Instruction manual

Instruction Manual PCD 650
104
7. APPENDIX
7.1 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 nonspecific 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 derives the term specific conductance or conductivity.
The relationship between conductance and specific conductivity is:
Specific Conductivity, S.C. = (Conductance) (cell constant, k) = siemens *
cm/cm
2
= siemens/cm
C is the Conductance (siemens) k is the cell constant, length/area or cm/cm
2
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 backwards - however, this term has been replace by the term
“siemen“. Conductivity measurements are reported as Siemens/cm, since the value
is measured between opposite faces of a cell of a known cubic configuration. With
most aqueous solutions, conductivity quantities are most frequently measured in
micro Siemens per cm (µS/cm) or mill Siemens per cm (mS/cm).