Formation constants for selected organo—metal (Al3+, Fe3+)—phosphate complexes

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Abstract
The possible formation of organo-metal-phosphate complexes (O-M-P) in aqueous solutions is examined by potentiometric titration analysis. The complex-forming solutions contain Al3+-ions or Fe3+-ions, organic acids (salicylic, phthalic, oxalic, and citric acids) and phosphate ions. The titrations are done within the precipitation-free pH-region from 2 to 4.5 (approximately) at 25.degree. C under nitrogen at an ionic strength of 0.2 M KCl. The calibration of the electrode pair (glass-electron, calomel-reference electrode) and the determination of the formation constants are facilitated by analyzing the pertinent curves with the ACBA-(Arena et al.) and MINIQUAD-(Gans et al.) computer programs. The results suggest (i) that O-M-P complexes are readily formed, and that the power of selected organic acids to form ternary mixed-ligand complexes with Al3+-ions and phosphate ions decreases in the order salycilate > citrate > oxalate > phthalate. Also, Fe3+-ions are found to be more strongly bound by salicylate and phosphate ions than Al3+-ions. The effect of O-M-P formation on the chemical speciation of Al in buffered aqueous solutions containing mineral nutrients for the growth of biological organisms is demonstrated for a special case study.