Page 51 - Geochemistry of Oil Field Waters

TITR IM ETR IC METHODS 39

Procedure. Calibrate the pH meter to pH 4 and 7 with appropriate buffer
solutions .and recheck the calibration often. Transfer an undiluted 50- or
100-ml sample to a beaker and determine the pH and record it. If the pH is
above 8.1, titrate it to 8.1 with 0.05N hydrochloric acid and record the titer
for the carbonate calculation. Continue the titration to pH 4.5 and record
the titer for the bicarbonate calculation. If salts of organic acids are in the
water sample, special precautions must be taken to separate the bicarbonate
titer from that required for the organic salts. This may be done by extracting
the acids, usually naphthenic, with a neutral organic solvent such as
petroleum ether. If the initial pH is below 4.5, titrate to pH 7 with 0.02N
sodium hydroxide and record the titer for the acidity calculation.
Next reduce the pH to 3.5 with 0.05N hydrochloric acid, and reflux the
sample 5 minutes. Remove the sample and immediately cool in an ice-water
bath. Carefully adjust the pH of the cooled brine to 7 with sodium hy-
droxide while nitrogen is aspirated gently over the top. Add 10 g of mannitol
and titrate the sample back to pH 7 with 0.02N sodium hydroxide. Record
this titer for the borate boron calculation. If more than 1 mg of boron is
present in the titrated sample, the results may be low.

Calculations. If the initial pH was more than 8.1, the titer for carbonate and
bicarbonate is determined:
m1 HC1 30y000 = mg carbonate per liter
ml sample

To convert carbonate to bicarbonate, multiply the carbonate value by
2.03.
If the initial pH is less than 8.1 but more than 4.5, only bicarbonate is
present :
ml HC1 x N x 61,000
ml sample = mg bicarbonate per liter
If the initial pH is below 4.5, the brine is acid:

ml NaOH x N x 50,000
ml sample = acidity as mg CaCO, per liter
If the total titration is equal to the titer found to pH 8.1, only hydroxide
is present:

ml HC1 x N x 50,000
mg sample = acidity as mg CaC0, per liter
The borate boron is calculated by using the titer for borate boron:
ml NaOH x N x 10,820
mg sample = mg borate boron per liter