Concentration

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Concentration

Description

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, c_B = [B] = n_B·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, dc_X, 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

Count concentration is the number of particles (entities, objects) per volume,

C_N = N_X·V^-1 [x·L^-1]

Amount concentration

c_N = n_X·V^-1 [mol·L^-1]

Charge density in electricity is charge per volume,

ρ = Q·V^-1 [C·L^-1]

Mass density is the reciprocal of specific volume,

ρ_X = m_X·V_X^-1 [kg·L^-1]

Volume density is equivalent to the volume fraction,

Φ_X = V_X·V^-1.

The entities X have to be specified in the text or indicated by a subscript or in parentheses: C_N_(X).

Total concentration

The total concentration of a substance (e.g. ADP) must be distinguished from the concentration of a specific ionic species (e.g. ADP^4-, MgADP^2-, MgHADP^-).
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).
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. p_O₂). The ratio of concentration, e.g. c_O₂ [µM], and partial pressure, e.g. p_O₂ [kPa], is the solubility (see oxygen solubility, S_O₂).