Using Conductivity Measurements to Determine Concentration of Dissolved Substances

Oct. 20, 2010 - PRLog -- Measuring cells and cell constants

A basic conductivity cell consists of a pair of electrodes between which the sample is situated. The ratio of the distance between the electrodes (d) and their surface area (A) is known as the cell constant, K.

K = d/A [cm-1]

The conductivity κ [S . cm-1] is given by the cell constant [cm-1] multiplied by the measured conductance, G [S].

Every measuring cell has its own particular cell constant. The occurrence of inhomogeneous boundary effects means that the cell constant cannot simply be calculated from the cell’s dimensions. It has to be determined experimentally through calibration with a standard solu-tion (of known conductivity) and is stored together with the adjustment in the conductivity meter.

Nowadays some sensors are delivered with certified cell constants and the user can enter the cell constant in the conductivity meter without having to measure the value beforehand. However, it is important to make absolutely sure that it is a certified cell constant, and not a nominal cell constant because this can vary greatly for production reasons.

To obtain the most accurate results, it is recommended that cell constant is determined using a standard. This then takes into account specific measurement conditions such as stirrer speed and the influence of CO2.

In contrast to a pH electrode, the measuring cell does not change with time, especially in the case of high-quality sensors that are used properly. The cell constant changes only if the surface of the electrode changes, for example through fingerprints, deposits, scratches, or enclosed air bubbles. If the sensor is contaminated, it must be cleaned with ethanol, suitable detergents or solvents, or possibly in an ultrasonic bath. Sensors made of graphite, platinum and steel can be carefully cleaned mechanically using a soft brush. It is of course extremely important not to damage the surface of a sensor during cleaning. Finally, the sensor is rinsed well with deion¬ized water. After thorough cleaning, sensors can be stored dry.

Air bubbles occurring on the surface of the electrode are a frequent source of error, but one that is often not fully realized. The bubbles should be removed by stirring the sample and, if necessary, by tapping. The release of air bubbles is observed as a sudden jump in conductivity.Since the accuracy of any measurement depends on the calibration, a fresh standard must always be used. Ideally, sample beakers and sensor should be rinsed two to three times with the sample because the presence of contaminants will give rise to errors in the conductivity results.

In general, one must make sure that electrode surfaces are always completely immersed in the solution. Conductivity samples and standard solutions should never be diluted because the effect of dilution is not linear.

The position of the sensor in the measurement beaker can influence the results (due to the occurrence of boundary effects outside the electrode surfaces). This of course depends on the design of the electrode. It is usually best to position the sensor in the middle.  Low conductivities such as occur in pure water should be measured in a flow cell. The reason for this is atmospheric CO2. This dissolves in water forming carbonic acid and leads to errors in the conductivity results.

The latter recommendation applies both to calibration and to measurements. The use of a flow cell prevents the sample coming into contact with air. The flow cell and tubing must be thoroughly rinsed before use.

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