Dinitrogen tetraoxide

Long-term toxicity to aquatic invertebrates

Description of key information

According to REACH Annex IX no testing is required if the results of the Chemical Safety Assessment (CSA) indicates no need to investigate further the effects of the substance on aquatic organisms. The CSA indicates no concern for daphnia. Furthermore, in accordance with REACH Annex XI, section 1.5 (Grouping of substances and read-across approach) the long-term toxicity test on daphnia does not need to be conducted as nitric acid with further dissociate into H+ ions (resulting in a pH decrease) and nitrate ions, and the endpoint can consequently be covered with a study done on sodium nitrate, which also dissociates into nitrate ions.
Finally, long-term effects to daphnia at environmentally relevant pH values are not expected to occur.
This is supported by a KC2 study from Belanger & Cherry study (1990) which shows that long-term effects of nitric acid were related to the pH. In a pH range that is common for testing (pH 6 - 8) no significant effects on mortality and reproduction were observed.
Furthermore there are two KC4 long term invertebrate studies with sodium nitrate (European Commission, 2000) and potassium nitrate (Reisk, 1970) indicating low long-term toxicity.

Additional information

In water, dinitrogen tetraoxide (EC 234-126-4) will decompose to nitrous acid and nitric acid, and the aquatic toxicity should be equal to the water concentrations of these substances due to the decomposition of the actual concentrations of dinitrogen tetraoxide. (European Commission). Nitrous acid molecules then combine to produce nitric acid, nitric oxide and water.

The overall reaction can be written as follows:

3 N2O4 + 2 H2O = 4 HNO3 + 2 NO

This reaction is known to be rapid (Masteller & Berman), with Larkin having made calculations assuming an instantaneous rate of reaction between dinitrogen tetraoxide and water.

Since nitric oxide is a gas, it is assumed that only nitric acid is relevant for the aquatic toxicity.

It is therefore considered appropriate to read across to nitric acid (EC 231-714-2).

Nitric acid in water immediately dissociates into H+ ions and nitrate ions. The H+ ions will cause a significant pH decrease. As regulatory ecotoxicity studies should be conducted at environmentally relevant pH values (usually pH 6-9), the pH of the nitric acid test solutions should be adjusted. Consequently the pH effect is disregarded and the effects potentially caused by nitrate ions should be examined. Nitrate salts are all well soluble and in water immediately dissociate into nitrate ions and its counterions, similar to nitric acid. The counterions, sodium and potassium, are considered not significant in respect of ecotoxicological properties. Therefore, the ecotoxicity studies on sodium nitrate and potassium nitrate can be used to read across.

The study from Belanger & Cherry (1990) with nitric acid focus on the pH effects caused by nitric acid. Different pH levels have been tested in Ceriodaphnia dubia.

In a pH range that is common for testing (pH 6 - 8) no significant long-term effects on mortality and reproduction were observed.

In addition, there are two KC4 long term invertebrate studies with sodium nitrate (European Commission, 2000) and potassium nitrate (Reisk, 1970) indicating low long-term toxicity.

It can thus be concluded that it is the low pH that is causing the toxic effects.

As regulatory ecotoxicity tests need to be conducted at pH 6 -9, nitric acid will not cause adverse effects to fish when in this pH range.

References:

1.European Commission, ESIS (2000) IUCLID Dataset, Dinitrogen tetraoxide (CAS #10544-72-6) p.10 (CD-Rom edition).

2.Masteller, R.D. & Berman, L.D. (1964) Evaluation of the Mechanism of Corrosion in Capillaries, Status report #1, Oxidiser Diffusion Studies DRS S 11047, ME #531. U.S. Department of Defense.