Nomenclature and SI Guidelines

SI (Système International) units should be used. Only symbols (not their subscripts, superscripts or description in brackets) of physical quantities should be written in italic. All physical quantities given in table columns or rows and corresponding table headings with units, or graphical plots and corresponding table headings with units, or graphic plots and corresponding axis labels should conform to the algebraic rules, i.e. physical quantity/unit=numerical value. Numerical values and their units must be written with one space between (e.g. 1 cm, 2 L, 3 g/L, 10 %, 20 °C).

For the mixtures of A (solute) and B (solvent) the content should be expressed with one of the physical quantities given in the table below (the content itself is not a physical quantity).

Name Symbol Definition SI unit
Mass ratio ζ ζ(A,B)=m(A)/m(B)  1
Volume ratio Φ Φ(A,B)=V(A)/V(B)   1
Amount (of substance) ratio  r r(A,B)=n(A)/n(B)   1
Number ratio R R(A,B)=m(A)/m(B)   1
Molality b b(A,B)=N(A)/N(B)   mol/kg
Mass per volume ratio m/V m(A)/v(B)  kg/m3


Mass fraction w w(A)=m(A)/[m(A)+m(B)]  1
Volume fraction φ φ(A)=V(A)/[V(A)+V(B)]  1
Amount fraction x x(A)=n(A)/[n(A)+n(B)]  1
Number fraction X X(A)=N(A)/[N(A)+N(B)]  1


Mass concentration γ γ(A)=m(A)/[V(A)+V(B)]  kg/m3
Volume concentration σ σ(A)=V(A)/[V(A)+V(B)]  1
Amount concentration c c(A)=n(A)/[V(A)+V(B)]  mol/m3
Number concentration C C(A)=N(A)/[V(A)+V(B)]  1/m3

The symbols w/w, v/v and w/v are also not recommended. Instead of these old symbols, SI recommends symbol for mass: m and volume: V. Besides, these older symbols are usually used for ratios but sometimes they are used as fractions and this can be ambiguous. Therefore, for unambiguous presentation either ratio or fraction should be stated. Ratio or fraction can be used either per unit or per 100 (percentage), per 103 (permillage), per 106 (ppm), or 109 (ppb), etc. units of denominator. Therefore, the symbol %=10–2, ‰=10–3, ppm=10–6, ppb=10–9, etc.

The principle to use as few characters as possible is recommended. In accordance with this the authors are encouraged to use units with SI prefixes instead of the basic SI unit (e.g. instead of 1.2·10–6 A, 1.2 μA should be used). For volume, the unit litre (1 L) or its decimal units are recommended as a special name for 1 dm3 volume unit (1 L=1 dm3, one character substitutes three characters). Following the same principle, although not recommended by IUPAC, the unit 1 M (or its decimal units) for amount concentration can be used (1 M=1 mol/L).

The IUPAC recommendations on chemical nomenclature should be followed (see


For the biochemical nomenclature including abbreviations, recommendations of the Nomenclature Committee of IUBMB and the IUPAC-IUBMB Joint Commission on Biochemical Nomenclature ( should be followed.


For gene nomenclature and symbols the Human Genome Nomenclature Database ( and Entrez Gene ( should be consulted.


Apart from the recommended nomenclature, the usual common terms are acceptable as is the use of the usual abbreviations within the text, particularly in cases of compounds of very long names.


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