Ammonium hydrogendifluoride

Endpoint summary

Additional information

Read-across justification

In aqueous solutions, ammonium hydrogendifluoride (AMBI) is completely dissociated into ammonium and hydrogendifluoride ions. In a second step, hydrogendifluoride (HF₂ ^-) is dissociated to the free fluoride (F^-) ion. Hydrogendifluoride (HF₂^-) is stable only in acidic media. At pH-values above 5 the fluoride ion is the main species. The equilibrium between ammonium and ammonia is determined by the base constant pK_bof 4.75. Therefore, in the environment the ammonium ion is expected to be the dominant species. Therefore it can be predicted that ammonium hydrogendifluoride dissociates rapidly and completely when released into waste water. Instead of the substance per se, ammonium and fluoride are released into the environment. Therefore, the environmental risk assessment is based on the ecological properties of the dissociation products. The acute toxicity of ammonium hydrogendifluoride was determined in fish. The 96 hour LC₀was 316 µg/l, and the 96 hour LC₁₀₀was 562 µg/l. The LC₅₀was estimated to be 422 µg/l, so a final 96 hour test was performed at this concentration, in which 3/10 fish died.

Aquatic toxicity of fluoride

All available studies were performed with sodium fluoride (NaF); read-across is appropriate as both NaF and HF are highly water soluble and therefore will dissociate into their constituent ions in the aquatic environment. The toxicity of both substances is essentially due to the fluoride ion. The EU RAR notes a clear relationship between the aquatic toxicity of sodium fluoride (and therefore hydrogen fluoride) and water hardness. Tests performed in soft water (<50 mg CaCO3/L) showed greater toxicity than those performed in hard water (>50 mg CaCO3/L) due to the precipitation of fluoride as CaF2. All endpoints are expressed in terms of concentrations of the fluoride ion (F-).

Short-term toxicity to fish

LC50 value of 107.5, 92.4, 118.5, 105.1, 64.1 ppm at 96, 120, 144, 168 and 192 h respectively are reported for rainbow tout (Camargo & Tarazona, 1991). The same authors report LC50 value for brown trout of 164.5, 135.6, 118.5, 105.1 and 97.5 ppm after 96, 120, 144, 168 and 192 h respectively. The EU RAR for hydrogen fluoride reports additional LC50 values of 299 mg/L (48h inLeuciscus idus); 51 mg/L in (96h inOnchorynkus mykiss) and 340 mg/L (96h inGasterosteus aculeatus). The RIVM Integrated Criteria Document reports additional data, with LC50 values ranging from 128 -460 mg/L (Sloofet al,1988).

Long-term toxicity to fish

In a 21 day test withOncorhynchus mykiss, reviewed in the ICD and EU RAR, an LC5 value of 4 mg/L is reported (actual concentration). This value is considered to be equivalent to the NOEC for mortality. The test was conducted in very soft (12 mg CaCO3/L) natural water with daily renewal of the test water.

Short-term toxicity to aquatic invertebrates

Camargo & Tarazona (1991) report that benthic larvae are sensitive to the concentration of fluoride, with EC50 values ranging from 26 -48 mg/L (actual concentration); however the study was performed in soft water. The EU RAR reviews and summarises the available data on short-term toxicity to aquatic invertebrates. The reported EC50 values for Daphnia sp. range from 97-352 mg/L and are based on nominal concentrations; EC50 values of 10.5 -39 mg/L are reported for marine invertebrates. The ICD summarises the available data on the short-term toxicity of sodium fluoride to aquatic invertebrates and reports EC50 values of 109 -340 mg/L; EC50 values of 30 -500 mg/L are reported for marine invertebrates.

Long-term toxicity to aquatic invertebrates

The EU RAR summarises the effects of two reproductive studies of sodium fluoride onDaphnia magna. The two studies report NOEC values of 3.7 and 14.1 mg/L, with an arithmetic mean of 8.9 mg/L. The ICD reports EC50 values in the range of 10 -48 mg/L

Toxicity to algae

The EU RAR reviews and summarises the available data on the toxicity of sodium fluoride to freshwater and marine algae species. The EC50 values for freshwater algae are reported to range from 43 to 122 mg/L. For marine algae the EC50 was 81 mg/L in a single study with Skeletonema costatum. NOEC values of 50 -249 mg/L and 50 -200 mg/L are reported for freshwater and marine algae, respectively.

Toxicity to microorganisms

An OECD 209 guideline-compliant Bayer study reports a 3 -hour NOEC of 510 mg/L for activated sludge. The EU RAR summarises and reviews the other available data on the toxicity of fluoride to aquatic microorganisms; NOEC values of between 7.1 -226 mg/L are reported.

Aquatic toxicity of ammonia

The toxicity of ammonia to aquatic organisms is highly dependent on physicochemical factors, most notably pH because of its importance in chemical speciation. The acute toxicity of ammonia is also influenced to a lesser degree by temperature, carbon dioxide, dissolved oxygen, and salinity. In aqueous solution, ammonia exists primarily in two forms, un-ionized ammonia (NH₃) and ammonium ion (NH₄⁺), which are in equilibrium. As pH increases, the fraction of the total ammonia which is un-ionized increases. It is this un-ionized ammonia which is generally considered to be the primary cause of toxicity in aquatic systems.