Importance of dialysis in the study of colloids

Dialysis has been utilised since the time of Graham for freeing a colloidal solution from the impurities introduced into it at the time of its preparation viz., the peptising agent and the other electrolytes formed as a result of double decomposition. The importance of the influence of dialysis on the various properties of colloidal solutions has not, however, been generally realised and as a result of this different investigators trying the same experiments on the same colloid and with the same electrolytes have frequently obtained apparently conflicting results. Thus Dhar and Ghosh working on prussian blue sol found that it showed abnormal behaviour to the dilution rule viz., that the greater the concentration of a colloid, the greater is the amount of an electrolyte necessary for coagulating it, when coagulated with KC1, while Weiser and Nicholas observed that the same sol when coagulated by the same electrolyte was normal to the dilution rule. Desai however showed that it is erroneous to classify colloids into two divisions according to their behaviour to the dilution rule as done by Dhar and collaborators because a suitable adjustment of the amount of the the peptising agent (by subjecting it to dialysis) seems to make a colloid normal or abnormal to the dilution rule.It is a general belief that the stability of a sol decreases with the progress of dialysis except in the case of colloidal gold where the stability with reference to KCl first increases and then decreases. Further it is customary to explain the decrease in stability on dialysis as being due to a decrease in the charge on the colloid particles although no direct measurements of charge are made to support this generalisation.According to V. Smoluchowski a sol with greater electric charge should show greater viscosity than a sol containing particles of feeble electric charge. The results of viscosity measurements of a number of colloids with the progress of dialysis and in the presence of electrolytes have however been explained by Dhar and co-workers on the assumption that the viscosity is related to the charge on the colloid and that the smaller the charge on the colloidal particles, the greater their hydration and the greater the viscosity of the sol.No systematic investigations seem to have been made to determine simultaneously the relation between the charge on the particles and the stability, viscosity and other related properties of colloidal solutions dialysed and diluted to diferent extents. In what follows is given a connected account and a discussion of the experimental results obtained with various colloidal solutions in the Physical Chemistry Laboratory of the Wilson College, Bombay (India) to show how changes in the purity of colloidal solutions and their concentration alter the charge on their particles and their stability, viscosity and conductivity. In addition simultaneous measurements of charge, stability and conductivity for certain of these colloidal solutions when dialysed for different periods and allowed to age or exposed to sunlight are also given