Thermo Fisher Scientific CyberScan CON 1500 - Manual (Page 64)

Instruction Manual CyberScan CON 1500
60
10. CONDUCTIVITY THEORY
Conductance is a metric 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
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 standardise 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 “mho:- ohm spelled