Tetrairon tris(pyrophosphate)

Hydrolysis

Description of key information

One key study is submitted to show the hydrolysis of a soluble pyrophosphate. 
The key study for the endpoint ‘hydrolysis as a function of pH’ (O’Connor BJ, 2010 ) has been selected on the basis that the study is conducted to the recommended OECD guideline and under the conditions of GLP and therefore meets the regulatory requirements for this endpoint. However the data does not necessarily reflect a ‘real world’ situation as phosphates and essential cations such as Na+ are rapidly assimilated by microorganisms in soil and waters. 
The key study reports the estimated half-life’s at 25°C of the test material were determined to be; 527 days at pH 4 and > 1 year at pH 7 and 9. Under the physiologically relevant conditions of pH 1.2, 37.0 ± 0.5°C, the half-life of the test material was determined to be 26.1 hours. The substance was shown to following the following mechanism of hydrolysis:
Pyrophosphate anion + water  →  2 x orthophosphate anion 
In reality and due to the pKa’s of orthophosphoric acid and the nature of the phosphate ion it is likely that in natural waters the hydrolysis product will be either orthophosphoric acid and monopotassium dihydrogen orthophosphate (which may also dissociate to phosphate ions and sodium ions). 
Results for the preliminary test indicate that the rate of hydrolysis increases with a decrease in pH.
For soluble pyrophosphates the rate of hydrolysis in natural waters is far greater in natural waters than in distilled water. Pyrophosphates will not persist in natural waters. Biotic degradation and assimilation by algae and/or microorganisms will occur at a faster rate than hydrolysis in distilled water. The breakdown products of such reactions are the ubiquitous orthophosphate anion. 

Key value for chemical safety assessment

Half-life for hydrolysis:

527 d

at the temperature of:

25 °C

Additional information

The hydrolytic half-life was determined for tetrapotassium pyrophosphate (EC No: 230 -785 -7). The ionic substance was assumed to undergo dissociation in aqueous solutions (resulting in potassium cations and pyrophosphate anions) therefore the cation was assumed to have a negligible influence on the rate of hydrolysis of the anion. This is expected to be replicated at environmentally relevant concentrations and on dissolution into complex environmental matrices. Although cation of the final matrix may retain some minor influence on the hydrolytic rate, such a relationship is considered beyond the scope of a standard test method and would not have been addressed or identified when testing in accordance with OECD method 111. Therefore, the rate of hydrolysis can be read across to the following substances:

- disodium dihydrogenpyrophosphate

- trisodium hydrogen diphosphate

- tetrasodium pyrophosphate

- calcium dihydrogenpyrophosphate

- dicalcium pyrophosphate

- tetrairon tris(pyrophosphate)

- copper (II) pyrophosphate

- magnesium pyrophosphate

- magnesium hydrogen pyrophosphate

 

No further testing is considered necessary.