See also: Quantities,
Units and Symbols in Physical Chemistry, 2nd edition (The Green
Book), Mills, I.; Cvitas, T.; Homann, K.; Kallay, N. and Kuchitsu,
K. Blackwell Science, 1993 [ISBN 0-63203-5838]
and per mils
T. Cvitas, February 2002
Although these are not units in the same sense as the units of dimensioned
quantities, they can be treated as such. The symbols % and ‰ are printed
with a space between the numerical value and the unit:
25 %, 3.2 ‰
The sign % represents a unit symbol equal to the submultiple 0.01
of the coherent SI unit 1. The notation including a space between
the numerical value and the unit is recommended by ISO 31-0: 1992.
Great care should be taken when using these symbols in compound derived
units. While some expressions, such as % s-1 appear unambiguous,
more complex ones involving powers (‰)2 or (%)-1
should be avoided. When listing percents in a table a column heading
such as 100 w for mass fractions in percents is preferred to
w / %.
Unit symbols should never be modified, as for instance: w/w %, vol.
%, ppmv, atom %, mol. %, ... . All % are equal in the same sense as
all metres are equal irrespective of whether we express heights, depths,
lengths, widths, diameters, ... .
We have to name the quantity we measure:
These are not concentrations in different types of percents, but
different types of fractions expressed by the same unit: % = 0.01
= 1/100 = 10-2.
Abbreviations such as ppb, ppt, ... are geography dependent (a billion
is 1012 in Europe and 109 in America) and should
be avoided or at least their meaning should be defined.
See also: T. Cvitas, Quantities describing compositions of mixtures,
Metrologia 33 (1996) 35-39. >