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It's common knowledge that the pressure of an environment influences many properties of a substance, like the melting point, boiling point, or triple point. Pyrophoric substances ignite spontaneously at temperatures below $\pu{55^\circ C}$, like caesium for example.

I wish to ask if the tendency for pyrophoric substances to ignite is influenced at all by pressure? Obviously, these substances would not spontaneously ignite in a vacuum, but would their ignition temperature change and/or be dependent on the pressure of their environment? Is there a pressure apart from a vacuum that could cause the substance to lost its pyrophoric properties?

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    $\begingroup$ How do you imagine changing triple point? Are you only asking about elements and phase changes or pressure induced reactions? $\endgroup$ – Mithoron Mar 7 '18 at 17:16
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Depending on the pyrophoric substance, there can be differeing effects from changing air pressure. Strange as it may seem, sometimes combustion is slowed at very high pressure. For example, ethane, which reacts explosively with oxygen at STP, oxidizes slowly in a series of steps at ~100 atmospheres (~107 Pascals).

That said, it depends on the substances involved, pressure and presence of catalysts or other substances. Aluminum in a ball mill produces pyrophoric dust, but adding graphite during milling suppresses ignition.

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