Combustion analysis is an analytical technique used to determine the elemental composition of an unknown substance. This method primarily focuses on organic compounds, which are molecules composed mainly of carbon and hydrogen. By measuring the amounts of elements within a sample, analysts can deduce the simplest whole-number ratio of atoms in the compound, known as the empirical formula.
The Underlying Chemical Reaction
The entire process hinges on the concept of complete combustion, which is a high-temperature reaction between a substance and excess oxygen. In this controlled environment, every atom of carbon and hydrogen in the original sample is quantitatively converted into stable, measurable gaseous products. Carbon atoms are transformed exclusively into carbon dioxide (\(\text{CO}_2\)), while hydrogen atoms are converted into water vapor (\(\text{H}_2\text{O}\)).
Any other elements present, such as nitrogen or sulfur, are also converted into their corresponding simple oxides, like nitrogen oxides (\(\text{NO}_x\)) or sulfur dioxide (\(\text{SO}_2\)). The introduction of excess oxygen ensures that the reaction proceeds fully, guaranteeing that all combustible elements are oxidized completely. The design of the analysis relies on the principle of conservation of mass.
Because of mass conservation, measuring the mass of the products formed allows calculation of the mass of the original elements they contain. This means every carbon atom found in the resulting \(\text{CO}_2\) must have originated from the initial sample, and similarly, all hydrogen in the \(\text{H}_2\text{O}\) came from the sample. This conversion of the sample into simple gas molecules enables the precise measurement of its constituent elements.
Performing the Measurement
The analysis is executed using specialized equipment, typically an automated elemental analyzer. A weighed sample, often only a few milligrams, is placed into a combustion chamber within a high-temperature furnace. High-purity oxygen is introduced, and the furnace temperature is raised, often to \(900^\circ\text{C}\) to \(1000^\circ\text{C}\), to ensure the complete and rapid breakdown of the sample.
The combustion products are swept out of the furnace by an inert carrier gas, such as helium, and travel through a series of chambers or detectors. In older, gravimetric methods, the gases passed through absorbent tubes that selectively trapped the products. Water vapor was trapped first, often using a hygroscopic agent like magnesium perchlorate, and the mass gain of this trap indicated the mass of \(\text{H}_2\text{O}\) produced.
The remaining gases then flowed into a second trap containing a strong base, such as potassium hydroxide, which absorbed the \(\text{CO}_2\). The mass increase of the second trap provided the mass of carbon dioxide. Modern, automated elemental analyzers replace these traps with specialized electronic detectors, using methods like Non-Dispersive Infrared (NDIR) detection to measure \(\text{CO}_2\).
These advanced instruments also often include a reduction stage, typically involving heated copper, to remove any unreacted oxygen and convert nitrogen oxides back into nitrogen gas (\(\text{N}_2\)) before detection. The resulting signals from the detectors are integrated and compared against calibration standards, providing the mass percentages of carbon, hydrogen, nitrogen, and sulfur directly. This automation allows for much faster analysis and the simultaneous determination of multiple elements, which is a significant improvement over the time-consuming gravimetric weighing process.
Calculating the Empirical Formula
The goal of the analysis is achieved by converting the measured masses of the gaseous products back to the mass of the original elements. Since every mole of \(\text{CO}_2\) contains one mole of carbon atoms, the mass of carbon in the sample is calculated directly from the mass of the collected \(\text{CO}_2\). Similarly, the mass of hydrogen is determined from the mass of the collected water (\(\text{H}_2\text{O}\)).
Once the masses of carbon and hydrogen are known, they are converted into moles using the respective atomic weights. These mole values represent the relative number of atoms of each element in the compound. To find the empirical formula, the mole amounts are divided by the smallest number of moles calculated, which establishes the simplest whole-number ratio of the elements.
If the compound is known to contain a third element, such as oxygen, its mass is determined by difference. The calculated masses of all other elements (C, H, N, S) are subtracted from the initial total mass of the sample. Any remaining mass is attributed to the oxygen content, which is then also converted into moles and included in the final ratio calculation.
Real-World Uses
Combustion analysis is used for quality control and research across several industries. In pharmaceutical and chemical research, it validates the structure and purity of newly synthesized organic molecules. Chemists must confirm that the novel compound they created has the expected elemental composition before proceeding with further testing.
The technique is used for purity testing of chemical intermediates and final products, ensuring that the material meets specifications. Environmental science relies on combustion analysis to measure trace amounts of carbon in various matrices, such as determining the total organic carbon (TOC) content in water or soil samples.
Additionally, the analysis is used in the energy sector to determine the sulfur content in petroleum products for regulatory compliance and environmental protection. By providing a measure of a substance’s elemental makeup, combustion analysis remains a standard and trusted tool in laboratories worldwide.