PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 362

11.3.2 Conductivity cell constant

Calculating conductivity from relations (11.23) and (11.24) is impractical in electrolytes due

to problems related to the determination of accurate values of the electrodes surface areaA

and the distance between them`. For this reason, conductivity is usually calculated using the

relation

κ=

C

R

. (11.25)

The quantityCis called theconductivity cell constant, and it is determined by calibration.
U Main unit:m−^1.

Example

The conductivity of a potassium chloride solution of the concentrationc = 0.1 mol dm/^3 is

κ= 1.288 S m−^1. The resistance of this solution was measured in a conductivity cell and the

valueR= 220Ωwas found. Calculate the conductivity cell constant.

Solution

From equation (11.25) we obtain

C=R κ= 220× 1 .228 = 540. 9 m−^1.

11.3.3 Molar electric conductivity.

Themolar electric conductivityΛ (or shortly molar conductivity) is defined by the relation

Λ =

κ

c

, (11.26)

wherecis the electrolyte concentration in the units mol m−^3.
U Main unit:S m^2 mol−^1.

Example

The resistance of a copper sulfate of the concentrationc= 0.02 mol dm−^3 isR= 1440Ω, and

the conductivity cell constant isC= 541 m−^1. Calculate the molar conductivity of the solution.