Page 40 - Geochemistry of Oil Field Waters

DETERMINATION OF Eh 29

TABLE 3.V
Performance characteristics of typical pH meters

Normal scale Expanded scale
~~~
Range 0-14 pH (+ 1,400 mV) 1 pH (* 100 mV)
Smallest scale division 0.1 pH (10 mV) 0.005 pH (0.5 mV)
Accuracy f 0.05 pH (5 5 mV) f 0.002 pH (+ 2% of reading)
Reproducibility f 0.02 pH (+ 2 mV) f 0.002 pH (+ 0.2 mV)
Temperature
compensation O-IOO~C (manual or
automatic)
Input impedance > 1014 > 1013
-
the pH probably will fall between 5 and 7, the standard pH buffer solutions
used could be for pH 5 and pH 7.
Standard buffer solutions, covering a range of pH, may be purchased from
almost any chemical supply house and are satisfactory for routine use. Table
3.IV gives a list of NBS buffers (easily made in the laboratory) and the
resulting pH at several different temperatures.
An idea of the effect of temperature on pH may be obtained by observing
temperature versus pH of various buffers shown in Table 3.IV. Theoretically,
the potential response of the electrode system changes 0.20 mV per pH unit
per degree Celsius. Since all pH meters measure potential but read out in pH,
a variable compensation is used. A rough rule of thumb is that temperature
compensation is about 0.05 pH unit per 5' increase in temperature. Perfor-
mance data of a conventional and an expanded scale pH meter are shown in
Table 3.V.

Determination of Eh

The Eh, called the oxidation-reduction potential or the redox potential,
is a measure of the relative intensity of oxidizing or reducing conditions in a
chemical system. It is expressed in volts, and at equilibrium it is related to
the proportions of oxidized and reduced species present. Standard equations
of chemical thermodynamics express the relationships (Collins, 1964).
Eo is the standard potential of a redox system when unit activities of
participating substances are present under standard conditions. Eo is related
to standard free energy change in a reaction by the equation:

AP = -nfEo

where n is the number of unit negative charges (electrons) shown in the
redox reaction and f is the Faraday constant in units that give a potential in
volts (94,484 absolute coulombs). Standard free energy values are given in
texts such as that of Latimer (1952).