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Dissociation constant

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Description of key information

The substance is a UVCB, dissociation constant is not applicable for a UVCB. Its representative component Ammonium sulfate has three dissociation constants at 25°C:
[sulfuric acid /hydrogen sulfate]: -3
[hydrogen sulfate /sulfate]: 1.92
[ammonium /ammonia]: 9.21
In aqueous solution, ammonium sulfate is completely dissociated into the ammonium ion (NH4+) and the sulfate anion (SO42-). Ammonia may however become a relevant form at basic environmental pH (8 to 9).

Key value for chemical safety assessment

Additional information

ECHA’s webinar “How to bring your registration dossier in compliance with REACH - Tips and Hints - Part 5 - Dissociation constant” (12 February 2014), indicates that “For complex mixtures (e.g. UVCBs*) estimation of the representative constituent’s pKa values, if appropriate, should be considered.” Indeed, the pKa of a component is a chemical constant (at a given temperature) which is independent of the other components present in the substance.

The dissociation constants of the representative constituent of the UVCB, ammonium sulfate, are known from peer-reviewed literature: OECD SIDS Initial Assessment Report - AMMONIUM SULFATE CAS N°: 7783-20-2, October 2004. These data respect all three conditions of Annex XI, § 1.1.1 (Use of existing data):

1) They are relevant for the registered substance since, as justified above, the pKa of Ammonium sulfate stays the same when this component is inside a UVCB; pKa value is not used in classification criteria; no exposure/risk assessment is required for the UVCB.

2+3) The data have been considered valid and sufficiently documented by the OECD SIDS experts (Advisory Committee on Existing Chemicals of the Association of German Chemists) who peer-reviewed it.

Therefore, the data on Ammonium sulfate adequately fill the data requirement on the UVCB.

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