Effect of the Removal of Reactive Surface Material on the Solubility of Synthetic Gibbsites

Abstract

Abstract The solubility of four different gibbsite preparations was measured, including two commercial hydrated aluminas produced by Alcoa, a Fisher ACS-grade Al(OH) 3 , and a laboratory preparation. The Alcoa samples and the laboratory-prepared sample had been studied previously by other investigators but without using a long-term acid treatment. Scanning electron microscopy showed globular surface material that was removed by a 14-day, 0.1 M HCl treatment. Solubility was determined at pH 4 from both over-and undersaturation with continuous agitation for 228 days. The acid treatment correlated with a decrease in log*K so [*K SO = (Al 3+ )/(H + ) 3 ] of about 0.5 units. The mean solubility of the three acid-treated commercial gibbsite samples was log*K so = 7.55 ± 0.055 (*K so = 3.5 × 10 7 ). The solubility of the acid-treated laboratory preparation was log*K so = 7.86 (*K so = 7.2 × 10 7 ). The greater solubility of the laboratory-prepared gibbsite is attributed to a greater concentration of structural defects in the crystals.