Page 76 - Geochemistry of Oil Field Waters

64 ANALYSIS OF OILFJELD WATERS

subject to few interferences except from calcium, but by using the chromate
precipitation, calcium is eliminated and barium is concentrated.

Reagents. The necessary reagents are barium standard solution, 1 mg/ml;
ammonium chromate solution (dissolve 10 g of ammonium chromate in
distilled water and dilute to 100 ml); 10% ammonium acetate aqueous solu-
tion; nitric acid (4N); n-propanol; acetic acid; and synthetic brine solution
(use carbon dioxide-saturated distilled water and dissolve the following
amounts of constituents in 1 liter of water: sodium bicarbonate, 0.4 g;
sodium chloride, 61 g; potassium. chloride, 5 g; calcium chloride, 19 g; mag-
nesium chloride, 12 g; and strontium chloride, 5 g).

Procedure. Transfer an aliquot of the sample containing 0.5-15 mg of
barium to a 100-ml beaker, add 1 ml of the ammonium acetate solution, 10
ml of the ammonium chromate solution, and adjust the pH to 4.6 using
acetic acid. Cover the beaker with a watchglass; heat the solution to near
boiling (90°C), remove from the hotplate, and allow to stand for 1 hour.
Filter the solution through a 0.45-pm membrane filter using vacuum. Take
care to transfer all of the precipitate from the beaker to the filter funnel. Use
ammonium chromate solution rather than distilled water to aid in this trans-
fer.
Wash the precipitate with 50 ml of ammonium chromate or until stron-
tium and calcium are absent. Wash the precipitate with 50 ml of hot water to
remove excess chromate.
Add 5 ml of 4N nitric acid to the filter and swirl the solution on the filter
gently to dissolve the precipitate. A clean test tube should be placed below
the filter to catch the dissolved precipitate. When all of the precipitate is
dissolved, turn on the vacuum and catch the solution in the test tube. Repeat
this procedure using an additional 5 ml of 4N nitric acid.
Transfer the solution from the test tube to a 50-ml volumetric flask.
Carefully wash the test tube with two 5-ml portions of water. Add 25 ml of
n-propanol, dilute to 50 ml volume with water, and mix the solution
thoroughly. Burn the sample in the flame spectrophotometer and record the
emission intensity at 873 mp and the background at 900 mp.
Prepare calibration curves by adding up to 25 mg of barium to 10 ml
portions of the synthetic brine followed by analysis according to the fore-
going procedure, and use in the calculation.
Calculation:

mg Ba x 1,000 = mg/l Ba+’
ml sample