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.
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.
