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Metal carboxylates are salts consisting of metal cation and carboxylic acid anion. Based on the solubility of zinc dioctanoate in water, a complete dissociation resulting in zinc and octanoate ions may be assumed under environmental conditions. The respective dissociation is in principle reversible and the ratio of the salt /dissociated ions is dependent on the metal-ligand dissociation constant of the salt, the composition of the solution and its pH.

However, under environmental conditions, a reunion of the dissociated ions is highly unlikely and it may reasonable be assumed that the respective behaviour of the dissociated zinc cations and octanoate anions in the environment determine the fate of zinc dioctanoate upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently the (eco)toxicological potential.

A metal-ligand complexation constant of zinc dioctanoate could not be identified. Data for zinc appear to be generally limited. However, zinc tend to form complexes with ionic character as a result of their low electronegativity. Further, the ionic bonding of zinc is typically described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions. 

Based on an analysis by Carbonaro et al. (2007) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as octanoate anions are not expected to bind strongly with zinc.Accordingly, protons will always out-compete zinc ions for complexation of monodentate ligands given equal activities of free zinc and hydrogen ions.

The metal-ligand formation constants (log KML) of zinc with other carboxylic acids, i.e. acetic and benzoic acid, ranging from 0.56 to 1.59 (Bunting & Thong, 1969), further point to a low strength of the monodentate bond between carboxyl groups and zinc.

The analysis by Carbonaro & Di Toro (2007) suggests that the following equation models monodentate binding to negatively-charged oxygen donor atoms of carboxylic functional groups:

log KML = αO * log KHL + βO; where

KML is the metal-ligand formation constant, KHL is the corresponding proton–ligand formation constant, and αO and βO are termed the Irving–Rossotti slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of octanoic acid of 4.89 results in:

log KML = 0.301 * 4.89 + 0.015

log KML = 1.49 (estimated zinc- octanoate formation constant).

Thus, it may reasonably be assumed that based on the zinc dioctanoate formation constant the respective behaviour of the dissociated zinc cations andoctanoateanions in the environment determines the fate of zinc dioctanoate upon dissolution. In the assessment of environmental fate of zinc dioctanoate, read-across to analogue substances and/or the assessment entities soluble zinc substances and octanoic acid is applied since the ions of zinc dioctanoate determine the environmental fate of zinc dioctanoate. Since zinc ions and octanoate ions behave differently in the environment, a separate assessment of the environmental fate of each assessment entity is performed. Please refer to the data as submitted for each individual assessment entity. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for zinc dioctanoate.

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