Limitation of Debye-Huckel-Onsager Theory:
On increasing the dilution the average distance between ions increases and, therefore, the retarding force decreases and there is an increase in Λm with increase in dilution. The above explanation, which is only a fragmentary reflection of a more detailed picture, leads to a relation between molar conductance and concentration of the same form as Kohlrausch’s equation.
Debye, Huckel and Onsager were able to calculate the constant A and B for many electrolytes in dilute solution. The calculated values were found to be in agreement with the values found from the slopes of the experimental data plotted as Am vs √C.
The theory which explains many other facts, however, fails when applied to solutions of moderate or higher concentration, i.e., 0.1 to 1 mol L-1 and above. Although no satisfactory theoretical treatment for such solutions is still available, mention may be made of the concept of the formation of ion-pairs or ions triples, primarily by Fuoss, Bjerrum and others.
Because of the proximity of the ions at high concentrations there may be specific attraction between the oppositely charged ions, so that two oppositely charged ions in an ion -pair will virtually behave like an undissociated molecule so far as conductance is concerned. The ion pairs cannot be regarded as molecules as these can exchange partners. The concept of ion-pair is of importance particularly in solutions of low dielectric constant.
Weak electrolytes; a weak electrolyte is one that is not fully dissociated. As such it has dissociation constant. The dissociation constant can be used to calculate the extent of dissociation and hence, make the essential correction needed to calculate activity coefficients.