Reversal of charge of lyophobic colloids by hydrolyzed metal ions. I. Aluminum nitrate

Abstract

Depending upon its concentration and the pH of the medium, aluminum nitrate may either coagulate or stabilize a lyophobic colloid consisting of negatively charged particles. The stabilization is caused by reversal of charge due to adsorption of positive ions on the colloidal particle and takes place only if a sufficient amount of hydrolyzed species are present in solution. In this paper, the entire [Al(NO 3) 3]-pH domain of stabilization due to such reversal of charge has been determined for silver bromide and silver iodide sols. No differences were observed between aged sols and sols in statu nascendi. The ligand composition of the hydrolyzed aluminum species can be determined from these data and agrees with the formulation Al 3(OH) 20 4+ determined earlier. Upon aging solutions of AI(NO 3) 3 by heating at 90°C. for 24 hours, hydrolyzed Al species are formed which remain stable even at low pH. Accordingly, there is a considerable shift of the stability limit towards lower pH's for solutions treated in this manner.