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EC number: 204-424-9
CAS number: 120-78-5
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.
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.
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.
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.
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