Corrosion

Laboratory Manual, Physical Chemistry, Year 1

Experiment 5

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EXPERIMENT 5

MOLAR CONDUCTIVITIES OF AQUEOUS ELECTROLYTES

Objective:

1

To determine the conductivity of various acid and the dissociation

constant, K a for acetic acid

Theory

1.1 Electrical conductivity in solutions

An electric current in solution is the result of the net movement of free ions in a

specific direction. The current may be determined by measuring the resistance R

between two similar inert electrodes immersed in the solution, as in the figure below

where the oval region represents the solution; A represents the electrode area and

l is the normal distance between the electrode planes. In actual practice an A.C.

current with a low frequency of the order of approximately 1000 Hertz is used (to

prevent electrolysis) in the measurement, and the components representing the

resistance R in the complex impedance Z for the circuit is determined. We will always

refer to this component (the real portion of the complex impedance) for what follows.

The resistance is also dependent on the frequency (Debye-Falkenhagen effect). The

theory and measurement here concentrates on low frequency measurements where

the Onsager equation is meaningful. The fully automated measuring apparatus has

been configured for low frequency measurement in accordance with the theory of

electrolytes.

Electrod

e

l

l =Distance

between electrode

A = Area of

electrode

A

Figure of conductivity circuit

According to Ohm’s law, the resistance R (unit Ohm, symbol Ω ) for the above circuit

l

is given by R = ρ

(1)

A

l

where ρ is the resistivity of the solution. The cell constant refers to

but there is no

A

need to refer to this quantity here. The electrical conductivity of the solution κ is

defined as κ =

1

where the

ρ

current density, j , is given by j = κΕ . The conductance

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