ASSOCIATION QUOTIENTS OF ALUMINUM OXALATE COMPLEXES IN AQUEOUS SOLUTIONS

Although a number of measurements of the association quotients of Al-oxalate species have been made at 25°C, relatively few measurements exist at higher and lower temperatures. To assess the significance of Al-oxalate speciation in geological systems we have measured equilibria of the form

Al³⁺ + nC₂O₄^2- = Al(C₂O₄)_n^3-2n n≤3

Formation constants were measured by potentiometric titration using a hydrogen electrode concentration cell. Measurements were made at 8 temperatures ranging from 5 to 150°C and at ionic strengths ranging from 0.1 mol-kg^-1 to 1.0 mol-kg^-1 in aqueous NaCl media. When I=0.6 mol-kg^-1, the values of Q₁, Q₂, and Q₃ increase from 10^5.33, 10^9.87, and 10^14.12, respectively, at 5°C to 10^7.41, 10^12.76, and 10^16.94, respectively, at 125°C. Values of Q₁, Q₂, and Q₃ at 25°C and I=0.6 mol-kg^-1 are in good agreement with those published previously after recalculation using more recent values of the dissociation of oxalic acid.