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Concentration [mol·L-1] or density is a volume-specific quantity, expressing the number of particles per volume, the amount of substance per volume, or generally matter in a variety of formats (count, amount, charge, mass, volume or energy) per volume of the system. In solution chemistry, amount concentration is amount, n, of B per volume, V, of the solution, cB = [B] = nB·V-1 [mol·dm-3]=[mol·L-1]. The standard concentration, c°, is defined as 1 mol·L-1 = 1 M. A change of concentration of an elementary entity, X, dcX, in isolated or closed systems at constant pressure is due to internal transformations (advancement per volume) only. In closed compressible systems (with a gas phase), the concentration of the gas changes, when pressure-volume work is performed on the system. In open systems, a change of concentration can additionally be due to external flow across the system boundaries.

Abbreviation: c [mol·L-1]; C [x·L-1]

Reference: IUPAC Gold Book

Communicated by Gnaiger E 2018-10-18 (last update 2020-05-18)

IUPAC definition

Concentration {quote}:
1. Group of four quantities characterizing the composition of a mixture with respect to the volume of the mixture (mass, amount, volume and number concentration).
2. Short form for amount (of substance) concentration (substance concentration in clinical chemistry).
{end of quote: IUPAC Gold Book}

Concentration in different formats

Concentration is a quantity, X, of objects, X, per volume, V, of the system. The entities X have to be specified in the text or indicated by a subscript or in parentheses: CN(X).
Concentration Symbol Definition Unit Note
Count concentration CN = NX·V-1 [x·L-1] 1
Amount concentration cn(X) = cB = nX·V-1 [mol·L-1] 2
Charge density ρe = QX·V-1 [C·L-1] 3
Mass density ρX = mX·VX-1 [kg·L-1] 4
Volume density ΦX = VX·V-1 [L·L-1] 5
  1. The IUPAC term 'number concentration' should be replaced by 'count concentration' (or 'number of entities concentration').
  2. Amount concentration is a counting concentration, converting the counting unit [x] into moles [mol] using the Avogadro constant.
  3. Charge density in electricity is charge per volume. Charge density is a counting concentration, converting the counting unit [x] into coulombs C using the elementary charge, or converting moles [mol] into coulombs [C] using the Faraday constant.
  4. Mass density is mass of X per volume of X, VX; ρX is the reciprocal of specific volume.
  5. Volume density is equivalent to the volume fraction.

Total concentration

  1. The total concentration of a substance (e.g. ADP) must be distinguished from the concentration of a specific ionic species (e.g. ADP4-, MgADP2-, MgHADP-).
  2. The total concentration of a substance must be distinguished from the free concentration, if unspecific binding occurs to membranes (e.g. TPP) or if a substance is bound to BSA (e.g. fatty acids).
  3. The concentration of a substance (c) differs from the activity (a), except for dissolved gases and at very high dilution. The activity of dissolved gases is expressed as the (relative) partial pressure, e.g. pO2). The ratio of concentration, e.g. cO2 [µM], and partial pressure, e.g. pO2 [kPa], is the solubility (see oxygen solubility, SO2).


Bioblast linkReferenceYear
Gnaiger Erich et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. Bioenerg Commun 2020.1. doi:10.26124/bec:2020-0001.v1.2020


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