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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.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

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Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
supporting 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:
no data
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
5.41
Temp.:
25 °C
Remarks on result:
other: Acid-base constant: Second Dissociation Constant pka2 (Hammer, 1945)
No.:
#2
pKa:
2.95
Temp.:
25 °C
Remarks on result:
other: pKa1: Kirk-Othmer, 2008
No.:
#3
pKa:
5.41
Temp.:
25 °C
Remarks on result:
other: pKa2: Kirk-Othmer, 2008
No.:
#4
pKa:
2.94
Temp.:
25 °C
Remarks on result:
other: pKa1: Lide, 2002
No.:
#5
pKa:
5.43
Temp.:
25 °C
Remarks on result:
other: pKa2: Lide, 2002
Hammer, 1945
Average value of pka2 at different temperature:

5.43 +/- 0.0007 ( 0 °C)
5.41 +/- 0.0008 (25 °C)
5.54 +/- 0.0009 (60 °C)
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:
Measurements of pH changes during titrations using glass electrodes
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
2.97
Temp.:
35 °C
Remarks on result:
other: pKa1
No.:
#2
pKa:
5.43
Temp.:
35 °C
Remarks on result:
other: pKa2
Dissociation constants (pka1, pka2), measured at different temperatures:
pka1 pka2
25 °C 2.76 4.92
2.63 4.73
3.05 4.89
35 °C 2.97 5.43
Executive summary:

The dissociation constants for phthalic acid are pKa1 of 2.97 and pKa2 of 5.43, both at 35°C (Serjeant, 1979).

Description of key information

The dissociation constants for phthalic acid are pKa1 of 2.97 and pKa2 of 5.43, both at 35°C (Serjeant, 1979).

Key value for chemical safety assessment

pKa at 20°C:
2.97

Additional information