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Assessment of Aquatic Toxicity Based on Assessment of Hydrolysis Products

Based on experiences with attempting to design and conduct appropriate studies to investigate the ecotoxicity of alkyl and aryl phosphites, it was determined that conducting aquatic toxicity studies on tris 2-propylheptyl phosphite (T2PHP) in algae, daphnia and fish would not be possible. This conclusion is consistent with OECD Guidance Document #23 entitled “Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures” (OECD 2000) because of the inherent physical/chemical properties of the test substance (i.e., extremely poor water solubility and hydrolysis). Based on the expert approach that was developed for other alkyl and aryl phosphites, it was concluded that, since direct measurements of the ecotoxicity of the parent test substance (T2PHP) would not feasible, the evaluation should focus on quantifying the toxicity of the combination of hydrolysis by-products of T2PHP– namely 2-propylehptanol and phosphorus acid.

 

It is possible to quantitatively predict the maximum theoretical concentration of hydrolysis by-products and resulting toxicity of the solution of these by-products based on the known toxicity of the individual by-products, assuming additive toxicity. To achieve this, measured and calculated aquatic toxicity values for the individual primary hydrolysis by-products of T2PHP (2-propylehptanol and phosphorus acid) were identified. Using the estimated water solubility of T2PHP of 2.1 x 10-8 mg/L, the maximum theoretical concentrations of the individual hydrolysis by-products in water were calculated. These maximum estimated concentrations were then compared to the aquatic toxicity values for the hydrolysis products and ratios for each were summed to develop a maximum theoretical solution toxicity. A derived solution toxicity value of 1.0 would be considered to be equivalent to a toxicity value for an “aged” (hydrolyzed) solution. The further this value is below 1, the lower the anticipated ecotoxicity hazard. 

 

Results

 

Based on the following stoichiometry:

 

one mole T2PHP (mw= 502.8 g/mole) yields two moles 2-propylheptanol (158.3 mw = g/mole) and one mole phosphorous acid (mw= 82.00 g/mole)

 

At its aqueous solubility limit of 2.1 x 10-11 g/L = 4.1 x 10 -14 moles/L (2.1 x 10-11 g/L ÷ 502.8 g/mole), T2PHP would hydrolyse to 1.2 x10 -13 moles/L 2-propylheptanol and 4.1 x 10-14 moles/L phosphorous acid. These molar concentrations equate to mass concentrations of 1.9 x 10-8 mg 2-propylheptanol/L and 3.4 x 10 -9 mg phosphorous acid/L respectively. 

These values are orders of magnitude below the NOECs and PNECs for 2-propylheptanol and phosphorus acid suggesting that T2PHP will not result in environmental hazards based on its hydrolysis products.

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