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What is Combustion Analysis?

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  • Written By: C. Martin
  • Edited By: Andrew Jones
  • Last Modified Date: 29 November 2016
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In chemistry, combustion analysis is a method that is used to determine the atomic make up of a substance. Using this method, a chemist can take an accurately measured mass of an unknown substance, and burn it under controlled conditions, in order to analyze the products of combustion, and thus determine the empirical formula of the sample. The empirical formula of a substance refers to the type of atoms that form the substance, and the relative numbers of each type of atom, expressed as the simplest ratio possible in whole numbers. For this reason, the empirical formula is sometimes referred to in organic chemistry as the simplest formula.

While combustion analysis methods can allow a scientist to determine the empirical formula of an unknown substance, it does not provide information on either the molecular formula or the structural formula of the substance. These two formulae provide additional information that cannot be ascertained by combustion analysis. The molecular formula is much like the empirical formula, with the exception that, in addition to showing the type and relative ratio of each type of atom, it also shows the absolute number of atoms in each molecule of the substance. The structural formula provides even more information, by showing exactly how the atoms in the molecule are linked with each other.

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Combustion analysis is mainly used to determine the empirical formulae of unknown carbon-based organic compounds. As the sample of the original compound is combusted, in the presence of oxygen, the carbon atoms from the sample are converted to carbon dioxide, and the hydrogen atoms are converted to water. Carbon dioxide is known as an “oxide” of carbon, because it is made up of a carbon atom joined with two oxygen atoms. Water is an oxide of hydrogen, as it consists of two hydrogen atoms joined with one oxygen atom. Similarly, other elements that are present in the original sample will be converted, during the combustion process, into oxides of the original elements.

A gas analyzer instrument used for combustion analysis typically consists of a heated combustion chamber, which is supplied with a stream of oxygen, and a number of traps. The stream of oxygen passes through the combustion chamber, and as the sample being analyzed is burned, this stream of oxygen carries with it the products of combustion. These products are in the form of gases, which are retained in the output traps, where they can be measured to complete the analysis.

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Charred
Post 2

@David09 - I can think of an application in chemical recycling where I believe that combustion analysis would make sense.

All sorts of chemicals get dumped into chemical recycling plants and unfortunately regular dump sites, creating hazards for toxic waste dumps.

I think if you burn up the chemicals in the way the article talks about you can have an idea of what you’re looking at.

I realize that you will have imperfect results, since it just shows the stuff at the atomic – not the molecular – level. However, to a trained observer it would still narrow the list of possible chemical substances used.

David09
Post 1

I wonder if combustion analysis is used in any capacity in the automotive industry. We are still, after all, relying on the internal combustion engine for most of our transportation needs.

Automotive manufacturers wouldn’t need to use such analysis to figure out the basic parts of the combustion – they already know that much.

But perhaps they can use it to determine how cleanly the engine burns fuel, and if there is any residue left over. I think they might be able to use this kind of analysis to improve the efficiency of their engines or the viscosity of the oil.

I’m not a mechanic, so I can’t speak with too much authority on the subject, but it seems to make sense.

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