Reaction mass of sodium hydrogensulfide and sodium carbonate

Endpoint summary

Additional information

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 data set for acute toxicity to aquatic organisms therefore not only contains toxicity tests in which Na₂S, NaHS or its respective hydrates have been used as test substance, but also contains toxicity tests in which H₂S was immediately dosed to the test medium. The toxicity of Na₂S and NaHS depends on the relative presence of H₂S in the test medium. In standard test systems - with a typical oxidizing nature - H₂S formation will be limited through partial oxidation of the sulfides to (eventually) sulfate, and high effect concentrations will be obtained when expressed as mg Na₂S/L, mg NaHS/L or mg S^2-/L (total dissolved sulfide, i.e. [H₂S]+[HS^-]+[S^2-]). In test systems especially adapted to sustain stabile H₂S exposure levels (e.g., high flow-through rates, hypoxic conditions, etc.), the relative presence of H₂S will be higher and much lower effect concentrations will be obtained when expressed as mg Na₂S/L, mg NaHS/L or mg S^2-/L. Consequently, Na₂S or NaHS concentrations are poor predictors of toxicity, and it is much more useful to consider effect concentrations expressed as H₂S concentrations. On the other hand, because in most natural aquatic systems the oxidizing conditions will counter the presence of H₂S through oxidation of sulfides to sulfate, the data set was extended with the minimal critical information for derivation of a PNEC_aquatic for sulfate. This information was extracted from the OECD SIDS for Na₂SO₄, which was considered the most relevant compound for read-across. However, it should be kept in mind that the observed toxicity of Na₂SO₄ is affected by the presence of both sodium ions and sulfate ions, and that recalculation of the observed effect concentrations from Na₂SO₄ concentration to sulfate concentration represents a worst case approach.

In oxidizing aquatic conditions, the alkalinity of the reaction mass (pH >12) and its NaOH content are likely to have the most significant ecotoxicological effects. Ecotoxicological data also are presented for both NaOH and Na₂CO₃. While the data is not extensive, the difference between the reported toxic concentration levels of NaOH and, on the other hand, those of Na₂CO₃ and Na₂SO₄ is clear.