Tetrakis(hydroxymethyl)phosphonium sulphate(2:1)

Hydrolysis

Description of key information

One reliable study with radiolabelled 14C-THPS performed according to standard guideline with GLP under aerobic conditions gave the following results (extrapolated at environmental relevant conditions for chemical risk assessment):

Half-lives (12°C, pH 5) = 2.8 days
Half-lives (12°C, pH 7) = 6.45 days

One reliable study with THPS 75% in aqueous solution performed according to standard guideline with GLP under anaerobic conditions gave the following results:
Half-lives (25°C, pH 5) = 131 days
Half-lives (25°C, pH 7) = 72 days
Half-lives (25°C, pH 9) = 7 days

In both conditions, THPS main ingredient hydrolyses to THPO (Tris(hydroxymethyl) phosphine oxide, CAS No. 1067-12-5) and formaldehyde (CAS No. 50-00-0).

A non-GLP reliable study with THPS performed with a scientifically valid method shows that at pH 7 and 10, no THPS has been detected after 5 minutes of incubation and is most stable at pH 1. The THPS half-life at pH 1 and at nominal concentration of 1 mg/l, 100 µg/l and

25 µg/l were estimated to be 5.3 hours, 2.4 hours and < 20 minutes, respectively.

Key value for chemical safety assessment

Half-life for hydrolysis:

6.45 d

at the temperature of:

12 °C

Additional information

Two studies of reliability 1 are available to assess the hydrolysis potential of the active substance THPS. These studies were performed on the THPS in solution, under anaerobic and aerobic conditions, and have been defined as key studies. One additional study of reliability 2 was performed with a scientifically valid method.

The study of Yeomans (2002) performed with radiolabeled 14C_THPS under aerobic conditions shows the effect of oxygen on the degradation of the test substance. In these conditions, THPS reacts with dissolved oxygen to form THPO and formaldehyde. The degradation rate constants are higher in presence of oxygen, and increase with decreasing pH.

The half-lives measured at 20°C were:1.5 days at pH 5 and 3.4 days at pH7. These half-lives values were recalculated at 12°C to be 2.8 days at pH 5 and 6.45 days at pH=7. This latest value was assess to be representative of environmental conditions and was chosen for environmental classification (CLP) and risk assessment.

The study of O'Connor (1992) has been performed under anaerobic conditions, as required by the test method when substances are known to oxidise. The degradation pathway is believed to proceed via the dissociation product THP (Tris (hydroxymethyl) phosphine, CAS No. 2767-80-8) to THPO (Tris(hydroxymethyl) phosphine oxide, CAS No. 1067-12-5) and formaldehyde (CAS No. 50-00-0). The formation of THP is favoured as the pH is raised, and the hydrolysis rate constant increases with increasing pH.

In anaerobic conditions, the half-lives measured at 25°C were: 131 days at pH 5, 72 days at pH 7 and 7 days at pH 9.

Further, a non-GLP study under aerobic conditions with a scientifically valid method was performed to assess:

- The stability of a 100 µg/l THPS main ingredient solution. The results showed that at pH 7 and 10, no THPS main ingredient has been detected after 5 minutes of incubation. THPS main ingredient is most stable at pH 1.

 - The THPS half-life at pH 1 and at nominal concentration of 1 mg/l, 100 µg/l and 25 µg/l. The results indicated that if more than twenty minutes was taken to prepare and inject the 25 µg/litre THPS solution, no response was observed.

Based on this data the half-life of 25 µg/litre THPS is estimated < 20 minutes. The half-life of THPS at 1mg/L and 100 µg/L were estimated to be 5.3 hours and 2.4 hours, respectively.