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Hydrolysis

Description of key information

Alkyl phosphites are expected to hydrolyze readily under most conditions. Data on tris alkyl phosphites of similar composition (triisodecyl phosphte (TDP) and triisotridecyl phosphite (TiTDP)) show half-lives ranging from 0.43 hr (pH 1.2) to 20.1 hrs (pH 9). Additional data on other phosphites also support the conclusion that this substance rapidly hydrolyzes.

Key value for chemical safety assessment

Half-life for hydrolysis:

17 h

at the temperature of:

23 °C

Additional information

It has been well established through a series of modified tests and practical handling experience that this class of alkyl phosphites readily hydrolyses. ECHA actually notes in its REACH Guidance R.11 on PBT assessment that phosphites are a class of substances that are known to hydrolyse in the environment.

Unfortunately, it is actually very hard to test phosphites for their hydrolysis rates in the standard OECD guideline (111) due to the extremely low water solubility of the substances. The registrant sought to place the ECHA requested OECD 111 guideline test on TiTDP with an accredited laboratory but, after much discussion with the laboratory, it was decided that the very low water solubility of TiTDP would render it unsuitable for this test method. 

 

A new comparative study of the hydrolysis of TiTDP and TDP (Carraz 2014) was just conducted to both draw a comparison of the relative rates of hydrolysis between the two phosphites and also to evaluate the rate of hydrolysis in simulated stomach acid (pH=1.2). In this study, it was found that using a co-solvent, 50% acetone in solution, greatly increased the reliability of the results likely because the co-solvent helped make the phosphites available to the aqueous phase allowing hydrolysis to occur. From this study,TiTDP with a t_1/2of 0.43 hour hydrolysed more rapidly than TDP with a t_1/2of 2.44 hours.

 

By comparison, the Reimer (2002) hydrolysis test of TDP, using 50% acetonitrile as a co-solvent, gave a half-life of 4.1 hours at pH 5, 17 hours at pH 7 and 20.1 hours at pH 9 at ambient temperature. The shorter half-life at lower pH is to be expected given that the hydrolysis of organophosphites is known to be acid-catalyised. 

 

The results from Carraz (2014) indicate that TiTDP has a similar, perhaps slightly faster, rate of hydrolysis than TDP. As such, these data support the use of the Reimer (2002) results on TDP for TiTDP dossier. The Carraz data also indicate that TiTDP and TDP will behave similarly in the stomach, further supporting the justification for using TDP toxicity data for TiTDP hazard characterization.