Triethoxy(methyl)silane

Dissociation constant

Link to relevant study record(s)

Reference

Reference 1

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)

Reference 2

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2008

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

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.

Reference 3

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 for cross-reference:

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

Description of key information

Dissociation constant (triethoxy(methyl)silane: not relevant
Dissociation constant (silanol hydrolysis product): approximately 10,

based on read-across from phenylsilanetriol

Additional information

The requirement to test the substance for dissociation constant is waived because in contact with water the substance rapidly hydrolyses to form methylsilanetriol and ethanol. There are no reliable measured data for the dissociation constant of methylsilanetriol. 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 methylsilanetriol 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.