Page 60 - Geochemistry of Oil Field Waters

48 ANALYSIS OF OILFIELD WATERS

On acidification in the presence of an iodide, the higher oxide of man-
ganese liberates a quantity of iodine stoichiometrically equivalent to the
quantity of dissolved oxygen present in the sample in the following manner:

MnO (OH), + 2H2 SO4 +. Mn(S04 )Z + 3H20
Mn(S04), + 2KI +. MnS04 + K2S04 + I2

The quantity of iodine liberated is determined by titrating an aliquot
portion of the sample with a standard solution of sodium thiosulfate
(Na2S203) using starch solution as an indicator, as shown by the equation:

2Na2 S, O3 + I, +. Na, S4 0, + 2NaI

The iodine modification of the Winkler method depends upon the conver-
sion of any hydrogen sulfide to hydrogen iodide and free sulfur by reducing
the iodine added to the brine. This reaction proceeds as follows:

H,S+I2 +2HI+S

Tests have shown that interfering substances other than hydrogen sulfide
that might be present in oilfield brines also are counteracted by the iodine
added.

Reagents. It is important to use sterile glassware or polyethylene bottles in
preparing and storing reagents for this test to prevent contamination and to
make longer storage of reagents possible without appreciable changes in their
normality.
Iodine solutions, 0.5N and 0.W.
Hydrogen sulfide water: saturate distilled water (which has been boiled
and cooled recently to drive off dissolved oxygen) with hydrogen sulfide gas.
Starch solution.
Manganous sulfate solution: dissolve 480 g of manganous sulfate
(MnS04 *4H2 0) or 400 g of manganous sulfate (MnS04 *2H2 0) in distilled
water, filter, and dilute to 1 liter.
Alkaline iodide solution: dissolve 700 g of potassium hydroxide (KOH) or
500 g of sodium hydroxide (NaOH) and 150 g of potassium iodide (KI), or
135 g of sodium iodide (NaI). in distilled water, and dilute the solution to 1
liter. If a white carbonate precipitate is formed, separate the precipitate by
settling and then siphoning off the supernatant liquid. The solution should
give no color with starch indicator when diluted and acidified, which indi-
cates the absence of nitrates, iodates, and ferric salts.
Sulfuric acid, concentrated.
Sodium thiosulfate solution, 0.1N.
Standard sodium thiosulfate solution, 0.025N.