ppm

Parts per million (ppm) is a unit of concentration that expresses a ratio of one part solute to one million parts of the surrounding medium. It is a dimensionless quantity used to describe very dilute concentrations in chemistry, environmental science, engineering, and related fields. ppm can denote different bases: by mass (ppm by mass), by volume (ppm by volume), or by mole (ppm by mole). Because of these bases, ppm must be interpreted from context.

In dilute aqueous solutions, 1 ppm is approximately equal to 1 milligram per liter (mg/L) since water’s density is near 1 g/mL. In air, 1 ppmv (ppm by volume) means one microliter of a substance per liter of air and corresponds to a mole fraction of about 1×10^-6. Thus, ppm by volume is common for gases, while ppm by mass or by mole are used in liquids or chemical reactions.

Common relationships include: 1% equals 10,000 ppm. Also, 1 ppm is 0.0001 percent. Regulators and scientists may report concentrations as mg/L or ppm (for water) or as ppmv (for air); conversions require knowledge of density and temperature, and whether the basis is mass, volume, or mole.

Measurement methods vary by medium. Gas analyzers and spectroscopic techniques are used for air, while chromatography, mass spectrometry, or electrochemical sensors are used for liquids and gaseous mixtures. The term ppm is context-dependent, and precise interpretation requires specifying the basis of measurement.