chapter-23.ppt

8/9/2019 chapter-23.ppt

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Potentiometric Methods A.) Introduction:

1.) Potentiometric Methods: based on measurements of the potential of electrochemical

cells in the absence of appreciable currents (i 0)

2.) Basic Components:

a) reference electrode: gives reference for potential measurement

b) indicator electrode: where species of interest is measuredc) potential measuring device


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B.) Reference Electrodes:

1.) Need one electrode of system to act as a reference against which potential

measurements can be made relative comparison.

esired !haracteristics:a) "nown or fi#ed potential

b) constant response

c) insensitive to composition of solution under study

d) obeys Nernest $%uation

e) reversible


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B.) Reference Electrodes:

&.) !ommon 'eference $lectrodes used in Potentiometry

a) !alomel $lectrode (g in contact with g&!l& *!l)

i. Saturated Calomel Electrode (+!$) very widely used

½ cell: g,g&!l& (satd)- *!l (#M)

½ reaction: g&!l& (s) / &e &g / &!l

ote: response is dependent on !l2

SCE


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b) +ilver,+ilver !hloride $lectrode

most widely used reference electrode system

3g electrode in *!l solution saturated with 3g!l

½ cell: 3g,3g!l (satd)- *!l (#M)

½ reaction: 3g!l (s) / e 3g(s) / !l

Ad!anta"e 4 one advantage over +!$ is that 3g,3g!l

electrode can be used at temperatures 5 60o!

#isad!anta"e 4 3g reacts with more ions

c) Precautions in the 7se of 'eference $lectrodes

need to "eep level of solution in reference electrode

above

level in analyte solution

need to prevent flow of analyte solution into reference electrode‚can result in plugging of electrode at junction erratic behavior

8ycor plug


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C.) Indicator Electrodes:

1.) etects or 'esponds to Presence of 3nalyte

9hree !ommon 9ypes:

a) Metallic ndicator $lectrodesb) Membrane ndicator $lectrodes

c) Molecular +elective $lectrode


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&.) Metallic ndicator $lectrode (;our Main 9ypes)

a) Metallic $lectrodes of the ;irst *ind

i. nvolves single reaction

ii. etection of cathode derived from the metal used in the electrodeiii. $#ample: use of copper electrode to detect !u&/ in solution

½ reaction: !u&/ / &e !u (s)

E ind "i!es direct measure of Cu2$:

$ind < $o!u 4 (0.0=>&,&) log a!u(s),a!u&/

since aCu%s) & 1:

$ind < $o!u 4 (0.0=>&,&) log 1,a!u&/

or usin" pCu & 'lo" aCu2$:

$ind < $o!u 4 (0.0=>&,&) p!u

i!. Problems: not very selective

many can only be used at neutral p metals dissolve in acids

some metals readily o#idi?e

certain hard metals (;e- !r- !o- Ni) do not yield reproducible

results

p@ versus activity differ significantly and irregularly from theory


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b) Metallic $lectrodes of the +econd *ind

i. etection of anion derived from the interaction with metal ion (Mn/)

from the electrode

ii. 3nion forms precipitate or stable comple# with metal ion (Mn/)iii. $#ample: etection of !l with 3g electrode

½ reaction: 3g!l(s) / e 3g(s) / !l $A < 0.&&& 8

E ind "i!es direct measure of Cl ':

$ind < $o 4 (0.0=>&,1) log a 3g(s) a!l,a 3g!l(s)since a A"%s) and a A"Cl%s)& 1

( E o & .222 *:

$ind < 0.&&& 4 (0.0=>&,1) log a!l

i!. 3nother $#ample: etection of $93 ion (BC) with g $lectrode

½ reaction: gB& / &e g(l) / BC $o < 0.&1 8

E ind responds to a+ ,':

$ind < $o 4 (0.0=>&,&) log ag(l) aBC,agB&

since a-"%l) & 1 and E o & .21 *:

$ind < 0.&1 4 (0.0=>&,1) log aBC,agB&


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c) Metallic $lectrodes of the 9hird *ind

i. Metal electrodes responds to a different cation

ii. Din"ed to cation by an intermediate reaction' Alread sa/ detection of E#0A -" electrode %2 nd ind)

ii. !an be made to detect other cations that bind to $93 affecting aBC

i!. $#ample: etect !a by comple# with $93

equilibrium reaction: !aB& !a&/ / BC

3here: f &

$ind < 0.&1 4 (0.0=>&,1) log aBC,agB&

aca&/ . aBC

a!aB&ayC < *f

. aca&/

a!aB&

ote: aBC and $ind now also changes with a!a&/


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d) Metallic 'edo# ndicators

i. $lectrodes made from inert metals (Pt- 3u- Pd)ii. 7sed to detect o#idation,reduction in solution

iii. $lectrode acts as e source,sin"

i!. $#ample: etection of !eE/ with Pt electrode

½ reaction: !eC/ / e !eE/

E ind responds to Ce,$:

$ind < $o 4 (0.0=>&,1) log a!eE/,a!eC/

!. Problems:

electrontransfer processes at inert electrodes are fre%uently

not reversible

do not respond predictably to F reactions in tables


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E.) Membrane ndicator $lectrodes

a) General

i. electrodes based on determination of cations or anions by the selective adsorption

of these ions to a membrane surface.ii. Aften called on +elective $lectrodes (+$) or pon $lectrodes

iii. esired properties of +$Hs

‚minimal soluilit 4 membrane will not dissolve in solution during

measurement

4 silica- polymers- low solubility inorganic compounds

(3g@) can be used‚eed some electrical conducti!it

‚Selecti!el inds ion of interest


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E.) Membrane ndicator $lectrodes

) p- Electrode

i. most common e#ample of an +$

‚ based on use of glass membrane that preferentially binds /

ii. 9ypical p electrode system is shown

‚ 9wo reference electrodes here

‚ one +!$ outside of membrane

‚ one 3g,3g!l inside membrane

‚ p sensing element is glass tip of 3g,3g!l electrode


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iii. p is determined by formation of boundary potential across glass membrane

3t each membranesolvent interface- a small local potential

develops due to the preferential adsorption of / onto the glass

surface.

Si A

Glass +urface


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iii. p is determined by formation of boundary potential across glass membrane

Ioundary potential difference ($b) < $1 &p log a/ (on exterior of probe or

in analyte solution)constant

Selecti!e indin" of cation %- $ ) to "lass memrane


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i!. 3l"ali $rror

‚ / not only cation that can bind to glass surface / generally has the strongest binding

‚ Get wea" binding of Na/- */- etc

‚ Most significant when /

2 or a/ is low (high p) usually p 111&

3t low a/ (high p)- amount of Na/ or

*/ binding is significant increases

the JapparentK amount of bound /


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!. 3cid $rror

‚ $rrors at low p (3cid error) can give readings that are too high

‚ $#act cause not "nown

usually occurs at p 0.=

c) 4lass Electrodes for 5ther Cations

i. change composition of glass membrane

‚ putting 3l&AE or I&AE in glass

‚ enhances binding for ions other than /

ii. 7sed to ma"e +$Hs for Na/- Di/- NC/


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d) Crstalline Memrane Electrodei. ;luoride $lectrode

‚ Da;E crystal doped with $u;&‚ mechanism similar to p electrode with potential developing at two

interfaces of the membrane from the reaction:

Da;E Da;&/ / ;

Solid

(membrane surface)

Solution

ˆ the side of the membrane with the lower a;

becomes positive relative to the other surface:

$ind < c 4 0.0=>& p;


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e) 6i7uid Memrane Electrode

‚ JMembraneK usually consists of organic li%uid (not soluble in sample) held by

porous dis" between a%ueous reference solution and a%ueous sample solution.

‚ Membrane has ability to selectively bind ions of interest

P

'A A

'A A!aA

P

AA'

A'

E8ample: !alcium dial"yl

phosphate Di%uid membrane

electrodes

3t solution,membrane interfaces:

('A)&PAA2&!a &('A&)PAA / !a&/

Organic

(membrane)

Organic

(membrane surface)

Solution

(aqueous sample)

ˆ the side of the membrane with the lower a!a&/

becomes negative relative to the other surface:

$ind < c 4 0.0=>&,& p!a


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e) 6i7uid Memrane Electrode

‚ !an design Di%uid Membrane $lectrodes for either cations or anions

cations use cation e#changers in membrane

anions use anion e#changers in membrane


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iii. $n?yme electrodes (or Iiocatalytic Membrane $lectrodes)

‚ General approach is to use an immobili?ed en?yme

en?yme converts a given molecular analyte into a species that

can be measured electrochemically

en?yme substrate E8amples:

/ p electrode

!A& !A& gas sensing electrode

NC/ NC

/ +$

‚$#ample 4 7rea $n?yme $lectrode

Principal: n presence of en?yme urease- urea (NC)&!A is

hydroly?ed to give NE and /

(NC)&!A / &&A / / &NC

/ / !AE

&NE / &/Monitor amount of - 9 produced

usin" - 9 "as sensin" electrode


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Example 18: The following cell was used for the determination of pCrO4:

SCE||CrO42- (!"# $g2CrO4 (sat%d"|$g

Calculate pCrO4 if the cell potential is -&')*'

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