Page 39 - Geochemistry of Oil Field Waters

28 ANALYSIS OF OILFIELD WATERS

electrode has a potential of +0.246 V. Electrical connection with the sample
is provided through porous fibers sealed into the immersion end. A
hydrogen-ion-selective glass electrode is normally used as an indicating elec-
trode. The glass electrode has several features that recommend it for pH
measurements. Among the most important are that it is not affected by
oxidizing or reducing substances in the sample and that it can be used to
measure the pH of turbid samples and/or colloidal suspensions. The basic
design is a silver-silver chloride or mercury-mercurous chloride electrode
immersed in a solution of known pH and the whole completely sealed in
glass.
The mechanism by which the glass membrane responds to hydrogen-ion
activity involves absorption of hydrogen ions on both sides of the membrane
proportionally to the activity of the hydrogen ions in solution. The cell for
measuring the pH of a solution is of the following type:

solution of glass solution of
Ago :AgC1 1 I known pH; membrane; unknown pH
glass electrode I 1 Hgo :HgC1

The voltage of the glass electrode is a logarithmic function of the differ-
ence in hydrogen-ion activity of the solutions on either side of the glass
membrane. To measure this voltage an electron-tube voltmeter is used be-
cause the resistance of the glass membrane is so great.
The pH should be determined at the time of sampling. A device similar to
that shown in Fig. 2.5, can be used, or the electrodes can be placed in a
container and then a stream of the sample allowed to flow from the oil-
water separator (Fig. 2.4.) into the container while the pH is measured. If
accurate results are desired, at least two pH buffer solutions should be used
to calibrate the pH meter and electrodes before determining the pH. Because

TABLE 3.IV

pH buffer solutions (pH values of NBS standards from 0-30°C)
Temperature 0.5M Potassium acid 0.05M 0.025M 0.01M
("C) potassium tartrate (sat. at potassium acid potassium dihydrogen sodium
tetroxalate 25OC) phthalate phosphate + 0.025M tetra-
sodium dihydrogen borate
phosphate

0 1.67 - 4.01 6.98 9.46
10 1.67 - 4.00 6.92 9.33
15 1.67 - 4.00 6.90 9.27
20 1.68 - 4.00 6.88 9.22
25 1.68 3.56 4.01 6.86 9.18
30 1.69 3.55 4.01 6.85 9.14