Instruction Manual CON 6/TDS 6
41
12 CONDUCTIVITY THEORY
Conductance is a quantity associated with the ability of primarily aqueous
solutions to carry an electrical current, I, between two metallic electrodes when
a voltage E is connected to them. Though water itself is a rather poor conductor
of electricity, the presence of ions in the water increases its conductance
considerably, the current being carried by the migration of the dissolved ions.
This is a clear distinction from the conduction of current through metal, which
results from electron transport.
The conductance of a solution is proportional to and a good, though non-
specific indicator of the concentration of ionic species present, as well as their
charge and mobility. It is intuitive that higher concentrations of ions in a liquid
will conduct more current. Conductance derives from Ohms law, E = IR, and is
defined as the reciprocal of the electrical resistance of a solution.
C = 1 / R
where C is conductance (siemens)
R is resistance (ohms)
One can combine Ohms law with the definition of conductance, and the
resulting relationship is:
C = I / E
where I is current (amps)
E is potential (volts)
In practice, conductivity measurements involve determining the current through
a small portion of solution between two parallel electrode plates when an AC
voltage is applied. Conductivity values are related to the conductance (and thus
the resistance) of a solution by the physical dimensions --- area and length --- or
the cell constant of the measuring electrode. If the dimensions of the electrodes
are such that the area of the parallel plates is very large, it is reasonable that
more ions can reside between the plates, and more current can be measured.
The physical distance between the plates is also critical, as it effects the
strength of the electric field between the plates. If the plates are close and the
electric field is strong, ions will reach the plates more quickly than if the plates
are far apart and the electric field is weak. By using cells with defined plate
