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9
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 V 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, V = 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 / V where I is current (amps), V 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 areas and separation
distances, it is possible to standardise or specify conductance measurements.
Thus derives the term specific conductance or conductivity.
The relationship between conductance and specific conductivity is:
Specific Conductivity, S.C. = (Conductance) (cell constant, k)
where C is the conductance (siemens), k is the cell constant, length/area or cm/cm
26
= siemens * cm/cm
= siemens/cm
2
2
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