Page 124 - Geochemistry of Oil Field Waters

112 ANALYSIS OF OILFIELD WATERS

Selenium adsorption on glassware can introduce a significant error. Much
of this adsorption can be eliminated by treating the glassware with a solution
of chlorosilane.

Reagents. Hydrobromic acid, 48%.
Selenium, stock solution: dry some selenium dioxide by placing it in a
desiccator over phosphorous pentoxide for 24 hours. Dissolve 0.141 g of the
dry selenium dioxide in water, add 80 ml of 48% hydrobromic acid, and
dilute to 1 liter with water. 1 ml of this solution contains 0.1 mg of
selenium. (Note: particles of red selenium may appear in this stock solution
after long standing as a result of reduction. When this happens, a new stock
solution must be prepared.)
Selenium solution: pipet 100 ml of the selenium stock solution into a
1-liter volumetric flask, add 80 ml of 48% hydrobromic acid, and dilute with
water. 1 ml of this solution contains 0.01 mg, or 10 pg of selenium.
Sulfur dioxide selenium free.
Hydrochloric acid, concentrated.
Sulfuric acid, concentrated.
3,3'-diaminobenzidine hydrochloride: dissolve 0.25 g of 3,3'-diaminoben-
zidine hydrochloride in 50 ml of water. Prepare a fresh solution each day.
Formic acid, 2.5M: dissolve 11.5 g of formic acid in water, and dilute to
100 ml with water.
Toluene, spectro-grade.
Ammonium hydroxide: dilute 10 ml of concentrated ammonium hy-
droxide to 100 ml with water.
Barium chloride solution: dissolve 5 g of barium chloride in 100 ml of
water.
EDTA solution, 0.1M: dissolve 37.225 g of disodium ethylenediamine-
tetraacetate in water and dilute to 1 liter.

Procedure. Pipet an aliquot of brine (50 ml or less) into a 100-ml volumetric
flask and dilute to volume with concentrated hydrochloric acid. If desired,
the detection limit can be increased by first concentrating the brine by
careful evaporation after adjusting the pH to 2 with hydrochloric acid. Mix
the solution and allow it to stand until most of the sodium chloride precipi-
tates.
Carefully withdraw 50 ml of the supernatant clear liquor into a 150-ml
beaker and add 10 ml of concentrated hydrochloric acid. Heat the mixture
to near boiling for 10 minutes. Place the beaker in an ice-water bath beneath
an exhaust hood, let the mixture cool to the temperature of the ice water,
and then bubble sulfur dioxide gas rapidly into the solution for about 8
minutes. If a heavy turbidity develops, filter the solution through a micro-
pore filter. Wash the precipitate with 20 ml of cold water if a 30-ml crucible
is used, or with 5 ml if a 1.5-ml crucible is used. Take care that no air is