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Physical & Chemical properties

Partition coefficient

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Description of key information

The partition coefficient of this substance is not considered to be relevant.

Key value for chemical safety assessment

Additional information

In accordance with Annex XI of Regulation (EC) No 1907/2006, this study does not need to be conducted for this substance, as it is considered to be neither scientifically necessary nor technically possible.

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 the substance in question here because of its inorganic nature.

For inorganic substances, the determination of a partition coefficient is not considered to be required (ECHA Guidance on information requirements and chemical safety assessment Chapter R.7a: Endpoint specific guidance, section R.7.1.8; November 2012); since the substance is similar to inorganic salts because of its ionic structure, waiving in accordance with Annex VII section 7.8 Column 2 of regulation (EC) 1907/2006) is also justified.

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 not allow the determination of an Pow by this method.

(ii) regarding the partitioning behaviour of fatty acid constituents, handbook data for myristic acid (C14, EC# 208-875-2, CAS# 544-63-8), palmitic acid (C16, EC# 200-312-9, CAS# 57-10-3) and stearic acid (C18, EC# 200-313-4, CAS# 57-11-4) report log Pows of 6.11, 7.17 and 8.23, respectively [D´Amboise, Hanai (1982) in Canadian National Committee for CODATA (CNC/CODATA) database: http://logkow.cisti.nrc.ca/logkow/]. The corresponding estimates for the log Pows of myristic, (C14), palmitic (C16) and stearic (C18) acid based on an atom/fragment contribution method are 5.98, 6.96 and 7.94, respectively (KOWWIN™ Program Version 1.68; EPI Suite™ v.4.11, 2012).

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