Page 109 - Geochemical Remote Sensing of The Sub-Surface

86 S.M. Hamilton

VOLTAIC CELLS

Since various authors have referred to the electrochemical mechanism occurring
around ore bodies as a geobattery or natural voltaic cell, it is appropriate to introduce the
concept of voltaic cells. Voltaic cells are man-made electrical circuits in which the
impetus for current flow comes directly from the chemical energy of partially-separated
reactants within the cell. All batteries are voltaic cells.
The basic components of a voltaic cell are a wire, two electrodes and two partially-
separated solutions. When the electrodes are placed in their respective solutions and the
wire is used to connect them, a spontaneous flow of electrons occurs in the wire from
one electrode to the other. The impetus for current flow comes from the difference
between the oxidation potentials of the electrodes and the solutions, or between the
electrodes themselves; or between the two solutions in which the electrodes are
immersed. A chemical redox reaction occurs between these separated species such that
the oxidation half of the reaction occurs in one solution and the reduction half occurs in
the other. The partial separation of the solution can be accomplished by a membrane or a
salt bridge, which allows an electrolytic connection but does not allow a general mixing
of the two solutions. Within the cell, electrical current moves in the form of free
electrons in the wire and as ions in the electrolyte.
At the electrodes, ions or neutral species change their oxidation state and either
accept electrons from, or discharge electrons into, the electrodes. At the anode, electrons
are discharged into the electrodes by conversion of the metallic electrode materials to a
positively-charged ion or by conversion of a negatively-charged species to a neutral
species, gas or ion with a higher oxidation state. At the cathode electrons are usually
discharged into solution by either conversion of a positively-charged ion to a neutral
species or gas, or by conversion of a neutral species to an anion with a lower oxidation
state. In all voltaic cells there must be reduction of species at the cathode and oxidation
at the anode and ions must be discharged, or attenuated, or both, at the electrodes. In
order to maintain electrical neutrality, the creation of an ion at an electrode must be
accompanied by the simultaneous removal of an oppositely-charged ion at the other
electrode. To prevent local charge imbalances this process also requires the net
migration of ions away from the electrode at which they are liberated into solution and
toward the electrode at which they are attenuated.
In general, anions move toward the anode and cations toward the cathode, and
therefore charge can be carried by reduced anions toward the anode and oxidised cations
toward the cathode (Hamilton, 1998). However, it is actually the movement of negative
and positive charge that results in electrical current between the electrodes and, in some
cases, reduced cations can move toward and deposit negative charge at the anode, or
oxidised anions can move toward and deposit positive charge at the cathode. Examples
of this include the movement of Fe 2+ toward an anode where it loses an electron to
become Fe 3+ and the movement of Ag(CN)2 toward a cathode where it is reduced to
Ag(s). This apparently paradoxical process of cations moving toward an anode and anions