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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Literature pKa values for acid and base components reported instead of experimental determination which was technically not possible due to multiplicity of pKa values for each component.
Qualifier:
no guideline required
Principles of method if other than guideline:
No determination of dissociation constant in water was performed by Method 112 of the OECD Guidelines for Testing of Chemicals, 12 May 1981, due to the availability of literature values for the individual components of the test material. Also, as the test material was a mixture of two components with multiple pKa's, an experimental determination would have proved technically very difficult. Each component had been previously investigated for their dissociating properties and the results reported in respected journals and text books. Journal reported literature values were obtained using the ACD/1-Lab Web service (ACD/pKa DB 7.02).
GLP compliance:
yes (incl. QA statement)
Dissociating properties:
yes
No.:
#1
pKa:
5.16
Remarks on result:
other: Organic component (protonation of ring nitrogen)
No.:
#2
pKa:
ca. 1
Remarks on result:
other: Pyrophosphoric acid
Remarks:
successive deprotonation of active hydrogen atoms
No.:
#3
pKa:
ca. 1.8
Remarks on result:
other: Pyrophosphoric acid
Remarks:
successive deprotonation of active hydrogen atoms
No.:
#4
pKa:
6.57
Remarks on result:
other: Pyrophosphoric acid
Remarks:
successive deprotonation of active hydrogen atoms
No.:
#5
pKa:
9.64
Remarks on result:
other: Pyrophosphoric acid
Remarks:
successive deprotonation of active hydrogen atoms

The following references were found:

(1) pKa of melamine - Albert A., Goldacre R., and Phillips J. N., J. Chern. Soc., 1948, 2240

(2) pKa of pyrophosphoric acid - Greenwood N. N., and Earnshaw A., Chemistry of the Elements, Butterworth-Heinemann, Oxford, 2001.

Conclusions:
The organic component (melamine) has one measurable pKa value of 5.16. Pyrophosphoric acid can dissociate by successive deprotonation resulting in four measurable pKa values, of ~1, ~1.8, 6.57 and 9.64.
However it is possible that MPP shows different physico-chemical properties from melamine and pyrophosphate because MPP is not inorganic salt of melamine or pyrophosphate but organic salt constituted by both of them. And according to these differences, it may also show different environmental and metabolic pathway from melamine and pyrophosphate. Therefore, dissociation constant will be conducted by MPP itself and its result will be used for conclusion.

Description of key information

The organic component has one measurable pKa value of 5.16. The inorganic component, pyrophosphoric acid, can dissociate by successive deprotonation resulting in four measurable pKa values, of ~1, ~1.8, 6.57 and 9.64

Key value for chemical safety assessment

Additional information

Read-across from supporting substances melamine and pyrophosphate

However it is possible that MPP shows different physico-chemical properties from melamine and pyrophosphate because MPP is not inorganic salt of melamine or pyrophosphate but an organic salt constituted by both of them. And according to these differences, it may also show different environmental and metabolic pathway from melamine and pyrophosphate. Therefore, dissociation constant will be conducted by MPP itself and its result will be used for conclusion.