Page 111 - Geochemical Remote Sensing of The Sub-Surface

88 S.M. Hamilton

Na + ions must move away from the anode to redress the loss of negative charge from
solution around it and the gain in negative charge around the cathode. In this particular
cell, there is a net increase in ionic strength around the anode and a decrease around the
cathode. The redox reaction would still occur without the membrane but, in the absence
of a physical separation of the solutions, Cu~s) would plate-out on the surface of the zinc
electrode and no current would flow through the wire.
An electrolytic cell is similar to a voltaic cell except the electrochemical reactions
involved do not occur spontaneously but require the input of current from an external
source. Wires connected to each end of a battery and submerged in a suitable electrolyte
can represent an electrochemical cell. As with voltaic cells, the creation and/or removal
of ions at the electrodes facilitates the transfer of current into and out of solution. If the
electrolytes in solution are redox-inert within the stability field of water (e.g., Na + and
CI ~ and the voltage is over 1.2 volts, the hydrolysis of water may transfer current at the
electrodes:
- at the anode 89 H20 zz> H + + 1/402(g) + e
- at the cathode H + + e ~ 89 H2(g)

Hydrogen ions are created at the anode and removed at the cathode and during the
process oxygen and hydrogen gases are produced at the respective electrodes. If the
voltage of the cell is significantly below 1.2 volts then water cannot be hydrolysed. If the
electrodes are inert and there are no redox-active electrolytes (i.e., those that cannot
change their oxidation state and thereby accept or lose electrons at the electrodes) in
solution, then virtually no current will flow in the cell.
Many voltaic cells can act as electrolytic cells if a power source is used to reverse the
direction of spontaneous electron flow. A lead storage battery, for instance, is a voltaic
cell when discharging and an electrolytic cell when being recharged. The two types of
cell also differ in a number of other ways. The most fundamental difference is that in a
voltaic cell conditions are more chemically reducing around the anode than around the
cathode whereas in the electrolytic cell the situation is reversed (Fig. 3-2). Electrons are
"pulled" into solution at the cathode of a voltaic cell by the oxidising agents in solution
whereas they are "pushed" in at the cathode of an electrolytic cell by an external power
source. Another difference is the nature of their electrical fields within the electrolyte. In
the electrolytic cell, the electrical field is applied; if the current is turned off, the field
will dissipate. In most voltaic cells, either the oxidants or reductants or both are
dissolved species in separate solutions. As such, the potential field between the two
electrodes is not applied but is semi-permanent and results from the differences in
oxidation potential between species in solution, or between these and the electrode
materials. Allowing the flow of current between the two electrodes will modify the shape
of equipotential lines in the field but does not create the field.
Table 3-I shows the standard electrode potentials of a number of redox half-reactions.
Tables of standard voltages can give an approximate idea of the likelihood of redox
reactions occurring spontaneously. Standard voltages are a measure of the oxidation