Registration Dossier

Data platform availability banner - registered substances factsheets

Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Physical & Chemical properties

Dissociation constant

Currently viewing:

Administrative data

Link to relevant study record(s)

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-xK will dissociate to form HEDP and potassium ions. Dissociation constants are obtained for the salt by valid read-across from the parent acid, HEDP-H. 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 

In a secondary source (Lacour 1999) to which reliability could not be assigned, four pKa values were reported for HEDP-H

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.