Parts Per Million

In-depth explanation of Parts Per Million

Parts per million, ppm, expresses very dilute concentrations as the number of parts of solute per million parts of solution. For dilute aqueous solutions it is approximately 1 mg of solute per kilogram or per litre of solution, since water has a density close to 1 kg/L. Parts per billion, ppb, is 1000 times smaller than ppm. These units are used for trace pollutants, mineral content in water and drinking water standards. At such low concentrations, very sensitive analytical methods are needed to measure the solute.

Going deeper: Parts Per Million

Key ideas: Formulas: ppm = (mass solute / mass solution) x 10^6, so 1 ppm = 1 mg/kg = 1 mg/L for water. ppb = 10^9 factor, equal to 1 microgram per litre. Convert to molarity by dividing mg/L by molar mass in mg/mol, e.g. 1 ppm of Pb is 4.8 x 10^-6 M. Air pollutant ppm is often by volume.

Common mistakes and connections: Students confuse ppm with percent (1 percent = 10,000 ppm) or use mg/L for non-aqueous solutions with different density. Another error is mixing mass and volume bases for gases. Link to molarity, dilution, environmental standards and sensitive analytical methods like ICP-MS.

Visualization of Parts Per Million

One ppm is 1 mg per litre of dilute water; drinking water limits sit at ppb to ppm levels, while seawater holds about 35,000 ppm of salts. The interactive lesson shows this as a live animation or chart.

Worked example and use case

The lead action level in drinking water is 15 ppb, which is equivalent to 15 micrograms per litre.

Where Parts Per Million fits in States of Matter & Solutions

Parts Per Million is topic 79 of 109 in States of Matter & Solutions.

Related topics

• MolarityBeginner
• MolalityIntermediate
• Mass PercentBeginner
• NormalityIntermediate
• DilutionBeginner
• Henry's LawIntermediate

Browse all 109 topics in States of Matter & Solutions