Density

high-resolution terminology - matching measurements at high-resolution

Density

Description

Density or mass density ρ = m·V^-1 [kg·m^-3] is mass m divided by volume V. Surface density ρ_A = m·A^-1 [kg·m^-2] (SI). For a pure sample S, the mass density ρ_S = m_S·V_S^-1 [kg·m^-3] is the mass m of pure sample S per volume V_S of the pure sample. With density ρ thus defined, the 'amount density' of substance B is ρ_B = n_B·V_B^-1 [mol·m^-3]. This is not a commonly used expression, but the inverse is defined as the molar volume of a pure substance (IUPAC), V_m,B = V_B·n_B^-1 [m³·mol^-1]. The amount concentration, c_B = n_B·V^-1 [mol·m^-3] is the amount n_B of substance B divided by the volume V of the mixture (IUPAC), and this is not called an 'amount density'. The term 'amount density' is reserved for an amount of substance per volume V_S of the pure substance. This explicit distinction between 'density' related to the volume of the sample and 'concentration' related to the total volume of the mixture is very helpful to avoid confusion. Further clarification is required in cases, when the mass density ρ_s of the sample in the mixture differs from the mass density ρ_S of a pure sample before mixing. Think of a sample S of pure ethanol with a volume of 1 L at 25 °C, which is mixed with a volume of 1 L of pure water at 25 °C: after the temperature of the mixture has equilibrated to 25 °C, the total volume of the mixture is less than 2 L, such that the volume V_S of 1 L pure ethanol has diminished to a smaller volume V_s of ethanol in the mixture; the density of ethanol in the mixture is higher than the density of pure ethanol. The volume V_s of sample s in a mixture is by definition smaller than the total volume V of the mixture. Sample volume V_S and system volume V are identical, but this applies only to the case of a pure sample. Concentration is related to samples s per total volume V of the mixture, whereas density is related to samples S or s per volume V_S = V or V_s < V, respectively (BEC 2020.1).

Abbreviation: ρ, C, D

Reference: BEC 2020.1

Communicated by Gnaiger E (2018-08-23) last update 2020-05-28

References

Bioblast link	Reference	Year
Bureau International des Poids et Mesures 2019 The International System of Units (SI)	Bureau International des Poids et Mesures (2019) The International System of Units (SI). 9th edition:117-216. ISBN 978-92-822-2272-0	2019
Cohen 2008 IUPAC Green Book	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.	2008
Gnaiger 2020 BEC MitoPathways	Gnaiger E (2020) Mitochondrial pathways and respiratory control. An introduction to OXPHOS analysis. 5^th ed. Bioenerg Commun 2020.2. https://doi.org/10.26124/bec:2020-0002	2020
Gnaiger 2020 MitoFit x	Gnaiger E (2021) The elementary unit — canonical reviewer's comments on: Bureau International des Poids et Mesures (2019) The International System of Units (SI) 9th ed. https://doi.org/10.26124/mitofit:200004.v2	2021
BEC 2020.1 doi10.26124bec2020-0001.v1	Gnaiger E et al ― MitoEAGLE Task Group (2020) Mitochondrial physiology. Bioenerg Commun 2020.1. https://doi.org/10.26124/bec:2020-0001.v1	2020