Indicators and Acid-Base Titrations | 酸碱指示剂与酸碱滴定

📚 Indicators and Acid-Base Titrations | 酸碱指示剂与酸碱滴定

Acid-base titrations are a core quantitative technique in A-Level Chemistry. They allow chemists to determine the concentration of an unknown acid or base by reacting it with a standard solution of known concentration. The choice of indicator is crucial because it must change colour exactly at or very near the equivalence point of the reaction.

酸碱滴定是 A-Level 化学中的核心定量技术。通过将未知浓度的酸或碱与已知浓度的标准溶液反应，化学家可以测定其浓度。指示剂的选择至关重要，因为它必须恰好在反应等当点或非常接近等当点时变色。

1. What Is an Acid-Base Titration? | 什么是酸碱滴定？

A titration is a controlled reaction between a solution of known concentration, called the titrant, and a solution of unknown concentration, called the analyte. In acid-base titrations, the reaction is a neutralisation between H₃O⁺ ions from an acid and OH⁻ ions from a base.

滴定是已知浓度溶液（滴定剂）与未知浓度溶液（待测液）之间的受控反应。在酸碱滴定中，反应是酸中的 H₃O⁺ 离子与碱中的 OH⁻ 离子发生的中和反应。

The progress of the titration is usually monitored by measuring the pH of the solution as the titrant is added. The resulting graph of pH against volume of titrant added is called a titration curve or pH curve.

滴定过程中通常通过测量加入滴定剂后溶液的 pH 来监控反应进程。以 pH 对加入滴定剂体积作图得到的曲线称为滴定曲线或 pH 曲线。

2. pH Curves and Key Features | pH 曲线及其关键特征

A pH curve shows how the pH of the analyte changes as the titrant is added. All acid-base pH curves share the same general shape: a gradual change in pH at first, a very sharp change near the equivalence point, and then a levelling off.

pH 曲线显示随着滴定剂的加入，待测液 pH 的变化。所有酸碱 pH 曲线具有相同的大致形状：起初 pH 缓慢变化，在等当点附近出现非常急剧的变化，然后趋于平稳。

The sharp vertical section of the curve is known as the inflection region. It contains the equivalence point, where the amount of acid and base present are exactly stoichiometrically equivalent.

曲线的陡直部分称为拐点区。该区域包含等当点，此时酸和碱的物质的量恰好按化学计量完全反应。

For a strong acid titrated with a strong base, the pH starts very low, remains low for most of the titration, then rises sharply from about pH 3 to pH 11 over a very small volume range.

对于强酸滴定强碱，pH 起始值很低，滴定过程中大部分时间保持低值，然后在很小的体积范围内从约 pH 3 急剧上升到 pH 11。

3. Equivalence Point vs End Point | 等当点与终点

The equivalence point is a theoretical point in the titration when the moles of H₃O⁺ equal the moles of OH⁻. At this point, neutralisation is exactly complete and no excess acid or base remains.

等当点是滴定中的理论点，此时 H₃O⁺ 的物质的量与 OH⁻ 的物质的量相等。在这一点，中和反应恰好完全，没有过量的酸或碱存在。

The end point is the experimental point at which the indicator changes colour. This is the signal that the titration should be stopped. A successful titration requires the end point to be as close as possible to the equivalence point.

终点是指示剂变色的实验点。这是滴定应停止的信号。成功的滴定要求终点尽可能接近等当点。

If the indicator is chosen poorly, the end point will differ significantly from the equivalence point, producing a systematic titration error.

如果指示剂选择不当，终点会明显偏离等当点，从而产生系统滴定误差。

4. How Indicators Work: Acid-Base Equilibria | 指示剂的工作原理：酸碱平衡

An acid-base indicator is a weak acid or weak base whose conjugate acid and conjugate base forms have different colours. The equilibrium can be written as:

酸碱指示剂是一种弱酸或弱碱，其共轭酸和共轭碱形式具有不同的颜色。该平衡可表示为：

HIn + H₂O ⇌ H₃O⁺ + In⁻

Here HIn represents the acid

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