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 Δ_cU 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.

Keywords:

• Bomb calorimetry
• Combustion enthalpy
• Energetic polyphosphazenes
• Fluorinated phosphoric acids

Notes

‡(Reddy et al., 1970); CFCl₃ was used as the internal reference.

†CFCl₃ was the ‘nominal’ instrumental ¹⁹F reference.

†Δ_cU values corrected for the energy of formation of aqueous HNO₃, taken (Roux et al., 2001) as Δ_fH° HNO_3(aq) = − 59.7 kJ mol⁻¹, and energy of dilution of crystalline H₃PO₄, taken (Bedford et al., 1960) as Δ_dilU° H₃PO_4(aq) = − 12.12 kJ mol⁻¹. The uncertainty intervals (not shown) were estimated as twice the standard deviation of the mean value of the replicate experiments and fluctuated between ±0.9 and ±2.0%.

‡Values rounded to the nearest 10 J g⁻¹.

^†SD: standard deviation. The uncertainty intervals were estimated as twice the standard deviation of the mean value of the replicate experiments (referred to as ‘average’). The mean Δ_cU values were rounded to 5 significant figures.

^†SD: standard deviation. The uncertainty intervals were estimated as twice the standard deviation of the mean value of the replicate experiments (referred to as ‘average’). The mean Δ_cU values were rounded to 4 significant figures.