1977 年 97 巻 11 号 p. 1228-1235
Ferric hydroxide was peptized by pouring the precipitate into an aqueous solution of various organic acids and boiling for 8 hr. The degree of peptization was evaluated by the concentration of ferric hydroxide in the sol thus formed by chelatometry after decomposition of the colloidal particles into ferric ions. The adsorption of these organic acids on ferric hydroxide particles was also measured. In the case of formic, acetic, and propionic acids, the degree of peptization increased with increasing amount of the organic acid adsorbed. However, no peptization took place in the case of butyric and valeric acids. The adsorption isotherms followed Langmuir's scheme, and the minimum acid concentration required for peptization was found to depend on the adsorption constant (k) in the case of formic, acetic, and propionic acids. The addition of a small amount of hydrochloric acid increased the degree of peptization by butyric acid, which can be ascribed to the decrease in dissociation of the latter. It was thus concluded that the adsorption of organic acids promotes, while that of their ions prevents, the peptization.