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Dissociation constant

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Key value for chemical safety assessment

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In accordance with column 2 of REACH Annex IX, the dissociation constant study (required in section 7.16) does not need to be conducted as the substance is a highly variable complex mixture and therefore the analytical method is unlikely to be sufficiently sensitive.

Furthermore, from the compositional description and the structural formula given, it can be concluded that this substance (EINECS: 310-154-3) is a potentially ionizable highly oil soluble alkylphenol of exceedingly low water solubility. Taken collectively, these properties may seem counter intuitive but are nonetheless sufficient to warrant a data waiver according to the assessment criteria normally applied during the evaluation of this physical property. Reversible ionization (deprotonation) is possible in the presence of strong aqueous and non-aqueous base (pKa ~10.0), however, within normal environmental or physiological pHs, the extent of dissociation is small enough to be neglected for all practical purposes.

Within the gut of animals and in typical (slightly acidic) aquatic environments, any water soluble fraction of phenate anion (parts per billion) will substantially revert to its neutral protonated phenolic state according to the understood equilibrium for these two species in natural aqueous media. The pKa of phenol itself is approximately 10.02, and the pKa of this alkylphenol is expected to be in the same region. Thus, at typical physiological and environmental pHs, it is reasonable to expect that the free phenolic species will be clearly dominate in such an aqueous acid-base media. Therefore slight pH changes (within the normal physiological pH ranges) will not significantly impact the position of this equilibrium with respect to acid-base pairings that are typical in such cases.

Apart from the phenolic functional group present, there are no other indicative structural alerts, Thus, it is reasonable to expect that this substance should not be considered ionizable in the normal sense of the meaning of the guidance given for assessing this endpoint. This empirical evaluation is furthermore confirmed by long term handling experience.

As such it is considered neither technically possible nor scientifically justified to conduct the study

Supporting information is available in the form of a predicted value derived using estimation software. The dissociation constant (pKa) of a representative struture of Phenol, (tetrapropenyl), derivative (C12 model compound) was estimated using the pKalc function of the PALLAS estimation software program (CompuDrug, version 3.6.2.2, 2006). The pKa was determined to be 9.87. A reliability rating of 4 was given according to Klimsch, 1997, as the variable nature of the substance may affect the outcome of this prediction, however the value is consistent with the justification for the data waiver.