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Dissociation constant

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Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Reason / purpose:
read-across source
Qualifier:
according to
Guideline:
other:
Version / remarks:
Estimation of the first dissociation constant was made using a procedure based on OECD method 107 and the relationship between partition coefficient, aqueous phase pH and dissociation constant.
Principles of method if other than guideline:
Estimation of the first dissociation constant was made using a procedure based on OECD method 107 and the relationship between partition coefficient, aqueous phase pH and dissociation constant.
Dissociating properties:
yes
No.:
#1
pKa:
9.6
Temp.:
22 °C
Remarks on result:
other: ±0.3
No.:
#2
pKa:
> 11
Temp.:
22 °C
Remarks on result:
other: Estimate
No.:
#3
pKa:
> 11
Temp.:
22 °C
Remarks on result:
other: Estimate

The results of each determination are shown in the following table:

Nominal test pH

Sample Number

Aqueous Phase pH

Partition coefficient (P’ow)

Dissociation constant (pKa)

8.0

1

7.99

0.874

8.7*

8.0

2

8.03

0.941

8.9*

8.5

3

8.58

0.883

9.3

8.5

4

8.58

0.883

9.3

9.0

5

9.08

0.879

9.8

9.0

6

9.08

0.837

9.7

9.5

1

9.21

0.563

9.3

9.5

2

9.26

0.648

9.5

10.0

3

9.78

0.408

9.6

10.0

4

9.91

0.404

9.7

10.5

5

10.48

0.182

9.8

10.5

6

10.49

0.178

9.8

* The results at pH 8 were excluded from the determination of the final pKa mean and range on the basis that the accuracy of the estimation method decreased as the distance between the dissociation constant and the aqueous phase pH increased, pH 8.0 being >1.5 units from the final estimation value. This is consistent with the approach used in the titration method in OECD method 112.

Conclusions:
A first dissociation constant of 9.6 for phenylsilanetriol was determined in a reliable study conducted in compliance with GLP.
Endpoint:
dissociation constant
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Please refer to additional information field in the endpoint study summary (Dissociation constant).
Reason / purpose:
read-across source
Dissociating properties:
yes
No.:
#1
pKa:
9.6
Temp.:
22 °C
No.:
#2
pKa:
> 11
Temp.:
22 °C
No.:
#3
pKa:
> 11
Temp.:
22 °C
Conclusions:
A first dissociation constant of 9.6 for phenylsilanetriol was determined in a reliable study conducted in compliance with GLP.

Description of key information

Dissociation constant: (chloromethyl)triethoxysilane) not relevant
Dissociation constant (silanol hydrolysis product): approximately 10, based on read-across from phenylsilanetriol

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 rapidly hydrolyses to form chloromethylsilanetriol and ethanol. There are no reliable measured data for the dissociation constant of chloromethylsilanetriol. 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 chloromethylsilanetriol 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, (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.