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Physical & Chemical properties

Vapour pressure

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Description of key information

Vapour pressure (1,1,1,3,3,3-hexamethyldisilazane): 2400 Pa at 25°C 
Vapour pressure (trimethylsilanol): 1300 Pa at 20°C (EU Method A.4)
Vapour pressure (ammonia): 861100 Pa at 20°C

Key value for chemical safety assessment

Vapour pressure:
2 400 Pa
at the temperature of:
25 °C

Additional information

Measured vapour pressures of 1900 Pa at 20°C, 2400 Pa at 25°C and 7400 Pa at 50°C were determined for the substance using a relevant method. The results are considered to be reliable and selected as key study. This is supported by a vapour pressure value of 10500 Pa at 50°C which was reported for the substance in a handbook or collection of reliable data which has been subject to peer-review and in which the original sources are traceable. Similarly, in other secondary sources to which reliability could not be assigned, vapour pressure values of 1200 - 3066 Pa at 20 - 25°C were reported for the substance. These values indicate that the substance is a moderately volatile substance. However, another study reported a vapour pressure of 10100 Pa at 20°C using the Carius tube method, this result is considered high for this substance considering the molecular weight (161.4 g/mol) and boiling point (125°C) of the substance. In addition, the presence of volatile impurities/reaction products (ammonia) may cause vapour pressure measurements to be higher than the true value for the substance. Therefore, the key study was selected as the study that gives a true indication of the vapour pressure of the substance with reference to other available data.

The substance hydrolyses very rapidly in contact with water to form trimethylsilanol and ammonia. Therefore, for the purpose of chemical safety assessment, the vapour pressure of the silanol hydrolysis products is also relevant. A vapour pressure value of 1300 Pa at 20°C and 1900 Pa at 25°C were determined in accordance with the standard guideline EC A4 for the silanol hydrolysis product, trimethylsilanol.

Ammonia has a vapour pressure of 861100 Pa at 20°C [OECD SIDS (2007). SIDS Initial Assessment Report for SIAM 24, Paris, France, 17-20 April 2007, Ammonia Category]