Dinitrogen tetraoxide

Short-term toxicity to fish

Description of key information

This end point is read across from the Lead Registrants dossier for nitric acid (EC # 231-714-2) and sodium nitrate (EC # 231-554-3).
The available studies show that the pH, rather than the anion (nitrate) is causing the toxic effect in fish. This is confirmed by studies with sodium nitrate and sodium potassium nitrate which all showed LC50 values > 100 mg/L.

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 (usuallt 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 two studies with nitric acid focus on the pH effects caused by nitric acid. Different pH levels have been tested in bluegill sunfish and rainbow trout.

The pH induced by nitric acid which caused 50% mortality was ca. 3.7 in rainbow trout and between 3.0 and 3.25 in bluegill sunfish.

No mortality was observed at pH 3.5 or above in bluegill sunfish after 96h exposure.

No mortality was observed at pH 4.0 in rainbow trout after 96h exposure, although 50% had died after 7 days of exposure.

The studies show that a pH caused by adding nitric acid roughly between pH 3 (or lower) and 4 is critical for fish. Furthermore, the Swift & Morgan study showed that the nitrate ion alone (used as control) is not causing any mortality in fish. In addition, several studies with sodium nitrate are all supporting each other in the fact that sodium nitrate has a high 96h-LC50 for fish.

Common bluegill LC50 was 12000 mg/L, fresh and saltwater studies with chinook and rainbow trout showed a lowest 96h-LC50 of 4400 mg/L (based on NO3 -), freshwater Gambusia affinis 96 hr LC50 was determined to be 6650 mg/L.

The 96h-LC50 to juvenile (19 months old) Topeka shiner was determined to be 1354 mg/L (as NO3 -N).

In addition, based on a reliable guideline study OECD 203, sodium potassium nitrate appeared to have a nominal LC50> 100 mg/L (confirmed by chemical analysis) in rainbow trout.

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