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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Principles of method if other than guideline:
No determination of the dissociation constants in water was possible using OECD Method 112. The test material did not demonstrate pH sensitive spectral profiles in the ultraviolet region of the electromagnetic spectrum. In addition, the weakly acidic nature of the anticipated dissociating groups was not suitable for investigation by either the titration of conductometric methods. An estimation of the first dissociation constant was made using a procedure based on OECD Method 107 (partition coefficient by the shake flask method) and the relationship between partition coefficient, aqueous phase pH and dissociation constant.
GLP compliance:
yes
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

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 at 22°C was determined for phenylsilanetriol in a reliable study conducted in compliance with GLP. The results are considered reliable.

Description of key information

Triethoxy(phenyl)silane: not relevant
Silanol hydrolysis product: pKa = approximately 10 at 22°C

Key value for chemical safety assessment

Additional information

The requirement to test the parent substance for dissociation constant is waived because in contact with water the substance rapidly hydrolyses to form phenylsilanetriol and ethanol.

Hydrolysis product (phenylsilanetriol): An estimated dissociation constant of approximately pKa =10 at 22°C was determined using a procedure based on OECD Method 107 (partition coefficient by the shake flask method) and the relationship between partition coefficient, aqueous phase pH and dissociation constant.