Sulfide, titrimetric, iodometric
Parameter and Code:
Sulfide, total, I-3840-85 (mg/L as S): 00745
1. Application
1.1 This method may be used to analyze water
and water-suspended sediment containing more
than 0.5 mg/L of sulfide.
1.2 Water-suspended sediment may be
analyzed by this method if sample is shaken
vigorously and a suitable aliquot of well-mixed
sample is rapidly withdrawn.
1.3 Water containing dissolved sulfides readily
loses hydrogen sulfide, particularly if the pH of
the sample is low. Oxygen destroys sulfides by
oxidation, particularly if the pH of the sample is
high. Aeration and agitation of the sample
should, therefore, be avoided. The addition of 2 g
of zinc acetate per liter of water will fix the
sample for several days. Acidic water must be
neutralized before addition of zinc acetate.
2. Summary of method
2.1 This iodometric method does not dif-
ferentiate the forms of the sulfide ion in solution.
2.2 Sulfide is reacted with an excess of iodine
in acid solution, and the remaining iodine is then
determined by titration with sodium thiosulfate,
using starch as an indicator (Kolthoff and others,
1969).
H
+1
S
-2
+ I
2
→
S + 2I
-1
H
+1
I
2
+ 2S
2
O
3
-2
→
S
4
O
6
-2
+ 2I
-1
A blank is treated exactly the same as the
samples. The sulfide concentration is calculated
from the difference between the volume of
thiosulfate required for the blank and the volume
used for the sample.
2.3 This method is similar to that in an arti-
cle published by the American Public Health
Association (1980).
3. Interferences
Reducing substances such as sulfites and
heavy-metal ions react with iodine, which con-
tributes to positive errors. Oxygen and other ox-
idants may react with hydriodic acid to liberate
iodine, which contributes to negative errors.
4. Apparatus
4.1 Buret, 10-mL capacity.
4.2 Flasks, Erlenmeyer, 250-mL capacity.
5. Reagents
5.1 Hydrochloric acid, concentrated (sp gr
1.19).
5.2 Iodine standard solution, 0.010N: Dissolve
6 g iodate-free KI in approx. 25 mL water. Add
1.2690 g resublimed I
2
. When solution is
complete, dilute to 1 L. Standardize with 0.010N
Na
2
S
2
O
3
, using starch as an indicator.
0.010 X mL Na
2
S
2
O
3
Normality of I
2
= ————————————
mL I
2
Adjust the normality of the iodine standard
solution, if necessary, to 0.010 by addition of
small quantities of demineralized water or iodine
as indicated by the first titration. Confirm the
normality by restandardization.
5.3 Potassium iodide, crystals, iodate-free: The
KI can be tested for IO
3
-1
by dissolving about 0.1
g in 5 mL water, acidifying with 1 or
2 drops concentrated H
2
SO
4
(sp gr 1.84) and
adding 2 to 3 mL starch indicator solution.
Immediate appearance of blue color indicates the
presence of IO
3
-1
; slow color formation is caused
by atmospheric oxidation.
5.4 Sodium thiosulfate standard solution,
0.010N: Dissolve 2.482 g Na
2
S
2
O
3
•5H
2
O in car-
bon dioxide-free water and dilute 1 L with car-
bon dioxide-free water. Standardize against KIO
3
as follows: Dry approx. 0.5 g KIO
3
for 2 h at 180
°C. Dissolve 0.3567 g in water and dilute to 1,000
mL. Pipet 25.0 mL KIO
3
solution into a 250-mL
Erlenmeyer flask, then add successively 75 mL
deionized water and 0.5 g iodate-free KI. After
solution is complete, add 10 mL HCl (sp gr 1.19).
Allow the stoppered flask to stand 5 min in the
dark and titrate with Na
2
S
2
O
3
solution, adding
starch indicator solution as the end point is
approached (light-straw color):
0.25
Normality of Na
2
S
2
O
3
= ——————
mL Na
2
S
2
O
3
Adjust the normality of the thiosulfate stand-
ard solution, if necessary, to 0.010 by addition of
small quantities of demineralized water or sodium
thiosulfate as indicated by the first titration.
Confirm the normality by restandardization.
5.5 Starch indicator solution, stable (NOTE 1).
NOTE 1. A convenient substitute for starch
indicator solution is the product thyodene, sold by
Fisher Scientific Co. It can be used in its dry form
and produces an end point similar to that of starch.
6. Procedure
6.1 Shake the sample vigorously and
immediately pipet a volume of sample with ZnS in
suspension containing less than 1.5 mg S
-2
(100.0
mL max) into a 250-mL Erlenmeyer flask, and
adjust the volume to approx. 100 mL.
6.2 Prepare a blank of approx. 100 mL de-
mineralized water, and carry it through the
procedure with the sample.
6.3 Add 10.0 mL 0.010N I
2
and mix.
6.4 Without delay add 10 mL concentrated HCl.
6.5 Immediately titrate the excess 12 with
0.010N Na
2
S
2
O
3
, adding 2 to 3 mL starch in-
dicator solution as the end point is approached
(light-straw color).
7. Calculations
1,000
S
-2
(mg/L) = ———————— X 0.1603
mL sample
X (mL blank titrant-mL sample titrant)
8. Report
Report sulfide, total (00745), concentrations as
follows: 0.5 to 10 mg/L, one decimal; 10 mg/L
and above, two significant figures.
9. Precision
Precision data are not available for this method.
References
American Public Health Association, 1980, Standard methods
for the examination of water and wastewater (15th ed.):
Washington, D.C., p. 448.
Kolthoff, I. M., Sandell, E. B., Meehan, E. J., and Brucken-
stein, S., 1969, Quantitative Chemical Analysis (4th
ed.): New York, Macmillan, p. 857.
