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Vapour pressure

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Description of key information

Vapour pressure (HEBMP-xNa): < 0.0038 Pa at 25 °C (OECD 104)

Key value for chemical safety assessment

Additional information

A measured vapour pressure of <0.0038 Pa at 25 °C was determined for HEBMP-xNa in accordance with OECD 104 test method and in compliance with GLP. The result is considered to be reliable and selected as key study. This data is read-across to the HEBMP.


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 each constituent must be understood and assessed separately in the chemical safety assessment. Since it is not possible to separate the constituents for testing individually, property prediction is used to complement experimental data for the equilibrium mixture.


Predicted vapour pressures of 2.7E-09 Pa and 5.5E-08 Pa were obtained for the linear HEBMP-H and cyclic condensation product of HEBMP-H (c-HEBMP) respectively at 25°C using an accepted calculation method (MPBPVP program, USEPA/Syracuse Research, v1.43). The results (Fisk 2010) are considered to be reliable. The value represents the major constituent only and demonstrates that it is involatile.