Page 59 - Geochemistry of Oil Field Waters

TITRlMETRIC METHODS 47

add three drops of ammonium molybdate solution, 0.5 g sodium fluoride (if
iron is present), and 0.5 g potassium iodide, mix until dissolved, and acidify
with 15 ml of 6N hydrochloric acid. Titrate with 0.OW sodium thiosulfate
using starch indicator. Disregard any return of blue color after the endpoint.
Record this titration for the bromide calculation.

Calculations. Iodide: ml of Na, S2 O3 for sample - ml of Na, S2 O3 for blank =
corrected ml of Na, S2 O3 :

(ml x N) Na2S203 x 21,150-
- mg/l I-
ml sample
Bromide: ml of Na2 S2 O3 for sample - ml of Na2 S2 O3 for blank = corrected
ml of Na2S203 :
(ml x N) Na, S203 x 13.320
ml sample - mg/l I- x 0.63 = mg/l Br-
The precision and accuracy of the method are about 3% and 676, respec-
tively, of the amounts of bromide and iodide present.

Oxygen

The solubility of a gas varies directly with pressure and inversely with
temperature and usually is reduced by the presence of dissolved minerals.
Most petroleum-associated waters contain little or no dissolved oxygen in
situ at depth. Knowledge of the dissolved oxygen content of waters that are
to be reinjected for waterflooding or disposal is needed to determine treat-
ment required to prevent corrosion. Instrumental and wet chemical methods
(American Petroleum Institute, 1968) are available for the determination of
dissolved oxygen. Instrumental methods usually are modifications of the
rotating platinum electrode method (Marsh, 1951), but with them the residual
current (when no oxygen is present) is difficult to determine. The modified
Winkler method probably is the most accurate wet chemical method available
(Watkins, 1954).
In the Winkler method for quantitatively determining dissolved oxygen in
water, a glass-stoppered bottle is completely filled with the water to be
tested. Manganous sulfate (MnS04 ) and potassium hydroxide (KOH) are
added, forming a precipitate of manganous hydroxide (Mn(OH), ) in accor-
dance with the following reaction:

MnS04 + 2KOH + Mn (OH), + K2S04
The manganous hydroxide combines with the oxygen dissolved in the
water to form a higher oxide of uncertain composition, assumed to be man-
ganese hydroxide (MnO(OH), ), as follows:

2Mn (OH), + 0, + MnO (OH),