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Read-across principle

In aqueous solution, the salts of the sulfur oxyacids dissociate into the counterions and the sulfur oxyacid anions. The subsequent reaction like disproportion or oxidation/reduction differs depending on the anion species and is part of the general sulfur cycle:



The dithionite ion is a strong reducing agent and disproportionates rapidly in aqueous media to form sulfite and thiosulfate (S2O32-) due to the weak S-S binding. The following principal reaction patterns can be described for dithionite in aqueous medium (Münchow, 1992) in dependence on the pH:

•3 S2O42-+ 6 OH-→5 SO32-+ S2-+ 3H2O

•2 S2O42-+ H2O→2 HSO3-+ S2O32-

•2 H2S2O4→3 SO2+ S + 2 H2O

•3 H2S2O4→5 SO2+ H2S + 2 H2O


Under aerobic conditions, dithionite will be oxidised with high oxygen depletion to finally form sulfate as the final oxidation/decomposition product.

•S2O42−+ O2+ 2OH→ SO32−+ SO42−+ H2O (in the presence of oxygen)

This environmental behaviour is also known in soils, where these anions are unstable and do not persist (Lindsay, 1979).



All sulfite, hydrogensulfite and metabisulfite substances are highly soluble in water. The substance readily dissociates and forms an equilibrium that depends on solution pH as follows (Holleman & Wiberg, 1995):

•SO2+ H2O ↔ H2SO3

•H2SO3↔ H++ HSO3↔ 2H++ SO32−

•2 HSO3↔ H2O + S2O52−


Under oxidised conditions, e.g., in surface waters, sulfite is oxidized to sulfate catalytically by (air) oxygen or by microbial action. A half-life of 77 hour was measured in deionized water, already suggesting substantial abiotic degradation.

•2SO32-+ O2→ 2SO42-(in presence of oxygen)



The structure of the thiosulfate ion is comparable to the sulfate ion with one oxygen replaced by a sulfur atom. It is considered to be metastable and has only moderate reducing properties (Cotton et al., 1999). In contrast to sulfite and dithionite, the hydrolysis/disproportion of thiosulfates occurs only under acid conditions but not in alkaline aqueous medium. The following reaction pattern is considered in acid medium:

•S2O32−+ 2 H+→ SO2+ S + H2O



Thus, thiosulfates, sulfites and dithionites may reasonably be considered as chemically unstable under conditions typical of most environmental test systems, being transformed into other chemical species being part of the sulfur cycle (i.e., sulfates and sulfites).

In sum, unrestricted read-across between the sulfites, thiosulfates and dithionite compounds is justified and it seems to be reliable to compare all available information and use the respectively lowest value for each trophic level to evaluate the risk for the individual compounds. For further comparison, the values will be presented as sulfite equivalent values.



Summary of acute toxicity data

For acute fish toxicity the lowest reliable 96-h LC50 amounts to 149.6 mg SO32-/L as measured by mortality of rainbow trout (O. mykiss) exposed to disodium disulfite. The LC50 values determined in the acute fish toxicity study with sodium dithionite is not considered due to the low oxygen saturation during the test.

Daphnia magna as representative of the freshwater invertebrates showed to be most sensitive against exposure to disodium disulfite. Respective 48-h LC50 as measured by immobilisation amounts to 74.9 mg SO32-/L.

For acute toxicity to microorganisms as measured by respiration inhibition of activated sludge was evaluated resulting in 3-h EC50 of 172.2 mg SO32-/L for sodium dithionite.

In acute aquatic toxicity, freshwater algae indicates to be the most sensitive of the tested trophic levels with a 72-h EC50 of 36.8 mg SO32-/L as measured by growth inhibition induced by disodium disulfite exposure.

Thus, the acute toxicity data indicates a minor potential for acute toxicity to aquatic organisms.



Summary of chronic toxicity data

For chronic fish toxicity, one reliable GLP-compliant study as measured by reproduction of Danio rerio is available for sodium sulfite. Respective 34-d NOEC amounts to 200.5 mg SO32-/L.

Since the chronic invertebrate toxicity as measured by reproduction of Daphnia is above or equal to the highest tested concentrations, the respective 21-d NOEC amounts to ≥ 8.41 mg SO32-/L.

The chronic toxicity of freshwater algae as measured by growth inhibition the respective lowest 72-h EC10 of 28 mg SO32-/L was measured in Desmodesmus subspicatus after exposure with disodium disulfite.

For chronic toxicity to microorganisms, the lowest reliable 3-h EC10/NOEC of 82.4 mg SO32-/L as measured by respiration inhibition of activated sludge was determined for sodium dithionite.

Therefore, the chronic toxicity data based on the read-across of sulfite-, thiosulfate- and dithionite compounds are well above respective test limits of 1 mg/L, indicating only a low potential for long-term toxicity to aquatic organisms.