Electrical Conductivity

The electrical conductivity of solutions is reported as the specific conductance. The fact that aqueous solutions of certain materials, called electrolytes, conduct an electric current provides the most direct evidence for the idea that ions capable of independent motion are present. More detailed studies of the electrical conductivity of such solutions provide information on the number and independence of these ions. Measurements of the conductivity of aqueous solutions are made with a conductivity cell and an electric circuit. When an alternating current is used to prevent buildup of charges of opposite sign near the two electrode surfaces, so that there is little electric resistance at the metal-solution interface, the conductivity cell obeys Ohm's law: The current flowing through the cell is proportional to the voltage across the cell. It is therefore possible to assign a resistance of so many ohms to such a cell, just as one assigns a resistance to a metallic conductor.

It is more convenient to focus on the conductance of an electrolytic solution rather than on its resistance. These quantities are reciprocally related, and the conductance L is calculated from the measured resistance as

L= 1/R

If R is the resistance in ohms, symbol Q. then L has the units of As for metallic conductors, the resistance and therefore the conductance depend on the cross-section area A and the length I of the region between the electrodes. As for a metallic conductor,

R= ρ (l/A)

Where p is the specific resistance and is the proportionality factor that corresponds to the resistance of a cell of unit cross-section area and unit length. One can also write

L= K (A/ l)

where K, the specific conductance, can be thought of as the conductance of a cube of the solution of electrolyte of unit dimensions.

The specific conductance can, in principle, be obtained from the measured value of R, which gives L = 1/R, and of l and A of the cell.

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