Fisher Scientific Accumet Basic AB30, User Manual

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9
CONDUCTIVITY THEORY
Conductance is a metric associated with the ability o 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 bring 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
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
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 affects 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. Thus comes
the term specific conductance or conductivity.
The relationship between conductance and specific conductivity is:
Specific Conductivity, S.C.= (Conductance) ( cell constant, k) =
siemens * cm/ cm2 = siemens/ cm
C is the Conductance (siemens)
K is the cell constant, length/ area or cm/ cm2
Since the basic unit of electrical resistance is the ohm, and
conductance is the reciprocal of resistance, the basic unit of
conductance was originally designated a Òmho:- ohm spelled