Conductivity
The resistance of a sample of an electrolytic solution is defined by
where l is the length of a sample of electrolyte and A is the cross sectional area. The symbol r is the proportionality constant and is a property of the solution. This property is called resistivity or specific resistance. The reciprocal of resistivity is called conductivity, k.
Since l is in cm, A in cm2 and R in ohms W, the units of k are W-1cm-1 or Scm-1 (Siemens per cm).
Conductivity is measured with a cell consisting of two platinum electrodes a fixed distance apart. These electrodes are coated with finely divided electrodeposited platinum black. The ratio l/A is a property of the cell and is called the cell constant. It is the resistance that is measured with an alternating current to minimize electrolysis. The catalytic properties of the platinum black allow what ever electrolysis there is to occur so rapidly in both directions that there is no buidup of products. The cell constant is not measured geometrically but by measuring the resistance of a sample solution of known conductivity, usually potassium chloride.