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When a chemist makes a brand-new compound, one of the first questions a journal reviewer asks is: does its composition match the formula you claim? The classic answer is elemental analysis, often called CHN analysis, which measures the percentages of carbon, hydrogen and nitrogen (and often sulfur) in a sample. It’s a quantitative technique that goes back two centuries and is still in daily use, now in automated instruments that need only a couple of milligrams.
The basic idea: burn it and weigh the products
Every organic compound burns in excess oxygen to give simple, measurable products:
- carbon → carbon dioxide, CO₂
- hydrogen → water, H₂O
- nitrogen → nitrogen gas, N₂ (after oxides of nitrogen are reduced)
- sulfur → sulfur dioxide, SO₂
If you know how much of each product forms from a known mass of sample, you can calculate the mass of each element, and hence its percentage.
A little history
In the early 1800s, Justus von Liebig perfected a combustion apparatus in which a sample was burned over hot copper(II) oxide. The water produced was absorbed in a tube of calcium chloride, and the carbon dioxide in a set of glass bulbs containing potassium hydroxide solution (the famous “Kaliapparat”). Weighing the tubes before and after gave the masses of water and CO₂. His method needed hundreds of milligrams and considerable skill, but it transformed organic chemistry by making accurate formulas possible.
In the early 1900s, Fritz Pregl scaled the method down to a few milligrams, using an extremely sensitive balance. He won the 1923 Nobel Prize in Chemistry for this microanalysis. The nitrogen method developed by Jean-Baptiste Dumas, which measures the volume of nitrogen gas, completed the picture.
How a modern CHN analyser works
Modern instruments automate the whole process:
- Weighing: about 1–3 mg of sample is weighed into a small tin capsule on a microbalance (reading to 0.001 mg). See using a balance.
- Flash combustion: the capsule drops into a furnace at around 900–1000 °C in a pulse of pure oxygen. The tin burns too, briefly raising the local temperature much higher and ensuring complete combustion.
- Oxidation and reduction: the gases pass over catalysts that complete oxidation, then over hot copper, which removes excess oxygen and reduces nitrogen oxides to N₂.
- Separation: the mixture of N₂, CO₂, H₂O (and SO₂) is carried by helium through a gas chromatography column, or through selective traps, that separates the gases.
- Detection: a thermal conductivity detector measures each gas as it emerges. Each gas changes the thermal conductivity of the helium carrier by an amount related to its quantity.
- Calibration: the instrument is calibrated with a pure standard compound of known composition, such as acetanilide (C₈H₉NO), analysed in the same run. See calibration curves.
A full analysis takes a few minutes.
What about oxygen?
Oxygen can’t be measured by burning in oxygen. It’s usually found by difference: 100% minus the sum of all other measured elements. Some instruments measure it directly by pyrolysis (heating without oxygen) over carbon, which converts the oxygen in the sample to carbon monoxide.
From results to formula
Elemental analysis gives percentages by mass, which lead directly to the empirical formula. See empirical vs molecular formula and percent composition.
Worked example 1: from percentages to empirical formula
A new compound gives C 62.04%, H 10.41%, N 0.00%. The rest is oxygen. Find the empirical formula.
- O = 100 − 62.04 − 10.41 = 27.55%
- Moles in 100 g:
- C: 62.04 ÷ 12.011 = 5.165
- H: 10.41 ÷ 1.008 = 10.33
- O: 27.55 ÷ 15.999 = 1.722
- Divide by the smallest (1.722): C 3.00, H 6.00, O 1.00
Empirical formula: C₃H₆O. With a molar mass of 58 g/mol from mass spectrometry, the molecular formula is C₃H₆O itself (propanone or propanal, among others; spectroscopy decides which).
Worked example 2: from product masses (classic combustion)
2.000 mg of a compound containing C, H, N and O burns to give 4.658 mg of CO₂ and 1.073 mg of H₂O. A separate nitrogen measurement gives 9.27% N. Find the empirical formula.
- Mass of C = 4.658 × (12.011 ÷ 44.009) = 1.271 mg → 1.271 ÷ 2.000 × 100 = 63.56% C
- Mass of H = 1.073 × (2 × 1.008 ÷ 18.015) = 0.1201 mg → 6.00% H
- N = 9.27%
- O by difference = 100 − 63.56 − 6.00 − 9.27 = 21.17%
Moles in 100 g:
| Element | Calculation | Moles | ÷ 0.662 |
|---|---|---|---|
| C | 63.56 ÷ 12.011 | 5.292 | 8.00 |
| H | 6.00 ÷ 1.008 | 5.95 | 8.99 |
| N | 9.27 ÷ 14.007 | 0.662 | 1.00 |
| O | 21.17 ÷ 15.999 | 1.323 | 2.00 |
Empirical formula: C₈H₉NO₂, which matches paracetamol (acetaminophen), M = 151.16 g/mol.
Notice how the mass fractions 12.011 ÷ 44.009 and 2.016 ÷ 18.015 convert product masses into element masses. They’re the same idea as in gravimetric analysis.
The ±0.4% rule
Many chemistry journals have traditionally required that measured C, H and N values fall within ±0.4 percentage points of the values calculated for the proposed formula. For example:
Proposed formula C₉H₈O₄ (aspirin). Calculated: C 60.00%, H 4.48%.
| Calculated | Found | Difference | |
|---|---|---|---|
| C | 60.00% | 59.84% | −0.16 ✓ |
| H | 4.48% | 4.52% | +0.04 ✓ |
Both are within 0.4, so the analysis supports the formula and suggests good purity.
Why a small error means a lot
A sample contaminated with 5% of a solvent can shift the carbon value by more than 0.4 percentage points, depending on the solvent. Water of crystallisation, trapped solvent from recrystallisation and inorganic residues (such as silica from chromatography) are the most common culprits. When ratios come out far from whole numbers, as in 1 : 1.27 : 0.43, the usual explanation is an impure or wet sample rather than a strange formula, so the chemist re-purifies, dries and re-analyses. Sometimes a result that’s off can be explained by a solvate (for example, the formula plus 0.5 H₂O), but that must be supported by other evidence.
Strengths and limitations
Strengths:
- small sample (milligrams)
- fast and inexpensive per sample
- quantitative check of bulk purity, which spectroscopy alone doesn’t always give
- detects non-obvious impurities such as inorganic salts or water
Limitations:
- destroys the sample
- can’t distinguish isomers (all isomers share the same percentages)
- says nothing about structure
- needs a pure, dry, homogeneous sample; a small weighing error or moisture uptake affects the result
- oxygen is usually only known by difference, which accumulates the errors of the other measurements
Metals and other elements are measured by different techniques, such as ICP-MS or atomic absorption.
Where CHN analysis is used
- confirming the purity and composition of newly synthesised compounds
- pharmaceutical quality control
- measuring carbon and nitrogen in soils, sediments and plants (for example, the carbon-to-nitrogen ratio of compost)
- fuels and coal, where carbon, hydrogen and sulfur content affect energy value and pollution
- food and feed protein estimation by the related Dumas method (see food analysis)
Key takeaways
- CHN analysis burns a weighed milligram sample and measures the CO₂, H₂O, N₂ and SO₂ produced.
- Modern analysers use flash combustion, reduction over copper, GC separation and a thermal conductivity detector.
- Oxygen is usually found by difference.
- The percentages give the empirical formula; results within ±0.4 of calculated values support purity.
- It can’t tell isomers apart, so it’s used alongside spectroscopy. See how chemists identify an unknown compound.
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