Cell Constant

Cell Constant

Specific conductivity is determined by placing a test solution in a conductivity cell and the resistance is measured by connecting to a Wheatstone bridge circuit. The conductivity cell does not have accurately known dimensions (area and distance between electrodes) but has a cell Constant (l/A) which is accurately known because the distance l and the area A are constant for a given conductivity cell.

From eqns (2) and (4) we may write;

L=K                                                                                                                            (6)

Or   K=(  ).L =(cell constant) x L                                                                              (7)

In practice the cell constant is deduced by measuring the resistance of a solution of known specific conductivity. Potassium chloride solution is usually used in the determination of cell constants. Once the cell has been calibrated, eqn (7) can be used to deduce K for an unknown solution from a measured value of L and a known value of the cell constant.

Table1: Specific Conductivities (K) of Some Materials at 25⁰C

Material	K (Ohm-1 cm-1)
Ag Fused NaCl 0.1M KCl 0.1MHAc H2O (l)	6.33 x 105 3.3 1.29 x 10-2 5.20 x10-4 4.0 x 10-8