Page 180 - Geochemistry of Oil Field Waters

PHYSICAL PROPERTIES 167

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).
When the system is not under standard conditions, the redox potential is
expressed by the Nernst equation:
R T (oxidized species)
Eh = Eo + - log (reduced species)
nf
where R is the gas content (1.987 cal. degree mole), and T is the temperature
in degrees Kelvin. Geochemical literature and biochemical literature, such as
that of Pourbaix (1950), present increasing positive potential values to repre-
sent 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.
Zobell (1946) established basic procedures for measuring the Eh of
geologic-related materials. The Zobell solution containing 0.003M potassium
ferrocyanide and 0.003M potassium ferricyanide in a 0.1M potassium chlo-
ride solution has an Eh of 0.428 V at 25OC. Minor temperature variations can
be calculated using the equation:

Eh = 0.428+).0022 (t - 25)

where t = temperature of the sample in degrees Celsius.
Garrels and Christ (1965) describe procedures for determining Eh
equilibria of mineral substances. Particularly useful are the procedures de-
scribed for constructing diagrams showing fields of stability for various
mineral substances as functions of pH and Eh. Fig. 5.19 is an Eh/pH
diagram. Such stability field diagrams might be constructed for the sub-
stances comprising petroleum and should be of considerable help in under-
standing the mechanisms of origin, accumulation, and chemical stability of
petroleum. Unfortunately, this approach does not yield simple results
because most oxidation reactions involving hydrocarbons and other
petroleum constituents are not reversible in the usual sense. Furthermore,
thermodynamic data are available for only a small fraction of the large
number of reactions and products that are possible.
Attempts to obtain useful results from Eh measurements in natural media
involve numerous difficulties. In a natural medium, such as petroleum-
associated water, there are many variables, none of which is controlled,
which individually or collectively may have little or great influence on Eh
measurements made on the water. Many chemical substances, such as ferric
or ferrous ions, various organic oxidation-reduction systems, sulfides, and
sulfates, may be present in the water in large or small amounts. Even con-
trolled systems in the laboratory often produce unaccounted-for variances.
In the field, the lack of knowledge of actual participating species may seri-
ously impair proper interpretation of Eh readings. Eh measurements made