Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 99)

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Appendix: Conductivity Theory
PC6000, CON6000, PC6500 and PCD6500 meters
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The four-cell electrode
Traditionally, conductivity measurements were made with a “two cell“ electrode. This electrode
used two metallic sensors, and 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, and 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.
Cell Constant Optimum Conductivity Range, 4-cell
0.1 Not Available
1.0 0.01 to 20 mS/cm
10.0 1 to 200 mS/cm
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, but the PC6000,
CON6000, PC6500 and PCD6500 meters lets the operator select any value between of 15°C and
30°C in the Setup menu option, set Reference Temperature.
The PC6000, CON6000, PC6500 and PCD6500 meters permits you to enter the temperature
coefficient which best suits your sample and use an ATC probe to automatically temperature
compensate back to the chosen reference temperature. Refer to the Setup menu option, set
Temperature Coefficient.