Page 41 - Geochemistry of Oil Field Waters

30 ANALYSIS OF OILFIELD WATERS

When the system is not under standard conditions, the redox potential is
expressed by the Nernst equation:

RT
Eh=Eo+ - log oxidized species)
nf (reduced species)
where R is the gas constant (1.987 calories per degree mole) and T is the
temperature in degrees Kelvin. Geochemical literature and biochemical litera-
ture such as that of Pourbaix (1949) use increasing positive potential values
to represent increasing oxidizing systems, and decreasing potential values to
represent reducing systems. The sign of Eh used in this manner is opposite to
standard American practice in electrochemistry.

Reagents. An Eh standard' which can be used is a solution of M/300
K3Fe(CN)6 and M/300 KqFe(CN), in M/10 KC1 (Zobell, 1946). The Eh of
this mixture is 0.430 V at 25'C.

Equipment. A pH meter equipped with a thermometer, a glass electrode, a
calomel electrode, a platinum electrode and a thermocompensating elec-
trode.
Eh flowchamber, a design similar to Fig. 2.5 can be used.

Procedure. Buff the platinum electrode lightly with a fine abrasive cloth
and wipe it carefully with a dry soft tissue. Install the glass electrode, the
calomel electrode, the platinum electrode, the thermocompensator, and the
thermometer in the flowchamber. Standardize the instrument using the Eh
standard.
Connect a line to the wellhead or waterline and install an oil-water
separator if oil and water both are present. Connect the flowchamber to the
waterline, allowing the water to flow into the bottom and out the top. Make
certain that all air bubbles are excluded at the top.
Take at least three readings of the Eh (in mV), and the temperature at
10-minute intervals. These readings should agree; if they do not, continue
making readings until three successive readings do agree. Make certain the
water is continually flowing, that there are no air bubbles in the flow-
chamber, and that the solution is being stirred. It may be necessary to
remove and rebuff the platinum electrode.

Calculation. Because a thermocompensator is used in determining the pH, a
temperature correction need not be made. However, if a thermocompensator
is not used, a temperature correction should be made.
The Eh value is obtained by algebraically adding the measured voltage E
and the voltage of the constant voltage reference electrode, which in this
case is the saturated calomel electrode. The potential of the saturated
calomel electrode at 25OC is 0.242 V. Therefore, if the millivolt reading of
the sample is +300: