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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
The salts are produced from the parent acid; [Hexane-1,6-diylbis[nitrilo(methylene)]]tetrakisphos-phonic acid. The read-across is therefore based on the presence of common functional groups in both the acid and the salt . The impurity profile of the salt is expected to be similar to that of the parent acid since the salt is prepared from the acid.
Reason / purpose:
read-across source
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

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 ion.Dissociation constant 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 apporpriate 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 1 = 3.25 
pKa 2 = 5.12 
pKa 3 = 5.68
pKa 4 = 6.23 
pKa 5 = 7.71 
pKa 6 = 11.82

 

An approximate experimental determination, using stocichiometric 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.