Octanoic acid, zinc salt, basic

Endpoint summary

Description of key information

Environmental fate and pathway studies with the substance octanoic acid, zinc salt, basic are not available. In the assessment of octanoic acid, zinc salt, basic read-across to analogue substances and/or the assessment entities soluble zinc substances and fatty acids is applied since the ions of the substance octanoic acid, zinc salt, basic determine its fate and toxicity in the environment. For details on the environmental fate and pathways of the individual moieties, please refer to the respective assessment entities. In brief, metal partition coefficients for distribution between different fractions e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) are the only parameter relevant for the fate of zinc in the environment and biotic degradation is the only fate process expected to substantially influence the fate of fatty acids in the environment. However, > 70 % of zinc is removed from the water column under reference conditions for EU regional waters (EUSES) (see section 5.6.: "removal from the water column") and the fatty acid moiety is rapidly biodegraded. Thus, for both moieties, bioconcentration/bioaccumulation is considered not relevant.

Additional information

Read across statement:

 

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the water solubility of octanoic acid, zinc salt, basic (321 mg/L at 20°C), a complete dissociation of octanoic acid, zinc salt, basic resulting in zinc cations and octanoate anions may be assumed under environmental conditions upon contact with water. 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.

 

A metal-ligand complexation constant of octanoic acid, zinc salt, basic could not be identified. Data for zinc appear to be generally limited. However, zinc cations 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. (2011) of monodentate binding of zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as octanoic acid 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 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 * 5.19 + 0.015

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

 

Thus, it may reasonably be assumed that based on the estimated zinc-octanoate formation constant, the respective behaviour of the dissociated zinc cations and octanoate anions in the environment determine the fate of octanoic acid, zinc salt, basic 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 its ecotoxicological potential.

In order to evaluate the environmental fate and toxicity of octanoic acid, zinc salt, basic, information on the assessment entities zinc cations and representative fatty acid anions were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for octanoic acid, zinc salt, basic.

 

Reference:

Carbonaro RF & Di Toro DM (2007) Linear free energy relationships for metal–ligand complexation: Monodentate binding to negatively-charged oxygen donor atoms. Geochimica et Cosmochimica Acta 71: 3958–3968.

Bunting, J. W., & Thong, K. M. (1970). Stability constants for some 1: 1 metal–carboxylate complexes. Canadian Journal of Chemistry, 48(11), 1654-1656.