Formation constants for mixed-metal complexes, K,,,, between uranium(vI), as the central metal ion, and each one of the metal ions iron(III), aluminium(rrI), indium(1rr) and copper(1r) have been determined using data from potentiometric measurements. In each case one of the hydroxycarboxylic acids, citric, tartaric or malic, was used as a ligand. At 298.15 K the values of -log K,,, for the investigated systems are uranyl iron@) citrate, 2.45 ; uranyl iron(II1) malate, 7.37 ; uranyl aluminium(II1) citrate, 8.21 ; uranyl aluminium(1rr) tartarate, > 12; uranyl aluminium(1n) malate, 8.06; uranyl indium(rr1) citrate, 1 1.3 ; uranyl indium(Ir1) tartarate, 7.88 ; uranyl indium(II1) malate, 7.45 ; uranyl copper(I1) citrate -1.41 ;uranyl copper(I1) tartarate, -1.17 ; and uranyl copper(I1) malate, -1.1 1.Previous investigations 1-3 have shown that at pH 4.0 uranium(v1) forms mixed-metal complexes with some other metal ions when a hydroxycarboxylic acid is present. The spectrophotometric data1 indicate formation of complexes with a metal-metal ratio of 1 : 1 and 1 : 2 with Al'I', In ''I and Cu" and a ratio of ligand to total metal ion of 1 : 1 with citric, tartaric and malic acids, have the predominant stoichiometry of 1 : 1 : 2. From calorimetric measurements2 it was concluded that Fe'" also forms mixed-metal complexes with uranium(v1) in the presence of these acids. The formation constants of uranyl indium(Ir1) systems with citric, tartaric and malic acids were reported pre-viously4* using potentiometric and solvent-extraction methods.In the present study a model for evaluation of formation constants for the mixedmetal complexes from potentiometric data is proposed, and the values of Kmmc are reported. ExperimentalAqueous solutions of Cu", AlII', In''' and Fe'" were prepared from the metals, of at least 99% purity (all Merck), by dissolving the weighted metals in HCl and HNO, (Merck). The solutions were then evaporated to dryness and diluted to final volume with distilled water. Indium and aluminium were determined gravimetrically as oxides, copper was determined electrogravimetrically and iron was determined complexometrically with EDTA (Merck). Aqueous solutions of uranium(v1) were prepared from UO,(NO,),, 6 H 2 0 (Merck) and standardized gravimetrically as U,O,. Fresh aqueous solutions of citric, tartaric and malic acids were prepared from analytical-grade reagents (Merck) and standardized potentiometrically, using standard NaOH (Merck) solutions.All solutions contained 0.5 mol dm-, NaNO, (Merck), to maintain a constant ionic strength. The pH of the solutions was measured with an El-Hama combined electrode t Represents part of the Ph.D. thesis submitted by Emanual Manzurola to Ben Gurion University of the Negev.