combustion analysis


Combustion analysis
Combustion analysis is a method
used in both organic chemistry and
analytical chemistry to determine the
elemental composition (more
precisely empirical formula ) of a pure
organic compound by combusting the
sample under conditions where the
resulting combustion products can be
quantitatively analyzed. Once the
number of moles of each combustion
product has been determined the
empirical formula or a partial
empirical formula of the original
compound can be calculated.
History
The method was invented by Joseph
Louis Gay-Lussac . Justus von Liebig
studied the method while working
with Gay-Lussac between 1822 and
1824 and improved the method in the
following years to a level that it could
used as standard procedure for
organic analysis.[1]
Combustion train
A combustion train is an analytical
tool for the determination of
elemental composition of a chemical
compound . With knowledge of
elemental composition a chemical
formula can be derived. The
combustion train allows the
determination of carbon and
hydrogen in a succession of steps:
combustion of the sample at high
temperatures with Copper(II)
oxide as the oxidizing agent ,
collection of the resulting gas in
an hygroscopic agent
( magnesium perchlorate or
calcium chloride ) to trap
generated water,
collection of the remainder gas in
a strong base (for instance
potassium hydroxide ) to trap
generated carbon dioxide.
Analytical determination of the
amounts of water and carbon dioxide
produced from a known amount of
sample gives the empirical formula.
For every hydrogen atom in the
compound 1/2 equivalent of water is
produced, and for every carbon atom
in the compound 1 equivalent of
carbon dioxide is produced.
Nowadays, modern instruments are
sufficiently automated to be able to
do these analyses routinely. Samples
required are also extremely small — 3
mg of sample is sufficient to give
satisfactory CHN analysis.
Modern methods
The water vapour, carbon dioxide and
other products can be separated via
gas chromatography and analysed
via a thermal conductivity detector .

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