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The activity (relative activity) is a dimensionless quantity related to the concentration or partial pressure of dissolved substances. The activity of a dissolved substance B equals the concentration, cB [mol·L-1], at high dilution divided by the unit concentration, c° = 1 mol·L-1:

aB = cB/c°

This simple relationship applies frequently to substances at high dilutions <10 mmol·L-1 (<10 mol·m-3). In general, the concentration of a solute has to be corrected for the activity coefficient (concentration basis), γB,

aB = γB·cB/c°

At high dilution, γB = 1.

For a dissolved gas G, the activity is the partial pressure, pG [Pa] (strictly: fugacity), divided by the unit partial pressure, p° = 1 Pa. The partial pressure is related to the concentration of the gas by the solubility, SG [Pa/mol] (see Oxygen solubility):

aG = cG·SG/p°

In general, the relative activity is defined by the chemical potential, µX

aX = exp[(µX-µ°)/RT]

Abbreviation: a

Reference: Cohen 2008 IUPAC Green Book

Communicated by Gnaiger E 2018-10-18

Activity in other contexts

Bq is the becquerel [s-1]


  1. Cohen ER, Cvitas T, Frey JG, Holmström B, Kuchitsu K, Marquardt R, Mills I, Pavese F, Quack M, Stohner J, Strauss HL, Takami M, Thor HL (2008) Quantities, Units and Symbols in Physical Chemistry, IUPAC Green Book, 3rd Edition, 2nd Printing, IUPAC & RSC Publishing, Cambridge. - »Bioblast link«

MitoPedia concepts: Ergodynamics 

MitoPedia topics: Substrate and metabolite