Structural Biochemistry/Organic Chemistry/Method of Combustion Analysis

Combustion analysis, also called elemental analysis, is one of the oldest analytic methods used to determine the empirical formula of a pure organic compound. A small portion of the compound undergoes combustion in the presence of excess oxygen resulting compounds containing carbon, hydrogen, and nitrogen, which is quantitatively analyzed.

History

Combustion analysis method was invented by Joseph Louis Gay-Lussac in 1810 and was later improved by Justus von Liebig, who studied under Gay-Lussac.

Combustion analysis: - The unknown compound is burned in pure O2 in an apparatus that consists of a combustion furnace and chambers containing compounds that absorb either H2O or CO2. All the H in the unknown is converted to H2O, which is absorbed in the first chamber, and all the C is converted to CO2, which is observed in the second. By weighing the absorbers before and after combustion, we find the masses of CO2 and H2O and use them to calculate the masses of C and H in the compound, from which we find the empirical formula. Many organic compounds also contain oxygen, nitrogen, or a halogen.

Reference: Silberberg, Martin S. Chemistry: The Molecular Nature of Matter and Change. 5th ed. 2009.

Current Method

The current method for combustion analysis is passing the combustion gases through a chromatography (gas chromatography) column, which detects the carbon dioxide, water, and nitrogen gas by thermal conductivity.

A compound containing carbon and hydrogen (CaHb) or carbon, hydrogen, and oxygen (CaHbOc) is burned completely to form H2O and CO2. The products are drawn through two tubes. The first tube absorbs water, and the second tube absorbs carbon dioxide.

Finding Empirical Formulas

The number of moles of each combustion product must be determined in order to find the empirical formula of the compound. The steps:

1. Weigh the sample being analyzed and place in the apparatus above.
2. Sample undergoes combustion.
3. Weigh the two tubes beforehand.
4. In each of the tubes, there is a substance that absorbs only water and only carbon dioxide.
5. The increased mass in the water tube is the mass of the water absorbed, and the increased mass in the carbon dioxide tube is the mass of the carbon dioxide absorbed.
6. Calculate the mass of carbon from the mass of the trapped carbon dioxide.
7. Calculate the mass of hydrogen from the mass of the trapped water.
8. If there is oxygen, calculate the mass of oxygen by subtracting the mass of carbon and hydrogen from the total mass of the original sample.
9. Now, use these mass to calculate the empirical formula of the pure organic compound sample.

Reference

1. http://en.wikipedia.org/wiki/Combustion_analysis
2. http://en.wikipedia.org/wiki/Joseph_Louis_Gay-Lussac
3. http://preparatorychemistry.com/bishop_combustion_analysis.htm
4. Frederic L. Holmes (1963). "Elementary Analysis and the Origins of Physiological Chemistry". Isis 54 (1): 50–81.

Silberberg, Martin S. Chemistry: The Molecular Nature of Matter and Change. 5th ed. 2009.