COUNTERION COMPLEXING AND SOL STABILITY .I. COAGULATION EFFECTS OF ALUMINUM SALTS IN PRESENCE OF FLUORIDE IONS

The coagulation and the reversal of charge of silver halide sols by aluminum ions in the presence of fluoride ions have been studied as a function of pH. Using known stability constants, the composition of the electrolyte environment was calculated. This included all aluminum fluoride complexes as well as the aluminum hydrolyzed species. With the increasing addition of fluoride ions larger amounts of aluminum perchlorate are required to coagulate the sol. The calculations show that at pH <4 the unhydrolyzed aluminum ion acts as the counterion while at pH >4 the hydrolyzed aluminum ion is the coagulant. The complexing of aluminum ions by fluoride produces lower charged species which have no effect upon the stability over the critical coagulation concentration range of the aluminum salt. The charge of the sols can be reversed by the hydrolyzed aluminum ions, but not by the fluoride complexes.