled poten-

tial electrolysis at concentrations of Se(IV) lower

than about 5104 M. At higher concentrations

of Se(IV) and vigorous stirring, Se(0) is formed,

possibly by reaction (25)

H2SeO3+2H2Se3Se+3H2O (25)

There is a competition between reactions (25)

and (23) involving Hg22+

or Hg2+

.The overall shape of the part of wave i1 more

positive than about 0.5 V is qualitatively simi-

lar for solutions of Se(IV) in HNO3, HClO4,

H2SO4 and HCl at comparable acidities [60,64],

but quantitatively the current at a given potential

depends on the nature of the acid [65]. Changes in

the limiting current of wave i3 observed in ammo-

niacal buffer pH 8.5 in wateralcoholic mixtures

[66] are either due to changes in viscosity or of

solvation of SeO2.

The decrease of current with increasing pHcorresponds to a decrease of the peak at about

0.6 V observed when DPP was used. The shape

of this peak and observed decrease of ip1 with

increasing scan rate [67] indicates the role of

adsorption.

Based on a comparison of the shape of i=f

(pH) plots with equilibrium pKa values [1] (Sec-

tion 2.1) for H3SeO3+, H2SeO3, and HSO3

and on

the kinetic character of i1, i2, and i3 when i0.15

id it was possible to show [68] that wave i1 corre-

sponds to the reduction of H3SeO3+, wave i2 to

that of H2SeO3 and wave i3 to that of HSeO3

The dianion SeO32 is not reducible within the

accessible potential range, similarly as SO32 is

not reduced. Limiting currents are controlled by

the rate of protonation, which can be accom-

plished by interactions with both the hydrogen

ions and acid buffer components [65,69]. The

shape of i1 at pH3 is complicated not only by

the additivity of the wave of reduction of H3SeO3+

to Se2 and the two anodic waves mentioned

above, but also by current due to a nonfaradaic

oxidation of mercury(0) by H3SeO3+ and reduc-

tion of mercury ions formed [70].

A.C. polarography of acidic solutions of Se(IV)

shows a round, kinetic peak at 0.1 V and anadsorption-desorption peak of HgSe at 0.65 V

[53,71]. Linear sweep and cyclic voltammograms

obtained on a single drop show three or more

cathodic peaks in acidic media resulting from

reduction of Se(IV) to selenide and reactions with

mercury. These peaks are replaced with increasing

pH first by peak ip2 and then by a small peak ip3Only the peak at about 0.65 V corresponding

to adsorptiondesorption of the mercury salt has

an anodic counterpeak [72].

3.1.1.1.Polarographic studies of Se(IV)in the pres-

ence of heay metal ions. Lingane and Niedrach

[47] observed that in ammoniacal buffer pH 8.4

polarographic waves of Cu2+ and Se(IV) are not

additive. With increasing concentration of Cu2+

the wave i3 of Se(IV) decreased. They correctly

concluded that Se2 ions formed at the electrode

surface react with Cu2+ transported toward the

electrode. As the reduction in wave i3 occurs at

negative potentials, where practically no mercury

ions are formed, the decrease of current in thepotential range where Se(IV) is reduced is actually

due to a decrease in Cu2+ ions reduced caused by

formation of slightly soluble CuSe. This is an

early example of a system, called by Kemula [73]

latent limiting currents. In acidic media addition

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of Cu2+ ions resulted in a decrease of the more

negative portion of wave i1, due to a precipitation

of CuSe [61]. The reaction of Cu2+ transported to

the electrode by diffusion with Se2 generated at

the electrode has not recently been considered [5]

in the interpretation of the variation of the cur-

rent of Cu2+ in the presence of excess of SeO32.

In acidic media, where any ZnSe formed is rela-

tively soluble, the addition of Zn2+ ions does not

affect the wave i1 of Se(IV). An addition of Hg2+

ions decreases the wave at 0.6 V and its pre-

wave at about 0.4 V, but has little effect on

current of Se(IV) at potentials more positive than

about 0.3 V [56].

Measurements of limiting currents (as obtained

by d.c., sampled, or normal pulse polarography)

as a function of the composition of the reaction

mixture containing varying ratio of Se(IV) and

Men+-ions is the simplest starting point for inves-

tigations of complex processes occurring in such

mixtures. Other techniques, such as a.c. polarog-raphy (ACP), differential pulse polarography

(DPP) or square-wave polarography (SWP) are

less suitable for this task. This is due to (a) the

difficulty to distinguish between cathodic and an-

odic processes, when these techniques are used.

This reflects the fact that in these techniques

principally derivatives of the iE curve are

recorded. But more importantly these techniques

are less suitable, because for ACP, DPP and SWP

a theoretical treatment for the transport phenom-

ena has not yet been developed for systems whereelectrolysis product may react with species trans-

ported from the bulk. In particular, such treat-

ment is needed for a situation where

electrogenerated selenide ions may be involved in

competing reactions with Hgn

2+ (formed at the

electrode) and Men+ (transported from the bulk

of the solution). In the investigated potential

range, which is several hundred millivolts more

negative than the standard potential of the Hgn

2+

+2eHg couple, the surface concentration of

Hgn2+

will be several orders of magnitude smallerthan that of the Men+ ion transported from the

bulk of the solution. Moreover, interactions be-

tween Se2 and metal cations in the vicinity of

the electrode will most probably be controlled by

kinetics rather than thermodynamics.

For these reasons various observations ob-

tained by ACP, DPP, and SWP in mixtures of

Se(IV) and Men+ are currently difficult to inter-

pret and thus only results will be reported here.

When ACP is used, Se(IV) yields in 0.03 M

HNO3, 0.08 M KNO3 an adsorption-desorption

peak at 0.64 V. Addition of Pb2+ results in a

decrease of this peak to practically zero at

[Se(IV)]= [Pb2+]. Simultaneously, an increase of

two new peaks at 0.68 and at 0.29 V is

observed. Both these peaks reach a limiting height

at [Se(IV)]= [Pb2+]. With a further increase in

[Pb2+] the peak at 0.29 V remains unchanged

but the peak at 0.68 V shows a decrease with

increasing [Pb2+]; moreover, the peak of the re-

duction of free Pb2+ ions at 0.47 V also in-

creases [71].

The ACP peak at 0.44 V in a solution of

Se(IV) (at concentration of the order of 107 M)

is a linear function of [Se(IV)], when 104 M

Cd(II) and Cu(II) are present. In a solution con-taining 0.1 M HClO4, 0.1 M NaClO4, 0.043 M

EDTA and 105 M Pb2+, Se(IV) yielded a single

ACP peak at 0.62 V, the height of which is a

linear function of [Se(IV)]. In the same solution

DPP yielded two peaks at 0.53 and 0.62 V

which both increased with increasing [Se(IV)] [74]

The addition of Hg2+ to a solution of Se(IV) in

0.1 M HClO4 results in a slight decrease of the

ACP peak at 0.6 V, but a marked decrease of

the peak at 0.0 V [63].

At low concentration of Se(IV) [6.3107

M]addition of excess Cu2+ ions results in a decrease

of DPP ip1 current, whereas at higher concentra-

tion of Se(IV) [1.3106 M] addition of Cu2+

results first in a decrease in ip1, followed by an

increase at higher [Cu2+] [75].

In ammonical buffers the peak ip3 is not af-

fected by the presence of Fe(III), Pb(II) and

Cu(II), which interfere with ip1 when added in

acidic media [75]. Similarly in BrittonRobinson

buffer pH 4 addition of Cu(II), Cd(II) and Pb(II)

resulted in a decrease of the DPP Se(IV) peak at0.6 V, but had no effect on ip2 at 1.3 V [76]

The addition of Pb2+ to a 3105 M solution

of Se(IV) in 0.1 M HCl results in a decrease of the

adsorption-desorption peak at 0.54 V. In the

presence of 3105 M Pb2+ the adsorption

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component of this peak is practically eliminated

and only a small, probably faradaic peak remains

at 0.54 V. Further increase in [Pb2+] results in

a small gradual decrease of the reduction peak at

0.54 V. In the absence of Pb2+, Se(IV) yields a

small peak at 0.33 V. The height of this peak

increases with increasing [Pb2+] as the peak at

0.54 V decreases. The maximum height is

reached when [Se(IV)]= [Pb2+]. This peak at

0.33 V is sharp and may be affected by adsorp-

tion. In the presence of excess [Pb2+], a peak of

free Pb2+ ions appears at 0.41 V. At a twofold

excess of Pb2+, the peak at 0.41 V is smaller

than the peak at 0.33 V.

The addition of Se(IV) solutions to a solution

of 3105 M Pb2+ results in a replacement of

the peak of free Pb2+ ions at 0.41 V by a peak

at 0.33 V and in an increase of the peak at

0.54 V. At an excess of Se(IV) the peak at

0.33 V does not increase with increasing

[Se(IV)]. This tendency to reach a limiting heightindicates effect of adsorption. This is further sup-

ported by the effect of temperature. An increase

of temperature from 25 to 45C results in an

elimination of the peak at 0.33 V. On cooling

to 25C the peak at 0.33 V reappears.

The addition of Se(IV) to a solution of Cd2+

ions that are reduced at 0.62 V results in a

decrease of this peak and formation of a new

peak at 0.41 V. Addition of Cu2+ ions has a

relatively small effect on the peak ip1 at 0.54 V

[77].In cyclic voltammetry carried out at a single

mercury drop [72] addition of Cu2+ results at pH

1.5 in a shift of the adsorptive peak at 0.7 V to

more positive potentials, increase in the small

peak at 0.3 V and formation of a new peak at

about 0.0 V. When Cu2+ was added at pH 6.0 the

sharp adsorptive peak at 0.8 V disappears and

new peaks at about 0.1 and 0.0 V are formed.

Addition of Hg2+ resulted in a decrease of peaks

at 0.7 and 0.3 V.

3.1.1.2. Applications of polarographic methods for

determination of Se(IV). For analysis of relatively

pure samples, recording of currentvoltage curves

in ammoniacal buffers has been recommended

[47,78]. As selenates are not reduced at the DME,

determination of Se(IV) in the presence of Se(VI)

is straightforward [64]. After a conversion of

Se(VI) into Se(IV) in 3.6 M HCl the total amount

of Se can be determined. Simultaneous determina-

tion of Se(IV) and Te(IV) is possible in support-

ing electrolytes containing 1 M ammonium salts

of various organic acids [79].

The most common interference in the determi-

nation of Se(IV) are Cu(II) ions. To determine

Se(IV) in the presence of Cu2+ ions the sum of

Se(IV) and Cu(II) was determined in solution of

perchloric acid at pH 23 and Cu2+ had to be

determined separately in an ammoniacal buffer

pH 8.39. From the difference the concentration

of Se(IV) can be obtained [80]. Alternatively, it is

possible to use the reaction of 4-chloro-1, 2-

phenylpiazselenol [81]. In a formate buffer pH 2.5

this compound yields two peaks at 0.11 and

0.62 V, which depend on [Se(IV)]. In the ab-

sence of Se(IV) in the solution of the diamine

ligand two other peaks at 0.41 and 0.97 Vare present. The peak at 0.97 V may corre-

spond to a catalytic hydrogen evolution, that at

0.41 V to an oxidation of the phenylenediamine

(as DPP does not allow distinguishing between

cathodic and anodic processes). But even this

procedure shows interferences by Cu2+, Sn2+

and Ni2+; these ions must be separated using a

Chelex-100 resin column.

Two main areas of applications of polaro-

graphic determination of Se(IV) are the analyses

of waters and soils. In a procedure using dcpolarography Se(IV) was chemically reduced to

Se(0), separated by filtration and dissolved in a

solution containing HBr and Br2. After removal

of excess of Br2 by CO2 Se(IV) was determined in

2 M HCl [82].

To determine Se(IV) in contaminated water

adsorption-desorption DPP peak at 0.63 V in

0.1 M HNO3 can be used after heavy metal ions

are removed on Chelex-100 resin [67]. When an

ammoniacal buffer containing Na2SO3is used [76]

the peaks at 0.6 and 0.8 V, attributed to thereduction of the SeSO ion, are not affected by

heavy metal ions which need not to be removed

prior to recording of the DPP iE curve. This

procedure was used for analyses of Asian river-

waters. A similar approach was used for analyses

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of waters from Chinese rivers [83]. When the

determination of Se(IV) was carried out in soil

using as a supporting electrolyte a buffer pH 8.0

consisting of 1.0 M ammonium acetate and 0.01

M EDTA, no serious inorganic interferences were

encountered [75].

Using a linear sweep voltammetry with a DME

for determination of Se(IV) in vegetables [84],

Se(0) obtained by reduction with ascorbic acid

was oxidized by bromine in hydrobromic acid.

After removal of excess bromine the peak at

0.35 V was measured.

In spite of the complex nature of the reaction

between Se(IV) and SO2 (Section 2.4), such reac-

tion between Se(IV) and SO2 in 3.04.0 M HCl

was reported to be fast and quantitative. From

the decrease in the DPP adsorption-desorption

peak at 0.5 V it was possible to determine SO2in air [85].

For the determination of Se(IV) in biological

material, a catalytic electrochemical processeshave recently been used. The sample is first

treated with permanganate in a mixture of sulfuric

and perchloric acid and left to react for 20 min.

Then a mixture of sodium sulfite, thiocyanate,

mandelic acid and potassium chlorate is added,

left to react for 1 h at 75C, some more thio-

cyanate is added and a peak at 0.2 V on DPP

current voltage curve measured [86]. It is as-

sumed that Se(IV) oxidizes mandelic acid to ben-

zaldehyde and Se(0) formed is reoxidized by

chlorate to Se(IV). Nevertheless, in the presenceof sulfite and CNS ions other reactions men-

tioned in Section 2.4 may be involved. In a

medium containing sodium sulfite and potassium

periodate Se(IV) yields a catalytic wave which can

be used for analysis of hair [87]. A similar reac-

tion has been carried out in a mixture containing

HNO3, HClO4, HCl, citric acid, Na2SO3 and

EDTA, which is added to a ammoniacal buffer

containing phenolphalein and potassium iodate

[88]. Peak currents obtained by LSV were mea-

sured at 0.8 V. Use of EDTA eliminates inter-ferences by many metals. The linear dependence

on mercury pressure (h) indicates that the current

depends on the surface area and is most probably

controlled by adsorption. This conclusion is sup-

ported by the decrease of ip in the presence of

surfactants. It is assumed [88] that the reaction of

Se(0) with SO32 yields SeSO3

2 which is consid-

ered to be the reducible species (26). Selenide

formed is then reoxidized by iodate (27):

SeSO32+2eSe2+SO3

2 (26)

Se2+IO3+12H+Se(0)+I2+6H2O (27)

Finally, a catalytic wave obtained by treatment ofthe blood serum with a mixture of H2SO4, HClO4and ammonium molybdate has been used to de-

termine Se(IV) in blood serum [89].

3.1.2. Behaior of Se(lV) on static mercury

electrodes

The majority of electrochemical studies with

mercury electrodes, the surface of which is not

renewed, involved the use of a hanging mercury

drop electrode (HMDE) and the most common

technique was the linear sweep (LSV) or cyclicvoltammetry (CV). The current-voltage curves ob-

tained by CV using HMDE in acidic media are

characterized by a sharp peak at about 0.6 V

the potential of which depends on pH and con-

centration of Se(IV) [53,63,72,90 94]. This peak

corresponds to an adsorption-desorption process

involving one or more layers of mercury selenide

which is accompanied by an oxidation reduction

of the couple Hgn

2+/Hg(0), as was indicated in

Section 3.1.1 and Eqs. (22) (24). This peak ap-

pears in the same potential range where in dcpolarography the most negative, sharp-edged an-

odic wave is observed, which corresponds to an

anodic dissolution of mercury and formation of a

slightly soluble mercury selenide at the electrode

surface. A corresponding anodic peak is also ob-

served on cyclic voltammograms in solutions of

H2Se, which on reverse sweep yields a sharp ca-

thodic desorption-reduction (further des-red)

peak. Nevertheless, the ratio icath:ianod which in

solutions of Se2 is close to 1.0, is in solutions of

Se(IV) usually smaller than 0.1 [90,93]. This indi-cates that during the scan from about 0.5 to

1.2 V and back to 0.5 V Se2 ions can

undergo competitive processes. This is supported

also by the dependence of the ratio oficath:ianod on

the rate of voltage scanning.

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Depending on concentration of Se(IV) the

sharp cathodic des-red peak can be split into two

or even three peaks. The number of peaks ob-

served by individual authors differs: in 1 3

106 M Se(IV) were reported two peaks [90,93],

at 1105 M Se(IV) two [93] or three [90] peaks,

in 1104 M Se(IV) two [63,90,92] or three [93]

peaks and in 1103 M Se(IV) one [93], two

[63], or three peaks [90]. The role of voltage scan

rate and drop-size cannot be excluded in explana-

tion of these differences.

At potentials more negative than about 0.8

V H2Se is the main reduction product and no

interaction with mercury takes place. The H2Se

formed at the electrode can react with Se(IV)

transported to the electrode and form Se(0) ac-

cording to reaction (25). This is demonstrated by

formation of a red, colloidal precipitate in the

vicinity of the electrode [47,50,72,90,92].

The iEcurve at potentials more positive than

about 0.6 V has a peculiar shape [47,63,9094]which indicates an overlap of several processes.

First indication that different processes play a role

in this potential range was the dependence of the

height of the stripping peak (Section 3.1.3.1) on

the accumulation potential [95], which indicated

different reactions occurring in three potential

ranges: (A) between +0.2 and 0.15 V; (B)

between 0.15 and 0.35 V, and (C) between

0.35 and 0.5 V. Only the accumulation in the

(A) and (C) range yielded a single, well defined

cathodic des-red peak [63].Repeated cyclic scans between 0.0 and 1.2 V

resulted in a marked decrease of current in the

range A and a small decrease in range C [92].

Increase in pH in acetate buffers at pH3.8 (in

the presence of 0.01 M EDTA) resulted in a

decrease in current larger in range A than in C

[92]. Current in range A increases markedly with

decreasing temperature, but shows little change in

range C [92]. When the adsorbate was generated

at +0.2 V, then chronoamperometric reduction

at potentials in the range A indicated complexkinetics, whereas at potentials in the range C the

decrease in current followed simple kinetics [93].

In range A also the formation of Se(0) has been

reported [50]. Whereas current in ranges B and C

was found to be independent of the nature of the

strong acid used, current in range A varied de-

pending on if 0.1 M HClO4, 0.1 M HNO3 and

most markedly if 0.1 M H2SO4 was used [90].

D.C. anodic waves of selenides with a sharp

edge [47], observed at about 0.6 V, are a linear

function of selenide concentration only below

about 1104 M. At higher concentration of

Se2 this wave reaches a limiting value and an-

other wave at more positive potentials increases.

The presence of two waves is attributed to a

formation of two different adsorbates. The pro-

cess occurring in the more positive wave may

correspond to one of the process in the potential

range B or A.

A microscopic observation of the surface of the

HMDE in the course of controlled potential elec-

trolysis of 1103 M Se(IV) indicated different

appearances of deposits obtained at different po-

tentials [90]. Under above conditions multilayer

deposits are formed. On top of such deposits

formation of some Se(0) was assumed [93], butexperimental evidence based on photocurrent

measurement is not unequivocal. The surface of

these films is namely black rather than red.

When the electrolysis of 1103 M Se(IV)

was carried out with HMDE for 4 min at 0.34

V in an unstirred solution, deposit corresponding

to about 40 monolayers was formed [90]. When

now some mercury was drawn from the drop, a

wrinkled bag was formed. This demonstrates that

the slightly soluble deposit is strongly adsorbed at

the surface of the mercury drop. When the chosenpotential for electrolysis was more negative than

about 0.7 V, removal of some mercury from

HMDE resulted in a smaller perfect sphere rather

than in a bag. In this potentials range all of HgSe

is desorbed and no deposit is formed.

Thus at potentials more positive than about

0.6 V at least three different processes take

place. The process occurring at potentials more

positive than about 0.15 V is limited by the rate

of a chemical reaction. This is indicated mainly by

its dependence on pH and temperature. Our pre-liminary results [70] suggest a reaction between

H3SeO3+ and metallic mercury. Other processes

involved result in a formation of several different

adsorbates of mercury selenides. The differences

between these adsorbates may be caused by for-

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mation of mono- and multilayers, of adsorbates

forming islands rather than a monolayer instead

of growing larger these islands can grow higher by

forming multilayers. When islands are formed,

electrochemical reactions can occur between these

islands or through these islands. The adsorbates

can also differ in orientation of monomers and/or

chemical composition of monomers, for example

in the oxidation state of mercury and/or stoi-

chiometry of slightly soluble species. To make

decision between these alternatives can be further

complicated by aging of adsorbates, both in solu-

tion and by solid state reactions.

The adduct of Se(IV) and glutathione (Section

2.4) yields in 0.04 M NaOH, 0.1 M KCl an

irreversible cathodic peak at 0.89 V. To convert

all Se(IV) into the adduct a tenfold excess of

glutathione is needed [38].

Differential pulse linear sweep voltammetry of

Se(IV) in 0.2 M HCl yielded very large des-red

peak at 0.45 V, smaller sharp peak at 0.07 Vand a very small peak at 0.14 V 96. Square-

wave voltammetry on HMDE yielded in the range

A to C one or two peaks depending on the

frequency of the square wave [91].

Controlled potential electrolysis at a mercury

pool electrode in a 1104 M solution of Se(IV)

at pH 4.2 yielded at potentials more positive than

0.6 V a black film and at more negative poten-

tials a thin layer of colloidal Se(0) formed by

reaction (25).

3.1.3. Stripping analysis using mercury electrodes

To increase the sensitivity and possibly also

selectivity of electroanalytical determination of

Se(IV), selenium is accumulated at the mercury

electrode, usually as a mercury selenide, by elec-

trolysis at a constant potential. The accumulated

mercury compound is then dissolved using a po-

tential sweep, using either a dc voltage (in LSV or

CV), differential pulse, square wave or ac super-

imposed voltage.

3.1.3.1. Principles. In spite of the wide use of

stripping determination of Se(IV) in various mate-

rials (Section 3.1.3.3) only limited attention has

been paid to a systematic evaluation of factors, on

which this determination is based. In cathodic

stripping voltammetry of Se(IV) the number

shape and height of reduction peaks depends on

deposition potential, time-period of accumulation,

and equilibration, the rate of stirring of the solu-

tion during the accumulation, composition of the

analyzed solution particularly on acidity, pres-

ence and concentration of complexing agents

presence of other cations and the concentration

range of Se(IV) as well as on conditions of the

stripping, such as the shape of the voltage ramp,

potential range and direction of the voltage scan

and duration of the period between application of

the starting potential and beginning of the record-

ing of the current voltage curve. Rarely the

choice of working conditions has been based on

systematic optirnization of the above parameters

Usually such conditions were selected empirically

using trial and error approach and all conse-

quences of the choice made were not fully

understood.Among the above factors perhaps the most

important is the choice of the deposition potential

(Ed). The electrodeposition must be carried out at

a potential at which the mercury selenide is

formed, for example in 0.1 M HClO4 at a poten-

tial more positive than about 0.6 V. In the

available scope of potentials it is possible to dis-

tinguish three ranges, as discussed in Section

3.1.2: (A) between +0.2 and 0.15 V; (B) be-

tween 0.15 and 0.35 V; (C) between 0.35

and 0.5 V.When Ed is chosen in the range A, a single

stripping des-red peak at about 0.65 V is ob-

served. The height of this peak varies with poten-

tial Ed between +0.05 and 0.15 V. The plot of

ip as a function of Ed is a bell-shaped curve with

a maximum at about 0.05 V [95,96]. The de-

pendence of ip on [Se(IV)] is non-linear with two

linear segments, one up to 2107 M Se and the

other above 3107 MSe [63].

The adsorbate formed under these conditions

adheres so strongly to the electrode surface, thatthe drop with the adsorbate can be transferred

into another solution and the stripping can be

carried out in a blank supporting electrolyte [94].

As an electrolyte either 0.1 M HClO4 ([63]) or

preferably 0.1 M HCl [94,95] are used. The peak

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on the ratio [Se(IV)]:[Cu(II)], on composition of

the supporting electrolyte, on the deposition po-

tential used, on the duration of accumulation and

on the stripping peak observed.

The increase of the height of the Se(IV) strip-

ping peak reaches with increasing concentration

of Cu(II) a limiting value, above which the peak-

height is independent of concentration of Cu(II).

In the presence of 3105 M Cu2+ and 8

103 M EDTA the height of the Se(IV) stripping

peak is independent ofEd from 0.0 V to 0.4 V

[105].

The independence of the peak of Se(IV) of a

sufficiently high concentration of Cu2 is the basis

of numerous recent analytical applications of the

Se(IV) stripping peak. At sufficient excess of

Cu(II) the height of the Se(IV) stripping peak is a

strictly linear function of [Se(IV)] and indepen-

dent of smaller variations in [Cu2+]. To samples,

which do not contain a sufficiently high concen-

tration of Cu2+, excess of a Cu(II) compound isadded.

Using square wave cathodic stripping voltam-

metry the influence of Cu2+ on stripping peaks of

Cu2+ has been studied in some detail [91]. Addi-

tion of Cu(II) ions to a solution of 3108 M

Se(IV) results at low concentrations of Cu2+ in a

stripping peak at about 0.5 V to be at concen-

tration larger than about 1106 M gradually

replaced by a sharp peak at 0.6 V. This latter

peak is at concentrations larger than 7105 M

Cu2+

the predominant one and is about four-times higher than the peak at concentration of

Cu2+ smaller than 1106 M. The potential of

the peak at 0.5 V is independent of concentra-

tion of Cu2+, that of the peak at 0.6 V is

shifted to more negative potentials with increasing

concentration of Cu2+. The peak current at 0.6

V is a linear function of the logarithm of the

frequency. The peak current at 0.6 V also

increases with scan increment and depends on the

amplitude of the square wave in a bell-shaped

curve [91].At higher concentrations of Se(IV), such as

1105 M, and at shorter deposition times,

different results are obtained. The peak height

does not increase with scan increment and no split

of the peak is observed at high amplitude. At

1105 M Se(IV) and deposition time of 2 s the

electrode reaction at 0.6 V is reversible both in

the absence and presence of Cu2+: product and

reactant are adsorbed in the absence, but only the

reactant in the presence of Cu2+. On the other

hand at 1107 M Se(IV) and much longer

deposition time (60 s) the electrode process is

reversible in the absence, but irreversible in the

presence of Cu2+. Under these conditions the

product and reactant are both adsorbed in the

absence, whereas only the reactant in the presence

of Cu2+.

Stripping peaks of Se(IV) and As(III) are non-

additive. Peak of As(III) in 0.18 M H2SO4 at

0.72 V increases with increasing concentration

of Se(IV), even if Se(IV) yields in this medium

using a linear voltage sweep a peak at 0.58 V

[106]. In 1 M HCl containing 2103 M Cu2+

and 3108 M As(II) addition of Se(IV) re-

sulted in an increase of the Se(IV) peak at 0.6

V and a decrease of the As(III) peak at 0.75 V[107].

In 0.1 M H2SO4 containing 1107 Rh(III) a

large, sharp stripping peak at about 1.0 V is

observed in the presence of 3109 M Se(IV)

after accumulation at 0.20 V for 60 s. This

peak, attributed to desorption-reduction of

Rh2Se3, is a nonlinear function of the scan rate.

Its peak current varies with Ed in a bell-shaped

curve with a maximum at 0.2 V and increases

by about 50% when the concentration of sulfuric

acid is increased from 0.01 to 0.2 M.Analytical method based on this peak is much

more (at least 10 times) more sensitive than the

determination of Se(IV) in the presence of Cu2+

and enables 1010 to 1011 M Se(IV) solutions to

be analyzed. No interference is observed for 100

fold excess of Cu(II), Zn(II), Pb(II), Cd(II)

Co(II), Ni(II), Bi(III), Mn(II), Fe(III), Tl(I)

As(III), Mo(VI), Ti(IV), V(V), Cr(VI), In(III)

Pd(II), U(VI), Pt(II), Zr(IV), Ge(IV), Nb(V) and

La(III). Only Te(IV) is limited to a fivefold excess

[108].

3.1.3.3. Applications of stripping analyses. In prac-

tical applications, the sensitivity of the determina-

tion of Se(IV), which is often present in ultralow

levels, is essential. This can be achieved by reduc-

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ing Se(IV) and accumulating mercury selenide in

one of the potential ranges mentioned in Section

3.1.3.1 and measuring the desorption reduction

current (des-red) usually at about 0.6 V. This

accumulation, together with a stripping procedure

using pulse and square-wave techniques and the

effect of some metal ions, allows to reach suffi-

cient sensitivity, in some cases up to 1011 M.

The reliability of such determinations is neverthe-less limited by changes in the measured signal

(peak current) in the presence of some metal ions,

as well as As(III) and Te(IV), in the sample. To

minimize these interferences several physical,

chemical, and electrochemical approaches have

been proposed.

In some instances it was possible to carry out

the stripping analysis directly in the analyzed

sample, probably because the samples contain a

sufficient excess of Cu2+. The stripping can be

carried out in HCl using a linear potential sweep[99,109112] as well as in 1 M (NH4)2SO4 con-

taining 0.4 M EDTA [113]. In the presence of an

excess of thioglycolic acid the observed des-red

peak of the reaction product was 23 times higher

than the peak of free Se(IV). Moreover, the peak

of the product is unaffected by a 100-fold excess

of Cd(II), Pb(II), Ni(II), Cr(III) and Sb(III) and

its height decreases only in the presence of Cu(II)

[35]. When a graphite wax electrode coated with

mercury [114] was used for the stripping at ED=

0.6 V, following the peak at 0.1 V enableddetermination of Se(IV) without interference from

Fe(III), Cu(II), and Pb(II) [115]. Differential pulse

stripping voltammetry has also been successfully

used for such direct analyses [116118].

In the presence of excess of Cu2+ ions, the

stripping can be followed using a linear voltage

ramp [98,100,109], possibly in the presence of

citrate as complexing agent [120]. Alternatively,

differential pulse [103,107] or square wave voltam-

metry [91] can be used. Stripping was also carried

out in the presence both of Cu

2+

and EDTA,using linear voltage ramp [97,121], differential

pulse [97] or ac voltammetry [121]. Differential

pulse voltammetry was also used for stripping in

the extremely sensitive procedure in the presence

of Rh(III) [108].

Anion-exchange resins were used to separate

Se(IV) from the sample [95] and cation-exchange

resins for removal of interfering cations [102]

Another example of pre-separation based on

physicochemical principles is the application of

extraction using pentanol [122,123] and accumula-

tion on a mercury plated graphite wax electrode

[114]. Esterification of selenious acid by the alco-

hol is assumed. When oxygen flask combustion

was used for sample decomposition, volatile Se

was absorbed in a persulfate sulfuric acid mix-

ture. Treatment with HCl enabled cleavage of the

excess of persulfate and conversion of Se(VI) into

Se(IV), which was determined by stripping analy-

sis [102]. Alternatively, after wet ashing selenium

was converted into 3, 4-diaminophenylpiazse-

lenol and extracted into benzene. Back extraction

into dilute acid enabled selective stripping analysis

[124,125].

In a unique approach, Se(IV) was reduced with

borohydride to Se2, converted into volatile H2Sewhich was separated by a stream of a gas from

the sample, trapped in an alkaline solution and

analyzed by differential pulse cathodic stripping

voltammetry [126].

Stripping peaks were also used in detectors for

HPLC [127] and flow injection analysis [128130].

Because of its physiological importance, numer-

ous stripping determinations of Se(IV) were used

for analyses of biological material. Thus analyses

were carried out of blood and serum

[99,109,115,123], urine and liver [110,118], andfood, in particular of milk [95,130], bovine liver

[95,110 112,118], animal muscle [95,110,118]

sausage and pig kidney [130], seafood and fish

[110 112], rapeseed oil [124] and brewers yeast

[103]. It had also been used for analyses of plants

[120], seed [124] and orchard leaves [110,118] as

well as of soils [102].

Another area of frequent applications are deter-

mination of Se(IV) in waters, such as rain and

snow [91,117,118], ground, well and river water

[108,128], tap water [119,128] and seawater[98,125].

Cathodic stripping can be used for analyses of

samples containing both Se and Te [97,121] and

was used for analyses of minerals, such as basalt

[107] and also of semiconductor alloys [113].

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3.1.3.4. Adsorptie stripping. The adsorptive strip-

ping is based either on adsorption of a compound

(usually organic) at the surface of an electrode or

on conversion of an ion into a bulky, usually

uncharged, hydrophobic species which is ad-

sorbed at an electrode. Such adsorption results in

an accumulation of the species at the electrode

surface. Potential sweep is then applied and the

peak current at the potential where the desorption

occurs is used for analysis.

For the determination of Se(IV) the adsorbed

species was in most instances a piazselenol,

formed in a reaction of an aromatic diamine with

Se(IV). As the reagent, 3, 3-diaminobenzidine

[101],o -phenylenediamine [131], or 2, 3-diaminon-

aphthalene [132] were used for this purpose. For

o-phenylenediamine in Britton Robinson buffer

pH 2.0 the peak stripping current at 0.62 V

changes with Ed in the bell-shaped curve with a

maximum at 0.45 V [131]. At 6106 M

Se(IV) a smaller peak at 0.13 V was observed,controlled by adsorption. For 3107 M Se(IV)

in the presence of 2, 3-diaminonaphthalene the

peak at 0.06 V was predominant and its peak

current in 0.1 M HClO4 or HNO3 and in acetate

buffer pH 4.5 was independent of Ed between

+0.2 and 0.0 V. In 0.1 M HCl the dependence of

ip on Ed had a maximum at +0.05 V [132].

Neither the mechanism of the process involved in

the desorption-reduction peak at 0.62 V nor

that at 0.06 V is understood, as the electro-

chemical reduction of piazselenol also is not un-derstood in sufficient detail, but it seems that in

the electroreduction of piazselenol HgSe is formed

which is adsorbed at more positive potentials, but

desorbed and reduced at 0.6 V. Similarly, HgSe

can be formed and stripped at about 0.65 V,

when Se(IV) is reacted with 2, 5-dimercapto-

1, 3, 4-thiadiazole [133].

3.2. Electroanalytical behaiors of Se(IV) on

other metals and their applications

Most attention has been paid to reduction of

Se(IV) and properties of its reduction products on

gold and carbon electrodes, but some reports deal

with electrochemistry of Se(IV) on other metals

and metal oxide electrodes.

3.2.1. Behaior on gold and siler electrodes

Contrary to an earlier report [134], the reduc-

tion of Se(IV) on gold (and similarly on carbon

and platinum, see below) electrodes is a six-elec-

tron process yielding Se2 [135]. Any formation

of Se(0) which can be observed, is due to a

chemical reaction between Se2 formed at the

electrode and Se(IV) transported to the electrode

by diffusion from the bulk of the solution [135],

following Eq. (25). The reaction takes place in

acidic solutions and Se(IV) acts as oxidant proba-

bly in the form of H3SeO3+.

Under comparable conditions, namely, similar

surface area, central rotation speed and scan rate

of 33 mV s1 [134] or 10 mV s1 [135] peak-

shaped current voltage curves were recorded in

0.1 M HClO4 containing 5105 M Se(IV)

[134], whereas in 5103 M solutions of Se(IV)

i n 1 M H2SO4 limiting currents were observed

[135]. In the latter case, a single wave with E1/2

about 0.4 V was observed, which appeared in asimilar potential range as corresponding waves

obtained with platinum or carbon electrodes. The

process at 0.4 V was earlier [134] erroneously

attributed to H2-evolution. In the 5105 M

solution a cathodic adsorption desorption peak

at +0.25 V and a wave at about 0.0 V were

observed [134]. On the reverse sweep appeared

three anodic peaks at +0.63, +0.8 and +1.15

V. Their attribution to oxidation of Se(0) in view

of the above observation is doubtful. On gold

plated glassy carbon electrode only an indistinctcathodic wave at 0.0 V occurred together with

anodic peaks at +0.67, +0.8 and +1.0 V on the

reverse sweep.

No interference was observed for Pb(II), Cd(II)

and even Cu(II) which was anodically stripped at

+0.1 V. No evidence for multiple peaks for Cu or

Se was reported. Determination of Se(IV) based

on anodic stripping using the peak at 0.8 V was

used for analysis of bovine liver and gave excel-

lent agreement with certificate value [134].

In a solution of 5104

M Se(IV) in 1 MH2SO4 in the presence of 0.1 M CdSO4, the

six-electron reduction of Se(IV) occurs at 0.5

V, followed by an increase in current at 0.7 V

due to the reduction of Cd2+. Reversal of the

scan-direction at 0.7 V yields a sharp anodic

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peak, which corresponds to a re-oxidation of Cd

superimposed on the Se(IV) reduction wave. The

potential of the Se(IV) reduction wave is shifted

to 0.4 V. This shift is caused by the reduction

of Se(IV) not on the gold surface, but on a

deposited layer of CdSe. The shift is attributed to

a high nucleation over-potential caused by the

layer of CdSe on the gold surface. Similar effect

was observed also on a carbon electrode [135].

At low concentrations of Se(IV) the CdSe film

formed contains only a small excess of Se(0). On

the other hand at concentrations of Se(IV) be-

tween 1 and 3103 M (in the same supporting

electrolyte as above), it is possible to deposit CdSe

films on both gold and carbon surfaces. Such

films contain usually larger amount of Se(0). This

is supported by a p-type behavior after illumina-

tion with white light.

When Se(IV) was deposited at 0.4 V on a

glassy carbon electrode plated with gold, a single

anodic stripping peak was observed at about +1.0 V, if the concentration of Se(IV) was lower

than about 5106 M. At higher concentrations

of Se(IV) another anodic peak increased at poten-

tials by about 0.2 V more negative. Using the

peak at 1.0 V it was possible to carry out determi-

nation of Se(IV) in the range from 2109 M to

2106 M. This procedure too showed an excel-

lent agreement with certificate value for determi-

nation of Se(IV) [136].

To determine selenium and tellurium in elec-

trolytic copper, the sample is dissolved in nitricacid. To eliminate interferences due to nitrite (lib-

erated during dissolution of the metal sample in

nitric acid), antimony and arsenic, hydrogen per-

oxide is added. After addition of ammonia, the

solution is passed through a Chelex-100 resin

column. From the eluate, Se and Te are deposited

on a gold film electrode at 0.1 V. On linear

sweep voltammogram on the anodic branch, the

oxidation stripping peak of Te is at +0.65 V that

of Se at +0.95 V. Analysis of certified samples of

electrolytic copper shows a very good agreementboth for the Se and Te content [137].

Investigations of the reduction of Se(IV) using

CV and electrochemical quartz crystal micro-

gravimetry were marred by using an unbuffered

solution of 0.5 M Na2SO4 as supporting elec-

trolyte [138]. Due to variations of pH during the

voltage scan the obtained results have no simple

meaning.

Anodic stripping voltammetry with a gold disk

electrode was used for determination of traces of

Se(IV) in bodily fluids [139]. Gold electrode

modified by poly(3, 3-diaminobenzidine) interacts

with Se(IV) to form piazselenol at the electrode

surface. Such electrode was used for selective ac-

cumulation and stripping analysis of Se(IV) [140].

Anodic stripping from a tubular gold electrode

[141] or an ultramicroelectrode [142] was used to

determine Se(IV) in a chromatographic effluent

[141] or in flow injection analysis [142].

The reduction of Se(IV) on a silver electrode

involves also a six-electron reduction, but Se2

ions formed interact with Ag+ ions and form

several types of adsorbates, similarly as on Hg

and Cu electrodes [135]. Accumulation of about

+0.35 V in the presence of potassium acetate at

pH 910 (where the buffer capacity is very small)and stripping at about +0.1 V is claimed to be

suitable for determination of Se(IV) [143].

3.2.2. Reduction on Carbon electrodes

An investigation of the reduction of Se(IV)

using a rotating disk carbon electrode indicated

that the reduction occurs in a six-electron transfer

[135]. On carbon electrodes no interaction of

Se2 with the material of the electrode material

takes place, similarly as for gold and platinum

electrodes. Any Se(0) observed results from areaction of Se2 with Se(IV) following Eq. (25)

[135].

The reduction of Se(IV) on stationary carbon

rod electrodes yielded well developed CV and

stripping voltammetry curves, when a soft

graphite electrode was used [144]. When a glassy

carbon electrode was used, only poorly defined

i-E curves were obtained. Authors concluded, that

red form of Se(0) is formed by a chemical reaction

(25) at some distance from the electrode and only

a small part of it is further reduced to Se2

Electrochemically generated grey form of Se(0) on

the other hand is assumed to be further reduced

to Se2. This form of Se(0) is assumed to be

generated only by reduction of the conjugate acid

form of Se(IV) (actually H3SeO3+) as its produc-

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tion decreases with increasing pH. At pH4 where

the reduction of H2SeO3predominates, Se2 is the

major product. But all these conclusions are based

on visual observations of some Se(0) formed, on

total charges that may be affected by adsorption

and on attribution of CV-peaks to consecutive

electron transfers rather than formation of different

adsorbates. Whereas these authors [144] claim that

neither cathodic nor anodic stripping using carbon

electrodes can be recommended [144], others [145]

used deposition at 0.6 V on such electrodes and

a stripping peak at +0.22 V for determination of

selenium in highly pure copper [145].

When solid Na2SeO3 was dispersed in a carbon

paste electrode, a well developed, probably adsorp-

tive cathodic peak was observed at 0.02 V,

followed by an ill developed one at 0.45 V. On

the reverse sweep anodic peaks at 0.11 V (the

shape of which indicates a role of adsorption

phenomena) were followed by peaks at +0.98 and

+1.07 V. The authors [146] did not consideradsorption and attributed individual peaks to con-

secutive electron transfers, involving formation of

Se(0) as an intermediate. When a 0.02 M solution

of selenite was dispersed into carbon paste, a

similar CV was obtained as with the solid selenite

[146].

A very similar pattern of CV curves was observed

when a potential scan 0 V+1.2 V0.8 V0.0

V was applied to a carbon paste electrode, in which

CuSe was dispersed [147]. In 2 M H2SO4 anodic

peaks were observed at +0.92 and +1.05 V andcathodic peaks at +0.05 and 0.35 V, so that

only the anodic peak at 0.11 V was missing,

when compared with the carbon electrode with

dispersed solid selenite [146]. As in the previous

case, attempts were made to attribute individual

peaks to consecutive electron transfers and the

possible role of adsorption was neglected. Authors

used total charges in their interpretation, but due

to the lack of consideration of the nonfaradaic

component of the current the conclusions are

doubtful. When the ratio Cu: Se was varied from2.0 to 3.55, several additional anodic peaks ap-

peared between 0.0 and +0.4 V [147]. A dispersion

of elemental Se(0) into graphite paste electrode

[148,149] was electroinactive and showed no oxida-

tion or reduction signals.

Adsorption desorption phenomena play evi-

dently an important role in processes leading to a

formation of copper selenide, which were followed

using cyclic voltammetry on a vitreous carbon

rotating disk electrode [150]. The deposition of

copper selenide occurs at potentials which are more

positive than required both for deposition of cop-

per and formation of selenide. The primary product

formed is Cu2Se, which is first oxidized to CuSe,

then to Cu2+ and Se.

3.2.3. Reduction on Platinum electrodes

At high concentration of Se(IV) (5 mM) the

reduction on a rotating platinum disk occurs in a

single six-electron step to Se2. Similarly as on gold

or carbon electrodes, selenide ions formed on

platinum electrodes do not undergo consecutive

reaction involving the metal of the electrode [135]

No experimental evidence is offered to support

assumption [150] that electroreduction of Se(IV) on

platinum electrodes yields in an electrochemicaprocesses Se(0). When the reduction of Se(IV) at

constant current on a rotating wire platinum elec-

trode in 1 M H2SO4was carried out in the presence

of a threefold excess of Cu(II), the stripping re-

sulted in a peak at +1.2 V which is proportional

to concentration of Se(IV) [151].

3.2.4. Reduction on other metals and metal oxides

Considerable attention has been paid [152159]

to the effect of Se(IV) on the reduction of Cu2+

ions on a copper electrode. On such electrode in 0.1M H2SO4 the reduction of Se(IV) occurs in two

steps [135,160,161]. In the first step, Se2 ions

formed in a six-electron reduction of Se(IV) react

with Cu ions to yield CuSe(or Cu2Se). This reduc-

tion current at potentials more positive than about

0.65 V is superimposed on current of the

anodic dissolution of copper following Cu

Cu2++2e. Due to additivity of these two cur-

rents, the limiting current at potentials between

0.1 and 0.65 V corresponds to a four-elec-

tron process. At potentials more negative than0.65 V such interaction between Cu2+ and

Se2 cannot take place, as the concentration of

Cu2+ ions is too low. Consequently, a six-elec-

tron limiting current is observed at potentials

more negative than 0.65 V. When a copper

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amalgam electrode is used, the current voltage

curve is complicated by adsorption peaks, corre-

sponding to a formation of mercury or mixed

selenides. Reductions of Se(IV) on nickel and tin

electrodes were unfortunately studied only in un-

buffered solutions of 0.1 M LiClO4 in water [160]

and mixed solvents [162] and due to changes in

acidity at the electrode surface cannot be simply

interpreted [160,162].

Using a copper microelectrode with 5 m di-

ameter and accumulation of Se at +0.1 V, ca-

thodic stripping yielded a sharp peak at 0.05 V,

which is a linear function of concentration of

Se(IV) [163].

This determination is unaffected by the pres-

ence of Cu2+ ions and enables determination of

Se(IV) between 5106 and 3105 M Se(IV).

A titanium electrode was used [42] to study the

electroreduction of Se(IV) in the presence of me-

thinonine. The reduction is manifested by two

pH-dependent peaks in acidic media, one between0.3 and 0.4 V, the other between 0.5 and

0.6 V.

In a solution containing 0.01 M H2SO4, 0.1 M

K2SO4 and 1103 M Se(IV) the reduction of

Se(IV) at a rotating tin oxide electrode occurs in a

single peak at about 0.5 V (SCE), superim-

posed on a limiting current. The limiting current

is about ten times smaller than the sixelectron

diffusion controlled current observed under the

same conditions for reduction of Se(IV) on a

rotating copper electrode. On the tin oxide elec-trode Cu(II) is reduced in a wave at about 0.15

V. Addition of Se(IV) does not practically affect

the limiting current of the Cu(II) wave, but results

in a wave at about 0.3 V. The height of this

wave increases with increasing concentration of

Se(IV) and reaches a maximum height at [Cu2+

]:[Se(IV)]1:0.9. Further increase in concentra-

tion of Se(IV) results in an increase in peak

current at about 0.5 V. This peak, on an elec-

trode covered with copper selenide is about ten

times higher than the peak of Se(IV) on a cleanSnO2 electrode. A reverse scan results in a forma-

tion of a single anodic peak at about 0.15 V

corresponding to a stripping of Cu2+ ions. Ex-

trapolation of currents to equilibrium conditions

results in a current-voltage curve showing a two-

electron reduction of Cu(II) withE1/2about 0.1

V and a six-electron reduction of Se(IV) at about0.2 V [164].

The product of electroreduction at 0.3 V wasidentified as Cu3Se2 by X-ray diffractometry. At

more negative potentials only Cu2Se is formed. Inthe presence of excess Se(IV), it reacts with copperselenide and forms a red suspension of Se(0) at

the surface.

On a stationary tin oxide electrode in 0.4 Mcitric acid containing 1103 M Cu2+ the depo-sition of Cu is observed at about 0.3 V (SCE)

with a sharp anodic peak at about 0.0 V afterreversal of the voltage scan [165]. An addition ofSe(IV) results in a small shift of the cathodic peak

of Cu(II) to about 0.25 V with peak currentremaining constant. With increasing [Se(IV)] isformed another increasing cathodic peak at about0.35 V. On the reverse scan the anodic peak,

corresponding to the oxidation of Cu to Cu2+

decreases in the presence of Se(IV) and its peakpotential is shifted by about 0.05 V to more

negative potentials. A new anodic peak is formedat about +0.15 V (SCE) the height of whichincreases slightly with increasing concentration ofSe(IV). An indistinct peak, probably correspond-

ing to an anodic stripping of a selenide, is ob-served at +1.0 V. With varying the ratio[Cu(II)]:[Se(IV)] and applied potential, forma

numbers of transferred electrons varied for Cufrom 2.0 to 0.95 and for Se from 4 to 5.3. Filmsformed at various potentials and at varying the

ratio [Cu(II)]: [Se(IV)] were analyzed using polar-ography and X-ray fluorescence. With increasingconcentration of Cu(II) in the solution, the com-position of the solid phase varied. The ratio

wCu:wSe in the solid phase was found at about 0.0V (SCE) varying from 0.14 to 1.87 and at 0.4 Vfrom 0.16 to 2.76. The composition of the solid

phase depends on initial concentrations of Cu(II)and Se(IV) in the solution. For ratio [Cu(II)][Se(IV)] between 0.0 and 1.5, CuSe and Se are ata predominating species, for the ratio between 1.5

and 2.2 a mixture of CuSe and Cu3Se2 was found,for the ratio from 2.2 to 2.9 the solid phase atelectrode surface contained Cu3Se2and Cu2Se and

at a larger excess of Cu(II) and ratio from 2.9 to4.0, the composition of the surface layer wasCu2Se and Se.

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Films prepared at 0.2 V and transferred to a

solution containing only 0.4 M citric acid yield

three anodic peaks-sharp peaks at about 0.0 and

+0.1 V. The shape of these peaks indicates role of

adsorption-desorption phenomena. The third peak

at +0.3 V is diffusion controlled. The peak at 0.0

V is attributed to an oxidation of Cu in the film

consisting of Cu2Se following (28) and peak at

+0.3 V to oxidation of Cu3Se (29) and of CuSe

(30):

Cu2SeCu3Se2+Cu(II)+2e (28)

Cu3Se22CuSe+Cu(II)+2e (29)

CuSeSe+Cu(II)+2e (30)

The composition of the deposit is controlled

between 0.2 and 0.4 V by diffusion, but at

potential more positive than 0.2 V also by the

kinetics of the charge transfer. At potentials more

negative than 0.4 V Cu2Se is the predominant

product in the solid phase, as CuSe and CuxSe(where X2) are not stable and are reduced to

Cu2Se [165].

Investigations of the reduction of Se(IV) Bi, Pb,

Cd, Zn and Nu amalgams [166168] did not offer

additional information.

4. Conclusions

Se(IV) is reduced in aqueous solutions in forms

H3SeO3+

, H2SeO3 and HSeO3

. All these threeforms are reduced in a six-electron process to Se2.

The dianion SeO32 is not reducible. Reductions of

H2SeO3and HSeO3, observed at pH5, occur at

such negative potentials that electrogenerated Se2

ions do not react with material of any studied

electrode. At pH4, where H3SeO3+ is the pre-

dominating reducible form, the reduction of Se(IV)

on Au, Pt, and carbon electrodes forms Se2 which

on these electrodes does not react with the material

of the electrode. Electrogenerated Se2 can react

in solution in a homogeneous reaction withH3SeO3+ and yield Se(0) which forms colloidal

species in the solution in the vicinity of the elec-

trode.

On Hg, Ag and Cu electrodes at pH4 at

potentials more positive than about 0.6 V (SCE),

the generated selenide ions can interact with elec-

trochemically dissolved metal ions and form

slightly soluble selenides. This composition can

vary with the metal ion-Se(IV) ratio and with

potential of generation of Se2. These selenides are

adsorbed at the electrode surface. Depending on

the potential applied and on concentration of

Se(IV) one, two or more types of adsorbates can

be formed. These adsorbates may differ in chemical

composition and/or physical properties. On the

surface of these adsorbates elemental Se(0) can be

formed. H3SeO3+ is a strong oxidizing agent and

can dissolve in a chemical reaction the material of

the electrode. Thus Se(IV) in acidic solutions reacts

with metallic Hg and resulting mercury ions are

reduced at positive potentials. In solutions contain-

ing in addition to Se(IV) also Cu2+, Cd2+, orP b2+

ions underdeposition is observed. Mechanism of

the involved process is not yet fully understood.

Electrochemical properties of Se(IV) enable use

of electroanalytical techniques for determination ofSe(IV) in various materials.

The choice of the technique depends on sample

composition and concentration levels. For ultra-

trace determination cathodic stripping methods

offer sensitivity enabling analyses of solutions con-

taining up to 1011 M Se(IV).

5. Note

After this paper was accepted, the authors weresent a manuscript by B. Lange and MG. van den

Berg submitted to Analytica Chimica Acta, de-

scribing determination of Se(IV) by cathodic strip-

ping voltammetry. The method is based on

measurement of a catalytic hydrogen wave at 1.5

V in the presence of 0.3 M HCl and 75 ppb Rh(III),

which enables to reach a detection limit of 2.4

1012 M Se(IV), when deposition at 0.2 V was

used. As organic substances can be surface and

catalytically active, they were eliminated by UV-di-

gestion, which also reduced Se(VI) to Se(IV)Cu(II), Pt(II), and Fe(III) increased the peak at

concentrations higher than 5109 M, As(III)

decreased it when present in concentration higher

than 3109 M. Method was used for determina-

tion of selenium in sea and lake waters.

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P. Zuman, G. Somer/Talanta 51 (2000) 645665 663

Acknowledgements

This publication was made possible by award-

ing the Senior Fulbright Fellowship to one of us

(P.Z.).

References

[1] A.S. Frank, P. Zuman, Anal. Lett. 15 (1982) 1609 1619.[2] H. Hagisawa, Bull. Inst. Phys. Chem. Res. (Tokyo) 18

(1939) 648656; CA 34 (1940) 49656.

[3] R.J. Gillespie, R. Kapoor, Ind. J. Chem. Sect. A 28A

(1989) 298301; CA 111 (1989) 65111a.

[4] Gmelins Handbuch der Anorganischen Chemie, eighth

ed., Selen B (Syst. No. 10), Gmelin Verlag, 1949, pp.

4472.

[5] G. Feroci, A. Fini, R. Badiello, A. Breccia, Microchem.

J. 57 (1997) 379388.

[6] R. Badiello, G. Feroci, A.J. Fini, Trace Elements Med.

Biol. 10 (1996) 156162; CA 126 (1997) 18213b.

[7] H.L. Clever, S.A. Johnson, M.E. Derrick, J. Phys.

Chem. Reference Data 14 (1985) 632654.[8] M. Koskenlinna, J. Valkonen, Acta Crystallogr. Sect. C

Cryst. Struct. Commun. C51 (1995) 1040 1042.

[9] M. Koskenlinna, J. Valkonen, Acta Crystallogr. Sect. C

Cryst. Struct. Commun. C52 (1996) 491493.

[10] Z. Micka, M. Uchytilova, M. Ebert, Chem. Zvesti 38

(1984) 759770.

[11] M.Z. Makhmetov, K.Z. Rakhimzhanova, Deposited

Doc. Viniti 3227 3280 (1980) 38 pp.; CA 96 (1982)

27656n.

[12] H. Aydin, G. Somer, Talanta 36 (1989) 723 726.

[13] W.C. Bray, G.M. Mackay, J. Am. Chem. Soc. 32 (1910)

914922.

[14] L.B. Agasyan, E.R. Nikolaeva, P.K. Agasyan, Zh. Anal.

Khim. 21 (1966) 14701474.[15] L.B. Agasyan, P.K. Agasyan, E.R. Nikolaeva, Vestn.

Mosk. Univ. Ser. II 21 (1966) 9394; CA66 (1967)

43449h.

[16] L.B. Agasyan, P.K. Agasyan, E.R. Nikolaeva, Vestn.

Mosk. Univ. Ser. II Khim 21 (1966) 9698; CA66 (1967)

8082w.

[17] L.B. Agasyan, E.R. Nikolaeva, P.K. Agasyan, Zh. Anal.

Khim. 22 (1967) 904908.

[18] G.A. Kitaev, N.M. Kondrateva, L.I. Roslyakova

(USSR) Deposited Doc. 1980, SPSTL 296 khp-D80;CA

97 (1982) 13899c

[19] P. Verma, V.K. Gupta, J. Ind. Chem. Soc. 63 (1986)

767768.[20] K. Rashid, H.R. Krouse, Can. J. Chem. 63 (1985)

31953199.

[21] V. Hovorka, Collect. Czechoslov. Chem. Commun. 4

(1932) 302318.

[22] S.A. Sadek, G.P. Basmadjian, R.D. Ice, Nuclear Med.

Commun. 3 (1982) 247250.

[23] G.S. Sastry, Acta Cienc. Ind. Chem. 11 (1985) 171 173

CA107 (1987) 88720w.

[24] S. Ball, J. Milne, Canad. J. Chem. 73 (1995) 716724.

[25] S. Rahim, J. Milne, Canad. J. Chem. 74 (1996) 753 759

[26] O. Foss, Acta Chem. Scand. 3 (1949) 435444.

[27] C. Milne, J. Milne, Canad. J. Chem. 74 (1996) 1889

1895.

[28] H.E. Ganther, Biochemistry 7 (1968) 2898 2905.

[29] J.L. Kice, T.W.S. Lee, S. Pan, J. Am. Chem. Soc. 102

(1980) 44484485.

[30] Kice, J.L., Selenium Biol. Med. in: Proc. Int. Symp.2nd, 1980, pp. 1732.

[31] W. Amaratunga, J. Milne, Canad. J. Chem. 72 (1994)

25062515.

[32] H. Afsar, R. Apak, I. Tor, Analyst 114 (1989) 1319

1321.

[33] Y. Saito, M. Chikuma, J. Chromatogr. 635 (1993) 71

74.

[34] Y. Saito, M. Chikuma, A. Iwado, Y. Saito, Fresenius, J

Anal. Chem. 348 (1994) 776777.

[35] R. Inam, G. Somer, Anal. Sci. 13 (1997) 653656.

[36] T. Nakagawa, Y. Hasegawa, H. Tanaka, M. Chikuma

H. Sakurai, M. Nakayama, Ganryu Aminosan 8 (1985)

5561; CA105 (1986) 186085w.[37] K. Pandian, S.S. Narayanan, Asian J. Chem. 9 (1997)

859862; CA128 (1998) 75656p.

[38] K. Pandian, S.S. Narayanan, Bull. Electrochem. 13

(1997) 303305.

[39] H. Xu, Y. Cheng, Z. Feng, Huazhong Ligong Daxue

Xuebao 19 (1991) 16; CA116 (1992) 146261w.

[40] A. Naganuma, J. Tabata, N. Imura, Res. Commun

Chem. Pathol. Pharmacol. 38 (1982) 291299; CA98

(1983) 12598n.

[41] K. Sumino, R. Yamamoto, T. Mio, S. Kitamura, Mina

matabyo ni Kansuru Sogoteki Kenkyu (1979) 27 29

CA97 (1982) 194109p.

[42] S.V. Ivanov, E.M. Tsikovkin, V.B. Zdolnik, Dopov

Nats. Akad. Ukr. 9093 (1995); CA124 (1996) 214444a

[43] M. Czauderna, K. Samochocka, Tetrahedron 38 (1982)

24212423.

[44] J.L. Kice, H. Slebocka-Tilk, J. Am. Chem. Soc. 104

(1982) 71237130.

[45] H.E. Ganther, Biochemistry 10 (1971) 40894098.

[46] L. Schwaer, K. Suchy, Collect. Czech. Chem. Commun

7 (1935) 2532.

[47] J.J. Lingane, L.W. Niedrach, J. Am. Chem. Soc. 70

(1948) 41154120; 71 (1949) 196204.

[48] K. Jonas, Vespremi Vegyipari Egyetem Kozlemenyie 3

(1959) 161167; CA55 (1961) 13134a.

[49] K. Jonas, Acta Chim. Hungar. 25 (1960) 379 389.

[50] M. Goto, D. Ishii, J. Electroanal. Chem. 40 (1972

303309.

[51] E.F. Speranskaya, Zh. Anal. Khim. 17 (1962) 347 354

[52] E.F. Speranskaya, Izv. Vyssh. Ucheb. Zaved. Khim

Khim. Tekhnol. 11 (1968) 11211124.

8/11/2019 216804

20/21

P. Zuman, G. Somer/Talanta 51 (2000) 645665664

[53] A.H. Shafiqul Alam, O. Vittori, M. Porthault, J. Elec-

troanal. Chem. 61 (1975) 191204.

[54] H.A. Azab, A. Hassan, R.M. Hassan, Ann. Chim.

(Rome) 76 (1986) 5159.

[55] S.A. El-Shatoury, A. Hassan, H.A. Azab, Bull. Fac. Sci.

Assiut Univ. 16 (1987) 1930.

[56] G.S. Deshukh, V.N. Choudhury, Fresenius Z. Anal.

Chem. 270 (1974) 205.

[57] A. Hasan, H.A. Azab, M.S. Saleh, A.B.M. El-Nady,

Bull. Poll Acad. Sci. Chem. 40 (1992) 89108.

[58] S.A. El-Shatoury, Aswan Sci. Technol. Bull. 16 (1995)8595; CA123 (1995) 357753r; Anal. Letters 29 (1996)

15631574.

[59] S.A. El-Shatoury, Bull. Fac. Sci. Assiut Univ. B 24

(1995) 17 30; CA124 (1996) 248965m; Collect. Czech.

Chem. Commun. 61 (1996) 992998.

[60] G.D. Christian, E.C. Knoblock, W.C. Purdy, Anal.

Chem. 35 (1963) 11281132.

[61] G.D. Christian, E.C. Knoblock, W.C. Purdy, Anal.

Chem. 37 (1965) 425427.

[62] M. Brezina, P. Zuman, Polarography in Medicine, Bio-

chemistry, and Pharmacy, Interface, New York, 1958,

pp. 470506.

[63] M. Zelic, L. Sipos, M. Branica, Croatica Chem. Acta 58(1985) 4355.

[64] Azab, H.A., Hassan, A., Hassan, R.M., Bull. Soc. Chim.

Fr. Chem. Ser. 1987, 2728.

[65] R. Inam, G. Somer, P. Zuman, J. Electroanal. Chem.,

submitted.

[66] Sharma, S.K., Gaur, J.N., Ind. J. Chem. 1980, 19A,

816817.

[67] G.E. Batley, Anal. Chim. Acta 187 (1986) 109 116.

[68] R. Inam, G. Somer, P. Hopke, P. Zuman, J. Electroanal.

Chem., submitted.

[69] P. Zuman, R. Inam, G. Somer, Electrochem. Commun.,

submitted.

[70] R. Inam, J. Somer, P. Zuman, unpublished results.[71] F. Umland, W. Wallmeier, J. Less-Common Met. 76

(1980) 245254.

[72] G.D. Christian, J. Buffle, W. Haerdi, J. Electroanal.

Chem. 109 (1980) 187194.

[73] W. Kemula, Z.R. Grabowski, Roczniki Chem. 26 (1952)

266280.

[74] A.M. Shefiqul Alam, O. Vittori, M. Porthault, Anal.

Chim. Acta 87 (1976) 437444.

[75] G.P. Bound, S. Forbes, Analyst 103 (1978) 176 179.

[76] H. Aydin, G.H. Tan, Analyst 116 (1991) 941 945.

[77] R. Inam, G. Somer, Anal. Sci. 14 (1998) 399 403.

[78] Y. Morimoto, F. Suzuki, J. Electrochem. Soc. Jpn. 23

(1955) 544547.

[79] A. Hassan, Monatsh. Chemie 122 (1991) 605 616.

[80] E. Hasdemir, G. Somer, Analyst 115 (1990) 297 299.

[81] A.G. Howard, M.R. Gray, A.J. Waters, A.R.

Oromiehie, Anal. Chim. Acta 118 (1980) 8791.

[82] R. C ervenka, M. Korbova, Chem. Listy CA49 (1955)

11581161; 49 (1955) 14566.

[83] W. Dunhu, Z. Diyang, L. Xiaoming, Analyst 114 (1989)

793797.

[84] P. Nangniot, J. Electroanal. Chem. 12 (1966) 187 193.

[85] G. Somer, A. Kocak, Analyst 118 (1993) 657 659.

[86] D. Wang, J. Pan, Anal. Proc. 24 (1987) 329.

[87] Y. Wan, Fenxi Shiyanshi 7 (1988) 32 33; CA111 (1989

3617c.

[88] Z. Shuliang, Y. Qing, Anal. Lett. 21 (1988) 159 167.

[89] X. Liu, L. Zhu, Y. Zhao, Y. Zhu, L. Yao, Z. Han

Lihua Jianyan, Huaxue Fence 27 (1991) 5456; CA115

(1991) 4531q.[90] G. Jarzabek, Z. Kublik, J. Electroanal. Chem. 137

(1982) 247259.

[91] M. Zelic, M. Branica, Electroanalysis 2 (1990) 455 462

[92] L. Campanella, T. Ferri, J. Electroanal. Chem. 165

(1984) 241249.

[93] G. Mattsson, L. Nyholm, L.M. Peter, J. Electroanal

Chem. 347 (1993) 303326.

[94] F. Vajda, Acta Chim. Acad. Sci. Hungar. 63 (1970

257265.

[95] S.B. Adeloju, A.M. Bond, M.H. Briggs, H.C. Hughes

Anal. Chem. 55 (1983) 20762082.

[96] P. Deldime, J.-P. Hartman, Anal. Lett. 13 (1980) 105

117.[97] G. Henze, P. Monks, G. Tolg, F. Umland, E. Wessling

Fresenius Z. Anal. Chem. 295 (1969) 16.

[98] C.M.G. Van Den Berg, S.H. Khan, Anal. Chim. Acta

231 (1990) 221229.

[99] B.X. Huang, H.C. Zhang, G.G. Pu, F. Yin, S.-Ch

Zheng, H. Yang, Anal. Lett. 18 (1985) 279285.

[100] H. Aydin, G. Somer, Anal. Sci. 5 (1989) 5155.

[101] V. Stara, M. Kopanica, Anal. Chim. Acta 208 (1988)

231236.

[102] S. Forbes, G.P. Bound, T.S. West, Talanta 26 (1979)

473477.

[103] W. Holak, J.J. Specchio, Analyst 119 (1994) 2179 2182

[104] G. Somer, M.S. Karacan, Electroanalysis 6 (1994) 527 530.

[105] V. Baltensperger, J. Hertz, Anal. Chim. Acta 172 (1985)

4956.

[106] W. Holak, Anal. Chem. 52 (1980) 2189 2192.

[107] G. Henze, Microchim. Acta [Wien] 2 (1981) 343 349.

[108] J. Wang, J.I. Lu, Anal. Chim. Acta 274 (1993) 219 224

[109] Z. Shen, R. Lu, Q. Shi, Yingyong Huaxue 1 (1984

7881; CA103 (1985) 19336f.

[110] S.B. Adeloju, A.M. Bond, M.H. Briggs, Anal. Chem. 56

(1984) 23972401.

[111] S.B. Adeloju, A.M. Bond, H.C. Hughes, Anal. Chim

Acta 148 (1983) 5969.

[112] R.B. Ahmad, J.O. Hill, R.J. Magee, Analyst 108 (1983)

835839.

[113] A.T. Russu, L.S. Kopanskaya, V.A. Voznesenskii, Zh

Anal. Khim. 30 (1975) 11531159; (Engl.) pp. 971976

[114] G. Somer, H. Aydin, Analyst 110 (1985) 631 633.

[115] M.S. Karacan, G. Somer, S. Kalayci, Microchim. Acta

127 (1997) 6770.

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