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Boiling point

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Description of key information

Boiling point (HEBMP-H): HEBMP-H will undergo thermal decomposition without boiling. 

Key value for chemical safety assessment

Additional information

In solution, HEBMP-H (CAS 5995-42-6) undergoes a reversible internal condensation reaction to form a pH-dependent equilibrium mixture with the cyclised structure 4-(Phosphonomethyl)-2-hydroxy-2-oxo-1,4,2-oxaza-phosphorinane (CAS 32422-02-9, c-HEBMP) and water. The precise proportion of the two forms is dependent on pH, temperature and time, and under acidic or neutral conditions, the proportion of c-HEBMP in the multi-constituent substance may reach proportions as high as ≥ 50% w/w. Spectroscopic evidence from 31P-NMR analysis suggests that in the pH range ca. 1 – 7 a high proportion (50-75% w/w) is present as the cyclised form. The two forms exist in equilibrium therefore it is extremely difficult to separate and purify both forms.The properties of the whole substance, in which both forms are present at representative levels, are used in the chemical safety assessment.

The requirement to test the boiling point of HEBMP-H was waived because the substance decomposes without boiling. This statement is supported by a recent, reliable DSC test which demonstrates that the salt melts with decomposition without boiling (Fox 2012). This is supported by secondary sources with data for HEBMP, to which no reliability could be assigned. Boiling point values of >100°C and 105°C with a decomposition temperature of >200°C after evaporation of water were reported for HEBMP-H acid in solution.