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8/20/2019 Spectrophotometric Determination of Hydrogen Sulfide
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732
A N A L Y T I C A L C H E M I S T R Y
sponsorship of the Bureau of Ships, Navy Departmen t, which
initiated this investigation. Thanks ar e expressed to James
McCambridge and Leonard Zoole, under whose supervision this
work was conducted, for their continued interest.
The views expressed by the authors are their own and are not
to be construed as representing the official views of t he Navy
Department. RECEIVEDeptember
15
1948
LITERATURE CITED
1)
Bureau
of Ships,
Navy Department,
~ p e c i f i c a t i o n 1-S-47(IXTT)
(2)
Hanawalt,
J. D.,
Rinn,
H. W.,
and Frevel,
L.
K., ISD. ENG.
(Oct .
1, 1947).
C H E Y . ,ANAL.
ED.,10
467-513 (1938).
Spect rophotomet r ic e terminat ion
o f
Hydrogen Sulf ide
Methylene Blue Method
J A J l E S
K . FOGO AND MILTON POPOWSKY
Southern California Gas Company, Los Angeles, Calif.
Hydrogen sulfide is absorbed from gases and precipitated as zinc sulfide. The
precipitate is then redissolved and allowed to react with p-aminod imethy laniline
in
the presenceof ferric chloride. The optical density of the resulting methylen e
blue solution is measured at 670-millimicron wave length and the corresponding
quantity of sulfide is read from a previously prepared calibration curve. The
method is sensitive to about 3 micrograms and the range up to about
500
micro-
grams.
The procedure is convenient for occasional as well as frequent use.
HE determination of hydrogen sulfide in gases has usually
T een accomplished by iodometric methods 2 , 6 ) . These give
accurate results on appropriate samples but ar e often too insensi-
tive for samples containing very lit tle hydrogen sulfide. A much
more sensitive method is th at of Field and Oldach S ) , n which the
sulfide is converted to bismuth sulfide which is determined photo-
metrically while in suspension. This method, although very
sensitive (1.4 micrograms), is not well suited for the occasional
user, because very rigid control of technique is said to be neces-
sary and all solutions must be protected against oxygen.
The method described herein is a refinement of the methylene
blue method 1, 5,
7 ) .
The technique has been improved by use
of opt imum conditions for the principal reaction and by applying
modern spectrophotomet ry to t he measurement of concentration.
The manipulation is simple and the results are not affected by
minor variations. Th e method has been in successful use in the
form given for several pears.
The hydrogen sulfide is absorbed from
a
stre am of gas in a
sus-
pension formed by adding sodium hydroxide to a solution of zinc
acetate. The stripped gas is then suitably metered. The suspen-
sion then containing the absorbed sulfide as zinc sulfide is treate d
with a n acid solution of p-aminodimethylaniline, followed by the
addit ion of a small amo unt of ferric chloride solution. Bfter time
has been allowed for the formation of the methylene blue, the
solution is diluted in
a
volumetric flask and an aliquot is trans-
ferred to the spectrophotometer for measurement. The corre-
spondin quan tity of sulfide is then determined from a previously
preparef calibration curve, plotted from similar measurements
on methylene blue solutions prepared in the same manncr with
known amounts of sodium sulfide
or
hydrogen sulfide.
The method is sensitive to abou t 3.5 micrograms of sulfide when
used as given. Greater sensitivity could be obtained fairly easily
by appropriate reductions in the volumes of solutions used. The
upper limit of the method as given is abou t 500 micrograms. The
1 Present addreas, Chemistry De part men t, University of Southern Cali-
fornia, Los Angeles, Calif.
precision at such high concentration is somewhat poorer than a t
abo ut 100 to 200 micrograms, where it is *3%.
APPARATUS
The list of appar atus includes the items necessary for taking
two samples simultaneously and thereafter treating them con-
secutively.
Two 250-ml. coarse sintered-glass type gas washing bottles.
(Those made by Corning Glass Works are suggested.)
Two test meters, either wet
or
dry type.
One pipet, 25-ml.
Two pipets, 5-ml.
One graduated cylinder, 250-ml.
One glass tubing cross, 8-mm.
Three tubing clamps, screw type .
Ten mete rs of 7-mm. Tygon tubing.
Three volumetric flasks, 250-ml.
One spectrophotometer or filter photometer.
REAGENTS
KO special care need be take n in the preparat ion of th e rea-
gents. Deviations up to 5y0 in the concentrations given are al-
lowable. If the diamine used produces a dark colored solution, a
fresh supply should be obtained.
Zinc acetate, c.P.
1%
solution in distilled Kater.
Sodium hydroxide, c.P. 12y0 olution in distilled water.
Ferric chloride, c.P. 0.023 molar solution in 1.2 molar hydro-
chloric acid.
gram in
500
ml. of
5.5
molar hydrochloric acid.
p-Aminodimethylaniline sulfate, Eastman white label, 0.5
SAMPLING
A dual sampling procedure in which two samples are obtained
simultaneously is recommended.
The absorption should be done
if
possible directly at the source. Gas samples brough t into the
laboratory in metal or rubber vessels usually give low results due
to th e reaction of hydrogen sulfide with the metal
or
its oxide or to
its solubility in rubber. The pressure at the source must be at
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V O L U M E 2 1 N O . 6, J U N E 1 9 4 9
least 50 mm. of mercury above atmospheric
or
a pump must be
used t o draw the sample through th e absorption bottle.
One arm of a glass cross is connected to t he source with Tygon
tubing. A 5-cm. length of t ubing is attached t o another ar m and
a screw clamp is placed on th e tubing. Each absorber is then
charged with 130 ml. of 1% zinc acetate solution an d 5 ml. of 12%
sodium hydroxide solution, and the solutions are mixed by swirl-
ing. The parts of the bottles are assembled with petrolatum and
fastened with rubber bands. The inlets of the bottles are con-
nected t o the remaining arms of the cross and screw clamps are
placed on the connecting tubing. A test meter is then connected
to t he outlet of each absorber.
M'ith all screw clamps open, the gas is turne d on a t th e source
a t a rate considerably in excess of the sampling rate. Then the
clamp on t he bleeder arm is slowly closed until gas passes through
the gas washing bottles at a ra te of about 170 liters per hour ( 6
cubic feet per hour). The rates through the two bottles may be
equalized by adjusting the screw clamps on the connecting tub-
ing. The amoun t of sample to be take n should be th at which
will conta in betiveen 35 and 350 micrograms. Where it is neces-
sary to use samples smaller than 50 liters, the sampling rate
should be reduced correspondingly.
If the method is being used to determine the amount of hydr o-
gen sulfide resulting from th e conversion of oth er sulfur com-
pounds to hydrogen sulfide for analysis 4),nly one gas washing
bottle is used and the test meter can be replaced by a simple
flowmeter.
PROCEDURE
After the sample has been passed through the gas-washing bot-
tle, the inlet and outle t of t he bottle a re closed by slipping the
ends of a 25-em. length of tub ing over them. Jus t before begin-
ning the methylene blue reaction the temperatu re of the bottle
and contents is adjusted to 24 3 C.; the temperature of the
diamine reagent should be similarly adjusted. Then the top of
the gas-washing bottle is rai;ed and
25
ml. of diamine reagent are
pipetted int o the bottle. The bottle is closed quickly and the
contents are sni rled until all the precipitate is dissolved. Then
by alternately applying slight pressure and suction on the inlet,
a
small amount of the solution is forced back and forth through the
sinter in order to dissolve any zinc sulfide that may have concen-
tra ted there. When all the sulfide is dissolved, the top is again
raised and 5 ml. of ferric chloride reagent are pipetted into the
bott le, followed by mixing as before. The use
of
pipets designed
forsho rt delivery time rat her t han great accuracy is recommended.
733
Table
I .
Specimen Calibration Data
(Coleman Universal spectrophotometer, wave length
650
millimicrons)
Micrograms/2SO
M I .
Optical Density
Sulfide Inserted,
6 4 . 8
6 4 . 5
129.6
129.6
2 59
239
389
359
0 2 1
0 .22
0.41
0.43
0.80
0 .82
1 .12
1 .16
After the closed bottle is allowed to stand for
10
minutes the blue
solution is transferred to a 250-ml. volumetric flask and diluted t o
the mark with distilled water. Before the optical density is meas-
ured, the solution should be allowed to stand at least 20 minutes
but not more th an 20 hours in a place out of direct sunlight.
A blank solution is made by mixing the same amoun ts of th e
four solutions used above in a 250-ml. volumetric flask and dilut-
ing to 250 ml. with distilled water. This solution should be al-
lowed to age for about 30 minutes before use in the spectropho-
tome ter; the solution may be stored for several days in a dark
or
dimly lighted place.
The optical density
or
transmit tanc e of th e test solution is de-
termined by making the initial adjust ment of the instrum ent
while the cell is filled with the blank solution. Sorma lly, an d for
highest sensi tivi ty, the measurements are made with light of 670-
millimicron nave length. Light of 750-millimicron wave length
may be used if t he solution is unusually opaque.
CA LIBR 4TION
If
the measurem ent of th e optical density of th e test solution is
to be useful, a calibration curve must be prepared by making up
several standards in the manner described above but using care-
fully measured quan titi es of sodium sulfide solution
or
hydrogen
sulfide in place of th e sample.
A
solution of sodium sulfide con-
taining about 20 micrograms of sulfur per milliliter is sati sfacto ry.
The lumps of sodium sulfide should be thoroughly washed imme-
diately before making the solution, in order to remove any
so-
dium sulfite. Oxvnen-free distilled water should be used in mak-
W VE
LENGTH-MILLIMICRONS
Figure 1.
Transmittancy of Methylene Blue Solution
157
microgram. of
sulfur
i n 250 m l .
Spectral band width, 2 to 3 millimicrons.
Cell thickneae, 1.00 cm
I
ing the solution. The solution is standa rdized
iodometrically. Care must be taken throughout
the preparation of the standa rds
t o
protect the
sodium sulfide solution from more than a mini-
mum am ount of contact with oxygen.
The calibration is completed bv measuring
the optical densities of the standard methylene
blue solutions and plotting the values obtained
against the corresponding mass
of
sulfide used
in preparing the 250-ml. solution. The result-
ing curve should be nearly linear in the lower
half of the useful range of concentrations. Speci-
men calibration data are given in Table
I.
Once
made the calibration may be used indefinitely.
Data should be obtained at 670 millimicrons and
also if possible at
710
and 750 millimicrons. The
apparent peak absorption wave-length may vary
somewhat from 670 millimicrons when instru-
ments
of
low spectral purity are used. For
ex-
ample, with the Coleman Universal spectro-
photometer the apparent peak is at 650 milli-
microns; this is apparently due to this instru-
ment's band width
of
about 35 millimicrons.
The absorption spectrum for a methylene blue
solution compared to a blank solution with
a
Beckman Model
DU
spectrophotometer using a
to 3
millimicron band width is shown in Figure
1.
EXPERIMENTAL
The amount
of
methylene blue finally formed
in the reactions involved is a function of the tem-
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934
A N A L Y T I C A L C H E M I S T R Y
Table 11. Effect of Temperature on Yield of Methylene
Temp erat ure, C. 5 20 25 30 40 5 5 75
Relative yield, 76 99 100 98 79 64 43
Blue
Table
111.
Effect
of
Acid Concentration of Diamine
Reagent on Optical Density of Methylene Blue Solutions
Mola rity (HC1) of
Optical density at
diamine reagent' 2 4 5 5 . 5 6 7
8 10
660 millimicrons 0.33
0 . 66
0.69 0 .71 0 .70 0 . 66
0 . 6 3
0.57
a -411solutions contained 222 micrograms of sulfide per 250 ml.
perature and other variables.
A t
higher temperatures the reac-
tion is rapid but greater amount s of hydrogen sulfide escape from
the acid solution into th e vapor space of the gas-washing bo ttle
before reacting; a t low temperatu res little hydrogen sulfide
escapes bu t th e methylene blue reaction becomes
so slow
tha t side
reactions occur to a greater extent . Th e over-all effect of tempera-
ture on the relative yields of methy lene blue fromide ntical reaction
mixtures is shown in Table
I1
Fortunately, the maximum yield
occurs at ab out 24 C. and a reasonable tolerance may be allowed.
Th e effect of final acid concentration on the optica l density of
a methylene blue solution formed from a given amo unt of hydro-
gen sulfide
was
investigated
by
preparing the solutions
as
de-
scribed above bu t with diamine reagents of various acid concen-
trations. All the solutions contained 222 micrograms of sulfide
per 280 ml.; the results are given in Table
111.
Th e effect is be-
lieved to be due largely t o the influence of acidity on the absorp-
tion spectrum of methylene blue rather tha n to influence on the
yield of the reaction.
When the di amine reagent is added to the suspension contain-
ing zinc sulfide, hydrogen sulfide is formed. Some of i t escapes
into the vapor space of th e bottle and is lost. The amount which
escapes is a function of t he solubili ty and the to tal amount pres-
ent . When only small amount s of sulfide were present no hydro-
gen sulfide was detectable over the solution an d this was arbitrar-
ily assumed to indicate complete conversion
t o
methylene blue.
Then solutions were prepared with greater amounts of sulfide and
these solutions were diluted with blank solution sufficiently so
that the diluted solution should have corresponded to the one in
which complete conversion was assumed.
Invariably the optical
densities of the dilu ted solutions were found to be less than t hat
of
the reference solution, indicating a loss.
The results of these ex-
periments are given in Table
\-.
Because corresponding losses
occur in preparing the calibration curve, this effect is not consid-
ered to be important for methylene blue solutions representing
less than
470
micrograms of sulfide in 250 ml. of solution.
This
effect accoun ts for the deviation of the calibration curve from
Beer's law.
Table
IV.
Recovery of Hydrogen Sulfide as Rlethylene
Blue
Sulfide inserted,
microgr ams 35 122 243 366 487 610 730 855
Recovery, 100 99 98 97 96 94 88 80
The reaction time of
30
minutes allowed in the procedure in-
cludes a considerable safety factor. Periodic determinations of
the opt ical density of a solution during the reaction period indi-
cated tha t the reaction
was
just completed after
10
minutes-that
is,
no further increase in the opt ical density was detected afte r 10
minutes. After about
20
hours a decrease due
t o
fading may be-
gin to be measurable.
LITERATURE
CITED
(1)
Almy,
J . Am . Chem.
Soc.,
47, 1381 (1925).
(2) Calif. Natural Gasoline Assoc.. Los Angeles, Calif., Determina-
tion of
Hydrogen Sulfide in Natural Gas, Bull. TS
413, 1943.
(3)
Field and Oldach, IND.KG. H E X . ,
N ~ L .
D.,
8,
665
(1946).
4)
Fog0 and Popowsky,
ATAL.C H E M . 1,734 (1949).
5 ) Mecklenburger and Rosenkranzer,
2 . anorg. Chem. 86, 143
(6) Shaw, ISD.ENG.HEM
N A L .
ED.,
2,
668
1940).
(7)
Sheppard and Hudson,
I b id . 2, 73 (1930).
(1914).
RECEIVE D ugust 30, 1948.
onversion of Sulfur ompounds t Hydrogen
Sulfide
n Ai r, Fuel
G a s , or
Mixtures
JAMES K. FOG01
AND
MILTON POPOWSKY
Southern California Gas Com pany,
Los
Angeles, Calif.
HE
sulfur content of fuel gases is usually determined either
T
y oxidation
or
by hydrogenation. Oxidation methods
5,
8
I O 11)
are capable of accurate results on gases conta ining
as lit tle as 114 micrograms of sulfur per cubic meter (0.005 grain
per
100
cubic feet), but the technique is cumbersome and the
apparatus is likely to be capricious.
Hydrogenation methods
2 ,
4,
, 9)
have some advantages but are limited in scope by the
interference of oxygen, which is plentiful in certain types of gases.
Even the oxidation methods cannot be applied to explosive
mixtures
or
nonflammable gases.
A
method that can be used
on any mixture
of
air and fuel gas became necessary
for
this labo-
ratory in order
t o
determine whether the natu ral gas present in
1 Present address, Chemistry ~ ~ ~ ~ ~ t ~ ~ ~ t ,niversity of southern tali-
Soil gases
was
th at normally present in the soil of certa in areas
or leakage from gas distribution lines which carry natural gas
ornia,
Los
Angeles, Calif.