Page 123 - Geochemistry of Oil Field Waters

COLORIMETRIC METHODS 111

Potassium nitrite solution: dissolve 10 g of potassium nitrite in 100 ml of
distilled water.
Sulfuric acid, 9N.

Procedure. Pipet a sample containing less than 3 mg of iodide into a separa-
tory funnel, and add three drops of the bromphenol blue solution and a few
drops of 9N sulfuric acid until the indicator turns yellow. Add 10 ml of car-
bon tetrachloride and 1 ml of a 10% aqueous potassium nitrite solution, and
vigorously mix the combined phases. Extract the carbon tetrachloride phase
into a glass-stoppered cylinder. A violet color in the carbon tetrachloride
indicates iodine. Repeat the extractions with 5-ml portions of carbon
tetrachloride until all of the iodine is extracted. Dilute the combined
extracts to 25 ml with carbon tetrachloride and measure the absorbance
using a spectrophotometer at a wavelength of 517 mp. Use a calibration
curve prepared with standard iodide solutions to determine the milligrams of
iodide in the sample.

Calculation:
mg I (from curve) x 1,000 = mg/l r
ml sample

Selenium

Selenium can be reduced to the elemental form with sulfur dioxids
(Collins et al., 1964), hydrazine, hydroxylamine hydrochloride, hypo-
phosphorous acid, ascorbic acid, and stannous chloride. From hydrochloric
acid solutions exceeding 8N, selenium is precipitated free of tellurium when
the reducing agent is sulfur dioxide. Both selenium and tellurium are precipi-
tated by sulfur dioxide from 3 to 5N hydrochloric acid solutions. Traces of
nitric acid should be removed before sulfur dioxide reduction. When precipi-
tating selenium, it is important that the temperature of the solution be kept
below 30°C because the volatile selenium monochloride easily can form and
be lost. A large excess of reducing agent helps to prevent loss of the mono-
chloride.
Selenium can be determined semiquantitatively by comparing the color of
the red amorphous form, or it can be adjusted to the quadrivalent form,
reacted with 3,3’-diaminobenzidine to form the monopiazselenol, and quan-
titatively determined spectrophotometrically. If sufficient selenium is
present, it also can be determined gravimetrically .
Selenate (VI) can be reduced to selenite (IV) by heating in concentrated
hydrochloric acid. Selenite is the only form that reacts with
3,3’-diaminobenzidine; the reaction is :

% N w - NH2 + H, SeO, + N = NH2 + 3Ha0
ii
H2 N NH2 SeN NH2