Studies on recrystallised aluminium trihydroxide precipitates: The energetics of dissolution by sodium hydroxide solutions

Summary

Aluminium hydroxide gels were recrystallised in high pH solution for 2–1000 hr to give a series of gibbsite powders of different surface areas, 1`11/2, P11/8, P11/48, P11/360 and P11/X; their average platelet lengths were 30, 60, 130, 260 and 450 nm. The dissolution equilibria of these powders in sodium hydroxide solutions of C=1-8 M were studied at 20° to 80°C.

The equilibrium constants (for formation of diaquo-tetrahydroxoaluminate anion) increased with temperature and with increasing surface area; the K value for P11/2 was about four times that of P 11/X at 20 °C and about three times (that for P 11/X) at 80 °C. The dissolutions were endothermic; however, the value over this temperature range increased from −29.5 kJ mol⁻¹ (for P11/X) to −24.7 kJ (for P 11/2). This increase was related to the appreciable surface energies of the high surface-area materials.

The (− ΔG°) values increased with increasing temperature and in turn with increasing surface area; the (− ΔG°) values for 1`11/2 dissolution at any temperature were about 4 kJ mol⁻¹ higher than those for P11/X dissolution.