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The absolute electrochemical potential of a single electrode cannot be measured. Instead, potentials are measured relative to a universally accepted reference. The Standard Hydrogen Electrode (SHE) is the primary reference, assigned a potential of exactly 0 volts under standard conditions (1 M [latex]H^+[/latex] concentration, 1 bar [latex]H_2[/latex] gas, 298 K). All other standard electrode potentials are reported relative to this zero point.
无法测量绝对单电极电位的原因是，任何测量都需要完整的电路，这不可避免地会引入第二个电极-电解质界面。测量的始终是两个电极之间的电位差。为了创建一个标准化、可比较的尺度，科学界一致同意了一个共同的参考点。
The Standard Hydrogen Electrode (SHE) serves this purpose. It consists of a platinum electrode, often coated with platinum black to increase its surface area and catalytic activity, immersed in a solution with a hydrogen ion activity of 1. Purified hydrogen gas is bubbled over the electrode at 1 bar pressure. The half-reaction is [latex]2H^+(aq) + 2e^- rightleftharpoons H_2(g)[/latex]. By international convention, the standard potential of this half-reaction is defined as 0.000 V at all temperatures.
To measure the standard potential of another half-cell, for example, a zinc electrode in a zinc sulfate solution ([latex]Zn^{2+}/Zn[/latex]), it is connected to the SHE to form a galvanic cell. The measured voltage of this cell is, by definition, the standard potential of the zinc half-cell. This process has been used to compile extensive tables of standard electrode potentials, which are invaluable for predicting the direction and cell potential of any redox reaction under standard conditions.
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