Page 64 - Geochemistry of Oil Field Waters

52 ANALYSIS OF OILFIELD WATERS

Reagents. The necessary reagents are potassium iodide, standard sodium
thiosulfate, 0.1N and 0.01N; standard iodine solutions, 0.1N and 0.01N; and
starch indicator solution.

Procedure. Collect the sample in a glass-stoppered bottle (approximately
200-mi capacity) in the manner previously described for dissolved oxygen.
Analysis should be made as soon as possible after sampling.
Pipet 5 ml of 0.1N or 0.01N standard iodine solution, depending upon the
hydrogen sulfide concentration expected, into each of two Erlenmeyer
flasks. It may be necessary to use a larger quantity of 0.01N solution if the
hydrogen-sulfide content of the sample is high.
Add approximately 1 g of potassium iodide crystals to each flask. (This
step usually may be omitted in determinations on brine samples because of
the high mineral content of the water.) Add 50 ml of distilled water to the
flask to be used for a blank determination, and pipet 50 ml of the water
sample into the other flask. Titrate both the distilled water blank and the
water sample with standard sodium thiosulfate solution of the same normal-
ity as the iodine solution used, adding 1 ml of starch indicator near the end
of the titration. Record the milliliters of thiosulfate used in each titration.

Calculation. Subtract the milliliters of thiosulfate solution used for the
sample from the milliliters used for the blank and use the difference in the
following formula:
(ml x N) I2 - (ml x N) Na2S203 x 17,000
ml sample = mg/l H2 S

Sulfur compounds
The redox potential of petroleum-associated waters indicates that sulfur
compounds other than sulfate and sulfide may exist in solution. When the
water is brought to the land surface, the change in pressure and temperature
will affect the redox potential and, if the sample is allowed to come into
contact with the atmosphere, the equilibria of the sample will start to change
immediately. Better methods are needed to determine the composition of a
water in situ. The following method can be used to gain a semiqualitative
estimation of the amomts of thiosulfate, sulfite, and sulfide in a water.

Reagents. Zinc carbonate suspension: add zinc acetate to a solution of
sodium carbonate, filter and wash the precipitate with several volumes of
cold water. Prepare the zinc carbonate suspension by vigorously shaking the
precipitate with water.
The other reagents are glycerol iodine, 0.01N; sodium thiosulfate, 0.01N;
starch indicator solution; glacial acetic acid; and formaldehyde.