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Effect of Different Anions on the Crystallization of Aluminum Hydroxide in Partially Neutralized Aqueous Aluminum Salt Systems¹

ABSTRACT

The effect of Cl^-, NO₃^-, and ClO₄^- ions on the crystallization of Al(OH)₃ was studied in 0.2M, 80% neutralized AlCl₃, Al(NO₃)₃, and Al(ClO₄)₃ systems at 25C. In the presence of 0.6M Cl^- no crystalline Al(OH)₃ formed during 2 year's reaction; instead, an aluminum hydroxychloride solid of composition Al(OH)_2.5Cl_0.5·xH₂O crystallized. In the presence of 0.6M NO₃^- a poorly crystalline aluminum hydroxynitrate of approximate composition Al(OH)_2.5(NO₃)_0.5 · xH₂O formed initially, but gibbsite crystallized eventually after 9 month's reaction. In the presence of 0.6M ClO₄^- nordstrandite initially crystallized and was present after 3 week's reaction. These results support the hypothesis that the rate of Al(OH)₃ crystallization is inversely related to the tendency of anion inclusion in aluminum hydroxide precipitates, and that this tendency decreases with an increase in size and structural complexity of monovalent inorganic anions.

The incorporation of anions, especially NO₃^-, in hydrous aluminum solids may have agronomic importance.

¹ Contribution no. 345 of Soil Research Inst., Canada Dept. of Agr., Ottawa, Ontario.

² Research Scientists.

Received for publication September 30, 1970. Accepted for publication January 25, 1971.