1. n. []

A unit of concentration, frequently abbreviated to ppm. For solid and liquid concentrations, ppm refers to weight (mass) units. For example,

• calcite in a ground barite sample may be 400 ppm
• calcium chloride in a water solution may be 250,000 ppm.

Note that the relationship of ppm to weight percentage is 10,000 ppm = 1 wt%. For gases, ppm refers to volume (or mole) units. For example, H₂S in an air sample may be 10 ppm (both by volume and by moles).

See related terms: dissolved solids,  epm,  LC₅₀,  milligrams per liter