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Let us describe how JIS (Japan Industrial Standard) standards deine pH.

Excerpt from JIS Z 8802-2006

Meaning of pH

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.

Notes: A particular solution, such as a buffer solution with less than 0.1 mol/l    with pH ranging from 3 to 10 is assumed to be as shown in formula(1)

Definition of the pH Scale

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 shows values for 2.3026 RT/F in mV at each temperature.

Table 1--Values of 2.3026RT/F

Temp(.C)

2.3026RT/F mV

Temp(.C)

2.3026RT/F mV

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℃

JIS and other standards and regulations relating to pH, such as JIS Z8802, will also be described on the pages dealing with Regulations and pH meters.