Page 128 - Geochemistry of Oil Field Waters

116 ANALYSIS OF OILFIELD WATERS

adding a few drops of hydrogen peroxide to the sample before heating to
near boiling, adding ammonium hydroxide dropwise, and stirring until the
odor of ammonia is faint but distinct. Heat to boiling to remove excess
peroxide and flocculate any precipitate, and then filter out the iron
hydroxide.
To an aliquot of iron-free filtered water (the water should be filtered even
if iron is not specifically removed) containing less than 500 mg of barium
and strontium, add acetic acid until the pH is 4.6. Then add 10 ml of
ammonium chromate and 1 ml of ammonium acetate. Readjust the pH to
4.6. The final volume should be about 200 ml. Boil the mixture for 5
minutes, stirring occasionally. Remove the mixture from the heat, cool it
fairly rapidly to room temperature, and allow it to stand at room tempera-
ture for 1 hour.
Filter the solution through a fine porosity filter. Wash the precipitate
from the beaker into the filter with the ammonium chromate wash solution.
Since a second precipitation is made, it is not necessary to police the beaker.
Wash the precipitate on the filter with 50 ml or more of ammonium
chromate wash solution or until calcium and strontium are absent.
Dissolve the precipitate in 3 or 4 ml of 4N nitric acid. Transfer the
dissolved precipitate back to the beaker and repeat the precipitation. The
same filter can be used, but make sure that it is acid-free.
Dry the second precipitate for 1 hour at llO°C or until it reaches a
constant weight.

Calcuhtions. Weigh the barium chromate and calculate barium as follows:
mgBaCr04 x 542 = mg/l Ba+ *
ml sample

Precision and accuracy. The precision and accuracy of this method with
optimum conditions are 1% and 2% respectively, of the amount of barium in
the sample.

OTHER METHODS

The approximate concentration of sodium in an oilfield water can be
calculated by using a knowledge of the amounts of other major cations and
anions in the sample. Likewise, the dissolved solids concentration in an
oilfield water can be calculated.

Sodium
The practice of determining sodium by calculation does not give an accu-
rate sodium value. For example, this value is calculated after determining all
of the major common anions plus two or more cations, usually calcium and