 Concentration

For other uses, see Concentration (disambiguation).
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types 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 homogeneous solutions.
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Qualitative description
Often in informal, nontechnical 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
There are four quantities that describe concentration:
Mass concentration
Main article: Mass concentration (chemistry)The mass concentration ρ_{i} is defined as the mass of a constituent m_{i} divided by the volume of the mixture V:
The SIunit is kg/m^{3}.
Molar concentration
Main article: Molar concentrationThe molar concentration c_{i} is defined as the amount of a constituent n_{i} divided by the volume of the mixture V:
The SIunit is mol/m^{3}. However, more commonly the unit mol/L is used.
Number concentration
Main article: Number concentrationThe number concentration C_{i} is defined as the number of entities of a constituent N_{i} in a mixture divided by the volume of the mixture V:
The SIunit is 1/m^{3}.
Volume concentration
Main article: Volume concentrationThe volume concentration ϕ_{i} (also called volume fraction) is defined as the volume of a constituent V_{i} divided by the volume of all consituents of the mixture V prior to mixing:
The SIunit is m^{3}/m^{3}.
Related Quantities
Several other quantities can be used to describe the composition of a mixture. Note that these should not be called concentrations.
Normality
Main article: Normality (chemistry)Normality is defined as the molar concentration c_{i} divided by an equivalence factor f_{eq}. Since the definition of the equivalence factor may not be unequivocal, IUPAC and NIST discourage the use of normality.
Molality
Main article: MolalityThe molality of a solution m_{i} is defined as the amount of a constituent n_{i} divided by the mass of the solvent m_{solvent} (not the mass of the solution):
The SIunit for molality is mol/kg.
Mole fraction
Main article: Mole fractionThe mole fraction x_{i} is defined as the amount of a constituent n_{i} divided by the total amount of all constituents in a mixture n_{tot}:
The SIunit is mol/mol. However, the deprecated partsper notation is often used to describe small mole fractions.
Mole ratio
Main article: Mixing ratioThe mole ratio r_{i} is defined as the amount of a constituent n_{i} divided by the total amount of all other constituents in a mixture:
If n_{i} is much smaller than n_{tot}, the mole ratio is almost identical to the mole fraction.
The SIunit is mol/mol. However, the deprecated partsper notation is often used to describe small mole ratios.
Mass fraction
Main article: Mass fraction (chemistry)The mass fraction w_{i} is the fraction of one substance with mass m_{i} to the mass of the total mixture m_{tot}, defined as:
The SIunit is kg/kg. However, the deprecated partsper notation is often used to describe small mass fractions.
Mass ratio
Main article: Mixing ratioThe mass ratio ζ_{i} is defined as the mass of a constituent m_{i} divided by the total mass of all other constituents in a mixture:
If m_{i} is much smaller than m_{tot}, the mass ratio is almost identical to the mass fraction.
The SIunit is kg/kg. However, the deprecated partsper notation is often used to describe small mass ratios.
Dependence on volume
Concentration depends on the variation of the volume of the solution due mainly to thermal expansion.
Concentration type Symbol Definition SIunit other unit(s) mass concentration ρ_{i} or γ_{i} m_{i} / V kg/m^{3} g/100mL (=g/dL) molar concentration c_{i} n_{i} / V mol/m^{3} M (=mol/L) number concentration C_{i} N_{i} / V 1/m^{3} 1/cm^{3} volume concentration ϕ_{i} V_{i} / V m^{3}/m^{3} Related quantities Symbol Definition SIunit other unit(s) normality UNIQ4e0e247d5b017e86math0000003EQINU c_{i} / f_{eq} mol/m^{3} M (=mol/L) molality m_{i} n_{i} / m_{solvent} mol/kg mole fraction x_{i} n_{i} / n_{tot} mol/mol ppm, ppb, ppt mole ratio r_{i} n_{i} / (n_{tot} − n_{i}) mol/mol ppm, ppb, ppt mass fraction w_{i} m_{i} / m_{tot} kg/kg ppm, ppb, ppt mass ratio ζ_{i} m_{i} / (m_{tot} − m_{i}) kg/kg ppm, ppb, ppt See also
References
 ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "concentration".
Articles related to solutions Solution Ideal solution · Aqueous solution · Solid solution · Buffer solution · FloryHuggins · Mixture · Suspension · Colloid · Phase diagram · Eutectic point · Alloy · Saturation · Supersaturation · Serial dilution · Dilution (equation) · Apparent molar propertyConcentration
and related quantitiesMolar concentration · Mass concentration · Number concentration · Volume concentration · Normality · Percentage solution · Molality · Mole fraction · Mass fraction · Mixing ratioSolubility Solubility equilibrium · Total dissolved solids · Solvation · Solvation shell · Enthalpy of solution · Lattice energy · Raoult's law · Henry's law · Solubility table (data) · Solubility chartSolvent Categories: Analytical chemistry
 Chemical properties
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