Thermodynamic study of gaseous sodium-phosphorus-oxygen ternary molecules by high temperature mass spectrometry

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Abstract
The equilibrium gaseous species over a condensed sodium tripolyphosphate melt were investigated by the high temperature mass spectrometric technique in the temperature range 1270–1567 K. Third law enthalpies and, where possible, second law enthalpies were measured for the following gas phase reactions involving the gaseous ternary species, NaPO or NaOP, monomeric sodium metaphosphite, NaPO2, and sodium metaphosphate, NaPO3: NaPO(g) =Na(g) + PO(g), NaOP(g) =Na(g) + PO(g), NaPO2(g) =Na(g) + PO2(g), and 2NaPO2(g) =NaPO3(g) + NaOP(g). The resulting enthalpies of reaction, ΔH°0 (in kJ), are given as 267.8±25, 281.4±25, 333.2±12, and 39.5±35, respectively. The atomization energies, ΔH°a,0, and standard heats of formation, ΔH°f,298, for the gaseous molecules NaPO, NaOP, NaPO2, and NaPO3 were derived from these reaction enthalpies in conjunction with appropriate literature data as follows: 861±30 and −176±30; 874±30 and −191±30; 1419±20 and −492±20; and 1925±45 and −753±45 kJ mol−1, respectively. The atomization energies of the ternary molecules have been discussed in terms of the bond additivity postulate.