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Dissociation constant

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Although the legal text of REACH (Annex IX, Column 2) clearly states that LogKow does not have to be determined for inorganics, a similar exclusion is not made for dissociation constant.  Since nickel sulphide is soluble, OECD guideline 112 (the recommended test guideline) is theoretically applicable.  However, possible dissociation in metal ion complexes is usually described by a stability constant. The calculations are similar but more involved. The result is usually expressed as LogK rather than pKa. OECD guideline 112 and the methods it contains do not cover these measurements and the stability constant issue is beyond the scope and requirements of OECD 112.  Additionally, because nickel sulphide is soluble, the pKa would be negative and negative pKa values are not used in a practical sense.  Therefore, data are waived because expressing the dissociation constant of nickel sulphide with a pKa value is impractical.  In addition, OECD guideline 112 is used to establish the dissociation of a chemical in water which is important in assessing its environmental impact.  The environmental impact of Ni and Ni-containing compounds has been extensively investigated and adequately reported under Sections 5 and 6 of this IUCLID database.

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