Concentration

Short description: Ratio of part of a mixture to the whole

Test tubes with liquid in which a blue dye is dissolved in different concentrations. In the test tube with dark blue liquid (in front), the blue dye is dissolved in a high concentration. In the following test tubes, the blue dye is dissolved in a lower concentration (and at the same time in a smaller amount, since the volume is approximately the same). Image is AI generated.

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] The concentration can refer to any kind of chemical mixture, but most frequently refers to solutes and solvents in solutions. 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.

## Contents

* 1 Etymology
* 2 Qualitative description
* 3 Quantitative notation
* 3.1 Mass concentration
* 3.2 Molar concentration
* 3.3 Number concentration
* 3.4 Volume concentration
* 4 Related quantities
* 4.1 Normality
* 4.2 Molality
* 4.3 Mole fraction
* 4.4 Mole ratio
* 4.5 Mass fraction
* 4.6 Mass ratio
* 5 Dependence on volume and temperature
* 6 Table of concentrations and related quantities
* 7 See also
* 8 References

## Etymology

Concentration-, concentratio, action or an act of coming together at a single place, bringing 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).[2]

## Qualitative description

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 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

There are four quantities that describe concentration:

### Mass concentration

Main article: Chemistry:Mass concentration

The mass concentration ρi is defined as the mass of a constituent mi divided by the volume of the mixture V:

ρi=miV.

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

### Molar concentration

Main article: Physics:Molar concentration

The molar concentration ci is defined as the amount of a constituent ni (in moles) divided by the volume of the mixture V:

ci=niV.

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

### Number concentration

The number concentration Ci is defined as the number of entities of a constituent Ni in a mixture divided by the volume of the mixture V:

Ci=NiV.

The SI unit is 1/m3.

### Volume concentration

The volume concentration σi (not to be confused with volume fraction[3]) is defined as the volume of a constituent Vi divided by the volume of the mixture V:

σi=ViV.

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

There seems to be no standard notation in the English literature. The letter σi used here is normative in German literature (see Volumenkonzentration).

## Related quantities

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

### Normality

Normality is defined as the molar concentration ci divided by an equivalence factor feq. 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

Main article: Chemistry:Molality

Not to be confused with molarity.

The molality of a solution bi is defined as the amount of a constituent ni (in moles) divided by the mass of the solvent msolvent (not the mass of the solution):

bi=nimsolvent.

The SI unit for molality is mol/kg.

### Mole fraction

Main article: Chemistry:Mole fraction

The mole fraction xi is defined as the amount of a constituent ni (in moles) divided by the total amount of all constituents in a mixture ntot:

xi=nintot.

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

### Mole ratio

Main article: Chemistry:Mixing ratio

The mole ratio ri is defined as the amount of a constituent ni divided by the total amount of all other constituents in a mixture:

ri=nintot−ni.

If ni is much smaller than ntot, 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

Main article: Chemistry:Mass fraction

The mass fraction wi is the fraction of one substance with mass mi to the mass of the total mixture mtot, defined as:

wi=mimtot.

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

### Mass ratio

Main article: Chemistry:Mixing ratio

The mass ratio ζi is defined as the mass of a constituent mi divided by the total mass of all other constituents in a mixture:

ζi=mimtot−mi.

If mi is much smaller than mtot, 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

Concentration type  | Symbol  | Definition  | SI unit  | other unit(s)

|
|
|
|

mass concentration  | ρi or γi | mi/V | kg/m3 | g/100mL (= g/dL)
molar concentration  | ci | ni/V | mol/m3 | M (= mol/L)
number concentration  | Ci | Ni/V | 1/m3 | 1/cm3
volume concentration  | σi | Vi/V | m3/m3 |
Related quantities  | Symbol  | Definition  | SI unit  | other unit(s)
normality  |  | ci/feq | mol/m3 | M (= mol/L)
molality  | bi | ni/msolvent | mol/kg  |
mole fraction  | xi | ni/ntot | mol/mol  | ppm, ppb, ppt
mole ratio  | ri | ni/(ntot−ni) | mol/mol  | ppm, ppb, ppt
mass fraction  | wi | mi/mtot | kg/kg  | ppm, ppb, ppt
mass ratio  | ζi | mi/(mtot−mi) | kg/kg  | ppm, ppb, ppt
volume fraction  | ϕi | Vi/∑jVj | m3/m3 | ppm, ppb, ppt

## See also

* Dilution ratio - Change in concentration when mixing two liquids
* Serial dilution - Step-wise dilution of a substance in solution
* Standard state § Solutes

## References

1. ↑ 1.0 1.1 IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006-) "concentration". doi:10.1351/goldbook.C01222
2. ↑ concentration (3rd ed.), Oxford University Press, September 2005, http://www.oed.com/view/Entry/38114 (Subscription or UK public library membership required.)
3. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006-) "volume fraction". doi:10.1351/goldbook.V06643

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Solutions

Solution|

* Ideal solution
* Aqueous solution
* Solid solution
* Buffer solution
* Flory–Huggins
* Mixture
* Suspension
* Colloid
* Phase diagram
* Phase separation
* Eutectic point
* Alloy
* Saturation
* Supersaturation
* Serial dilution
* Dilution (equation)
* Apparent molar property
* Miscibility gap

Concentration
and related quantities|

* Molar concentration
* Mass concentration
* Number concentration
* Volume concentration
* Normality
* Molality
* Mole fraction
* Mass fraction
* Isotopic abundance
* Mixing ratio
* Ternary plot

Solubility|

* Solubility equilibrium
* Total dissolved solids
* Solvation
* Solvation shell
* Enthalpy of solution
* Lattice energy
* Raoult's law
* Henry's law
* Solubility table (data)
* Solubility chart

Solvent|

* (Category)
* Acid dissociation constant
* Protic solvent
* Inorganic nonaqueous solvent
* Solvation
* List of boiling and freezing information of solvents
* Partition coefficient
* Polarity
* Hydrophobe
* Hydrophile
* Lipophilic
* Amphiphile
* Lyonium ion
* Lyate ion

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