In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Furthermore, in chemistry, four types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration.^[1] The term concentration can be applied to any kind of chemical mixture, but most frequently it refers to solutes in solutions.

Qualitative description[link]

These glasses containing red dye demonstrate qualitative changes in concentration. The solutions on the left are more dilute, compared to the more concentrated solutions on the right.

Often in informal, non-technical language, concentration is described in a qualitative way, through the use of adjectives such as "dilute" for solutions of relatively low concentration and "concentrated" for solutions of relatively high concentration. To concentrate a solution, one must add more solute (for example, alcohol), or reduce the amount of solvent (for example, water). By contrast, to dilute a solution, one must add more solvent, or reduce the amount of solute. Unless two substances are fully miscible there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve, except in certain circumstances, when supersaturation may occur. Instead, phase separation will occur, leading to coexisting phases, either completely separated or mixed as a suspension. The point of saturation depends on many variables such as ambient temperature and the precise chemical nature of the solvent and solute.

Quantitative notation[link]

There are four quantities that describe concentration:

Mass concentration[link]

The mass concentration Failed to parse (Missing texvc executable; please see math/README to configure.): \rho_i

is defined as the mass of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): m_i
divided by the volume of the mixture Failed to parse (Missing texvc executable; please see math/README to configure.): V

Failed to parse (Missing texvc executable; please see math/README to configure.): \rho_i = \frac {m_i}{V}

The SI-unit is kg/m³.

Molar concentration[link]

The molar concentration Failed to parse (Missing texvc executable; please see math/README to configure.): c_i

is defined as the amount of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): n_i
divided by the volume of the mixture Failed to parse (Missing texvc executable; please see math/README to configure.): V

Failed to parse (Missing texvc executable; please see math/README to configure.): c_i = \frac {n_i}{V}

The SI-unit is mol/m³. However, more commonly the unit mol/L (= mol/dm³) is used.

Number concentration[link]

The number concentration Failed to parse (Missing texvc executable; please see math/README to configure.): C_i

is defined as the number of entities of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): N_i
in a mixture divided by the volume of the mixture Failed to parse (Missing texvc executable; please see math/README to configure.): V

Failed to parse (Missing texvc executable; please see math/README to configure.): C_i = \frac{N_i}{V}

The SI-unit is 1/m³.

Volume concentration[link]

The volume concentration Failed to parse (Missing texvc executable; please see math/README to configure.): \phi_i

(also called volume fraction) is defined as the volume of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): V_i
divided by the volume of all constituents of the mixture Failed to parse (Missing texvc executable; please see math/README to configure.): V
prior to mixing:

Failed to parse (Missing texvc executable; please see math/README to configure.): \phi_i = \frac {V_i}{V}

The SI-unit is m³/m³.

Related quantities[link]

Several other quantities can be used to describe the composition of a mixture. Note that these should not be called concentrations.

Normality[link]

Normality is defined as the molar concentration Failed to parse (Missing texvc executable; please see math/README to configure.): c_i

divided by an equivalence factor Failed to parse (Missing texvc executable; please see math/README to configure.): f_{eq}

. Since the definition of the equivalence factor may not be unequivocal, IUPAC and NIST discourage the use of normality.

Molality[link]

(Not to be confused with Molarity)

The molality of a solution Failed to parse (Missing texvc executable; please see math/README to configure.): b_i

is defined as the amount of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): n_i
divided by the mass of the solvent Failed to parse (Missing texvc executable; please see math/README to configure.): m_{solvent}
(not the mass of the solution):

Failed to parse (Missing texvc executable; please see math/README to configure.): b_i = \frac{n_i}{m_{solvent}}

The SI-unit for molality is mol/kg.

Mole fraction[link]

The mole fraction Failed to parse (Missing texvc executable; please see math/README to configure.): x_i

is defined as the amount of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): n_i
divided by the total amount of all constituents in a mixture Failed to parse (Missing texvc executable; please see math/README to configure.): n_{tot}

Failed to parse (Missing texvc executable; please see math/README to configure.): x_i = \frac {n_i}{n_{tot}}

The SI-unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole fractions.

Mole ratio[link]

The mole ratio Failed to parse (Missing texvc executable; please see math/README to configure.): r_i

is defined as the amount of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): n_i
divided by the total amount of all other constituents in a mixture:

Failed to parse (Missing texvc executable; please see math/README to configure.): r_i = \frac{n_i}{n_{tot}-n_i}

If Failed to parse (Missing texvc executable; please see math/README to configure.): n_i

is much smaller than Failed to parse (Missing texvc executable; please see math/README to configure.): n_{tot}

, the mole ratio is almost identical to the mole fraction.

The SI-unit is mol/mol. However, the deprecated parts-per notation is often used to describe small mole ratios.

Mass fraction[link]

The mass fraction Failed to parse (Missing texvc executable; please see math/README to configure.): w_i

is the fraction of one substance with mass Failed to parse (Missing texvc executable; please see math/README to configure.): m_i
to the mass of the total mixture Failed to parse (Missing texvc executable; please see math/README to configure.): m_{tot}

, defined as:

Failed to parse (Missing texvc executable; please see math/README to configure.): w_i = \frac {m_i}{m_{tot}}

The SI-unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass fractions.

Mass ratio[link]

The mass ratio Failed to parse (Missing texvc executable; please see math/README to configure.): \zeta_i

is defined as the mass of a constituent Failed to parse (Missing texvc executable; please see math/README to configure.): m_i
divided by the total mass of all other constituents in a mixture:

Failed to parse (Missing texvc executable; please see math/README to configure.): \zeta_i = \frac{m_i}{m_{tot}-m_i}

If Failed to parse (Missing texvc executable; please see math/README to configure.): m_i

is much smaller than Failed to parse (Missing texvc executable; please see math/README to configure.): m_{tot}

, the mass ratio is almost identical to the mass fraction.

The SI-unit is kg/kg. However, the deprecated parts-per notation is often used to describe small mass ratios.

Dependence on volume[link]

Concentration depends on the variation of the volume of the solution due mainly to thermal expansion.

Table of concentrations and related quantities[link]

or Failed to parse (Missing texvc executable; please see math/README to configure.): \gamma_i

See also[link]

• Serial dilution
• Dose concentration

References[link]