A method of reporting the "actual" weight (milligrams) of an ion or substance in a given volume of water (liter). For dilute solutions, mg/l and ppm are equivalent. For example, a 1,000 mg/l (ppm) sodium chloride solution would result in a residue of 1,000 mg of NaCl after evaporation of one liter of water. RO chemists use mg/l frequently in the calculation of TDS.