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Physical & Chemical properties

Dissociation constant

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Description of key information

The dissociation constants of magnesium hydrogenorthophosphate were estimated on the basis of literature values for phosphoric acid.

Literature values of pka at 25°C:

2.16

7.21

12.32

Key value for chemical safety assessment

Additional information

No experimental determination of the dissociation constants in water was performed for the test material as it was anticipated that on performance of the test procedures, as detailed in Method 112 of the OECD Guidelines for Testing of Chemicals, 12 May 1981, that the resulting dissociation constants determined would be that of the parent anions only, for which literature values are available. For example, it is anticipated that different types of orthophosphate will demonstrate significantly different pH values in water due to increasing numbers of protons being substituted with the particular cation on titration with acid or base as appropriate. The actual dissociation constants determined for each compound would be common, i.e. that of the triprotic acid anion. Titrations would also be expected to be similar irrespective of the counter ion.

Thus, the dissociation constants of magnesium hydrogenorthophosphate were estimated on available literature data for phosphoric acid. Read-across is justified on the basis that phosphoric acid is the parent acid for all inorganic phosphates. The literature values for phosphoric acid are shown in the following table.

 No. pKa  Temperature (°C)
 #1  2.16  25
 #2  7.21  25
 #3  12.32  25