Abstract

Comparison of the standard free energies of formation at 25°C of corundum, diaspore, boehmite, gibbsite, and bayerite estimated from carefully selected aqueous solubility data with those of calorimetric origin permits selection of a set which is more consistent with stabilities observed in field and laboratory than the calorimetric set alone. The selection process involves acceptance of —378.2 and —116.0 kcal per mole for ∆G°f of αAkOa and Al3+ (aq) respectively and assignment of —311.0 kcal per mole for the AG°t of Al(OH)i (aq). Selected values for the solids are:

kcal/Mole
diaspore –219.5 ±0.5 
boehmite –218.7 ±0.2 
gibbsite –275.3 ± 0.2 
bayerite –274.6 ±0.1 
kcal/Mole
diaspore –219.5 ±0.5 
boehmite –218.7 ±0.2 
gibbsite –275.3 ± 0.2 
bayerite –274.6 ±0.1 

Solubility-pH curves based on these assignments agree with independent solubility data only if it is assumed that slight supersaturation results during many experiments and leads, upon aging toward equilibrium, to precipitation of gibbsite or bayerite, depending on pH. One set of data which suggests this interpretation is discussed.

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