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Environmental fate & pathways

Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
Study is peer-reviewed in EU-draft risk assessment
Principles of method if other than guideline:
redox reaction in buffer system at pH 6.5: oxidative environment with the presence of oxygen; reductive environment with the presence of glutathione
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
- after 2 h reaction time in the experiments in pure buffer
- after 10 min when glutatione was added to the buffer solution
Buffers:
0.5 M phosphate buffer
Details on test conditions:
- stock solution: 5 mM MBTS in Tetrahydrofuran (THF)
- test tubes: 0.2 ml stock solution and 1.8 ml 0.5 M phosphate buffer
- addition of glutathione: dissolving 100 µmol solid substance in 1.8 ml 0.5 M phosphate buffer at pH 6.5 immediately before adding 0.2 ml the stock solution of test substance.
Duration:
2 h
pH:
6.5
Initial conc. measured:
0.5 mmol/L
Transformation products:
yes
No.:
#1
Details on hydrolysis and appearance of transformation product(s):
Stable in phosphate buffer at pH 6.5 with the presence of atmospheric oxygen for 2 hours of reaction time, however under reductive condition with the presence of gluthathione MBTS was fully converted to MBT in 10 mins.
pH:
6.5
Remarks on result:
other: Stable in phosphate buffer with the presence of atmospheric oxygen for 2 hours of reaction time

EU-draft risk assessment:

Hansson & Agrup (1993) analyzed test solutions containing 500 µM (170 mg/l) MBTS in 0.5 M phosphate buffer and 10% tetrahydrofuran at pH 6.5. After 2 hours reaction time a sediment of MBTS was formed and only trace amounts of MBTS were converted to MBT. After addition of the reducing agent gluthathione, MBT was formed quantitatively within 10 minutes. When the same experiment was run with a MBT solution, 60% were converted to MBTS without gluthathione after 2 hours, while after gluthathione addition MBT was stable. The results reveal that the equilibrium between MBT and MBTS is largely influenced by the redox status of the medium.

Conclusions:
MBTS was stable in phosphate buffer at pH 6.5 with the presence of atmospheric oxygen for 2 hours of reaction time, however under reductive condition with the presence of gluthathione MBTS was fully converted to MBT in 10 mins.
Executive summary:

MBTS was stable in phosphate buffer at pH 6.5 with the presence of atmospheric oxygen for 2 hours of reaction time (Hansson & Agrup, 1993), however under reductive condition with the presence of gluthathione MBTS was fully converted to MBT in 10 mins.

Description of key information

In this endpoint, the hydrolysis potential of MBTS is discussed under weak oxidative condition with the presence of atmospheric oxygen. According to expert justification, the conversion of MBTS to MBT takes place only under reductive condition and hence with the presence of oxygen MBTS is considered as stable in aqueous solution. This has been proved by Hasson & Agrup (1993) with negligible dissipation of MBTS observed in the buffer solution of pH6.5. Increased dissipation of MBTS with metabolites like MBT (149-30-4), BT (95-16-9) and BTon (934-34-9) was observed under increased temperature and pH (e.g. 6.7 % dissipation of MBTS at pH 9.8 and 58 °C in 65 hours) and decreased particle size present in the aqueous solution. In conclusion, MBTS is considered as hydrolytically stable under environmental relevant pH and temperatures with the presence of atmospheric oxygen in aqueous solution.

Key value for chemical safety assessment

Additional information

37% dissipation of MBTS converting into MBT within 7 days at pH 7 and ambient temperature was observed. The result matches the negligible dissipation of MBTS observed by Hansson & Agrup from 1993 under similar conditions and hence can be used as supporting study. However, so high dissipation rate of MBTS can not be expected under oxidative condition with the presence of atmospheric oxygen, according to expert judgment, and therefore this study is not used as key study for this endpoint. The abiotic degradation of MBTS was assessed in the appendix B of EU-Risk Assessment for CBS (CAS: 95-33-0) as rapidly hydrolyzable. According to up-to-date results and assessment, this statement from EU-Risk Assessment is no more valid.