Calorimetric studies on the thermal decomposition of tri n-butyl phosphate-nitric acid systems

Abstract

Thermal decomposition of neat TBP, acid-solvates (TBP·1.1HNO3, TBP·2.4HNO3) (prepared by equilibrating neat TBP with 8 and 15.6 M nitric acid) with and without the presence of additives such as uranyl nitrate, sodium nitrate and sodium nitrite, mixtures of neat TBP and nitric acid of different acidities, 1.1 M TBP solutions in diluents such as n-dodecane (n-DD), n-octane and isooctane has been studied using an adiabatic calorimeter. Enthalpy change and the activation energy for the decomposition reaction derived from the calorimetric data wherever possible are reported in this article. Neat TBP was found to be stable up to 255 °C, whereas the acid-solvates TBP·1.1HNO3 and TBP·2.4HNO3 decomposed at 120 and 111 °C, respectively, with a decomposition enthalpy of −495.8 ± 10.9 and −1115.5 ± 8.2 kJ mol−1 of TBP. Activation energy and pre exponential factor derived from the calorimetric data for the decomposition of these acid-solvates were found be 108.8 ± 3.7, 103.5 ± 1.4 kJ mol−1 of TBP and 6.1 × 1010 and 5.6 × 109 S−1, respectively. The thermochemical parameters such as, the onset temperature, enthalpy of decomposition, activation energy and the pre-exponential factor were found to strongly depend on acid-solvate stoichiometry. Heat capacity (C p ), of neat TBP and the acid-solvates (TBP·1.1HNO3 and TBP·2.4HNO3) were measured at constant pressure using heat flux type differential scanning calorimeter (DSC) in the temperature range 32–67 °C. The values obtained at 32 °C for neat TBP, acid-solvates TBP·1.1HNO3 and TBP·2.4HNO3 are 1.8, 1.76 and 1.63 J g−1 K−1, respectively. C p of neat TBP, 1.82 J g−1 K−1, was also measured at 27 °C using ‘hot disk’ method and was found to agree well with the values obtained by DSC method.