Ultrafiltration I. Ultrafiltration of Electrolytes from Alkali

Ultrafiltration is an important research tool in colloid chemical and in certain types of biological investigation. However, in spite of the many years that ultrafiltration has been used as a research tool, the phase of the subject concerning the ultrafiltra tion of electrolytes from systems containing electrically charged colloids has been lacking in both experimental data and a theoretical basis for the interpretation of the process. The widely current view is that an ultrafiltration of this kind is a simple filtration and the diffusible electrolytes ultrafilter through in the same concentration that they are present in the free solvent of the colloidal solution. Differences that are found by gross analysis are ascribed to such effects as solvation of the colloid or to adsorption. On the basis of such an assumption for ultrafiltration, the amount of peptizing electrolyte adsorbed by a colloid has been evaluated by subtracting the concentration in the ultrafiltrate from the total of the peptizing electrolyte in the colloidal solution. In a similar way what has been taken to be the hydration of certain colloids has been calculated from the increase of the concentration in the filtered liquid of an added electrolyte reference substance. In this way in recent years ultrafiltration has been employed to evaluate the constitution of stannic oxide sols peptized by potassium hydroxide, chromic oxide sols peptized by ammonium chlo-