Hydrogen electrode

Japanese: 水素電極 - すいそでんきょく（英語表記）hydrogen electrode

A type of reference electrode, it is made by immersing the lower half of a platinum electrode in an acid of a certain concentration and exposing the upper half to hydrogen gas. Usually, a liquid containing hydrogen ions is placed in a glass container, and a platinum plate coated with platinum black (black platinum powder) is half-inserted into it, and hydrogen gas is pumped from the bottom of the container so that it bubbles into the liquid. If the hydrogen pressure is P _H2 at this time, the hydrogen dissociates and is adsorbed onto the platinum, and these adsorbed hydrogen atoms quickly reach equilibrium with the hydrogen ions in the liquid (activity a _H ⁺ ) due to the catalytic action of the platinum. In other words,

At 25°C, the potential E is

When P _H2 = 1 (atmosphere) and a _H ⁺ = 1, it is called a standard hydrogen electrode, and its potential E , or E _0, is guaranteed to be 0 volts regardless of temperature.

[Toda Genjiro]

Hydrogen electrode structure
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Hydrogen electrode structure

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

基準電極の一つで、白金電極の下半分を一定濃度の酸に浸し、上半分を水素の気体に接触させてつくったもの。普通、ガラス容器に水素イオンを含む液を入れ、この中によく白金黒（白金の黒色粉末）をつけた白金板を半分挿入し、容器の下部から水素ガスを送って液中に泡として出てくるようにする。このとき水素の圧力をP_H2とすると、水素は解離して白金上に吸着し、この吸着水素原子は液中の水素イオン（活動度a_H⁺）と白金の触媒作用により速やかに平衡となる。つまり、

の平衡が成り立ち、25℃でその電位Eは

となる。P_H2＝1(気圧),a_H⁺＝1としたときを標準水素電極standard hydrogen electrodeといい、その電位EすなわちE₀は温度に無関係に0ボルトと約束する。