NUMBAT OER - Open Educational Resources

2. Units

Concentration is the amount of one substance expressed in relation to the amount of a second substance in which the first is dissolved or dispersed. It is possible to express concentration using different units, depending how the amounts of the two substances are measured:

Concentrations of solutions are typically measured as the mass of solute per unit volume of solution. Such units might be mg l-1 or kg m-3. Note the use of superscripts to indicate that mass is divided by volume, rather than mg/l – there is a separate NuMBerS helpsheet covering units if you are unfamiliar with this type of notation.

Sometimes solutions are expressed in relation to the mass of the solution rather than its volume. For instance, older methods for the determination of the salinity (salt content) of seawater expressed the result as a value for grammes of salt per kilogramme of seawater. Strictly speaking, this is dimensionless - it has no units and is just the ratio between two masses. Modern salinity determinations based on electrical conductance are expressed in PSU (practical salinity units) which are equivalent to salt concentration as described earlier.

Mixtures of liquids and gases are typically expressed in terms of volume, describing the proportion of the overall mixture accounted for by one or more of the ingredients. An example would be the oxygen concentration in air, where oxygen is 21% by volume. Note again that this is a ratio, and concentration expressed in this way has no units.