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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 [triethoxy(3-isocyanatopropyl)silane]: not relevant due to very rapid hydrolysis in contact with water

Dissociation constant [3-aminopopyl(triethoxy)silane] (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 very rapidly forming an intermediate hydrolysis product, 3-aminopropyltriethoxysilane (CAS 919-30-2). The ultimate hydrolysis product under dilute conditions is 3-aminopropylsilanetriol (CAS 58160-99-9). Condensation of 3-aminopropylsilanetriol is important above 1000 mg/L (see water solubility section). The other products of hydrolysis are ethanol and carbon dioxide.

There are no reliable measured data for the dissociation constant of 3-aminopropylsilanetriol. 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 (CAS 58160 -99 -9) from phenylsilanetriol (CAS 3047-74-3). 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.

The amino group 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 and Lide 2010). The amino group present in 3-aminopropyltriethoxysilane 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.

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