How to Find Purity in Chemistry: 5 Methods

Updated 6 November 2026 · Standard formula, shown below

Finding the purity of a substance tells you how much of your sample is the desired compound versus impurities. This guide covers the most common lab techniques—melting point, boiling point, chromatography, spectroscopy, and titration—so you can choose the right method and interpret results correctly. You’ll need a basic understanding of moles, concentrations, and lab safety before starting.

What You’ll Need

Sample of the substance to test
Reference standard of the pure compound (if available)
Melting point apparatus or capillary tube and thermometer
Boiling point apparatus (for liquids)
TLC plates, developing chamber, and UV lamp (for chromatography)
UV-Vis or IR spectrometer (for spectroscopy)
Burette, pipette, flask, and appropriate titrant/indicator (for titration)
Analytical balance
Solvents (appropriate for your method)
Personal protective equipment: lab coat, gloves, safety goggles

Step-by-Step

Identify the likely impurities. Check the sample’s source, storage, and any previous processing. Common impurities include water, unreacted starting materials, by-products, or solvents. Knowing what to look for helps you choose the best method.
Choose a purity test based on your sample’s state and quantity. For solids, melting point is quick and requires little sample. For liquids, boiling point works. If you need quantitative results, use chromatography or spectroscopy. For a known acid/base or redox reaction, titration is accurate.
Prepare a clean, dry sample. Remove any visible contamination. If the sample is hygroscopic, dry it in a desiccator. For liquids, ensure no suspended solids. Weigh an accurate amount if you’ll do quantitative analysis.
Measure the melting point (for solids). Pack a small amount (about 1-2 mm high) into a capillary tube. Place it in the melting point apparatus. Heat slowly (1-2°C per minute near the expected melting point). Record the temperature range from first liquid to complete melting. Compare to the literature value for the pure compound.
Measure the boiling point (for liquids). Use a distillation setup or a Thiele tube with a capillary. Heat the liquid and record the temperature when it starts to boil steadily. Compare to the literature boiling point at your atmospheric pressure (adjust for altitude if needed).
Run thin-layer chromatography (TLC). Spot a dilute solution of your sample and a reference standard on a TLC plate. Develop in a suitable solvent system. Visualize under UV light or with a staining agent. Compare the Rf values and spot patterns. A pure compound shows a single spot matching the standard.
Use spectroscopy for structural confirmation. For UV-Vis, measure the absorption spectrum and compare to the standard. For IR, look for characteristic peaks and check for extra peaks from impurities. NMR is powerful but requires more sample and expertise.
Perform a quantitative titration (if applicable). Dissolve a known mass of sample in an appropriate solvent. Add a few drops of indicator. Titrate with a standard solution of known concentration until the endpoint. Record the volume of titrant used.
Calculate purity from titration data. Use the stoichiometry of the reaction to find the moles of the active substance. Divide by the moles of sample (from mass and molar mass) and multiply by 100 to get percent purity. Example: if 0.500 g of a monoprotic acid requires 25.0 mL of 0.100 M NaOH, then moles of acid = 0.0250 L × 0.100 mol/L = 0.00250 mol. Mass of pure acid = 0.00250 mol × molar mass (e.g., 204.22 g/mol for KHP) = 0.5106 g. Purity = (0.5106 g / 0.500 g) × 100 = 102% (which indicates an error or impurity that reacts).
Interpret the results. For melting/boiling point: a pure substance has a sharp range (≤1-2°C) and matches the literature value. A depressed or broad range indicates impurities. For TLC: a single spot at the same Rf as the standard indicates purity. For spectroscopy: match the reference spectrum; extra peaks indicate impurities. For titration: purity near 100% (within experimental error) indicates high purity.

Common Mistakes

Heating too fast during melting point, causing a falsely broad range.
Not calibrating the thermometer or apparatus, leading to inaccurate readings.
Using a contaminated or wet sample, which alters results.
Overloading the TLC plate, causing streaking instead of distinct spots.
Choosing a solvent system that doesn’t separate the components.
For titration, using an unstandardized titrant or wrong indicator.
Ignoring the need to correct boiling point for atmospheric pressure.

Tips

Always run a reference standard alongside your sample for comparison.
For melting point, do a fast preliminary run, then a slow, accurate run.
For TLC, use a small amount of sample—too much causes poor separation.
For titration, do at least three trials and average the results.
If you suspect water as an impurity, use Karl Fischer titration or dry the sample and re-measure.
For quantitative analysis, use an analytical balance and volumetric glassware for accuracy.

Troubleshooting

Melting point is lower than expected but sharp: The sample may be a different polymorph or contain a small amount of impurity that forms a eutectic. Try recrystallization and re-measure.
Melting point range is wide (>2°C): Impurities are present. Purify by recrystallization or sublimation.
Boiling point is higher than literature: Check your thermometer calibration and atmospheric pressure. If pressure is higher, boiling point increases. Also, the sample may be impure.
TLC shows multiple spots: The sample is a mixture. Separate components by column chromatography if you need pure material.
TLC shows streaking: Reduce the sample concentration or choose a different solvent system.
Spectrum doesn’t match: The sample may be a different compound or contain a strong impurity. Check the sample preparation and reference spectrum.
Titration gives purity over 100%: This indicates a calculation error, an impure titrant, or the sample reacts with something else. Recheck your stoichiometry and standardize the titrant.

Frequently Asked Questions

What is the simplest way to test purity?

For solids, melting point is quickest. For liquids, boiling point or refractive index. These are qualitative and require a pure reference.

How accurate is melting point for purity?

It’s a good indicator but not precise. A sharp melting point near the literature value suggests high purity, but it can’t detect all impurities, especially those with similar melting points.

Can I use TLC to quantify purity?

TLC is mainly qualitative. For quantitative analysis, use HPLC or GC, which are more accurate and can measure peak areas.

What does ‘percent purity’ mean in titration?

It’s the mass of the active substance divided by the total mass of the sample, times 100. It assumes the impurity doesn’t react with the titrant.

Why does my boiling point differ from the literature?

Atmospheric pressure affects boiling point. At higher altitudes, it’s lower. Also, impurities can raise or lower it. Correct for pressure using a nomograph.

Is there a universal purity test?

No. The best method depends on the substance and the type of impurities. Often, a combination of methods (e.g., melting point plus TLC) gives confidence.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.