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Environmental fate & pathways

Hydrolysis

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Description of key information

Not susceptible to hydrolytic degradation.

Key value for chemical safety assessment

Additional information

The substance as registered is an aqueous solution and is stable in this solution (at high pH). Tripotassium propylsilanetriol(ate) and potassium hydroxide contain no chemical groups that are susceptible to hydrolytic degradation.

Hydrolysis of the read-across substance trimethoxypropylsilane (CAS 1067 -25-0)

Data for the substance trimethoxypropylsilane (CAS 1067-25-0) are read-across to the submission substance tripotassium propylsilanetriolate for appropriate endpoints (see Section 1.4 of the CSR).The hydrolysis half-life and the silanol hydrolysis product of the read-across substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trimethoxypropylsilane, hydrolysis half-lives at 20-25°C of 0.2 h at pH 4, 2.6 h at pH 7 and 0.1 h at pH 9 were determined using a validated QSAR estimation method.

For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

kobskH3O+[H3O+]

At pH 4 [H3O+]=10-4mol dm-3and at pH2 [H3O+]=10-2mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

t1/2(pH 2) = t1/2(pH 4) / 100

The calculated half-life of trimethoxypropylsilane at pH 2 is therefore 0.002 hours (7.2 seconds).

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT50(XºC) = DT50(T) * e(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for trimethoxypropylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 1 hour. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the predicted half-life is 2.6 seconds. However,it is not appropriate or necessary to attempt to predict accurately when the half-life is less than 5-10. As a worst-case it can therefore be considered that the half-life for the substance at pH 2 and 25°C is approximately 5 seconds.

The hydrolysis products are propylsilanetriol and methanol.

Hydrolysis of the read-across substance trimethoxymethylsilane (CAS 1185-55-3)

Data for the substance trimethoxymethylylsilane (CAS 1185-55-3) are read-across to the submission substance tripotassium propylsilanetriolate for appropriate endpoints (see Section 1.4 of the CSR).The hydrolysis half-life and the silanol hydrolysis product of the read-across substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trimethoxymethylsilane, hydrolysis half-lives at 25°C of <0.0033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2004).

In a supporting reliable study, the stability of the substance in aqueous  media under physiological conditions was investigated. The rates of 

hydrolysis of 1000 ppm trimethoxy(methyl)silane were determined in water at  pH 5.7, 0.15 molar (M) sodium-phosphate buffer (PBS), and 10% rat  serum in  0.15M PBS at pH 7.4 and 37.4°C in soft glass reactors. In this study, the substance was hydrolysed in water, PBS, and PBS plus 10% rat serum at pH 7.4 and 37°C with half-lives of 24, 6.7 and 8.6 minutes respectively.

For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

kobskH3O+[H3O+]

At pH 4 [H3O+]=10-4mol dm-3and at pH2 [H3O+]=10-2mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

t1/2(pH 2) = t1/2(pH 4) / 100

The calculated half-life of trimethoxymethylsilane at pH 2 is therefore 0.000033 hours (0.12 seconds). However, it is not appropriate or necessary to attempt to predict accurately when the half-life is less than 5-10. As a worst-case it can therefore be considered that the half-life for the substance at pH 2 and 25°C is approximately 5 seconds.

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT50(XºC) = DT50(T) * e(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for trimethoxymethylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 0.8 hours. At 37.5ºC and pH2 (relevant for conditions in the stomach following oral exposure), it is not appropriate to apply any further correction for temperature to the limit value and the hydrolysis half-life is therefore approximately 5 seconds.

The hydrolysis products are methylsilanetriol and methanol.

Hydrolysis of the read-across substance triethoxyisobutylsilane (CAS 17980-47-1)

Data for the substance triethoxyisobutylsilane(CAS 17980-47-1) are read-across to the submission substance tripotassium propylsilanetriolate for appropriate endpoints (see Section 1.4 of the CSR).The hydrolysis half-life and the silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For triethoxyisobutylsilane, hydrolysis half-lives at 20 -25°C of 0.7 h at pH 4, 30.1 h at pH 7 and 0.4 h at pH 9 were determined using a validated QSAR estimation method.

For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

kobskH3O+[H3O+]

At pH 4 [H3O+]=10-4mol dm-3and at pH2 [H3O+]=10-2mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

t1/2(pH 2) = t1/2(pH 4) / 100

The calculated half-life of triethoxyisobutylsilane at pH 2 is therefore 0.007 hours (25 seconds).

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT50(XºC) = DT50(T) * e(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for triethoxyisobutylsilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 11 hours. At 37.5ºC and pH2 (relevant for conditions in the stomach following oral exposure), the half-life is approximately 11 seconds.

The hydrolysis products are (2-methylpropyl)silanetriol and ethanol.