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The very useful information profile on n-hexane
on page 587 of the May 2001 issue of the
Journal would benefit from inclusion of the flash point
of this substance. This is especially so in view of
the fact that the information is largely directed at
students and teachers. If there is a spillage of
an organic chemical in the teaching laboratory,
emergency responses are all the more urgent if the chemical is above its flash point. However, n-hexane provides an example of the anomaly that
a liquid can be too far above its flash point to form
an ignitable mixture.
Authoritative sources including the SFPE
Handbook give the closed-cup flash point of
n-hexane as 251 K (-22 °C); therefore one would certainly,
on those grounds, expect the compound to ignite when spilt at room temperature if the vapor is contacted by a flame. Roughly speaking, the
flash point of any liquid is the temperature at which
the equilibrium vapor pressure is such that in a
mixture with air, total pressure one atmosphere, the
proportion of the vapor corresponds to the lower
flammability limit. Now n-hexane burns in air according to
C6H14 + 9.5O2
(+ 35.7N2)  6CO2 +
7H2O (+ 35.7N2)
We are told in the information profile in the
Journal that the vapor pressure of n-hexane at 120 °C
is 124 torr, so at this temperature, with a total
pressure of one atmosphere, a mixture of air and
n-hexane at its equilibrium vapor pressure
would have a percentage
(124/760) x 100 = 16.3% (molar or volume basis)
of n-hexane. This is well in excess of the
upper flammability limit of 7.4% (SFPE
Handbook), so on those grounds the n-hexane would not ignite if
spilt at room temperature and contacted by a flame.
Of course, the assumption of equilibrium between
the two phases of the hydrocarbon compound might not be justified following accidental spillage, so I certainly do not recommend reliance on the
conclusion that the n-hexane/air mixture is too rich
to ignite. Nevertheless, this does illustrate
another facet to the understanding and application of
flash points: that a liquid can in principle be too far
above its flash point to form a flammable mixture with air.
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