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Physical & Chemical properties

Partition coefficient

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partition coefficient
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is inorganic

Description of key information

The partition coefficient of zinc dipropionate is not considered to be relevant.

Key value for chemical safety assessment

Additional information

Zinc dipropionate is an inorganic substance in accordance with the definitions of the International Union of Pure and Applied Chemistry (IUPAC) because of its ionic structure. In accordance with Regulation (EC) No 1907/2006, Annex VII section 7.8, column 2, a study on the partition coefficient n-octanol/water does not need to be conducted if the substance is inorganic.

The octanol/water partition coefficient, Pow, is defined as the ratio of the equilibrium concentrations of a dissolved substance in each of the phases in a two-phase system consisting of octanol and water. It is a key parameter in studies of the environmental fate of organic substances, indicating the potential for bioaccumulation and soil absorption. Whereas this may also be applicable for organometallic substances, it does not pertain to zinc dipropionate because of its inorganic nature.

Instead of the determination of a Pow value, the environmental fate and distribution of the dissociation products of this substance in water are better assessed according to the dissociation products in water as follows:  

(i) the mechanisms for partitioning of Zn2+ in environmental media, including the adsorption and/or absorption by organic matter and living cells, are understood to be different from those traditionally attributed to carbon-based substances (see IUCLID section 5.4 for information on the partitioning of zinc in the environment). Thus, octanol/water partitioning has little relevance to ionic zinc. In order to measure an octanol/water partition coefficient, it is necessary to determine the concentration in each phase (as in OECD method 107), or to conduct an HPLC assay (as in OECD method 117). However, zinc is a metallic element that exists only in an ionic form in solution. The solubility of divalent zinc cations in water is low and can safely also be expected to be low in organic solvents such as octanol. Because of the unlikely partitioning of zinc cations into the octanol phase, it is not appropriate to determine the partition coefficient by direct quantification of zinc in both phases. Similarly, any aqueous HPLC mobile phase will cause dissociation of inorganic zinc compounds, and thus does not allow the determination of a Pow by this method.

(ii) regarding the partitioning behaviour of propionic acid, the reliable handbook of Hansch, C., Leo, A.J. and Hoekman, D. (1995) reports a log Pow of 0.33 [in OECD SIDS Propionic acid, UNEP Publications (2007)]. The corresponding estimate for the log Pow based on an atom/fragment contribution method is 0.58 (KOWWIN™ Program Version 1.68; EPI Suite™ v.4.11, 2012).

In conclusion, the conduct of further experimental verification for zinc dipropionate is considered to be neither technically nor scientifically feasible, and for the reasons stated above derogation from testing is hereby applied for.