Magnesium dimetaphosphate

Hydrolysis

Additional information

Solubility of Magnesium dimetaphosphate (CAS 13573-12-1) in water is limited and regulated by the pH under the environmental conditions. In release to water the dissolved substance dissociates to magnesium cations and metaphosphate anions that undergoes hydrolysis step by step forming phosphate species. A determination of the hydrolysis as function of pH according to OECD guideline 111 was not conducted as ionic magnesium is not subject to hydrolysis and hydrolysis of phosphate complexes as a function of pH has been well studied.

The kinetic of hydrolysis of cyclic sodium trimetaphosphate (P3O92-) was investigated at acidic and alkaline solution, and at temperatures between 50 and 70 °C, by Healy & Kilpatrick (1955). The hydrolysis was rapid in acid solution, less rapid in alkaline solution, but negligible at pH9. The kinetic data showed that although H3P3O9 has been classified as strong tribasic acid. It is necessary to take into account that the solution reactivity increases with increase in (positive) charge of species involved and hydrolysis spread up in present of metal ions, such as Ca, Mg, Ba and Al. The experimental data were in good agreement with the following mechanism and gave a pseudo first-order reaction law and the hydrolysis may be written as:

P3O92- + H2O → P3O105- → PO43- + P2O74- + 2H

P2O74- + H2O →2 O43- + 2H+

The kinetics of hydrolysis of triphosphate and pyrophosphate were also studied in sterile lake water and sterile algal culture media and in non-sterile media at 25 °C by Clesceri and Lee (1965a, 1965b),and compared to published results obtained in distilled water. The results showed that triphosphate and pyrophosphate were hydrolysed in orthophosphate in a period of several days. Addition of glucose increased the rate of hydrolysis, indicating that microbial activity was one of the primary mechanisms of hydrolysis.

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
		pH4 pH 7	14.5 days > 1 year
		pH 9	> 1 year

References:

Healy R.M., Kilpatrick M.L., (1955) A Kinetic Study of the Hydrolysis of Trimetaphosphates, J. Am. Chem. Soc., 1955, 77 (20), pp 5258–5264

Clesceri N.L. and Lee G.F. (1965a) Hydrolysis of Condensed Phosphates – II : Sterile Environment, Int. J. Air Wat. Poll. 9, 743-751.

Clesceri N.L. and Lee G.F. (1965b) Hydrolysis of Condensed Phosphates – I : Non-Sterile