Ahstract4toichiometric stability constants for the formation of scandium(II1) chlorate and scandium(III) bromate complexes have been measured at several temperatures by using a liquid cation exchanger. Relations between ligational enthalpy and entropy changes are discussed in COMCXiOn with the formation of outer-sphere ion pairs and inner-sphere complexes. R&um&Mesure des constanks de stabilitd stoechiom&riques propres B Ia formation des complexes du chlorate de scandium(II1) et du bromate de scandium(III), & diverses temp&ratures, en utilisant un 6changeur de cation liquide. Les relations entre les variations d'enthalpie et d'entropie de coordination sont discut&s en liaison avec la formation de paires d'ions p&iph&iques et de complexes intemes. Zusammenfassung-Die stiichiometrischen Stabilitftskonstanten fiir die Bildung von Scandium (III)-chlorat und Scandium(II@bromat wurden bei verschiedenen Temperaturen unter Verwendung eines I%.issigkeitsionenaustauschers gemcssen. Die Aenderung der Ligandenthalpie und -entropic wird im Zusammenhang mit der Bildung von Ionenpaaren in der &usseren Sphlre und von Komplexen in der inneren Sphlre besprochen. AS PART of a wide investigation of the formation of wholly inorganic complexes in aqueous solution, a quantitative study has been made of the stability of scandium(II1) chlorate and scandium(II1) bromate complex species. Scandium-46 has been used as a tracer to follow the solvent extraction of SC?+ in the presence of varying amounts of halate ion. The extractant, dinonyl naphthalene sulphonic acid (HD) in solution in n-heptane, has been shown to have the properties of a liquid cation-exchanger.l In the experiments the hydrogen-ion concentration and ionic strength p of the aqueous complex solutions was held constant and fairly high (l-00 mol/dm3). This ensured that hydrolysis of Sc3+ (hydrated) was suppressed and, although it precluded the dependable estimation of thermodynamic stability constants valid for ,u = 0, the stoichiometric equilibrium constants & = [SC(AO,)~~-~)+]IISCS~] [AO,]j (A = Cl, Br: j = 1, 2) obtained can be collated with corresponding data obtained for other systems by the same procedure. Scandium-46 decays with a half-life of 84 days [S-O-36 (ca 100 %), 1.48 (0.004 %) MeV; y O-89 (100x), 1-12 ( ca 100 %) MeV] to the stable nuclide *=Ti. Values of the distribution ratio D Count rate of lsSc per cm5 of equilibrium organic phase = Count rate of "SC per cm3 of equilibrium aqueous phase = Concentration of scandium(III)(mol/dms) in organic phase -Concentration of scandium(III)(mol/dms) in aqueous phase were determined by y-ray counting. METHOD The method employed to derive the stability constants p, is inherently similar to that used by Morris, Hedger and WatsonB *