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


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