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Description of key information

Key value for chemical safety assessment

Additional information

Trimagnesium bis(orthophosphate) (CAS 7757-87-1) is well soluble in water and dissociates to magnesium cations and phosphate species. A determination of the hydrolysis of the magnesium and the phosphate as function of pH according to OECD guideline 111 was not conducted since both ions have no potential mechanism for further hydrolysis or hydrolytic degradation.

The inorganic phosphates are normally found in different forms. In dilute aqueous solution, phosphate exists in four forms (see attachment). In strongly-basic conditions, the phosphate ion (PO43−) predominates, whereas in weakly-basic conditions, the hydrogen phosphate ion (HPO42−) is prevalent. In weakly-acid conditions, the dihydrogen phosphate ion (H2PO4−) is most common. In strongly-acid conditions, aqueous phosphoric acid (H3PO4) is the main form.

In a study evaluating the abiotic degradation of the anions of specified phosphates, pyrophosphates and triphosphates the hydrolytic half-live has been addressed by read across from a single example for each chemical class, where applicable. The ionic substances were assumed to undergo dissociation in aqueous solution and the resulting cation to have negligible influence on the hydrolysis rate of the anion within the buffer solutions used as instructed by Method 111 of the OECD Guidelines (Harlan, 2011).

The available data demonstrate that triphosphates and tripolyphosphates are hydrolytically stable under environmental conditions with a half-life > 1 year.

Chemical Class

Substance Used for Experimental Determination

Anticipated Half-Life at 25°C


Phosphates Orthophosphates

Not applicable, no possible mechanism for hydrolysis

pH4 > 1 year pH 7 > 1 year pH 9 > 1 year

Pyrophosphates Diphosphates

Tetrapotassium Pyrophosphate

CAS 7320-34-5

pH4 > 1 year pH 7 > 1 year

Triphosphates Tripolyphosphates

Pentapotassium Triphosphate

CAS 13845-36-8

pH 9

> 1 year


pH 7

14.5 days > 1 year

pH 9

> 1 year