This cell constant (K) is a function of the electrode areas, the distance between the electrodes and the electrical field pattern between the electrodes.
Often, for considerations having to do with sample volume or space, a cell’s physical configuration is designed differently. Cells with constants of 1.0 cm-1 or greater normally have small, widely spaced electrodes. Cells with constants of K = 0. 1 or less normally have large closely spaced electrodes. Since K (cell constant) is a “factor”which reflects a particular cell’s physical configuration, it must be multiplied by the observed conductance to obtain the actual conductivity reading.
For example, for an observed conductance reading of 200 µS using a cell with K = 0. 1, the conductivity value is 200 x 0. 1 = 20 µS/cm.
The cell constant is defined as the ratio of the distance between the electrodes, d, to the electrode area, A. This however neglects the existence of a fringe-field effect, which affects the electrode area by the amount AR. Therefore K = d/(A + AR). Because it is normally impossible to measure the fringe-field effect and the amount of AR to calculate the cell constant, K, the actual K of a specific cell is determined by a comparison measurement of a standard solution of known electrolytic conductivity.
The most commonly used standard solution for calibration is 0.01 M KCl. This solution has a conductivity of 1412 µS/cm at 25oC
The Effect of Temperature
The conductivity of a solution with a specific electrolyte concentration will change with a change in temperature. The temperature compensated conductivity of a solution is the conductivity which that solution exhibits at the reference temperature. This temperature is chosen to be either 25oC or 20oC. A measurement made at reference temperature, therefore, needs no compensation. Generally for most aqueous samples, a coefficient of 2.1 % per degree Celcius is used in temperature compensation, with the apparent value being 2.1 % high for each degree C above 25oC or conversely the apparent value being 2.1 % low for each temperature for measurement is 25oC. A useful algorithm for temperature correction is:
CT = C25 [1 + 0.021 (T – 25)]
Where CT = the measured conductivity of a solution at sample temperature; C25 = the conductivity of the solution at 25oC and T = the sample temperature(oC).
Many conductivity meters today automatically compensates for temperature if the conductivity probe includes a Thermistor. However, as will be explained later, this can be a major source of error in analysis if the Thermistor is not accurate or if the instrument is improperly calibrated.
Note the two following examples to explain the effect and compensation of the fringe-field effect and temperature.
