Solubility of simple aluminium compounds expected in soils. I. Hydroxides and oxyhydroxides

Results from dissolution equilibria with four aluminium hydroxide and oxyhydroxide compounds in M/100 K2SO4 over pH range 3.5-10, when examined by plotting the negative logarithm of the aluminium ion concentration against the pH, showed that AlOH2+, Al(OH)2+, and Al(OH)4-, hydrated and possibly polymeric, were formed. For pH>7, precipitated aluminium hydroxide (containing mostly boehmite) gave bayerite equilibria while gibbsite, diaspore, and bauxite gave gibbsite equilibria. For p H t 7 amorphous A1(OH)3 and corundum equilibria were observed. Using accepted free energy values, the following were estimated at 2.5°: KS1 = log [(AlOH2+)(OH-)2] = -23.31 and KS2 = log [(Al(OH)2+)(OH-)] = - 14.04 for amorphous Al(OH), and KS1 = -24.41 and KS2 = -15.14 for corundum and also ÄF° values of 164.9, -215.1, and -313.9 kcal for A10H2+, Al(OH)2+, and Al(0H)-4 respectively in aqueous solution.