Registration Dossier

Physical & Chemical properties

Dissociation constant

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

Dissociation constant (HEBMP-H): HEBMP-H acid has four ionisable protons. Dissociation constant (HEBMP-H cyclic): HEBMP-H cyclic has three ionisable protons.

Key value for chemical safety assessment

Additional information

Four dissociation constants as listed below were obtained for the parent acid using an appropriate estimation method. The result is considered reliable and selected as key study.

The non-cyclic constituent has four dissociation c onstants as listed below:

pKa1 = 1.3 pKa2 = 2.7 pKa3 = 5.9 pKa4 = >8


Similarly, the cyclic constituent has three dissociation constants as listed below:

pKa1 = 1.3

pKa2 = 2.7

pKa3 = 7.3


An approximate experimental determination, usingstoichiometrictitration and pH determination of aqueous solutions, has been conducted. This is considered to provide suitable and practical information, which is fit for purpose in the context of REACH chemical safety assessment. This data is reported in section 4.20.


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.