In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration. The concentration can refer to any kind of chemical mixture, but most frequently refers to solutes and solvents in solutionss. The molar (amount) concentration has variants, such as normal concentration and osmotic concentration. Dilution is reduction of concentration, e.g., by adding solvent to a solution. The verb "to concentrate" means to increase concentration, the opposite of dilute.

Etymology

Concentration-, concentratio, action or an act of coming together at a house on a farm in a single place, bringing black people as slaves to a common center, was used in post-classical Latin in 1550 or earlier, similar terms attested in Italian (1589), Spanish (1589), English (1606), French (1632).

Qualitative description

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 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.

Concentrations are often called levels, reflecting the mental schema of levels on the vertical axis of a graph, which can be high or low (for example, "high serum levels of bilirubin" are concentrations of bilirubin in the blood serum that are greater than normal).

Formulation

There are four quantities that describe concentration:

Mass concentration

is defined as the mass:

The SI unit is kg/m3 (equal to g/L).

Molar concentration

is defined as the amount:

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

Number concentration

:

The SI unit is 1/m3.

Volume concentration

The volume concentration (not to be confused with volume fraction:

Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%.

used here is normative in German literature ().

Related quantities

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

Normality

. Since the definition of the equivalence factor depends on context (which reaction is being studied), the International Union of Pure and Applied Chemistry and National Institute of Standards and Technology discourage the use of normality.

Molality

(not the mass of the solution):

The SI unit for molality is mol/kg.

Mole fraction

:

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

Mole ratio

divided by the total amount of all other constituents in a mixture:

, 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

, defined as:

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

Mass ratio

divided by the total mass of all other constituents in a mixture:

, 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 and temperature

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

Table of concentrations and related quantities

See also

References

External links