Page 126 - Geochemistry of Oil Field Waters

114 ANALYSIS OF OILFIELD WATERS

add additional concentrated hydrochloric acid if necessary, and bubble
sulfur dioxide gas into the solution for 3 minutes. If the solution remains
clear, there is less than 25 pg of selenium present. Filter the solution through
a 5-ml micropore filter.
Compare the color in the crucible with a series of color standards com-
prising 3-20 pg of selenium. These cofar standards are prepared with known
amounts of selenium and will give the fobwing colorations (in pg of
selenium): 3 - very pale yellow; 6 - very pale orange; 10 - pale orange; 15
- orange; and 20 - red orange.
Barium

Qualitative test

This test can be used to detect barium and strontium in an oilfield brine.
It is possible to detect barium and strontium individually by using chromate
to precipitate the barium.
Transfer an aliquot of brine to a test tube, add a few millimeters of 0.5%
aqueous sodium rhodizonate solution, stopper the tube, and shake the
mixture vigorously. Barium and/or strontium is present if a bright red, a
brownish-red, or a yellow-red precipitate forms. The deeper brown indicates
barium, while the lighter yellow may indicate strontium. In any event, if a
precipitate forms, barium and/or strontium is present. A series of standards
can be prepared to help in determining the approximate amounts present.
To differentiate between barium and strontium, a few milliliters of a 10%
aqueous solution of ammonium chromate can be added to a sample brine
30-60 minutes before the sodium rhodizonate solution is added. The more
soluble strontium chromate will react with the rhodizonate while the less
soluble barium chromate will not.

GRAVIMETRIC METHODS
Gravimetric methods involve isolating a compound and determining its
weight. Their use can involve considerable time because preliminary separa-
tions often are necessary to remove interfering elements; e.g., to determine
barium as the sulfate, all strontium should be removed before the final
precipitation of the barium sulfate. One constituent present in most oilfield
waters that has resisted development of a good instrumental method of
analysis is sulfate, and perhaps the most accurate method to determine
sulfate in oilfield waters is still the gravimetric method.

Sulfate

Sulfate is precipitated as barium sulfate from an acid solution. The preci-
pitate is baked, cooled, and weighed.