What is the electric potential of a reference electrode? Types of reference electrodes include the mercury sulfate electrode and mercury oxide electrode as well as the well-known calomel electrode and silver-silver chloride electrode. How do we convert the electromotive force measured using a certain reference electrode into a value corresponding to use of another reference electrode (for example, ORP measured in combination with a platinum electrode; for details of ORP, please refer to the description given in the relevant separate section)?
To do this, it is necessary to determine the relative value existing between the two reference electrodes on the basis of a certain standard. Such relative values, which are known as the electric potentials of reference electrodes, are determined using a standard hydrogen electrode (abbreviated as S.H.E. or N.H.E.)  as the standard. The electric potential of the S.H.E. is defined as 0 mV at 25℃ by such organizations as the International Union of Pure and Applied Chemistry (IUPAC). An S.H.E. is formed by dipping a platinized electrode into the hydrochloric acid obtained by bubbling saturation of hydrogen gas at 1 atm, giving a mean activity of 1 (approximately 1.2-N HCl).
The following table shows the electric potentials known for the calomel electrode (Hg-Hg2Cl2) and silver-silver chloride electrode (Ag-AgCl), which are well-known reference electrodes. These values were determined in comparison with the electric potential of an S.H.E.

Temperature

Ag/AgCl/KCl 

Hg/Hg2Cl2/KCl 

3.5M-KCl(25℃)

Standard KCl

3.5M-KCl(25℃)

Standard KCl

10

0.215

0.214

0.256

0.254

15

0.212

0.209

0.254

0.251

20

0.208

0.204

0.252

0.248

25

0.205

0.199

0.250

0.244

30

0.201

0.194

0.248

0.241

35

0.197

0.189

0.246

0.238

40

0.193

0.184

0.244

0.234

(All values show volts versus S.H.E.)

D. T. Sawyer, A. Sobkowiak, J. L. Roberts, Jr., “Electrochemistry for Chemists 2nd ed.” p. 192 (1995), John Wiley.
Excerpt from “Bunseki Kagaku Binran (Analytical Chemistry Handbook)” edited by the Japan Society for Analytical Chemistry, p. 723, Maruzen Co., Ltd., 2001

In recent years, the Hg-Hg2Cl2  electrode is rarely used, since the use of mercury and its compounds is avoided; the Ag-AgCl electrode is mainly used instead. The values in the table clearly show that the electric potentials of the electrodes vary depending on temperature and concentration of the KCl solution. For all our reference electrodes, we employ the Ag-AgCl electrode with 3.33M KCl solution, which saturates at 0℃. We do this because 3.33M KCl solution has the advantage of not inducing KCl saturation in any operating temperature range above 0℃ and not exhibiting concentration changes other than those caused by the changes in volume accompanying variations in temperature. That is to say, using this solution minimizes the change in electric potential caused by changes in KCl concentration accompanying temperature variations.

The electric potential of the Ag-AgCl standard hydrogen electrode (S.H.E.) with 3.33M KCl solution is not shown in handbooks etc. Therefore, we have performed our own measurements of the electric potentials of various 3.33M KCL Ag-AgCl electrodes and have obtained the following formula describing their behavior.



The measured values have been published in the form research papers, including the references used in establishing the above formula.

“Free Diffusion Calomel Electrodes”
Shinzo Okada, Tomota Nishi, and Hiroshi Takahashi (Matsushita), Kougyou-kagaku Zasshi (Industrial Chemistry Magazine), Vol. 61, No. 8, pp. 922 to 925, 1958
“Silver-Silver Chloride Electrode prepared using Molten Silver Chloride”
H. Matsushita, H. Maruyama, T. Aomi and N. Ishikawa, Memoirs of Chubu Institute of
Technology, Vol.10-A, 117 to122, 1974

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