JOURNAL OF THE BRAZILIAN CHEMICAL SOCIETY

Summary of IUPAC Recommendations on Units and Symbols

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The recommendations of IUPAC should be followed.
Their basis is the “Système Internationale d'Unités” (SI).
A detailed treatment is given in the so-called Green Book: Quantities, Units and Symbols in Physical Chemistry (Blackwell Scientific Publications, Oxford, 1993 ed.), from which the following guidelines were taken.

For detailed information about Nomenclature and Symbols go to http://www.iupac.org/general/FAQs/ns.html

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Guidelines for the publications of SBQ.

An author will not be denied any reasonable usage, but if non-SI units are used for critical data or for quantities measured to a high  order of accuracy (as opposed to the rough physical conditions of an experiment), the definitive values will be expressed in SI units as well.

The following will be the guidelines used:

(a) SI will be the standard usage.

(b) The units used to record the definitive values of “critical data” or quantities measured to a high degree of accuracy will be SI.

(c) When non-SI units are used they must be adequately explained unless their definition is obvious (e.g. ºC, mmHg, g, h). The derivation of derived non-SI units will be indicated.

(d) Equations involving electrical quantities should normally be those appropriate for use with SI (rationalized mks) units. If authors wish to use equations suitable for esu or emu, the lack of consistency with SI units must be explicitly noted.

(1) Base-units. The SI base-units are given in Table 1.

Table 1: Base-units

Physical quantity	Name of base-unit	Symbol for unit
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electrical current	ampere	A
Thermodynamic temperature	kelvin	K
Luminous intensity	candela	cd
Amount of substance	mole	mol

(2) Supplementary units. The SI also includes two `supplementary' dimensionless units as follows:

Physical quantity	Name of unit	Symbol for unit
plane angle	radian	rad
solid angle	steradian	sr

(3) Multiples and sub-multiples. In the SI there is one and only one basic unit for each physical quantity. Decimal fractions and multiples of these basic units may, however, be constructed by use of certain prefixes (see Table 2). They may also be used with derived SI units.

Table 2: Prefixes

Fraction	Prefix	Symbol	Multiple	Prefix	Symbol
10^–1	deci	d	10	deca	da
10^–2	centi	c	10^2	hecto	h
10^–3	milli	m	10^3	kilo	k
10^–6	micro	µ	10^6	mega	M
10^–9	nano	n	10^9	giga	G
10^–12	pico	p	10^12	tera	T
10^–15	femto	f
10^–18	atto	a

The combination of a prefix and a unit symbol constitutes a new single unit symbol; compounding of prefixes is not permitted. Although it will not always be possible, particularly in Tables, the general principle should be to choose a unit (i.e. including multiple or sub-multiple) such that the resulting numerical value is between 0.1 and 1000.

(4) Derived units. Some derived units have special names and symbols, and these are given in Table 3. Others do not (Table 4).

Table 3: Derived units with special names and symbols

Physical quantity	Name of SI  unit	Symbol for SI unit	Definition of SI unit
energy	joule	J	kg m^2 s^–2
force	newton	N	kg m s^–2 = J m^–1
power	watt	W	kg m^2 s^–3 = J s^–1
electric charge	coulomb	C	A s
electric potential difference	volt	V	kg m^2s^–3 A^–1 = J A^–1 s^–1
electric resistance	ohm	W	kg m^2 s^–3 A^–2 = V A^–1
electric capacitance	farad	F	A^2 s^4 kg^–1 m^–2 = A s V^–1
magnetic flux	weber	Wb	kg m^2 s^–2 A^–1 = V s
inductance	henry	H	kg m^2 s^–2A^–2 = V A^–1 s
magnetic flux density	tesla	T	kg s^–2 A^–1 = V s m^–1
luminous flux	lumen	lm	cd sr
illumination	lux	lx	cd sr m^–2
frequency	hertz	Hz	s^–1

Table 4: Derived units with no special names or symbols

Physical quantity	SI unit	Symbol for SI unit
area	square metre	m^2
volume	cubic metre	m^3
density	kilogram per cubic metre	kg m^–3
velocity	metre per second	m s^–1
angular velocity	radian per second	rad s^–1
acceleration	metre per second squared	m s^–2
pressure	newton per square metre	N m^–2
kinematic viscosity, diffusion coefficient	square metre per second	m^2 s^–1
dynamic viscosity	newton second per square metre	N s m^–2
electric field strength	volt per metre	V m^–1
magnetic field strength	ampere per metre	A m^–1
luminance	candela per square metre	cd m^–2

(5) Symbol. The symbol for a unit will be printed in roman (upright) type, remains unaltered in the plural and does not take a full point, i.e. 5 cm not 5 cm. or 5 cms or 5 cms.
The symbol will be separated from the numerical value by a thin space.

(6) Decimal fractions and multiples of SI units having special names. These names are not part of the SI, but for the time being their use in SBQ's publications may continue. The list given in Table 5 is not exhaustive.

Table 5: Fractions and multiples of units with special names

Physical quantity	Name of unit	Symbol for unit	Definition of unit
length	ångström	Å	10^–10 m = 10^–1nm
length	micron	µm	10^–6 m
area	barn	b	10^–28 m^2
volume	litre	L	10^–3 m^3 = dm^3
mass	tonne	t	10^3 kg = Mg
force	dyne	dyn	10^–5 N
pressure	bar	bar	10^5 N m^–2
pressure	pascal	Pa	N m^–2
energy	erg	erg	10^–7 J
kinematic viscosity, diffusion coefficient	stokes	St	10^–4 m^2 s^–1
dynamic viscosity	poise	P	10^–1 kg m^–1 s^–1
magnetic flux	maxwell	Mx	10^–8 Wb
magnetic flux density (magnetic induction)	gauss	G	10^–4 T
conductance	sîemens	S	W^–1

NB: concentration in mol L^-1

(7) Units defined in terms of the best available experimental values of certain physical constants. These units are not part of the SI. The factors for conversion of these units to SI units are subject to change in the light of new experimental measurements of the constants involved. Their use outside the restricted contexts to which they are appropriate should be discouraged. The following list is not exhaustive.  

Physical quantity	Name of unit	Symbol for unit	Conversion factor
energy	electronvolt	eV	eV = 1.6021 x 10^–19 J
mass	unified atomic mass unit	u	u = 1.66041 x 10^–27 kg

(8) Other units now exactly defined in terms of the SI units. These units are not part of the SI. It is recognised that their use may be continued for some time but it is recommended that except in special circumstances they should be progressively abandoned in conformity with international recommendations. The list given in Table 6 is by no means exhaustive. Each of the definitions given in the fourth column is exact.

Table 6: Units defined in terms of SI units

Physical quantiy	Name of unit	Symbol for unit	Definition of unit
length	inch	in	2.54 x 10^–2 m
mass	pound (avoirdupois)	lb	0.453 592 37 kg
time*	minute	min	60 s
time*	hour	h	3600 s
force	kilogram-force	kgf	9.806 65 N
force	pound-force	lbf	9.806 65 x 0.453 592 37 N
pressure	atmosphere	atm	101 325 N m^–2
pressure	conventional millimetre of mercury	mmHg	13.5951 x 9.806 65 N m^–2
pressure	torr	Torr	(101 325/760) N m^–2
pressure	pound-force per square inch	lbf in^–2	(9.806 65 + 4 535.9237)  / 6.4516 Nm^–2
energy	kilowatt	hour  kW h	3.6 x 104 J
energy	thermochemical calorie	cal(thermo calorie    chem.)	- 4.184 J
energy	international calorie	calIT	4.1868 J
thermodynamic temperature	degree Rankine	ºR	(5/9) K
radioactivity	curie	Ci	3.7 x 1016 s^–1

 * Use of other common units (min, h, day) may continue in normal expressions of intervals of time.

Acknowledgement

We thank the managing editor of J. Chem. Soc. Dalton Trans for permission to adapt the original document available from http://www.rsc.org/publishing/books/pub/siunits.asp