Heptasodium bis[4-hydroxy-5-[(2-hydroxy-1-naphthyl)azo]-3-[(2-hydroxy-3-nitro-5-sulphophenyl)azo]naphthalene-2,7-disulphonato(5-)]cobaltate(7-)

Toxicity to aquatic plants other than algae

Description of key information

ErC50 (7 d, Lemna minor) > 63.9 mg/l, based both on frond number and on dry weight (mean measured concentration; equivalent to 100 mg/l nominal)

Key value for chemical safety assessment

EC10 or NOEC for freshwater plants:

7.6 mg/L

Additional information

No data was available regarding toxicity of the test item to aquatic plants or algae, thus a read-across approach was adopted. In particular, available data on Similar Substance 01 was used, i.e. cobalt complex with 2 phenyl-azo-naphtyl ligands functionalised by similar groups. A detailed description of the read-across approach is attached in section 13.

The toxicity of Similar Substance 01 was assessed in a study conducted according to the OECD Guideline 221 (2006). Lemna minor was exposed to nominal test concentrations of 1.56, 3.12, 6.25, 12.5, 25, 50 and 100 mg/l, which corresponded to measured concentrations of 1.36, 2.77, 5.41, 10.34, 19.80, 37.92, and 63.93 mg test item/l (63.9 % and 88.8 % of nominal concentrations). As not all values fell within 80 -120 % of the nominal concentrations, the test endpoints were calculated based on geometric mean measured concentrations. The test was conducted as static test. After an exposure time of 7 days, effect levels for growth rate based on frond number and dry weight were determined:

- frond number: E_rC₅₀ > 63.9 mg/l (mean measured concentration; equivalent to 100 mg/l nominal)

- dry weight: E_rC₅₀ > 63.9 mg/l (mean measured concentration; equivalent to 100 mg/l nominal).

For completeness of information, the following data is referred to Similar Substance 02.

The toxicity of the test item to aquatic plants was evaluated in an experimental study according to the OECD Guideline 221 (2006). Lemna gibba was exposed to the test item for 7 days and monitored for change in frond number and dry weight. Test item concentrations tested were 0.3, 0.87, 2.52, 7.32, 21.22 and 61.53 mg/l test item in test medium and a negative control of test medium only, based on a preliminary range-finding study. Three of the six concentrations (0.3, 7.32 and 61.53 mg/l) were monitored for test item concentration at the start end end of the study period. The concentration which inhibits 50 % of growth (E_rC₅₀) was calculated using the average specific growth rate, and the concentration which inhibits 50 % of yield (E_yC₅₀) was calculated as change in biomass, based on frond number and frond dry weight. Toxicity of a reference substance, 3,5-Dichlorophenol, to Lemna gibba was evaluated in a separate study.

As values of measured concentrations fell within 80 % to 120 % of the nominal test item concentrations, results are reported using the nominal values. From the average specific growth rate calculation, the E_rC₅₀ was calculated to be 28.31 mg/l based on frond number (confidence limits: 27.16 and 29.51 mg/l) and 24.22 mg/l based on frond dry weight (confidence limits: 23.24 and 25.24 mg/l).