The ppm is an acronym for “parts per million”, a unit of measure that describes how much of one substance (called the “solute”) is dissolved in a sample of water (called the “solvent”). This is a measurement of concentration (or density), which can often be helpful in our daily lives. The ppm can be used to measure the concentration of many substances, such as the minerals in our drinking water or the oxygen in a fish aquarium. Although the ppm is commonly used for concentration measurements, it only provides us with a ratio of the solute’s mass to the mass of the water, without specifying the water’s total volume. But, more often we need to know the total amount of solute dissolved in the water. This is extremely important, for example, when determining the therapeutic dose for medicine or the ingested level of a toxin. Without including the size of the container we are testing, which tells us the amount of water our solute is dissolved in, the ppm ratio by itself does not tell us how much solute the water contains. To convey this information, scientists use a more appropriate unit of measure, one which specifies the solute’s concentration in units of “mass per unit volume”. The unit commonly used is the “milligram per liter”, abbreviated mg/L. The mg/L always references the solute’s mass relative to a fixed volume, one liter.

Note: Frequently, the ppb (parts per billion) is used to measure solute concentration. Although the ppb is not used in this article, 1 ppm = 1000 ppb