On Colloidal Electrolytes. An Attempt to Account for the Effect of Dilution on their Osmatic Pressure, Conductivity and Counter Ion Activity

Department of Chemistry, University College of Science, Calcutta-9

Received 24 June 1971; accepted for publication 9 February 1972

On the basis of certain assumptions the following equations have been deduced to account for the variation, with dilution, of osmotic pressure and conductance of congo red and of hydrogen ion activity and conductance of cetyl sulphonic acid sols

                                             \(P_c = {P_m} {C \over C+K} ..(1)\)

                                             \(X_c- X_o = X_m[1 \beta(C-C_d)]{(C-C_o) \over K+(C-C_o)} ..(2)\)

                                             \(H_c-H_o = a_m[1 \beta(C-C_d)]{(C-C_o) \over K+(C-C_o)} ..(3)\)

 In equation (1) \(P_c\) represents the osmotic pressure at the concentration C, of congo red and \(P_m\) and K are constants. \(P_m\) represents the osmotic pressure of the counter ions within. the diffuse double layer at the plane of contact with the osmometer membrane. Its biological significance has been discussed.

In equation (2) X represents the specific conductivity at the concentration, C, of the micelle forming electrolyte, Co, its c.m.c., \(C_d\), the concentration at which the diffuse double layers overlap, X₀ the specific conductivity corresponding to C₀ and X_m, β, and K, are constants.

In equation (3) \(H_c\) represents the hydrogen ion activity at the concentration, C, of cetylsulphonic acid, C₀ , the c.m.c. and H₀ , the hydrogen ion activity corresponding to C₀. The other terms have the same significance as in equation (2).

The equations have been found to agree well with the experimental results on congo red obtained by Donnan and Harris and on cetyl sulphonic acid obtained by McBain and Williams. The minimum in equivalent conductivity at about C = 0.094 N has been shown to be mainly due to the suppression of ionisation of the molecularly dispersed cetylsulphonic acid at the c.m.c. level which latter is also lowered to some extent.