Potentiometric studies in the formation and stability of colloidal solutions

Abstract

1. Potentiometric titrations of acidified and unacidified ferric chloride solutions have been conducted against carbonate-free sodium hydroxide, with and without the addition of neutral salts. 2. The titration curves show that the range of stability of ferric oxide sols is between the pH limits 3·20–4·45, the precipitation of ferric oxide commences in the neighbourhood of pH 4·5, and is complete at pH 6·0. 3. The composition of the precipitate of ferric oxide is found to vary with the neutral salt added. The molar ratio between ferric chloride and sodium hydroxide varies between the limits 1:2·69 to 1:2·85, the theoretical ratio being 1:3·0. The molar ratio at the end point follows the series SO4″ < Cl′ < NO3′ < water. 4. The different stages in the formation of micelles of ferric oxide and their subsequent coagulation have been elucidated. 5. The variations in pH of ferric oxide sols with dilution have been studied, and the results support the view proposed by the author that there is an equilibrium between the ions in the micelle and those in the intermicellary liquid. 6. An explanation has been offered for the development of alkalinity on addition of neutral salts to ferric oxide sols. The liberation of OH− ions on addition of salts has been correlated with the lower molar ratio obtained in the titrations of ferric chloride against alkali.