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Physical & Chemical properties

Partition coefficient

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Description of key information

Not appropriate, environmental partition coefficients (Kp or Kd) to be used instead 

Key value for chemical safety assessment

Additional information

This endpoint is covered by the category approach for soluble iron salts (please see the section summary for the category justification/report format).

Testing for this endpoint has been waived in accordance with column 2 restrictions. The solubility of iron salts in organic solvents such as octanol is low. The iron ions undergo a series of reactions in aqueous solution which result in the formation of highly insoluble oxo-hydroxides. However, the initial solubility in water is high, but testing is technically not feasible due to the hydrolytic instability of the submission item.

Anyhow the Octanol/water partition coefficient is a property of interest and relevance for organic substances only and not appropriate for inorganics such as metals and measured environmental partition coefficients (Kp or Kd) should be used instead (ECHA 2008, p 12). Generally only the metallic dissociation products (i.e. the iron forms) are of relevance in the context of Chemical Safety Assessment because the remaining dissociation products are considered irrelevant (see category justification in the Endpoint Summary for physical and chemical properties).

The Log Kow is used in two ways for assessment and should be substituted as follows:

1) As a surrogate for fugacity properties such as bioconcentration factor and soil-water partition coefficient: Given the biochemical importance of iron, all physiological processes are controlled by highly-evolved mechanisms, and therefore the octanol-water partition coefficient is not needed for this purpose. Due to the active transport of metal kations, the fugacity based approach is not applicable for the assessment of the likelihood for enrichment in the food chains of biota. The assessment on enrichment in biota can be based on the comparison of measured concentrations in biota of increasing trophic levels (see section on bioaccumulation).

2) To help model fate and effects: This is approached in a different way for naturally-occurring and biologically significant inorganic compounds. For that purpose a Koc measurement is often replaced by estimation on the basis of Kow. Koc can be replaced using Kd data. Values are available and reported in the section on adsorption / desorption.

  • ECHA European Chemicals Agency (2008). Guidance on information requirements and chemical safety assessment. Appendix R.7.13-2: Environmental risk assessment for metals and metal compounds. Version 1. Self-published, Helsinki, Finland, in July. 78 p.