Registration Dossier

Physical & Chemical properties

Dissociation constant

Currently viewing:

Administrative data

Link to relevant study record(s)

Referenceopen allclose all

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 (incl. certificate)
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
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 [trimethoxy(octyl)silane]: Not relevant due to rapid hydrolysis in contact with water (half-life <12 hours)


Dissociation constant [octylsilanetriol]: pKa approximately 10

Key value for chemical safety assessment

Additional information

The requirement to test the substance for dissociation constant is waived because the registered substance will ionise rapidly in contact with water. The hydrolysis products in this case are octylsilanetriol and methanol.

There are no reliable measured data for the dissociation constant of octylsilanetriol. 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 octylsilanetriol 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. Additional information is given in a supporting report (PFA 2013b) attached in Section 13.

Reference:

PFA (2013b). Peter Fisk Associates, Dissociation constant Analogue report, PFA.300.001.001