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Physical & Chemical properties

Dissociation constant

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Description of key information

Dissociation constant (1,1,1,3,3,3-hexamethyldisilazane): Not relevant
Dissociation constant (trimethylsilanol): ca. 11
Dissociation constant (ammonium ion): 9.25

Key value for chemical safety assessment

Additional information

The requirement to test the substance for dissociation constant is waived because in contact with water the substance hydrolyses very rapidly to form trimethylsilanol and ammonia. Silanols are not expected to undergo significant dissociation within an environmental relevant range. This is supported by measured data for trimethylsilanol which is reported in secondary literature to have a first dissociation constant pKa of approximately 11. Additional information is given in a supporting report (PFA 2013b) attached in Section 13 of the REACH technical dossier.

The pKa of the ammonium ion, NH4, is given in a reliable handbook as 9.25 ( Haynes W. M, Lide D.R. (ed.) 2010. CRC Handbook of Chemistry and Physics. 91stEdition. Boca Raton: CRC Press).