📚 Standard Electrode Potential Measurement | 标准电极电势的测量方法
In electrochemistry, the standard electrode potential (E°) measures the tendency of a half-cell to gain electrons relative to the standard hydrogen electrode (SHE). It is a fundamental concept in A-Level Chemistry that explains the direction of electron flow, reactivity of metals, and the voltage of electrochemical cells. This article explains how standard electrode potentials are defined and measured, step by step, with attention to the conventions expected in CIE examinations.
在电化学中，标准电极电势（E°）用于衡量某一半电池相对于标准氢电极（SHE）获得电子的倾向。它是A-Level化学中的重要概念，用来解释电子流动方向、金属活泼性以及电化学电池的电压。本文将逐步解释标准电极电势如何定义和测量，并关注CIE考试所要求的规范。
1. What Is Standard Electrode Potential? | 什么是标准电极电势？
A half-cell cannot exist by itself as a complete electrical circuit; its electrical potential can only be measured relative to another electrode. Therefore, chemists define the standard electrode potential of a half-cell as the potential difference measured when that half-cell is connected to the standard hydrogen electrode under standard conditions.
单独的半电池不能构成闭合电路，因此其电位只能相对于另一个电极进行测量。为此，化学家将半电池的标准电极电势定义为：在该半电池与标准氢电极相连，且处于标准条件下时，所测得的电位差。
Standard conditions for electrode potential measurements are usually:
标准电极电势测量的标准条件通常为：
- 298 K (25 °C) | 298 K（25°C）
- 1 mol dm⁻³ for all dissolved ions | 所有溶解离子的浓度为1 mol dm⁻³
- 1 atm (≈100 kPa) for gases | 气体分压为1 atm（约100 kPa）
- Pure solid electrodes | 电极为纯固体
The half-reaction is always written as a reduction, with electrons on the left-hand side. For example:
半反应总是写成还原形式，电子位于左侧。例如：
Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) E° = +0.34 V
2. The Standard Hydrogen Electrode (SHE) | 标准氢电极（SHE）
The standard hydrogen electrode is the universal reference electrode. It is arbitrarily assigned an electrode potential of exactly 0.00 V under standard conditions. This allows all other half-cells to be compared on a common scale.
标准氢电极是通用的参比电极。在标准条件下，它的电极电势被规定为恰好0.00 V。这样，所有其他半电池都可以在同一标度上进行比较。
The SHE consists of a platinum electrode coated with very fine platinum powder, called platinum black. The platinum electrode is immersed in a 1 mol dm⁻³ solution of H⁺, usually hydrochloric acid, and hydrogen gas at 1 atm is bubbled continuously over the platinum surface.
SHE由镀有极细铂粉（即铂黑）的铂电极构成。铂电极浸入1 mol dm⁻³的H⁺溶液中（通常为盐酸），并持续在铂表面通入1 atm的氢气。
The equilibrium that establishes the potential is:
建立电势的平衡为：
2H⁺(aq) + 2e⁻ ⇌ H₂(g)
Platinum is chemically inert, but it acts as a catalyst and provides a surface where H₂ molecules can dissociate and exchange electrons. The platinum black increases the surface area, so the equilibrium is reached quickly.
铂化学性质稳定，但作为催化剂为H₂分子解离和电子交换提供表面。铂黑增大了表面积，使平衡能快速建立。
3. Constructing a Half-Cell | 构建半电池
Two common types of half-cells are used in electrode potential measurements:
电极电势测量中常用两类半电池：
Type 1: Metal / metal-ion half-cell
A strip of pure metal is dipped into a 1 mol dm⁻³ aqueous solution containing its own ions. For example, a zinc electrode is placed in Zn²⁺(aq), or a copper electrode in Cu²⁺(aq).
第一类：金属/金属离子半电池
将纯金属片浸入含有其自身离子的1 mol dm⁻³水溶液中。例如，锌电极浸入Zn²⁺(aq)，铜电极浸入Cu²⁺(aq)。
The metal surface must be cleaned, usually with emery paper, and rinsed with deionised water. Any oxide layer or grease will interfere with electron transfer and cause unstable voltmeter readings.
金属表面必须清洁，通常用砂纸打磨，再用去离子水冲洗。氧化层或油脂会干扰电子转移，导致电压表读数不稳定。
Type 2: Inert electrode / ion half-cell
When the redox couple involves ions only, such as Fe³⁺/Fe²⁺ or Cl₂/Cl⁻, an inert platinum electrode is dipped into a solution containing both the oxidised and reduced forms.
第二类：惰性电极/离子半电池
当氧化还原电对仅涉及离子时，如Fe³⁺/Fe²⁺或Cl₂/Cl⁻，将惰性铂电极浸入同时含有氧化态和还原态物质的溶液中。
The relevant half-reaction for the iron(III)/iron(II) system is:
铁(III)/铁(II)体系的相关半反应为：
Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq)
4. Connecting Half-Cells: The Salt Bridge | 连接半电池：盐桥
To measure an electrode potential, the test half-cell and the standard hydrogen electrode must be connected to form a complete circuit. The two half-cells are connected by a salt bridge, which contains a concentrated solution of an inert electrolyte.
要测量电极电势，必须将待测半电池与标准氢电极连接形成闭合电路。两个半电池通过盐桥相连，盐桥内含有惰性电解质的浓溶液。
Common salt bridge electrolytes include KNO₃ and KCl. The ions in the salt bridge must not react with the half-cell solutions or form precipitates with them.
常用盐桥电解质包括KNO₃和KCl。盐桥中的离子不得与半电池溶液反应，也不能形成沉淀。
KNO₃ is often preferred because both K⁺ and NO₃⁻ are electrochemically inert and do not interfere with most half-cells.
KNO₃常被优先选用，因为K⁺和NO₃⁻在电化学上均为惰性，不会干扰大多数半电池。
The salt bridge serves two essential purposes:
盐桥具有两个关键作用：
- It completes the electrical circuit by allowing ions to migrate between the two half-cells. | 它通过允许离子在两个半电池之间迁移来构成完整电路。
- It prevents a build-up of electrical charge in either half-cell, which would otherwise stop the cell from producing a stable potential. | 它防止任一半电池中电荷积累；否则电荷积累会使电池无法产生稳定电势。
5. Measuring the Potential with a Voltmeter | 用电压表测量电势
The two half-cells are connected through a salt bridge, and a voltmeter is connected between the two electrodes. The voltmeter reading gives the cell potential, from which the standard electrode potential can be obtained.
两个半电池通过盐桥相连，电压表连接在两个电极之间。电压表读数即为电池电动势，由此可得到标准电极电势。
It is important to use a high-resistance voltmeter. Such a voltmeter draws only a very small current, so the concentrations of ions in the half-cells remain effectively unchanged. If a large current were allowed to flow, the half-cell equilibrium would be disturbed and the measured potential would drift.
必须使用高电阻电压表。这种电压表只吸取极微小的电流，因此半电池中离子浓度几乎保持不变。若允许较大电流通过，半电池平衡将被破坏，测量电势会发生漂移。
The sign of the voltmeter reading indicates which electrode is positive. By convention, the cell diagram places the negative electrode on the left and the positive electrode on the right. For example, the cell used to measure the copper electrode potential is written as:
电压表读数的正负表示哪个电极是正极。按照惯例，电池符号将负极放在左侧，正极放在右侧。例如，用于测量铜电极电势的电池写作：
Pt(s) | H₂(g) | H⁺(aq) || Cu²⁺(aq) | Cu(s)
The double vertical line ‘||’ represents the salt bridge, while the single vertical line ‘|’ represents the phase boundary between an electrode and a solution.
双竖线’||’表示盐桥，单竖线’|’表示电极与溶液之间的相界面。
6. Standard Conditions and Conventions | 标准状态与符号约定
For a measured potential to be called a standard electrode potential, every species must be in its standard state:
只有所有物种均处于标准态，测量得到的电势才能称为标准电极电势：
- 1 mol dm⁻³ for all dissolved species | 所有溶解物种浓度为1 mol dm⁻³
- 1 atm (≈100 kPa) for gases | 气体分压为1 atm（约100 kPa）
- Pure solids and liquids | 纯固体和液体
- Temperature 298 K | 温度298 K
Electrode potentials are always quoted as reduction potentials. The half-reaction is therefore always written in the reduction direction, with electrons added to the left-hand side.
电极电势一律以还原电势表示。半反应总是写成还原方向，即电子加在左侧。
For a general electrochemical cell:
对于一般电化学电池：
A(s) | Aⁿ⁺(aq) || Bᵐ⁺(aq) | B(s)
By convention, the standard cell potential is the right-hand electrode potential minus the left-hand electrode potential:
按照惯例，标准电池电动势等于右侧电极电势减去左侧电极电势：
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