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Concentration

## Description

Concentration [mol·L-1] is a volume-specific quantity for diluted samples s. In a concentration, the sample is expressed in a variety of formats: count, amount, charge, mass, energy. In solution chemistry, amount concentration is amount of substance nB 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. Count concentration CX = NX·V-1 [x·L-1] is the concentration of the number NX of elementary entities X, for which the less appropriate term 'number concentration' is used by IUPAC. If the sample is expressed as volume Vs (e.g., VO2), then the 'volume-concentration' of Vs in V is termed 'volume fraction', Φs = Vs·V-1 (e.g., volume fraction of O2 in dry air, ΦO2) = 0.20946). Density is the mass concentration in a volume VS of pure sample S.

A change of concentration, dcX, in isolated or closed systems at constant volume 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: BEC 2020.1

```Communicated by Gnaiger E (2018-10-18) last update 2022-04-03
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## 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 and density

Concentration and density in different formats

Concentration is an extensive quantity divided by volume V, or a count divided by volume V. The elementary entities X (or B) or the sample type s have to be specified in the text or indicated by a subscript or in parentheses. Examples: cell-count concentration Cce; count concentration of protons CH+; molar concentration of protons cH+. Density is not only 'mass density ρ, but is used for many extensive quantities divided by volume or area.
Concentration Symbol Definition Unit Note
Count concentration CX = NX·V-1 [x·L-1] The IUPAC term 'number concentration' should be replaced by 'count concentration' (or 'number of entities concentration').
Amount concentration cB = nB·V-1 [mol·L-1] Amount concentration is a counting concentration, converting the elementary unit [x] into moles [mol] using the Avogadro constant.
Charge concentration CQX = QX·V-1 [C·L-1] Charge concentration is a counting concentration, converting the elementary unit [x] into coulombs C using the elementary charge, or converting moles [mol] into coulombs [C] using the Faraday constant. Charge density in electricity is 'charge per volume'.
Mass concentration of X CmX = mX·V-1 [kg·L-1] Mass concentration CmX is mass of entities X per volume V of the mixture.
Mass density of S ρS = mS·VS-1 [kg·L-1] Mass density ρS is mass of the pure sample S per volume VS of the pure sample; ρS is the reciprocal of specific volume.
Volume concentration of X CVX = VX·V-1 [L·L-1] Volume concentration CVX is the volume of entities X per volume V of the mixture.
Volume density ΦX = VX·V-1 [L·L-1] 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).