The four-cell electrode
Traditionally, conductivity measurements were made with a Òtwo
cellÓ electrode. This electrode used two metallic sensors, an anode
and a cathode to which ions migrated. Under the influence of DC
current the electrodes quickly became polarized. In this situation,
molecules formed at the electrode surfaces and ions migrating to
the area collect around the respective anode or cathode and
actually screen it from other ions. In essence the flow of ions
stops, and current ceases to flow. Polarization and associated
errors can be minimized by using AC voltage, the appropriate cell
constant, and a large electrode surface area. The influence of
polarization can also be minimized by the use of a four-cell
electrode.
The four cell configuration consists of two cells, an outer cell and
an inner cell. Voltage is applied to the sensors of the outer cell,
which in turn generates a voltage across the sensors of the inner
cell. The inner cell is connected to a high impedance circuit and,
unlike the outer cell generates no current. Since no current is
generated across the inner cell, polarization cannot occur at the
inner cell. By measuring the voltage of the inner cell, which is
adjusted to match the reference voltage by increasing or
decreasing the current through the inner cell, one obtains a true
picture of conductivity minus the influence of polarization.
Conductivity and Temperature
Conductivity in aqueous solutions reflects the concentration,
mobility, and charge of the ions in solution. The conductivity of a
solution will increase with increasing temperature, as many
phenomena influencing conductivity such as solution viscosity are
affected by temperature.
The relationship between conductivity and temperature is
predictable `and usually expressed as relative % change per
degree centigrade. This temperature coefficient (% change per
degree) depends on the composition of the solution being
measured. However, for most medium range salt concentrations in
water, 2% per degree works well. Extremely pure water exhibits a
temperature coefficient of 5.2%, and concentrated salt solutions
about 1.5%.
Since temperature effects the conductivity measurement so
profoundly, the usual practice is to reference the conductivity to
some standard temperature. This is typically 25¡C; therefore,
measurements are reported as if the sample were at 25¡C.
The accumet AB30 permits you to enter one of four temperature
coefficients (0.00, 1.5, 2.0, or 5.2%) and use an ATC probe to
automatically temperature compensate back to a reference
temperature of 25¡C.
The meter requires no regular maintenance, but it is recommended
to occasionally wipe down the front with a damp cloth. If there are
any further questions regarding maintenance, call the Fisher Lab
Equipment Technical Support Hotline at 800/943-2006 or
412/490-6260.
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