Page 58 - Geochemistry of Oil Field Waters

46 ANALYSIS OF OILFIELD WATERS

Iron, manganese, and organic matter can interfere but are removed in the
procedure. Fluoride is added to mask interference from any remaining traces
of iron.

Reagents. The necessary reagents include a 2% ammonium molybdate solu-
tion; glacial acetic acid; calcium hydroxide; calcium carbonate; 0.05N
hydrochloric acid; 6N hydrochloric acid; potassium iodide; sodium fluoride;
starch indicator solution; 0.OlN sodium thiosulfate (standardize prior to
use); 3.8M sodium formate (prepare fresh daily); saturated bromine water;
and methyl red indicator solution.

Equipment. The necessary equipment includes a mechanical shaker, 200-ml
bottles, a hot-water bath, flasks, pipets, and microburets.
Procedure. To remove iron, manganese, and organic matter from the sample,
add exactly 100 ml of sample to a stoppered bottle. Add 1 g of calcium
hydroxide, and place the mixture in a shaker for 1 hour. Allow the mixture
to stand overnight and filter through a dry folded filter, discarding the first
20 ml that comes through. Brines with specific gravities of less than 1.009
may be filtered without standing overnight. Prepare a blank in the same
manner.
Transfer an aliquot of the filtrate containing 1-2 mg of iodide to a
250-ml Erlenmeyer flask. Add sufficient water to make the total volume 75
ml, and three drops of methyl red indicator. Add 0.05N hydrochloric acid
until the mixture is just slightly acid, add 10 ml of sodium acetate solution,
1 ml of glacial acetic acid, and 4 ml of bromine water, and allow to stand for
5 minutes. Next add 2 ml of sodium formate solution, blow out any bromine
vapor from the neck of the flask, and wash down the sides with water.
When the solution is completely colorless, add 0.2 g of sodium fluoride
and 0.5 g of potassium iodide. Mix until dissolved and add 15 ml of 6N
hydrochloric acid. Titrate with 0.01N sodium thiosulfate using starch indica-
tor. Disregard any return of blue color after the endpoint. Record this titra-
tion volume for the iodide calculation.
Transfer another aliquot of the filtrate containing 1-2 mg of bromide to a
250-ml Erlenmeyer flask and add sufficient water to make the total volume
75 ml. Add 10 ml sodium hypochlorite solution and approximately 0.4 g of
calcium carbonate (or enough so that approximately 0.1 g will remain after
the next step). Adjust the pH of the solution with 3N hydrochloric acid to
between 5.5 and 6.0 and heat in a water bath to 90°C for 10 minutes. (A
small amount of undissolved calcium carbonate should remain at this point.)
Remove the flask and cautiously add 10 ml of sodium formate solution,
return the flask to the water bath, and keep the contents hot for 5 minutes
more and observe the timing very closely. Rinse down the inside of the flask
with a few milliliters of distilled water and allow the solution to cool to
room temperature. (Do not use a cold water bath.) To the ambient solution