Concentration

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.^[1] A concentration can be any kind of chemical mixture, but most frequently solutes and solvents in solutions. The molar (amount) concentration has variants such as normal concentration and osmotic concentration.

Qualitative description[edit]

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.

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).

Quantitative notation[edit]

There are four quantities that describe concentration:

Mass concentration[edit]

The mass concentration ${\displaystyle \rho _{i}}$ is defined as the mass of a constituent ${\displaystyle m_{i}}$ divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle \rho _{i}={\frac {m_{i}}{V}}.}$

The SI unit is kg/m³ (equal to g/L).

Molar concentration[edit]

The molar concentration ${\displaystyle c_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ (in moles) divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle c_{i}={\frac {n_{i}}{V}}.}$

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

Number concentration[edit]

The number concentration ${\displaystyle C_{i}}$ is defined as the number of entities of a constituent ${\displaystyle N_{i}}$ in a mixture divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle C_{i}={\frac {N_{i}}{V}}.}$

The SI unit is 1/m³.

Volume concentration[edit]

The volume concentration ${\displaystyle \phi _{i}}$ (not to be confused with volume fraction^[2]) is defined as the volume of a constituent ${\displaystyle V_{i}}$ divided by the volume of the mixture ${\displaystyle V}$:

${\displaystyle \phi _{i}={\frac {V_{i}}{V}}.}$

Being dimensionless, it is expressed as a number, e.g., 0.18 or 18%; its unit is 1.

Related quantities[edit]

Several other quantities can be used to describe the composition of a mixture. Note that these should not be called concentrations.^[1]

Normality[edit]

Normality is defined as the molar concentration ${\displaystyle c_{i}}$ divided by an equivalence factor ${\displaystyle f_{\mathrm {eq} }}$. Since the definition of the equivalence factor depends on context (which reaction is being studied), IUPAC and NIST discourage the use of normality.

Molality[edit]

(Not to be confused with Molarity)

The molality of a solution ${\displaystyle b_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ (in moles) divided by the mass of the solvent ${\displaystyle m_{\mathrm {solvent} }}$ (not the mass of the solution):

${\displaystyle b_{i}={\frac {n_{i}}{m_{\mathrm {solvent} }}}.}$

The SI unit for molality is mol/kg.

Mole fraction[edit]

The mole fraction ${\displaystyle x_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ (in moles) divided by the total amount of all constituents in a mixture ${\displaystyle n_{\mathrm {tot} }}$:

${\displaystyle x_{i}={\frac {n_{i}}{n_{\mathrm {tot} }}}.}$

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

Mole ratio[edit]

The mole ratio ${\displaystyle r_{i}}$ is defined as the amount of a constituent ${\displaystyle n_{i}}$ divided by the total amount of all other constituents in a mixture:

${\displaystyle r_{i}={\frac {n_{i}}{n_{\mathrm {tot} }-n_{i}}}.}$

If ${\displaystyle n_{i}}$ is much smaller than ${\displaystyle n_{\mathrm {tot} }}$, 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[edit]

The mass fraction ${\displaystyle w_{i}}$ is the fraction of one substance with mass ${\displaystyle m_{i}}$ to the mass of the total mixture ${\displaystyle m_{\mathrm {tot} }}$, defined as:

${\displaystyle w_{i}={\frac {m_{i}}{m_{\mathrm {tot} }}}.}$

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

Mass ratio[edit]

The mass ratio ${\displaystyle \zeta _{i}}$ is defined as the mass of a constituent ${\displaystyle m_{i}}$ divided by the total mass of all other constituents in a mixture:

${\displaystyle \zeta _{i}={\frac {m_{i}}{m_{\mathrm {tot} }-m_{i}}}.}$

If ${\displaystyle m_{i}}$ is much smaller than ${\displaystyle m_{\mathrm {tot} }}$, 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[edit]

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

Table of concentrations and related quantities[edit]

Concentration type	Symbol	Definition	SI unit	other unit(s)
mass concentration	${\displaystyle \rho _{i}}$ or ${\displaystyle \gamma _{i}}$	${\displaystyle m_{i}/V}$	kg/m3	g/100mL (= g/dL)
molar concentration	${\displaystyle c_{i}}$	${\displaystyle n_{i}/V}$	mol/m3	M (= mol/L)
number concentration	${\displaystyle C_{i}}$	${\displaystyle N_{i}/V}$	1/m3	1/cm3
volume concentration	${\displaystyle \phi _{i}}$	${\displaystyle V_{i}/V}$	m3/m3
Related quantities	Symbol	Definition	SI unit	other unit(s)
normality		${\displaystyle c_{i}/f_{\mathrm {eq} }}$	mol/m3	N (= mol/L)
molality	${\displaystyle b_{i}}$	${\displaystyle n_{i}/m_{\mathrm {solvent} }}$	mol/kg
mole fraction	${\displaystyle x_{i}}$	${\displaystyle n_{i}/n_{\mathrm {tot} }}$	mol/mol	ppm, ppb, ppt
mole ratio	${\displaystyle r_{i}}$	${\displaystyle n_{i}/(n_{\mathrm {tot} }-n_{i})}$	mol/mol	ppm, ppb, ppt
mass fraction	${\displaystyle w_{i}}$	${\displaystyle m_{i}/m_{\mathrm {tot} }}$	kg/kg	ppm, ppb, ppt
mass ratio	${\displaystyle \zeta _{i}}$	${\displaystyle m_{i}/(m_{\mathrm {tot} }-m_{i})}$	kg/kg	ppm, ppb, ppt

See also[edit]

• Dilution ratio
• Dose concentration
• Serial dilution
• Wine/water mixing problem

References[edit]