Conductance in electrolytic solutions

Conductivity (or specific conductance) of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter (S/m).

Conductivity measurements are used routinely in many industrial and environmental applications as a fast, inexpensive and reliable way of measuring the ionic content in a solution.

The SI unit of conductivity is S/m and, unless otherwise qualified, it refers to 25 °C.

Often encountered in industry is the traditional unit of μS/cm. 106 μS/cm = 103 mS/cm = 1 S/cm.

The values in μS/cm are lower than those in μS/m by a factor of 100 (i.e., 1 μS/cm = 100 μS/m).

Sometimes encountered is mho (reciprocal of ohm): 1 mho/m = 1 S/m.

Historically, mhos antedate Siemens by many decades; good vacuum-tube testers, for instance, gave transconductance readings in micromhos.

Resistance, R, is proportional to the distance, l, between the electrodes and is inversely proportional to the cross-sectional area of the sample, A (noted S on the Figure above). Writing ρ (rho) for the specific resistance .

R = I / A * p

In practice the conductivity cell is calibrated by using solutions of known specific resistance, ρ*, so the quantities l and A need not be known precisely

R* = C*P*

The specific conductance, κ (kappa) is the reciprocal of the specific resistance.

k = 1 / ρ = C / R

Conductance = Specific conductance (K) = Equivalent conductance ( λ)

If the solution is diluted to say (9 cm3) (9 mL), the conductance of the solution will be the same but specific conductance becomes 1/9th as it contains nine cubes.

The conductance is also equal to the equivalent because the solution still has 1 g equivalent of the electrolyte.

Equivalent conductance (λ ) = 9 × k

In general,

λ = k × V

where V is the volume in mL containing 1 g equivalent of the electrolyte.