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HEBMP-H is a multi-constituent substance with two constituents that are the linear and cyclised forms of the same basic structure.

In solution, the linear from, [[(2-hydroxyethyl)imino]bis(methylene)]-bisphosphonic acid (and its anions) (l-HEBMP (CAS 5995-42-6)) undergoes a reversible internal condensation reaction to form a pH-dependent equilibrium mixture with the cyclic form, 4-(phosphonomethyl)-2-hydroxy-2-oxo-1,4,2-oxaza-phosphorinane and its anions (CAS 32422-02-9, c-HEBMP) and water. The precise proportion of the two constituents is dependent on pH, temperature, concentration and time.

Spectroscopic evidence from 31P-NMR analysis suggests that in the pH range ca. 1 – 7, a high proportion (typically 50%, range 50-75% w/w) is present as the cyclised constituent. A higher proportion of the linear form is expected at higher pH values.

The two constituents exist in equilibrium. The properties of the whole substance, in which both constituents are present at representative levels, are used in the chemical safety assessment.

The anhydrous form of HEBMP-H is a solid under ambient conditions, however, commercial samples as supplied in Europe are typically manufactured in situ as an aqueous solution (liquid). HEBMP-H will undergo thermal decomposition when heated, the substance has specific gravity values in the range 1.26 - 1.39 g/cm³ under ambient conditions for various commercial solutions. l-HEBMP and c-HEBMP have predicted vapour pressure values of 2.7E-9 Pa at 25°C and 5.5E-8 Pa at 25°C, predicted log Kow values of -4 and -3.4 respectively. 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.

l-HEBMP has four P-OH groups and c-HEBMP has three P-OH groups that can be ionised in solution. The ionised species present is dependent on the pH of the solution. The pKa values of both the l-HEBMP and c-HEBMP are in the range 1.3 to >8, the results indicate that HEBMP will be ionised at environmentally relevant pH values.

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008 based on a preliminary screening study performed according to EU Method A.10. There is evidence of relative self-ignition observed below 400°C based on EU Method A.16. It is not explosive and not oxidising based on structural examination.

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