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EC number: 234-126-4 | CAS number: 10544-72-6
Short-term toxicity to aquatic invertebrates
Administrative data
Link to relevant study record(s)
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).
Median lethal pH (48h) 4.4-4.7 for Ceriodaphnia dubia (US EPA guideline). This study shows that the pH rather than the anion (nitrate) is causing the toxic effects in daphnids. This is confirmed by two additional studies with sodium nitrate (24h EC50 8610 mg/L for Daphnia magna (similar to OECD TG 202) - Müller & Büchs (1983)) and sodium nitrate (48h EC50 3581 mg/L) and potassium nitrate (48h EC50 490 mg/L for Daphnia magna (no guideline followed) - Dowden & Bennett (1965)).
Key value for chemical safety assessment
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.
In the Belanger & Cherry study (1990), the water flea Ceriodaphnia dubia was exposed to nitric acid pH-adjusted test solution for 48 hours with test solution renewal at 24 hours. The 48-hour LC50 was determined after testing at two pH ranges, 3.8 -8.1 and 3.2 -8.0. The grand probit 48 hour LC50 (combined analyses) was 4.6 pH standard units.
Similar to the other aquatic toxicity endpoints, it is shown that adverse affects due to nitric acid exposure are caused by the decreased pH and not by the nitrate anion. This finding is strengthened by the Müller & Büchs (1983) study which shows a high EC50 value to Daphnia exposed to sodium nitrate (24h LC50 = 8609 mg/L) and Dowden and Bennett (1965) study which shows a high EC50 value to Daphnia exposed to sodium nitrate (48h EC50 3581 mg/L) and potassium nitrate (48h EC50 490 mg/L).
As regulatory ecotoxicity tests need to be conducted at pH 6 -9, nitric acid will not cause adverse effects to daphnids 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.
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