Debye huckel onsager equation, limitation

Debye huckel Onsager equation is a mathematical expression that relates the variation of equivalent conductance with the concentration of electrolyte solutions.

Debye huckel onsager equation

Debye and Huckel initially derived an equation to describe how the equivalent conductance of electrolytic solution changes on dilution. Later this equation was further improved by Onsager and now this equation is known as Debye huckel Onsager equation.

Mathematically, Debye huckel Onsager equation can be expressed as,

debye huckel onsager equation

Where, Λc= equivalent conductance at a given concentration

Λo= equivalent conductance at infinite dilution

D= Dielectric constant for the medium or solvent

η= Viscocity of the solvent

T= Absolute temperature

C= concentration

For a particular solvent at a particular temperature, the debye huckel Onsager equation can be written as

Λco-(A+BΛo ) C1/2 ……………………………………(ii)

Where A and B are constant at a given temperature, Whose values can be calculated as

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If water is solvent and at a temperature of 25°C, the value of A is taken as 60.20, and the value of B is taken as 0.229.

Debye huckel onsager equation graph

When Λc is plotted against C1/2 ( Under root C), a straight line having a negative slope (A+BΛo) and intercept Λo is obtained. Therefore, from the slope, we can determine the values of equivalent conductance of strong electrolytes at infinite dilution.

Debye huckel onsager equation graph

Limitations of debye huckel onsager equation

The graph of Λc versus C1/2 shows that the equivalent conductance of weak electrolyte decreases exponentially with concentration so that equivalent conductance at infinite dilution ( Λo) for weak electrolyte can not be determined from the above curve on extrapolation. Therefore, the Onsager equation is limited only for strong electrolytes and invalid for weak electrolytes.

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