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Physical & Chemical properties

Water solubility

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Description of key information

Water solubility (HEBMP-H): The substance is very soluble in water and has a water solubility value of 75.7 - 77.2% w/w of solution at 20.0 ± 0.5 °C  and at pH= 4.2 -4.3 (approximately 750 g/l at 20°C) (OECD 105)

Key value for chemical safety assessment

Water solubility:
750 g/L
at the temperature of:
20 °C

Additional information

In solution, HEBMP (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, a combination of read-across, evidence from supplier's literature of extremely high solubility is used to complement experimental data for the equilibrium mixture.


The substance HEBMP-H (and its analogues) are known to be highly hydrophilic and extremely water-soluble. A measured water solubility of 75.7 - 77.2% w/w of solution at 20.0 ± 0.5°C (approximately 750 g/l) was determined for HEBMP-xNa in accordance with OECD 105. The result was considered to be reliable and selected as key study. This value is supported by water solubilities in the range 450 - 820 g/l for HEBMP-H and HEBMP-xNa in secondary sources to which reliability could not be assigned. Aqueous solutions at 50-60% w/w are typical for HEBMP commercial products as supplied. The supporting studies are in agreement with the key study that the substance is highly soluble in water.


The aqueous forms as supplied in EU are miscible in water.