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Ecotoxicological information

Toxicity to aquatic algae and cyanobacteria

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Description of key information

Key value for chemical safety assessment

Additional information

Read-across Hypothesis

The reaction mass is composed of the following substances: sodium sulphite, sodium thiosulphate, sodium polysulphide and disodium sulphide, with residual amounts of sodium sulphate.

The reaction mass can be considered as a mixture of different salts which fully dissociate in aqueous systems. Because they are chemically related their effects are considered to be additive.

Therefore, the aquatic toxicity of the mixture can be determined based on respective data/classification of the single compounds and the quantitative composition of the reaction mass.

Applicability domain

Reaction mass of sodium sulphite, sodium thiosulphate, sodium polysulphide and disodium sulphide

Target substances, molecular formulas:

Source substances, molecular formulas:

sodium sulphite (NA2SO3)

disodium disulphite (Na2S2O5)

sodium thiosulphate (NA2S2O3)

ammonium thiosulphate (NH4)2S2O3

sodium polysulphide (NA2Sn)

sodium sulphate (Na2SO)4

disodium sulphide (NA2S)

hydrogen sulphide (H2S), sodium sulphate (Na2SO4)

Purity / Impurities

sodium sulphate, Na2SO4

The impurity sodium sulphate is a naturally occurring inorganic compound and not environmentally classified or labelled. Since sodium sulphate is also used as read-across substance for sodium polysulphide and disodium sulphide, it is taken into account for the classification of the reaction mass.


Category justification

The constituents of the reaction mass consist of sodium as cation and sulfur compounds as anion. They all have a high water solubility and fully dissociate in water.

All compounds are registered under REACH and detailed ecotoxicological information can be found in the respective dossiers. Because the reaction mass can be considered as a mixture of its compounds, this data is used to determine the aquatic toxicity of the reaction mass.


Conclusions per endpoint for C&L

Toxicity studies investigating the toxicity of Na2S or NaHS to aquatic organisms have focused in most cases on the toxicity of H2S, which is the most toxic sulfur species that can be formed upon dissolution of Na2S and NaHS.

The study by Breteler et al., 1991 was considered as key study for H2S toxicity. The available studies only reported reliable results for marine algae. No reliable information was identified for freshwater algae. The effects of sulphide on photosynthesis of the planktonic diatom Skeletonema costatum were investigated by exposing the diatoms for 4 h to a series of sulphide concentrations in the presence of a 14C-labeled carbon source and measuring 14C photoincorporation. The NOEC was determined to be 0.77 mg S2-/L, which corresponds to a H2S concentration of 0.041 mg H2S/L. The EC50 was 1.5 mg S2-/L, which corresponds to 0.104 mg H2S/L.

In oxic environments, released sulphides will be oxidized to - eventually - sulphate. In these cases, the risks entailed by the released sulfur should be evaluated using toxicity data for sulphate. Na2SO4was identified as the most relevant substance for this purpose.

For sulphate toxicity, the study of Patrick et al. (1968) was identified as a key study. The 120-h EC50 (based on reduction in number of cells produced compared to the control) of Na2SO4 for the freshwater diatom Nitzschia linearis was reported to be 1900 mg/L. Recalculation to SO42- yields an EC50 of 1285 mg/L.

An OECD 201 test was conducted under GLP with ammonium thiosulphate. Nominal test concentrations ranged between 1 and 100 mg test item/L. No statistically significant differences were found between the control and any of the test concentrations. The EC50 and NOEC were > 100 mg ammonium thiosulphate/L, respectively, which corresponds to ≥75.7 mg S2O32-/L (endpoints: biomass yield and growth rate). Data can be used for the environmental assessment of ammonium thiosulphate and other thiosulphate compounds (read-across principle). 

For the assessment of sodium sulphite toxicity, a 96-h algal growth inhibition test was conducted with disodium disulphite as test compound. A 72-h EC10 of 33.3, and an EC50 of 43.8 mg test item/L were derived, which corresponds to 36.8 and 28.0 mg SO32-/L, respectively. This data can be used for the environmental assessment of sodium disulphite and other sulphite compounds (read-across principles).



The environmental classification of the reaction mass is determined as described in Regulation (EC) No 1272/2008 (Part 4: Environmental hazards, The classification is derived with the summation method (, In principle, the summation method is based on the summation of the respective concentration of its classified components. Existing M-factors are included in the calculation.

Sodium polysulphide and disodium sulphide are both classified as Acute Aquatic 1, without M-factors, according to Annex VI of Regulation (EC) No 1272/2008. Summed up, their concentration in the reaction mass is >25 %. If the sum of the concentrations of all components of a mixture classified as Acute Aquatic 1, multiplied by their corresponding M-factors is >25%, the classification of the mixture results in Acute Aquatic 1 (Regulation (EC) No 1272/2008, Therefore, the summation method leads to a classification of the reaction mass as Acute Aquatic 1, H400.