Ammonium mercaptoacetate

Short-term toxicity to aquatic invertebrates

Link to relevant study record(s)

Reference

Endpoint:

short-term toxicity to aquatic invertebrates

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

key study

Justification for type of information:

REPORTING FORMAT FOR THE CATEGORY APPROACH

1. HYPOTHESIS FOR THE CATEGORY APPROACH
This scenario covers the category approach for which the hypothesis is based on transformation to a common compound. For the REACH information requirement under consideration, the effects obtained in studies conducted with different source substances are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect is observed for the different source substances; this may include absence of effects for every member of the category. No relevant differences in strengths of effect are observed for several source substances.
There are no relevant variations in properties among source substances and the same potency is predicted for all target substances. This corresponds to Scenario 5 of the RAAF (ECHA, 2017). The substances ATG, MEATG, KTG, CaTG, and NaTG are different inorganic salts of a common acid, thioglycolic acid (TGA; synonym: 2-mercaptoacetic acid). They dissociate rapidly in aqueous media to the common thioglycolate anion and to their different counter ions. The water solubility of all category members is high.
This approach serves to use existing data on aquatic toxicity endpoints for substances in this category.
It can be predicted with high confidence that the target substances will display the same mode of action and lead to the same type and strength of effects as observed with the source substances.


2. CATEGORY APPROACH JUSTIFICATION
For details, refer to Justification for read-across attached to Iuclid section 13

Reason / purpose for cross-reference:

read-across: supporting information

Key result

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

45.26 mg/L

Nominal / measured:

meas. (geom. mean)

Conc. based on:

other: read-across, molecular weight correction

Basis for effect:

mobility

Description of key information

48 h EC50 = 45.26 mg/L (Daphnia magna, OECD TG 202; geom. mean TGA, MeaTG)

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates

Dose descriptor:

EC50

Effect concentration:

45.26 mg/L

Additional information

Experimental data on short-term toxicity to aquatic invertebrates are available for TGA and MeaTG.

 

TGA

Two reliable studies on short-term toxicity of Thioglycolic acid to Daphnia magna are available.

 

The acute toxicity (inhibition of mobility) of Thioglycolic acid for Daphnia magna was assessed according to OECD Guideline 202.

Daphnia magna (10/concentration) were exposed for 48 hours to an aqueous solution of thioglycolic acid at different nominal concentrations of 0.20 - 0.39 - 0.78 - 1.56 - 3.13 - 6.25 - 12.5 - 25 and 50 mg/L.

15% immobilisation of the test organisms was recorded after 24h of exposure at the highest test concentration of 47 mg/L.

The 48 h EC 50 was determined to be 38 mg/L, based on measured concentrations. Even though it is known that thioglycolic acid can be easily oxidized, the conditions of the test surely prevented such kind of chemical action (lack of catalysts, buffer, pH, etc). This explained the stability of the concentrations tested throughout the test even in static conditions.

Immobilisation in the control did not exceed 10 % at the end of the test. Concentration of dissolved oxygen in the test vessels remained above 2 mg/L at the end of the test and pH did not vary by more than 1 unit. The concentrations of the test substance have been maintained to within 80 % of the initial concentration throughout the duration of the test.

 

The acute toxicity of Thioglycolic acid to Daphnia magna has been investigated in a test in accordance with OECD Guideline 202 (1984) under semi-static conditions. The Daphnia were exposed to test chemical at nominal concentrations of 4.8 mg/L, 8.6 mg/L, 15.4 mg/L, 27.8 mg/L, 50.0 mg/L, 90.0 mg/L (Measured concentrations control (0.0), <0.2 mg/L, 1.1 mg/L, 1.6 mg/L, 17.2 mg/L, 41.0 mg/L, 84.6 mg/L).

The tested substance concentration in the test solution was measured at the start of exposure and 24 hours later. The test substance concentration at the start of the test was 0.5 to 85.3 mg/L (the set concentration was 4.8 to 90.0 mg/L), and the proportion relative to the set concentration was 10.4 to 94.8%. The test substance concentration at the 24 hours after the start of the test was < 0.1 to 84.0 mg/L (the set concentration was 4.8 to 90.0 mg/L), and the proportion relative to the set concentration was < 1.3 to 93.3%. Because the measured value was outside of ± 20% of the set concentration, the geometric mean of the measured concentration was used to calculate the test results.

EC 50 - 48h for immobilisation determined was 35.8 mg/L.

 

Supporting data are available for the oxidation product Diammonium dithioglycolate. Diammonium dithioglycolate is the main degradation product of thioglycolic acid when released in the environment occuring few hours after the release (Devaux, 2003 - Smolin and Fölsing, 2009).

 

Acute toxicity of Diammonium dithioglycolate to Daphnia magna has been investigated through a test in accordance with OECD Guideline 202 and GLP requirements.

Daphnia magna were exposed for 48 hours to an aqueous solution of Diammonium Dithiodiglycolate at a nominal concentration of 100 mg/L.

The 48 h EC 50 was determined to be >100 mg/L.

 

MeaTG

A study was performed to assess the acute toxicity of the test material to Daphnia magna. The method followed that described in the OECD Guidelines for Testing of Chemicals (April 2004) No 202, "Daphnia sp, Acute Immobilisation Test" referenced as Method C.2 of Commission Directive 92/69/EEC (which constitutes Annex V of Council Directive 67/548/EEC).
Following a preliminary range-finding test, twenty daphnids (2 replicates of 10 animals) were exposed to an aqueous solution of the test material at concentrations of 10, 18, 32, 56 and 100 mg active ingredient (ai)/l for 48 hours at a temperature of approximately 20°C under dynamic test conditions. The number of immobilised Daphnia were recorded after 24 and 48 hours.
A positive control conducted approximately every six months used potassium dichromate as the reference material.
The 48-Hour ECso for the test material to Daphnia magna based on nominal test concentrations was 49 mg ai/l with 95% confidence limits of 41 - 61 mg ai/l. The No Observed Effect Concentration was 18 mg ai/l.
Given that the relevant test concentrations (18, 32, 56 and 100 mg ai/l) of the test material MEATG were within the 80% to 120% acceptance limits, it was considered justifiable to base the results on nominal test concentrations only.
The 48-Hour EC50 for the reference material to Daphnia magna based on nominal concentrations was 0.75 mg/l with 95% confidence limits of 0.56 — 1.0 mg/l.

 

Conclusion

Overall, all three results are combined in a geometric mean value after adjustment to thioglycolate.The Overall 48 h EC50 for ATG is 45.26 mg/L