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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis: Half-life << 1 min at 25°C and pH 4, 7 and 9 (OECD 111) 

Key value for chemical safety assessment

Additional information

Measured hydrolysis half-lives of ≤0.04 min, ≤0.5 min, and ≤0.1 at pH 4, 7 and 9 and 1.5°C respectively were determined for the substance in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable and selected as key study. This is supported by another reliable study according to EEC Guideline C10 with measured half-lives of <150 minutes at pH 4, 7 and 9 and 50°C. The supporting study is in agreement that the substance reacts very rapidly.

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 1,1,1,3,3,3 -hexamethyldisilazane (HMDZ) at pH 2 and 1.5°C is therefore <0.0004 minutes (0.02 seconds). However, it is likely that factors such as diffusion become rate-determining when the half-life is less than 5-10 seconds. As a worst-case it can therefore be considered that the half-life for HMDZ at pH 2 and 1.5°C is <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 HMDZ the predicted hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is <2 seconds. As discussed above, it is appropriate to consider that the half-life at pH 7 and 37.5 ºC is <5 seconds. At 37.5ºC and pH 2 (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 <5 seconds.

The hydrolysis products for the present substance are trimethylsilanol and ammonia.

Ammonia (NH3) exists in solution in equilibrium with the ammonium ion. (NH4+). Under normal environmental conditions, ammonium ions (NH4 +) predominate.