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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Principles of method if other than guideline:
not stated
GLP compliance:
no
Dissociating properties:
yes
Remarks on result:
other: Acid-base constant: Acid-base-titration
No.:
#1
pKa:
1.7
Temp.:
25 °C
Remarks on result:
other: ± 0.2
No.:
#2
pKa:
2.47
Temp.:
25 °C
Remarks on result:
other: ± 0.04
No.:
#3
pKa:
7.28
Temp.:
25 °C
Remarks on result:
other: ± 0.06
No.:
#4
pKa:
10.29
Temp.:
25 °C
Remarks on result:
other: ±0.07

HEDP has four possible ionisations. In a recognised data source for information on pKa values and stability constants, pKa values of HEDP are reported of:

pKal = 1.7 
pKa2 = 2.47 
pKa3 = 7.28 
pKa4 = 10.29 

These were measured by titration in 0.1 M potassium chloride. The original source is cited in the data book. These data are from a recognised handbook.

Conclusions:
Dissociation constant values of 1.7, 2.47 , 7.28, 11.41 are reported at 25°C in a handbook which has been subject to peer-review and in which the original data sources are traceable. The result is considered to be reliable.

Description of key information

Dissociation constant: HEDP acid has four ionisable phosphonate groups.

Key value for chemical safety assessment

Additional information

In aqueous solution, HEDP (2-3Na) will ionise to form HEDP and sodium ions. Dissociation constants are obtained for the salt by valid read-across from the parent acid. Four dissociation constants as listed below were obtained for the parent acid using an appropriate method (Martell 1968). The result is considered reliable and selected as key study.

pKa1 = 1.7 
pKa2 = 2.47 
pKa3 = 7.28
pKa4 = 10.29 

Four pKa values were determined in a second reliable study (Lacour 1999) and are listed below.

pKa1 = 1.6
pKa2 = 2.7
pKa3 = 6.9
pKa4 = 11.0

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.