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Physical & Chemical properties

Dissociation constant

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Dissociation constants for the components:
1) Measured complex formation constant calcium phosphate - humic acid complex: log K = 3.07
2) pKa calcium phosphate (handbook data):
pKa Calcium (II) ion: 12.6 (25 °C), pKa values phosphoric acid: 2.16, 7.21, 12.32 (25 °C)
3) pKa calcium sulfate (published): 4.35 at 298.15 K.
4) Measured pKa values for humic acid: ca. 4 (carboxylic groups) and ca. 9-10 (phenolic and amino groups).

Key value for chemical safety assessment

Additional information

Data about the dissociation constants of all components of the submision substance is available:

The complex formation constant of a model complex similar to the submission substance was determined by the the AOAC Official Method 983.03 (1983). The complex dissociates at pH=8 with the apparent stability constant log K=3.07, i.e. the ratio between free humic acid (or free calcium phosphate) to the complex is ca.1:35.

The Handbook of Chemistry and Physics provides pKa values for calcium phosphate:

pKa Calcium (II) ion: 12.6 (25 °C); pKa values phosphoric acid: 2.16, 7.21, 12.32 (25 °C)

Published data on the dissociation of calcium sulfate in water is presented: Corti and Fernandez-Prini (1984), Messnaoui and Bounahmidi (2006). The pK value for calcium sulfate is 4.35 at 298.15 K.

Titration measurements of humic acid samples reveal two distinct pKa values: ca. 4 (carboxylic groups) and ca. 9-10 (phenolic and amino groups).

The component rock phosphate or calcium fluoroapatite is insoluble and therefore does not dissociate.