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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Remarks:
This is an accepted calculation method supported by a limited amount of additional validation.
Justification for type of information:
See attached document
Principles of method if other than guideline:
Predictive method
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
1.3
Temp.:
20 °C
No.:
#2
pKa:
2
Temp.:
20 °C
No.:
#3
pKa:
3.4
Temp.:
20 °C
No.:
#4
pKa:
4.1
Temp.:
20 °C
No.:
#5
pKa:
5.9
Temp.:
20 °C
No.:
#6
pKa:
6.6
Temp.:
20 °C
No.:
#7
pKa:
8
Temp.:
20 °C
No.:
#8
pKa:
> 10
Temp.:
20 °C

HMDTMP was determined to have eight pKa values, of: 
1.3
2
3.4
4.1
5.9
6.6

8

>10

Conclusions:
HMDTMP acid was determined to have eight dissociation constants using a relevant estimation method.

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 -7K) will ionise to form HMDTMP acid and potassium 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 available 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.