Page 120 - Geochemistry of Oil Field Waters

108 ANALYSIS OF OILFIELD WATERS

aqueous solution of Brucine hydrochloride), acid, place in a dark area and
allow to cool to 30°C. Determine the absorbance of the sample with a
spectrophotometer at a wavelength of 410 mp.

Arsenic

The determination of arsenic in brines has received little attention despite
its toxic relationship to fish and animals. The arsenic content of sea water
was first investigated by Gautier (1903), who found inconsistent variations.
He attributed the higher amounts found at great depth to volcanic in-
fluences, and the higher amounts found at the surface to evaporation and
disturbances caused by marine animals.
Rakestraw and Lutz (1933) and Gorgy et al. (1948) also studied arsenic in
sea water. They conclude that 50-60% of the arsenic is in the arsenite form,
with 8--10% each of arsenate, dissolved organic arsenic, and arsenic sus-
pended in particulate matter. Smales and Pate (1952) used an activation
analysis method to determine submicrogram quantities of arsenic in sea
water. They found an average of 2.6 pg of arsenic per liter, with a range of
1.6-5.0 pg/l. The water analyzed is believed representative for Atlantic
Ocean water.
The Gutzeit method can be used to analyze a petroleum-associated water
for arsenic (Collins et al., 1961). Arsenic is reduced to arsine with zinc in
acid solution. A yellow to brown stain is produced when AsH3 passes through
paper impregnated with mercuric chloride or mercuric bromide. The color-
ation is produced by AsH(H~B~)~ -
- yellow, As(H~B~)~ brown, and
As2Hg3 - black. By comparing unknowns with a series of standard papers
prepared with known amounts of arsenic, a quantitative estimation can be
made. Papers prepared from mercuric bromide can be preserved for several
months in a dark, dry atmosphere.

Arsenic silver diethyldithiocarbamate method

Arsine gas is liberated from arsenic compounds upon the addition of zinc
in an acid medium (Stratton and Whitehead, 1962). The arsine gas is passed
through a lead acetate scrubber and into an absorbing tube containing silver
diethyldithiocarbamate solution. The arsine and the silver diethyldithiocar-
bamate solution react forming a red color that can be measured spectro-
photometrically.

Apparatus. Arsine generator, scrubber, and absorber.
Spectrophotometer set at the following operating conditions: wavelength
- 535 mp; cells - 10 mm; phototube -blue sensitive; and slit width - 0.02
mm.

Reagents. Standard arsenious oxide solution: dissolve 1.320 g of As203 in