Description
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 [molยทL-1], at high dilution divided by the unit concentration, cยฐ = 1 molยทL-1:
aB = cB/cยฐ
This simple relationship applies frequently to substances at high dilutions <10 mmolยทL-1 (<10 molยทm-3). In general, the concentration of a solute has to be corrected for the activity coefficient (concentration basis), ฮณB,
aB = ฮณBยทcB/cยฐ
At high dilution, ฮณB = 1.
For a dissolved gas G, the activity is the partial pressure, pG [Pa] (strictly: fugacity), divided by the unit partial pressure, pยฐ = 1 Pa. The partial pressure is related to the concentration of the gas by the solubility, SG [Pa/mol] (see Oxygen solubility):
aG = cGยทSG/pยฐ
In general, the relative activity is defined by the chemical potential, ยตX
aX = exp[(ยตX-ยตยฐ)/RT]
Abbreviation: a
Reference: Cohen 2008 IUPAC Green Book
Communicated by Gnaiger E 2018-10-18
Activity in other contexts
- Free activity
- Activity of a radioactive substance, A = โ dNB/dt [Bq] [1]
- Bq is the becquerel [s-1]
References
- โ Cohen ER, Cvitas T, Frey JG, Holmstrรถm 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. - ยปBioblast linkยซ
MitoPedia concepts:
Ergodynamics
MitoPedia topics:
Substrate and metabolite