Thermo Fisher Scientific CyberScan CON 1500 - Manual (Page 67)

Instruction Manual CyberScan CON 1500
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The 4-cell electrode
Traditionally, conductivity measurements were made with a “2-cell”
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 polarised. 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. Polarisation and associated errors can be minimised by using
AC voltage, the appropriate cell constant, and a large electrode
surface area. The influence of polarisation can also be minimised 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, polarisation 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 polarisation.
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 CyberScan CON 1500 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, contact Eutech Instruments
at (65) 6778 6876.