Registration Dossier

Data platform availability banner - registered substances factsheets

Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Physical & Chemical properties

Dissociation constant

Currently viewing:

Administrative data

Link to relevant study record(s)

dissociation constant
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is hydrolytically unstable (half-life less than 12 hours)

Description of key information

Dissociation constant [1,1,1,3,3,3-hexamethyldisilazane]: not relevant due to very rapid hydrolysis in contact with water

Dissociation constant [trimethylsilanol]: pKa approximately 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 a 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.

The pKa of the ammonium ion, NH4+, is given in a reliable handbook as 9.25 (Haynes W. M, Lide D.R. (ed.) 2010).


Haynes W. M, Lide D.R. (ed.) 2010. CRC Handbook of Chemistry and Physics. 91st Edition. Boca Raton: CRC Press

PFA (2013b). Peter Fisk Associates, Dissociation constant Analogue report, PFA.300.001.001