The formation of orange red chelate between hexavalent uranium and 8-hydroxy quinoline-7-sulfonic acid has been studied in aqueous medium spectrophotometrically. The chelate is stable in the pH range 5 to 6.5. The composition of the chelate has been determined by three different methods and found that a stable complex is formed between one mole of uranium(VI) and two moles of the reagent. The stability constant in aqueous medium determined by the method of Banerji and Dey and by the mole-ratio

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... ethod was found to be log K = 8.9±0.06 and the free energy of formation —1229 k.cal/mole at 25°. The effect of ionic strength and temperature on the chelate has been investigated. The enthalpy change and entropy change are —3 k.cal/mole and 59 e.u. respectively. The interference of various ions in the measurement of colour and the probable structure of the chelate in the solution are also suggested.