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Physical & Chemical properties

Dissociation constant

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dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2010-02-10 to 2010-07-12
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
comparable to guideline study
according to guideline
other: Laboratory method (CIPAC method MT 22, 1994).
Principles of method if other than guideline:
To obtain a value for dissociation constant (pKa) which quantitatively expresses the extent to which a substance dissociates in solution. The higher the value of pKa, the less dissociation of the species in solution. This value varies with temperature, ionic strength, and the nature of the solvent. The test is carried out using the titration method.

A test solution of the sample in water is prepared to a concentration of 0.1 M. The solution is then titrated with a standard base or acid solution as appropriate, measuring the pH after each addition of titrant. About 0.1 mL to 2 mL of titrant is added at a time. Where possible, about 10 incremental additions should be made before the equivalence point is reached.
At the end of the titration, a titration curve is obtained by plotting the pH of the test solution after each addition of titrant vs. volume of titrant added.
The equivalence point or point of inflection of the titration is obtained by plotting the ratio of change in pH of test solution (after each addition of titrant) to change in volume (of titrant) vs. mean volume of titrant.
The mid-point of the equivalence point is where a well buffered region exists, and the concentration of the undissociated species is approximately equal to the concentration of dissociated species. The pH of the solution at this mid-point provides the pKa of the test material.
Using this estimate, the titration is repeated as before. At approximately 1 mL of titrant to the mid-point of the equivalence point, 0.1 mL increments are added at a time. The pH of the test solution, when half of the volume of titrant required to reach the equivalence point has been added, determines the pKa of the sample.
GLP compliance:
Specific details on test material used for the study:
Component of a Polyurethane System
Polyoxypropylene triamine
(39423-51-3) 60-100%
Dissociating properties:
20 °C
The test substance was demonstrated to have a pKa of 10.25.

Description of key information

The test substance was demonstrated to have a pKa of 10.25.

Key value for chemical safety assessment

pKa at 20°C:

Additional information