Page 63 - Geochemistry of Oil Field Waters

TITR IMETR IC METHODS 51

Reagents. The necessary reagents are 0.05N sodium carbonate solution and
phenolphthalein indicator solution.

Procedure. Collect the water sample in the same manner used in taking the
sample to be analyzed for dissolved oxygen. Pipet 100 ml of the water into a
flask and add five drops of phenolphthalein indicator. If the sample turns
red, no free carbon dioxide is present; if it remains colorless, titrate the
sample with the standard sodium carbonate solution to a red endpoint.

Calcula tion :
ml Na2 CO, x N x 22,000
ml sample = mg/l C02

Sulfide

As mentioned above, the bacterial reduction of sulfate causes some
petroleum-associated waters to contain appreciable concentrations of hydro-
gen sulfide. Knowledge of the amount of dissolved sulfide present is neces-
sary for corrosion and water compatibility studies (Watkins, 1954).
The following method depends upon the reduction of iodine by the hy-
drogen sulfide in the brine, as shown by the following equation:

H2S+I2 +2HI+S

Because of the unstable nature of the hydrogen sulfide in solution in waters
and brines, the sulfide is not titrated directly. To prevent the loss of hydro-
gen sulfide to the air, an excess of iodine solution is added, and the sample is
back-titrated with standard sodium thiosulfate solution, in accordance with
the following equation:

2Na2 S2 0, + I2 +. Na2 S4 O6 + 2NaI

Experiments conducted by the US. Bureau of Mines indicate that residual
reducing agents that cannot be removed by aeration or boiling are present in
some oilfield brines. Brine from the Arbuckle (siliceous) Limestone forma-
tion originally containing 96 mg/l hydrogen sulfide showed such residual
reducing agents to equal 9 mg/l of hydrogen sulfide after air has been
bubbled through the brine for 28 hours. This dropped to 4 mg/l after
standing another 24 hours. Further tests in which the hydrogen sulfide was
driven off by boiling indicated the presence of 5 mg/l residual reducing
agents. When the brine was neutralized with hydrochloric acid (using methyl
orange indicator) before boiling, residual reducing agents equal to 2 mg/l
hydrogen sulfide remained.