Mass concentration (chemistry)
In chemistry, the mass concentration ρi (or γi) is defined as the mass of a constituent mi divided by the volume of the mixture V:
For a pure chemical the mass concentration equals its density (mass divided by volume); thus the mass concentration of a component in a mixture can be called the density of a component in a mixture. This explains the usage of ρ (the lower case Greek letter rho), the symbol most often used for density.
The volume V in the definition refers to the volume of the solution, not the volume of the solvent. One liter of a solution usually contains either slightly more or slightly less than 1 liter of solvent because the process of dissolution causes volume of liquid to increase or decrease. Sometimes the mass concentration is called titer.
The notation common with mass density underlines the connection between the two quantities (the mass concentration being the mass density of a component in the solution), but it can be a source of confusion especially when they appear in the same formula undifferentiated by an additional symbol (like a star superscript, a bolded symbol or varrho).
Mass concentration depends on the variation of the volume of the solution due mainly to thermal expansion. On small intervals of temperature the dependence is :
where ρi,T0 is the mass concentration at a reference temperature, α is the thermal expansion coefficient of the mixture.
The sum of the mass concentrations of all components (including the solvent) gives the density ρ of the solution:
Thus, for pure component the mass concentration equals the density of the pure component.
The sum of products between these quantities equals one.
The SI-unit for mass concentration is kg/m3 (kilogram/cubic metre). This is the same as mg/ml and g/l. Another commonly used unit is g/100 mL, which is identical to g/dL (gram/decilitre).
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