In this standard, pH means a value determined based on the definition of the pH scale. It does not have any strict physical and chemical meaning.



When representing the pH values of two solutions, with solution X and solution S at the same temperature, for pH (X) and pH (S), the difference between those pH values is defined by formula (2)



where Ex is the electromotive force of a battery with a glass electrode and reference electrode placed in solution X, and Es is the electromotive force of a battery with a glass electrode and reference electrode placed in solution S.
R: gas constant of 8.3144J/.C¥ mol
T: absolute temperature of t.C+ 273.15
F: Faraday constant of 96495 C/g-equiv.
In formula (2), the same units of measure must be used in the denominator and numerator.


Table 1--Values of 2.3026RT/F
Temp(.C)
2.3026RT^FmV
Temp(.C)
2.3026RT^FmV
0
54.19
50
64.11
5
55.19
55
65.11
10
56.18
60
66.10
15
57.17
65
67.09
20
58.16
70
68.08
25
59.15
75
69.07
30
60.15
80
70.07
35
61.14
85
71.06
40
62.13
90
72.05
45
63.12
95
73.04


The definition represented by formula (2) means that the pH of all solutions are measured in reference to the known pH of a reference solution. That solution is 0.05 mol/l phthalate solution. the pH of which is 4.000 at 15.C.