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Key Study:

In an OECD 111 study, conducted according to GLP, transformation of 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione was observed at environmental relevant pHs (pH 4, 7 and 9), however, transformation was not considered to originate from true hydrolysis, i.e. the reaction with addition of water to the substance. Significant, extremely rapid transformation occurred prior to analysis.5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione is expected to undergo non-hydrolytic transformations (Harlan Laboratories Ltd, 2016).

Two reaction pathways must be considered in any experiments involving 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione:

• Tautomerization; and

• Disulfide exchange.

Tautomerization should rapidly produce an equilibrium mixture of thione and thiol forms of the test substance upon exposure to a protic solvent (such as water). The position of the equilibrium will depend on temperature, solvent, and pH of the solution.

Disulfide exchange, involving the cleavage and reformation of the disulfide bridge, may be slower and the rate will depend on exposure to heat, light, and reducing or oxidizing species. Both the dimer and monomer present during this dynamic exchange will undergo tautomerization.