Nova-Tech SALT 6+ - Manual (Page 32)

Instruction Manual COND 6+, TDS 6+, SALT 6+
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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 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
areas and separation distances, it is possible to standardize or specify conductance
measurements.