Oxygen Combustion Calorimetry of Samples Containing Phosphorus and Fluorine: Formation of Fluorinated Phosphoric Acids as Non-Ideal Combustion Species

Abstract

Whilst measuring the energy of combustion of a series of novel energetic polyphosphazenes which contained both phosphorus and fluorine in varying ratios, the non-ideal aqueous combustion species monofluoro-, difluoro- and hexafluoro-phosphoric acids were observed, in addition to the expected H3PO4 and HF. The same aqueous products were also observed to arise from combustion of physical mixtures of P- and F-containing compounds. A quantitative analytical method based on 19F NMR spectroscopy and Ion Chromatography of the bomb solutions was developed. However, since monofluoro- and difluoro-phosphoric acids are hydrolytically unstable, it was necessary to replace the water normally added to the bomb at the outset of the calorimetric experiments with an aqueous buffer solution to inhibit hydrolysis and hence obtain meaningful analytical data. Despite the relatively low precision ΔcU data obtained with the available adiabatic, static bomb calorimeter, the magnitude of the corrections to standard states accounting for the energy of (aqueous) hydrolysis of the fluorinated phosphoric acids were estimated. Two potential secondary standards for the oxygen combustion calorimetry of samples containing phosphorus and fluorine were identified and calorimetrically assessed.