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The Free Energy of Formation of Gibbsite and Al(OH)₄^- from Solubility Measurements¹

ABSTRACT

Solubility determinations on two particle sizes of crystalline gibbsite, from both undersaturation and supersaturation in the acid pH range, gave a K_SO of –34.03 ± 0.05. Using the same gibbsite in the alkaline pH range and extrapolating values from supersaturation to infinite time, a *K_S4 of –15.30 ± 0.05 was obtained. From these values a G_f⁰ (gibbsite) of –274.20 ± 0.06 kcal/mole of Al(OH)₃ and a G_f⁰ [Al(OH)₄^- aq] of –310.03 ± 0.07 kcal/mole were calculated. The value for gibbsite confirms previous work on the stability of gibbsite and the new Al(OH)₄^- value eliminates a discrepancy between gibbsite stabilities derived from solubility measurements in the acid and in the alkaline pH range.

¹ Scientific Paper no. 2779, College of Agriculture, Washington State University, Pullman. Project 1489. The author wishes to express his appreciation to Dr. H. H. Cheng and to Dr. C. D. Moodie for their helpful criticism of the manuscript.

² Associate Professor of Soils.

Received for publication March 21, 1966. Accepted for publication June 16, 1966.