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Physical & Chemical properties

Dissociation constant

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dissociation constant
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The registration substance is an inorganic well water soluble substance. When dissolving the substance in water, the pH of the solution drops as the RE3+ ion will extract OH- from the aqueous solution (formation of RE(OH)2+). Determination of the pKa requires increasing the pH of the aqueous solution (addition of an alkaline substance) and determining the pH at which the protonated and non-protonated species are present in equal amounts. When increasing the pH of an aqueous solution of the registration substance, further RE hydroxides (RE(OH)3) will be formed; these RE hydroxides are in nature not soluble in water. Determination of the accompanying stability constants of the complexes formed at different pH is therefore technically not feasible and outside the scope of the OECD 112 test guideline (which describes the specific determination of acid/base dissociation constants).  In addition, the purpose of a dissociation constant is described in ECHA guidance on information requirements and chemical safety assessment (R.7a): “This property is important for ionisable organic substances, since it indicates which chemical species will be present at a particular pH” and “The endpoint is important as it indicates partitioning behaviour of the substance in the environment and what chemical species are likely to be present in the environment and in vivo”. However the concept of parameters influenced by the dissociation of a substance (octanol/water partitioning coefficient and soil adsorption/desorption) is not applicable for inorganic compounds (R.7.13-2). Partitioning / adsorption is driven by other parameters including the presence of different ligands in the environment and in for instance the gastrointestinal tract. Following the above rationale it is concluded that the determination of the dissociation constant is scientifically unjustified for the registration substance.

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