Equilibrium and Structural Studies of Silicon(IV) and Aluminium(III) in Aqueous Solution. 20. Composition and Stability of Aluminium Complexes with Propionic Acid and Acetic Acid.

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Abstract
Equilibria in the title systems were studied in 0.6 M Na(Cl) medium at 25.degree. C using potentiometric (glass electrode) measurements supplemented with quantitative 27Al NMR measurements. All aluminium complexation data, covering the ligand/metal ratios 1/3 to 30 and metal concentrations 0.001-0.012 M can be explained on the basis of the single complex Al2(OH)2L3+. The corresponding equilibrium constants, defined according to the reaction: 2Al3+ + HL + 2H2O .dblarw. Al2(OH)2L3+ + 3H+ are log.beta.-3,2,1 = -8.038 .+-. 0.008 and -7.979 .+-. 0.017 for propionic and acetic acid, respectively. The ligand dissociation constants for the reaction HL .dblarw. H+ + L- were studied in separate titrations and were found to be log.beta.-1,0,1 = -4.6058 .+-. 0.0009 and log.beta.-1,0,1 = -4.4868 .+-. 0.0007 for propionic and acetic acid, respectively. All errors given are three times the standard deviations, 3.sigma.(log.beta.p,q,r). Data were analysed using the least-squares computer program LETAGROPVRID.