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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 tri isotridecyl phosphite (TiTDP) 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 (TiTDP) would not feasible, the evaluation should focus on quantifying the toxicity of the combination of hydrolysis by-products of TiTDP – namely isotridecanol 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-product 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 TiTDP (isotridecanol and phosphorous acid) were identified (robust summaries of these data are presented in Section 6.1 of the IUCLID dossier). Using the estimated water solubility of TiTDP of 6.29 x 10-7mg/L (the higher of the two estimates) 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. 




Based on the following stoichiometry:


one mole TiTDP (mw= 629.05 g/mole) yields three moles isotridecanol (200.36 mw = g/mole) and one mole phosphorous acid (mw= 82.00 g/mole);


At its aqueous solubility limit of 6.29 x 10-7mg/L = 6.29 x 10-10g/L = 1 x 10-12 moles/L (6.29 x 10-10g/L ÷ 629.05 g/mole), TiTDP would hydrolyse to 3 x10-12 moles/L isotridecanol and 1 x 10-12moles/L phosphorous acid. These molar concentrations equate to mass concentrations of 6.01 x 10-7mg isotridecanol/L and 8.2 x 10-8mg phosphorous acid/L respectively. These concentrations were then compared to the aquatic toxicity values for isotridecanol and phosphorus acid.




The lowest aquatic toxicity value was a NOEC of 0.04 mg/L for 16-day exposure to daphnia or approximately 5 orders of magnitude higher than the highest estimated theoretical concentration for isotridecanol from TiTDP.


Phosphorus Acid        


The EC50 values for phosphorus acid are 383 mg/L for acute fish, 387 mg/L for acute daphnia, and 230 mg/L for algae. These values are all approximately 9-10 orders of magnitude higher than the highest estimated theoretical concentration for phosphorus acid from TiTDP. 




These findings indicate that the mixture of hydrolysis by-products arising from a standard solution of TiTDP would not be toxic to aquatic organisms as the estimated concentrations from the aged (hydrolyzed) TiTDP are much, much lower than the toxicity values of the hydrolysis products. This is due, in essence, to the incredibly low solubility of TiTDP. While the soluble portion of TiTDP does hydrolyze rapidly to substances that have measured aquatic toxicity values the fact that TiTDP is so incredibly insoluble in water means that the solution can never achieve levels of hydrolysis products that come anywhere close to those at which aquatic toxicity would be observed. As such, it is concluded that TiTDP is not toxic to aquatic life. 

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