A Potentiometric, Spectrophotometric and Pitzer Ion-Interaction Study of Reaction Equilibria in the Aqueous H+-Al3+, H+-Oxalate and H+-Al3+-Oxalate Systems up to 5 mol⋅dm−3 NaCl

Abstract

The aqueous speciation of the mixed H+-Al3+-oxalate (L) system at equilibrium was determined by potentiometry in solutions of 0.1, 1.6 and 3.0 mol⋅dm−3 NaCl and by spectrophotometry in solutions of 5.0 mol⋅dm−3 NaCl at 298.2 K. These experiments enabled the description of the ionic strength dependence of the AlL+,AlL2− and AlL33− species and the formulation of a Pitzer ion-interaction model. The accuracy of our aqueous speciation calculations for this mixed system was also strengthened by experimental validation of previously-established models for the protonation of oxalate in the H+-L system and of the hydrolysis of Al3+ in the H+-Al3+ system. The latter system also enabled the formulation of a new Pitzer ion-interaction model for the heptavalently-charged Al13O4(OH)247+ Keggin ion to 3.0 mol⋅dm−3 NaCl. The proposed set of parameters can be used to predict chemical speciation in the mixed H+-Al3+-L system in aqueous solutions of NaCl up to 5.0 mol⋅dm−3 (5.6 mol⋅kg−1 NaCl) at 298.2 K.