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Specific Conductance

In some chemistry or physics class, you undoubtedly observed that a light bulb would not light when an electrical current was passed through a circuit which had two separate electrodes immersed in pure water; no current passed through the water. When salt was added to the water, the current passed and the bulb glowed. The resistance of the water to electrical current was reduced by the addition of ions of salt and the electron flow was increased. Specific conductance (conductivity) of fresh water is based on the same principle. The purer the water (i.e., the fewer the dissolved electrolytes in water), the greater will be the resistance to electrical current.

By definition, specific conductance of an electrolyte is the reciprocal of the specific resistance of a solution expressed in micro-Siemens per distance (µS/cm) or as the reciprocal of ohms (µmhos/cm). The amount of current conducted is proportional to the concentration of ions in solution and, hence, to both the concentration and extent of dissociation of the dissolved salts. The standard conductance cell consists of two black platinum electrodes 1 cm2 in surface area and 1 cm apart.

The temperature of the electrolyte affects ionic velocities and must be controlled carefully. Conductance increases 2-3 % per degree C. When samples are not maintained at 25ºC, an appropriate factor for temperature compensation must be applied.

It has been found that the specific conductance of water in bicarbonate-dominated lakes and streams is closely proportional to the concentrations of major cations (Ca2+, Mg2+, Na+, K+). Once the concentrations of these cations are known for an individual body of water, changes in the specific conductance can be used to estimate the proportional concentrations of the major cations with relatively small error.

Procedure (Specific conductance)

1. Insert the conductivity probe into any of the analog ports on the LabQuest interface and turn on the power.

2. Rinse the tip of the conductivity probe with distilled water.

3. Insert the tip of the probe into the sample to be tested. Important: Be sure the electrode surfaces in the elongated cell are completely submerged in the liquid.

4. While gently swirling the probe, wait for the reading to stabilize and record the reading.

5. Rinse the probe with distilled water before taking another measurement.

Note : This conductivity probe is automatically compensated between temperatures of 5-35ºC. Readings are automatically referenced to a conductivity value at 25ºC which is standard temperature. Therefore, no additional factor for temperature compensation need be applied.