Guide for the Use of the International System of Units (SI)

8.5 Temperature interval and temperature difference 
 
As discussed in Sec. 4.2.1.1, Celsius temperature (t) is defined in terms of thermodynamic 
temperature (T) by the equation t = T − T
0
, where T
0
= 273.15 K by definition. This implies that the 
numerical value of a given temperature interval or temperature difference whose value is expressed in the 
unit degree Celsius (ºC) is equal to the numerical value of the same interval or difference when its value is 
expressed in the unit kelvin (K); or in the notation of Sec. 7.1, note 2, {Δt }ºC = {
ΔT}
K
. Thus temperature 
intervals or temperature differences may be expressed in either the degree Celsius or the kelvin using the 
same numerical value. 
Example: The difference in temperature between the freezing point of gallium and the triple point of 
water is Δt = 29.7546 ºC = ΔT = 29.7546 K. 

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Guide for the Use of the International System of Units (SI) 
 
25 
8.6 Amount of substance, concentration, molality, and the like 
 
The following section discusses amount of substance, and the subsequent nine sections, which are 
based on Ref. [6: ISO 31-8] and which are succinctly summarized in Table 12, discuss quantities that are 
quotients involving amount of substance, volume, or mass. In the table and its associated sections, symbols 
for substances are shown as subscripts, for example, x
B
, n
B
, b
B
. However, it is generally preferable to place 
symbols for substances and their states in parentheses immediately after the quantity symbol, for example 
n(H
2
SO
4
). (For a detailed discussion of the use of the SI in physical chemistry, see the book cited in Ref. 
[6], note 3.)