Page 127 - Geochemistry of Oil Field Waters

GRAVIMETRIC METHODS 115

Reagents. Hydrochloric acid, concentrated.
Barium chloride solution, 10% aqueous.

Procedure. Use an aliquot that will produce no more than 100 mg of precipi-
tated barium sulfate. Dilute the aliquot to 250 ml with distilled water and
add 1 ml of hydrochloric acid. If the sample volume itself is larger than 250
ml, add 1 ml of hydrochloric acid per 250 ml of volume. Heat to boiling and
add an excess of hot, 10% barium chloride solution while stirring. Cover the
solution and allow it to stand for about 4 hours at a temperature of about
85OC. Filter through a very retentive filter paper such as Munktells No.OOH
or Whatman No.42, and wash with hot water until the filtrate is chloride
free. Place the filter plus precipitate in a tared crucible, char slowly without
igniting, and bake at 800°C for 1 hour. Place the crucible in a desiccator to
cool and then weigh.

Calculation:

Barium

Interest in the knowledge of the barium concentration in most
petroleum-associated waters has spurred the development of several types of
methods to determine barium. Perhaps the most rapid but least accurate
method is the turbidimetric method, which measures the turbidity of the
sample caused by precipitated barium sulfate after addition of excess sulfate.
The gravimetric method also measures precipitated barium sulfate or more
preferably barium chromate. It is a more time-consuming method than the
turbidimetric method, but will yield more accurate results. The double
precipitation as chromate reduces the interference from calcium and stron-
tium.

Reagents. Ammonium chromate solution: dissolve 10 g of ammonium
chromate in water, dilute to 100 ml, and filter.
Ammonium chromate wash solution: dissolve 5 g of ammonium chromate
in water and dilute to 1 liter. Adjust the pH of this solution to 4.6 with
ammonium acetate or acetic and filter.
Ammonium acetate solution: dissolve 30 g of ammonium acetate in water,
dilute to 100 ml, and filter.
Nitric acid, 4N: cautiously add 30 ml of concentrated nitric acid to 90 ml
of water.
Ammonium hydroxide, concentrated.
Hydrogen peroxide, 30% solution.

Procedure. Because iron will interfere if present, it should be removed by