Combustion analysis is a method for figuring out what elements make up an unknown compound, and in what proportions. The technique works by burning a small sample in excess oxygen, then measuring the gases produced. Because carbon always converts to carbon dioxide and hydrogen always converts to water during combustion, chemists can work backward from the mass of those gases to determine how much carbon and hydrogen were in the original sample. It remains one of the most widely used ways to confirm the identity and purity of organic compounds.
How Combustion Analysis Works
The core idea is straightforward. An organic compound, something containing carbon and hydrogen, is placed in a furnace and burned completely in the presence of excess oxygen. Every carbon atom in the sample ends up as part of a carbon dioxide molecule. Every hydrogen atom ends up as part of a water molecule. If the sample also contains nitrogen, that element gets converted to its own gas product as well.
The key insight is that this is a one-way, complete conversion. If you can capture and weigh all the carbon dioxide produced, you know exactly how many carbon atoms were in your original sample. The same logic applies to water and hydrogen. From there, you can calculate the ratio of elements and determine the compound’s empirical formula, which is the simplest whole-number ratio of atoms in the molecule.
The Apparatus and Trapping Chemicals
In a traditional setup, the combustion gases flow through a series of tubes, each packed with a chemical that selectively absorbs one product. Water vapor is trapped first by anhydrone, a granular form of anhydrous magnesium perchlorate that pulls moisture out of the gas stream. Carbon dioxide is trapped next by ascarite, which is sodium hydroxide coated onto silica. Each absorption tube is weighed before and after the experiment. The increase in mass tells you exactly how much water or carbon dioxide was produced.
The order matters. Water must be removed before the gas reaches the carbon dioxide trap, because ascarite would absorb water too, throwing off your measurements. This sequential design is what makes the technique precise enough for quantitative work.
Modern Automated Analyzers
Today, most labs use automated instruments called CHNS or CHNSO analyzers (named for the elements they measure: carbon, hydrogen, nitrogen, sulfur, and oxygen). These instruments still burn the sample in oxygen, but instead of using absorption tubes that need to be weighed by hand, they separate the combustion gases using a technique called gas chromatography and measure them with thermal conductivity detectors. The entire process is faster and requires very little sample material, often just a few milligrams.
The result is a printout showing the percentage by mass of each element in the sample. Chemists compare these percentages against the values predicted by a proposed molecular formula. If the numbers match, the formula is confirmed.
How the Math Works
The calculation relies on simple stoichiometric ratios. One molecule of carbon dioxide contains exactly one carbon atom, so every mole of carbon dioxide produced corresponds to one mole of carbon in the original sample. One molecule of water contains two hydrogen atoms, so every mole of water produced accounts for two moles of hydrogen.
Here’s what that looks like in practice. Say you burn a sample and collect 69.00 milligrams of carbon dioxide. Carbon dioxide has a molar mass of about 44.01 grams per mole, and carbon itself is 12.01 grams per mole. You convert the mass of carbon dioxide to moles, then multiply by the molar mass of carbon to find that the sample contained 18.83 milligrams of carbon. You do the same thing for water: 11.30 milligrams of water translates to 1.264 milligrams of hydrogen, after accounting for water’s molar mass (18.02 g/mol) and the fact that each water molecule carries two hydrogen atoms.
If the compound contains oxygen or another element that isn’t directly measured, you subtract the mass of carbon and hydrogen from the total sample mass. Whatever is left over is attributed to that remaining element. Once you have the mass of each element, you convert them all to moles and find the simplest whole-number ratio. That ratio is your empirical formula.
Measuring Nitrogen Content
For compounds that contain nitrogen, a variation called the Dumas method has been used since the 19th century. The sample is burned in the presence of copper oxide, which ensures complete combustion, and the nitrogen in the compound is released as gas. Carbon dioxide is then absorbed by a strong alkaline solution, leaving only the nitrogen gas behind, which is collected and measured by volume.
Getting accurate nitrogen results requires careful control of the burn rate. If the sample ignites too quickly, a sudden burst of gas can push through the system and escape measurement. To prevent this, the sample is sometimes mixed with calcium carbonate, which slows the combustion. The copper oxide packing is typically heated to around 650°C by an electric furnace, while the sample itself is ignited more gradually, starting cold and ramping up to about 550°C with gas burners. Modern CHNS analyzers handle all of this automatically, but the underlying chemistry is the same.
Accuracy and Purity Standards
Combustion analysis isn’t just for identifying unknowns. It’s also the standard way to prove that a newly synthesized compound is pure enough for publication. The majority of chemistry journals require that the measured percentages of carbon, hydrogen, and nitrogen fall within ±0.4% of the values calculated from the proposed molecular formula. So if your formula predicts 20.14% nitrogen, your measured value needs to land between 19.74% and 20.54% to pass. Some journals are stricter, expecting agreement within ±0.3%.
An international study published in ACS Central Science evaluated how well labs around the world actually meet this standard. A data point was labeled “Fail” if it fell outside that 0.4% window and “Acceptable” if it stayed within it. This benchmark exists because hitting ±0.4% generally confirms at least 95% sample purity, which is the minimum threshold for most research applications. High-resolution mass spectrometry can sometimes substitute for combustion analysis, but many journals still prefer or require elemental analysis data for new small molecules.
What Combustion Analysis Cannot Do
The technique tells you the ratio of elements in a compound, but not how those atoms are arranged. Two completely different molecules can share the same empirical formula. Ethanol and dimethyl ether, for instance, both contain two carbons, six hydrogens, and one oxygen, but they behave very differently. To pin down the actual structure, chemists pair combustion analysis with other methods like mass spectrometry or nuclear magnetic resonance spectroscopy.
Combustion analysis also assumes complete conversion of all elements to their expected gas products. Compounds containing halogens like chlorine or bromine, or metals, require modified procedures and specialized equipment to handle the additional combustion products. Without those modifications, the trapped gases won’t account for all the mass in the sample, and the results will be off.