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Dissociation constant

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dissociation constant
Type of information:
experimental study
Adequacy of study:
supporting study
4 (not assignable)
Rationale for reliability incl. deficiencies:
secondary literature
Principles of method if other than guideline:
No details are available on the guideline used to determine this result.
GLP compliance:
Dissociating properties:

In a source giving no experimental details, HMDTMP is described as having six pKa values, of: 

Description of key information

Dissociation constant: HMDTMP acid has eight ionisable phosphonate groups.

Key value for chemical safety assessment

Additional information

In aqueous solution, HMDTMP (4 -7Na) will ionise to form HMDTMP acid and sodium ions. Dissociation constants are obtained for the salt by valid read-across from the parent acid. Eight 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.

pKa1 = 1.3
pKa 2 = 2
pKa 3 = 3.4
pKa 4 = 4.1
pKa 5 = 5.9
pKa 6 = 6.6

pKa 7 = 8

pKa 8 = >10

Similarly, in a secondary source to which reliability could not be assigned, six dissociation constant values as listed below are reported for the substance.

pKa 3 = 3.25 
pKa 4 = 5.12 
pKa 5 = 5.68
pKa 6 = 6.23 
pKa 7 = 7.71 
pKa 8 = 11.82


An approximate experimental determination, using stoichiometric titration 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.