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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.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

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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
No.:
#1
pKa:
< 2
Temp.:
25 °C
No.:
#2
pKa:
< 2
Temp.:
25 °C
No.:
#3
pKa:
4.3
Temp.:
25 °C
Remarks on result:
other: ± 0.05
No.:
#4
pKa:
5.46
Temp.:
25 °C
Remarks on result:
other: ±0.05
No.:
#5
pKa:
6.66
Temp.:
25 °C
Remarks on result:
other: ±0.04
No.:
#6
pKa:
12.3
Temp.:
25 °C
Remarks on result:
other: ±0.1

ATMP has six possible ionisations.

pK1 <2
pK2 <2
pK3 4.3
pK4 5.46
pK5 6.66
pK6 12.3

These were measured in 1 M potassium nitrate.
These data are from a recognised data source for information on pKa  values and stability constants (Martell and Sillen).

Conclusions:
Dissociation constant values of <2, <2, 4.3, 5.46, 6.66, 12.3 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 [ATMP-H]: ATMP acid has six ionisable phosphonate groups.

Key value for chemical safety assessment

Additional information

Six dissociation constants as listed below were reported for ATMP-H from a handbook or collection of reliable data. The result (Martell 1968) is considered reliable and selected as key study.

pKa1 = <2
pKa 2 = <2
pKa 3 = 4.3
pKa 4 = 5.5
pKa 5 = 6.7
pKa 6 = 12.3


In Lacour 1999, dissociation constant values at 25°C were reported for ATMP-H, using a potentiometric method.

pKa1 = 0.5

pKa2 = 1.6

pKa3 = 4.6

pKa4 = 5.9

pKa5 = 7.2

pKa6 = 12.5

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

pKa1 = 2.2
pKa2 = 2.2 
pKa3 = 4.5
pKa4 = 5.9 
pKa5 = 7.3 
pKa6 = 11.4

 

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.