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Dissociation constant

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Description of key information

Dissociation constant [parent substance]: Not relevant
Dissociation constant (silanol hydrolysis product): ca. 10

Key value for chemical safety assessment

Additional information

In accordance with Section 1 of REACH Annex XI, the study does not need to be conducted because there are no groups present in the molecule that are expected to ionize within an environmentally-relevant range. Testing would also be technically difficult because the substance has very low water solubility and is hydrolytically unstable.

In contact with water the substance hydrolyses rapidly. The hydrolysis products are (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol for the S2 constituent and (3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol for the S3 constituent; the other hydrolysis product in each case is ethanol. There are no reliable measured data for the dissociation constant of the silanol hydrolysis products. 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 the silanol hydrolysis products 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 ca.10 in a reliable study using a relevant test method, (reliability 1; read-across conclusion is given reliability 2). Additional information is given in a supporting report (PFA 2013b) attached in Section 13 of the REACH technical dossier.

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