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Short-term toxicity to fish

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Description of key information

Median lethal pH (96h) 3-3.25 for Lepomis macrochirus (no guideline followed)
All available studies show that mortality is caused by low pH values. When adjusted to environmentally relevant pH values, phosphoric acid does not cause acute adverse effects.

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Additional information

The study from Ellgaard and Gilmore 1982, focuses on the pH effects caused by phosphoric acid. Different pH levels have been tested in bluegill sunfish.

The pH induced by phosphoric acid which caused 50% mortality was between 3.0 and 3.25, while no mortality was observed at pH 3.5 or above after 96h exposure. At neutral pH 7.5 no fish died.

Similar results were obtained in the Alkahem study: a median lethal pH of 3.58 in Aphanius dispar. The Korean study showed a 96h-LC50 of phosphoric acid in Oryzias latipes of 75.1 mg/L, but the pH was not adjusted to relevant environmental values. No mortality was observed at 100 mg/L when the pH was adjusted.

The studies show that a pH caused by adding phosphoric acid roughly between pH 3 (or lower) and 4 is critical for fish. The Gueylard and Duval study showed that survival times increased with increasing pH, and that this increase was more rapid at pH values above 3.5. It can thus be concluded that it is the low pH which is causing the toxic effects. Wallen et al. cited by Von Burg as well described a study in which fish (Gambusia affinis) were exposed to both phosphoric acid and turbidity (600 ppm of clay particles), and in which phosphoric acid precipitated turbidity to levels below 25 ppm. The median tolerance limit after 24, 48 and 96h exposure was 138 ppm (i.e. about 138 mg/L). Townsend and Cheyne (1944) showed that exposure of Oncorhynchus kisutch fingerlings to 20 ppm of phosphoric acid (i.e. pH 6.75) induced no mortality at relatively normal dissolved oxygen concentrations (4.95 -8.15 ppm). However, when the dissolved oxygen concentration was lowered to 1.50 ppm, phosphoric acid at 12.0 and 20.0 ppm induced 100% mortality, which is obviously caused by the combination of the low dissolved oxygen concentration and the presence of increased hydrogen ion concentrations.

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