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Dissociation constant

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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)
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. QA statement)
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.

Description of key information

Dissociation constant [3-(trimethoxysilyl)propyl isocyanate]: not relevant due to very rapid hydrolysis

Dissociation constant [3-(trimethoxysilyl)propylamine] (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 silanol hydrolysis product, 3-(trimethoxysilyl)propylamine (CAS 13822-56-5). 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 methanol 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 of 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 2010). The amino group present in 3-(trimethoxysilyl)propylamine 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.