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Dissociation constant

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Description of key information

Not relevant

Key value for chemical safety assessment

Additional information

This endpoint is covered by the category approach for soluble iron salts (please see the section for physical and chemical properties for the category justification/report format). Testing for this endpoint has been waived in accordance with column 2 restrictions. In addition a study is considered scientifically unjustified. The overall dissociation constant is not relevant and the environmental fate assessment bases on rapidly formed transformation products. The behaviour of iron ions in solution is dependent on the conditions. Key influences include oxygen content, pH and the presence of potential ligand anions with which the cationic iron might associate. When the ferric ion is resulting from oxidation of the ferrous ion or directly released to water the hexa-aquo Kation is formed. This is strongly acidic with a pKa of 3.05 (Cotton & Wilkinson 1972). Thus:

[Fe(H2O)6]3+ -> [Fe(H2O)6](OH)2+ + H+(aq)

Nonetheless the Fe-hexa-aquo-kation is only minor species due its limited solubility, short half-life, finally precipitation and eventually polymerisation in the environment.

  • Cotton FA, Wilkinson G eds (1972). Advanced Inorganic Chemistry. New York, U.S.A. John Wiley and Sons