Sodium 2-mercaptoethanolate

Short-term toxicity to fish

Description of key information

The 96-h LC50 value is 32.3 mg/L for fish (L. idus).   

Additional information

No data on aquatic toxicity is available for sodium 2 -mercaptoethanolate (target chemical). Read across can however be made to 2- mecaptoethanol (CAS 60 -24 -2). The (sodium) 2-mercapthoethanolate ion is a conjugated base, the read across substance 2-mercaptoethanol its undissociated acid. With a pKa value of ca. 9.5 for both substances, in an aqueous environment with pH 7, they will be present almost exclusively as 2-mercaptoethanol. Further, no effects on aqueous toxicity from the sodium cation is to be expected.

Reliable data on the toxicity of 2 -mercaptoethanol to fish is available from two different sources.

The first one describes a short-term fish toxicity study in accordance with DIN 38412 (part 15) (BASF, 1988). In this study groups of 10 golden orfe (L. idus) were exposed to nominal test concentrations of 0 (control), 10, 21.5, 46.4, 100, 215 and 464 mg/L for 96 hours under static conditions. Analysis of test substance stability was performed in a separate test under similar conditions as the fish toxicity test. Based on an arithmetic mean test substance recovery of 54.3%, 'measured concentrations' were determined at 0, 5.4, 11.7, 25.2, 54.3, 116.7 and 251.8 mg/L. Mortality and general symptoms of toxicity were recorded at 1, 4, 24, 48, 72 and 96 h after the start of exposure. Apathy was observed in the 464 mg/L test group after 4 hours exposure. No further symptoms are described at any exposure period or concentration. No mortality was observed up to a analytical concentration of 25.2 mg/L. At an analytical test concentration of 54.3 mg/L all fish died 48 hours after exposure. At higher test concentrations 100% mortality was recorded already after 24 hours exposure. Based on these findings the 96-h LC50 value is determined at 25.2 - 54.3 mg/L or rather 25.2 mg/L as worst case approach. After correction for molecular weight difference (for read-across) the 96 -h LC50 is 32.3 mg/L.

The second one is a report of the Japanese authorities (National Institute of Technology and Evaluation (NITE; former MITI)) of 2010. In this OECD 203 study the toxicity of 2-mercaptoethanol to fish is examined in a 96-h test. Although the used test organisms is not specified, normally in fish toxicity studies performed by the Japanese Authorities orange-red killifish (O. latipes) are used. The 96-h LC50 is reported to be 29 mg/L. After correction for molecular weight difference (for read-across) the 96-h LC50 is 37.2 mg/L.

The BASF study (1988) is considered as key study as it provides the lowest effect concentration for fish. Also, golden orfe are considered to be the most relevant species for European risk assessment.