| Summary: | The enthalpy of transfer (DeltaHt°) of uranyl nitrate hexahydrate from 0.492 M nitric acid and 0.5 M tri-n-butyl phosphate (TBP) in cyclohexane solutions has been measured by (i) direct calorimetric, and (ii) temperature coefficient methods. The present work applies the calorimetric measurements to aqueous and organic phases separately, in order to obtain DeltaHt° (uranyl nitrate hexahydrate) from an aqueous to an organic phase. In the calorimetric method the aqueous phase contained nitric acid and was presaturated with the organic phase, and the organic phase had itself the composition derived from presaturation with aqueous nitric acid in order that conditions for the determination of DeltaHt°should be those prevailing in determinations by the temperature coefficient method. The composition of presaturated aqueous and organic phases has been obtained by analytical measurements. The standard state was that of infinite dilution of uranyl nitrate in the specified aqueous phase and infinite dilution of uranyl nitrate in the specified organic phase. In order to obtain DeltaHt° by the temperature coefficient method, the variation of the distribution coefficient of uranyl nitrate hexahydrate in the nitric acid-TBP system, as a function of temperature (25-60°C) has been studied employing cyclohexane as a diluent, at four concentrations of TBP (0.25, 0.50, 0.75 and 1.00 mol l-1) and at four concentrations of aqueous nitric acid (0.492, 1.5, 2.5 and 4.0 mol l-1). All the data obtained showed that the extraction of U(VI) from nitric acid into TBP-cyclohexane is exothermic in nature. The thermodynamic functions for the extraction reaction: have been calculated. It is clearly seen from the data in the table that the DeltaHt° values are similar, within the experimental error. Variation of the heat of solution of UO2(NO3)26H2O at different concentrations in different concentrations of nitric acid has also been measured, and the percentage of nitrate complex UO2NO3+ at different nitric acid concentrations has been calculated. The standard heat of solution of UO2(NO3)26H2O in water has been calculated (DeltaHs° = 4995 +/- 80 cal mol-1) from these measurements. A critical evaluation of thermodynamic quantities in this and closely related systems is offered.
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