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Phthalic acid is a weak to medium strong acid and partly dissociating in water causing a pH shift (pKa1: 2.97; pKa2: 5.43). The tolerance of water organisms towards pH is diverse. Recommended pH values for test species listed in OECD guidelines are between 6 and 9.

In cases where no reliable ecotoxicity test for phthalic acid on aquatic organisms are available a read-across approach of phthalic anhydride was carried out. Phthalic anhydride degraded by hydrolysis to phthalic acid, the hydrolysis half-life was determined to be ca. 30 seconds. In all likelihood in the aquatic toxicity tests the observed effects were caused by degradation product phthalic acid. To describe a worst case scenario no conversion to molecular weight of phthalic acid, which would lead to less sensitive effect values, was carried out.

A test conducted according to the Draft Guideline OECD 210 is regarded as an acute toxicity test of phthalic anhydride to fish; as developmental stages in the life cycles of fish have been shown to be relatively sensitive to the effects of chemicals rather than to adults leading to more meaningful results with respect to acute toxicity. The embryolarval stages of Brachydanio rerio (new name: Danio rerio) were exposed to phthalic anhydride for 7 days in buffered medium under semistatic conditions yielding a LC50 of 560 mg/l (Van Leeuwen, 1990).

In a test, the fresh water alga Desmodesmus subspicatus was exposed to phthalic acid for 72 hours. The experiment was conducted according to EU method C.3 as a limit test in two media, once with and the other without pH adjustment. The results of both tests clearly indicate that the toxic effects are not due to substance inherent properties but to a function of pH. The test with pH adjustment gave a NOEC of > 100 mg/l (Bayer Industry Services, 2004).

A test with respect to the acute toxicity towards invertebrates (Daphnia magna) yields in a 48 h-EC50 of > 640 mg/l (Adams, 1986). For long-term toxicity of phthalic acid to fish a read-across approach of phthalic anhydride was used. An early life stage toxicity test according to the Draft Guideline OECD 210 was carried out in buffered medium using Salmo gairdneri (new name: Oncorhynchus mykiss) as test species. A NOEC of 10 mg/L after 60 days was received(Van Leeuwen, 1990).

Long-term toxicity of phthalic anhydride towards aquatic invertebrates (Daphnia magna) was investigated in a reproduction test conducted according to OECD Guideline 211. After 21 days of exposure a NOEC-reproduction of 16 mg/l was determined (MITI, 2003). The result is regarded with caution, since it cannot be excluded that the toxicity observed was due to pH effects based on the formation of phthalic acid during the experiment causing a pH shift.