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Influence of Citric Acid on the Hydrolytic Reactions of Aluminum¹

ABSTRACT

Hydrolytic reactions of Al were studied at the initial Al concentrations of 1.10 x 10^–4M and 1.10 x 10^–3M and at varying degrees of hydrolysis as influenced by the presence of 1.0 x 10^–6M and 1.0 x 10^–4M citric acid. The effect of citric acid on the formation of the readily extractable Al and moderately extractable Al by the 8-quinolinolate method varied with the initial concentrations of Al and citric acid and OH/Al molar ratios of the system. Hindrance to the hydrolytic reaction of Al by the acidity of citric acid and its complexation with Al ions were two counteracting reactions governing its effect. Citric acid substantially promoted the formation of the nonextractable Al fraction. The precipitation of the solid phase products was invariably hindered by citric acid.

Electron diffraction analysis showed that citric acid, at the concentrations used, hampered the formation of even microcrystalline bayerite and gibbsite. The data obtained from X-ray diffraction and thermal analyses indicate that on aging, the degree of order in the solid phase improved upon the replacement of the citrate in the structure by the hydroxyl. Citric acid evidently hampered the hydroxyl bridging mechanism in the hydrolytic reaction of Al and thus imparted a rough fluffy surface to the solid phase as opposed to well-formed crystals in its absence.

The present study clearly illustrated the important role of citric acid in influencing the hydrolytic reaction of Al and the nature of the resulting solid phase reaction products which should be of concern in the study of pedogenesis and other soil physico-chemical reactions.

¹ Contribution no. R163, Saskatchewan Institute of Pedology, Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, Canada. This investigation was supported by the National Research Council of Canada Grant A3248- and E1770-Huang. Presented before Div. S-2, Soil Sci. Soc. of Am., Houston, Texas, 1 Dec. 1976.

² Graduate Student and Associate Professor of Soil Science, respectively.

Received for publication September 16, 1976. Accepted for publication February 22, 1977.