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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
secondary literature
Qualifier:
no guideline followed
Principles of method if other than guideline:
No details provided how the endpoint was measured.
GLP compliance:
not specified
Dissociating properties:
yes
No.:
#1
pKa:
2.38
Temp.:
25 °C
Conclusions:
The dissociation constant for MPA was reported to be 2.38 at 25 °C.

Description of key information

Since only secondary literature is available a weight of evidence approach was used to cover this endpoint

Key value for chemical safety assessment

pKa at 20°C:
2.12

Additional information

In 1953 Crofts and Kosolapoff published two dissociation constant values for methyl phosphonic acid that have been reviewed by Kingery and Allan in 1995. Crofts and Kosolapoff published a pKa of 2.38 and 7.74.

Harris et al. published also two dissociation constant values at pKa of 2.2 and 7.7 in 1964, that have been also reviewed by Kingery and Allan in 1995.

Serjant and Dempsey published pKa values of 2.12 and 7.29 in 1979 and Munro et al. published a pKa of 2.38 at 25 °C in 1999.

Taking these data together it seems that the slight differences observed in the lower dissociation constant might be due to different temperatures these constants were measured. Since in most cases temperatures are not reported it is difficult to estimate the pKa value at 20 °C. Therefore the value of 2.12 is used for the temperature of about 20 °C, assuming that this value was measured at a room temperature of about 20 °C.