Definition of pH

This discovery definitely marked a great step forward. However, since activity cannot be determined by actual direct measurement, it cannot be used as the basis for theoretical calculation of pH value. Accordingly, actual pH is determined by comparison with the pH of a specific solution, a standard solution that tends to maintain its pH value. Methods for measuring pH are defined by ISO and JIS.

JIS (Japanese Industrial Standards) Regulations

When pH measurement became commonplace, differences between values measured for the same sample emerged as a problem. As a result, it became necessary to give a clear definition of pH and establish a method for selecting standard solutions. Hence, it became a pressing requirement that an effort be made to establish JIS regulations for pH measurement methods as soon as possible. Through various types of research and study, and with the cooperation of various types of organizations, JIS for pH measurement was established in March, 1957.
When drafting the JIS, the authors also referred to the standards for pH measurement in the United States, United Kingdom, and France. This is because the unit of pH is used internationally as well as in Japan. If individual countries created independent definitions, these would be unacceptable as international standards and would cause problems for academic and commercial activity.

For more detailed information on pH, please see the references below.
The following page gives an unmodified excerpt of part of “JIS Z 8802: Methods for determination of pH of aqueous solutions”. The JIS were established for industrial use. In order to gain a further, more precise understanding of pH, we recommend you to read - or reread - them through.
Kameyama: “Denki-kagaku no Riron oyobi Ouyou—Joukan I (Theory and Applications of Electrochemistry, First Volume I),” Maruzen Co., Ltd., 1963
Yoshimura, Matsushita, Morimoto: “pH no Riron to Sokutei-hou (Theory of pH and its Methods of Measurement),” Maruzen Co., Ltd., 1968
Kishimoto, Matsushita: “pH Sokutei to Jidouseigyo (pH Measurement and Automatic Control),” Nikkan Kogyo Shimbun, Ltd., 1968
Bates: “Determination of pH,” John Wiley & Sons, 1964
Ives, Janz: “Reference Electrodes,” Academic Press, 1961
“JIS Z 8802: Methods for determination of pH of aqueous solutions,” Japanese Standards Association
“JIS Z 8805: Glass electrodes for measurement of pH,” Japanese Standards Associatio

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℃

Principles of the Glass-Electrode Method

In the glass-electrode method, the known pH of a reference solution is determined by using two electrodes, a glass electrode and a reference electrode, and measuring the voltage (difference in potential) generated between the two electrodes. The difference in pH between solutions inside and outside the thin glass membrane creates electromotive force in proportion to this difference in pH. This thin membrane is called the electrode membrane. Normally, when the temperature of the solution is 30 ℃, if the pH inside is different from that of outside by 1, it will create approximately 60 mV of electromotive force.

The liquid inside the glass electrode usually has a pH of 7. Thus, if one measures the electromotive force generated at the electrode membrane, the pH of the sample can be found by calculation.
A second electrode is necessary when measuring the electromotive force generated at the electrode membrane of a glass electrode. This other electrode, paired with the glass electrode, is called the reference electrode. The reference electrode must have extremely stable potential. Therefore, it is provided with a pinhole or a ceramic material at the liquid junction.

In other words, a glass electrode is devised to generate accurate electromotive force due to the difference in pH. And a reference electrode is devised not to cause electromotive force due to a difference in pH

Detector (Glass Electrode)

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Glass Electrode

A glass electrode consists of an electrode membrane that responds to pH, a highly isolating base material to support the unit, solution inside the glass electrode, an internal electrode, a lead wire, and a glass electrode terminal.
The most critical item in this system is the electrode membrane. First, the membrane glass must generate a potential that accurately corresponds to the pH of the solution. Second, even though it must be accurately sensitive to acidity and alkalinity, it must not be damaged by them. Third, the electric resistance of the membrane itself must not be too large. Fourth, too large a difference in potential (asymmetric difference in potential) must not be generated between the solutions inside and outside the electrode when the electrode is immersed in a solution of identical pH to that of the solution inside of the electrode. Another requirement is that the glass membrane be resistant to shock and chemical reactions.

Generally, silver chloride is used as the material for the internal electrode. Potassium chloride solution maintained at pH 7 is usually used as the internal solution.

Principle figure of glass electrode method

Birth and History of the Glass Electrode

In 1906, Cremer blew the tip of a glass tube into a bubble and measured the difference in potential between two kinds of solutions (0.6% NaCl + diluted H₂SO₄ and O.6% NaCl + diluted NaOH). This is considered the birth of the glass electrode. In 1909, Habert and Klemensiewicz measured the difference in potential between a silver chloride electrode and a mercurous chloride electrode, and found that they could obtain a titration curve similar to that of a hydrogen electrode. They called this a glass electrode. So, the glass electrode took its first step toward becoming a practical pH electrode. However, early glass electrodes had large electrical resistance and very thin glass membranes. Therefore, they were very fragile and difficult to handle.
Later, with the introduction of glass containing lithium, which is chemically strong and has low electric resistance and with development of technology for fabricating electronic parts and insulation materials, the glass electrode made rapid progress after the Second World War. Now it is widely used as the standard for measuring pH.

In Japan, Professor Tatsuzo Okada of Kyoto University launched a study on lithium glass electrodes right after the end of the war. Also, studies on reference electrodes and amplifiers were carried out by people in various fields. HORIBA Radio Laboratory (the predecessor of HORIBA, Ltd.) introduced and integrated these technologies and developed the first glass-electrode pH meter in Japan in 1950.