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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
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 [3-(trimethoxysilyl)propylamine] (RNH3+ ¿ RNH2): pKa approximately 10

Dissociation constant [3-aminopropylsilanetriol] (triol group): pKa approximately 10

Dissociation constant [3-aminopropylsilanetriol] (amino group): pKa approximately 10

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 rapidly. The hydrolysis products in this case are 3-aminopropylsilanetriol and methanol. The amino group present in [3-(trimethoxysilyl)propylamine] is basic and has a predicted pKa of 10.4. There are no reliable measured data for the dissociation constant of 3-aminopropylsilanetriol.

The amino group present in the silanol hydrolysis product is basic. The pKa is not known but an indicative read-across substance would be ethanolamine. This has a basic pKa of 9.5 (Haynes 2010). The amino group present in 3-aminopropyltrimethoxysilane has a predicted basic pKa of 10.4. The read-across and the prediction are in agreement that the pKa of this group is approximately 10.

The typical dissociation constants for the hydroxyl protons of silanetriols are available from studies in other silanetriols. It is appropriate to read-across dissociation constant data for 3-aminopropylsilanetriol from phenylsilanetriol. This suggests that silanols are not expected to undergo significant dissociation within the range of pH relevant in the environment or in vivo.

Phenylsilanetriol has a measured first dissociation constant approximately 10 in a reliable study using a relevant test method. Additional information is given in a supporting report (PFA 2013b) attached in Section 13 of the REACH technical dossier.