Page 49 - Geochemistry of Oil Field Waters

TITRIMETRIC METHODS 37

Knowledge of the specific gravity of the sample is necessary to convert the
analytical data determined for the sample from milligrams per liter to parts
per million. In addition, the specific gravity will give an indication of the
amount of dissolved solids present in the sample, as indicated in Table 3.VI.

TITRIMETRIC METHODS

Acidity, alkalinity, and borate boron
If the pH of the water is less than 4.5, the water possesses what is called
“mineral-acid acidity”. The acidity of a petroleum-associated water may
indicate a contaminant because of acid treatment of the well or it could
indicate the presence of various dissolved gases and salts. Most petroleum-
associated waters contain little or no acidity. If a water contains acidity, it
does not contain alkalinity.
The acidity of a water is determined by adding a standard base such as
0.02N sodium hydroxide to the water until the pH of the water is 4.5
(Collins et al., 1961) as monitored with a pH meter. To obtain a value close
to natural conditions, the acidity should be determined at the sampling
point.
The alkalinity of a water is determined by adding a standard acid such as
0.05N hydrochloric acid to the water and recording the volume used to
neutralize it to pH 8.1 and pH 4.5. The amounts of hydroxide, carbonate,
and/or bicarbonate can then be calculated using the relationships shown in
Table 3.VII. Because the alkalinity will change when the sample is exposed
to the atmosphere, the alkalinity should be determined as rapidly as possible
after sampling.

TABLE 3.VII

Relationships for determining alkalinity after neutralization with a standard acid

Volume of standard acid used
OH co3 HC03

P=O 0 0 T
P = < 1/2T 0 2P T- 2P
P = 1/2T 0 2P 0
P> 1/2T 2P-T 2(T-P) 0
P=T T 0 0

P = volume used to titrate to pH 8.1; T = volume used to titrate to pH 4.5 plus
volume used to titrate to pH 8.1.