定义Definition
酸碱指示剂是颜色随溶液 pH 改变的物质，通常本身是弱酸 HIn：HIn ⇌ H⁺ + In⁻，HIn 和 In⁻ 颜色不同。溶液 pH 约在 pKIn ± 1 范围内时，指示剂发生可见的颜色变化。
An acid–base indicator is a substance whose colour depends on pH, usually itself a weak acid HIn: HIn ⇌ H⁺ + In⁻, with HIn and In⁻ differently coloured. The visible colour change happens roughly over pH = pKIn ± 1.
常见指示剂Common indicators
甲基橙：pH 3.1–4.4，红 → 黄。甲基红：4.4–6.2，红 → 黄。溴百里酚蓝：6.0–7.6，黄 → 蓝。酚酞：8.2–10.0，无色 → 粉红。石蕊：约 5–8，红 → 蓝。通用指示剂由几种指示剂混合而成，在较宽 pH 范围内呈现一系列颜色。
Methyl orange: pH 3.1–4.4, red → yellow. Methyl red: 4.4–6.2, red → yellow. Bromothymol blue: 6.0–7.6, yellow → blue. Phenolphthalein: 8.2–10.0, colourless → pink. Litmus: about 5–8, red → blue. Universal indicator mixes several to give a series of colours over a wide pH range.
滴定中的选择Choosing one for a titration
指示剂的变色范围应落在滴定曲线等当点附近的 pH 突跃范围内。强酸–强碱：甲基橙或酚酞均可。强酸–弱碱：突跃在酸性区，用甲基橙。弱酸–强碱：突跃在碱性区，用酚酞。弱酸–弱碱没有明显突跃，不宜用指示剂。
The colour-change range must fall within the steep part of the titration curve at equivalence. Strong acid–strong base: methyl orange or phenolphthalein. Strong acid–weak base: the jump is acidic, so methyl orange. Weak acid–strong base: the jump is alkaline, so phenolphthalein. Weak acid–weak base has no sharp jump and suits no indicator.
年表Timeline
- 1664
玻意耳记录植物色素在酸碱中变色
Boyle records plant pigments changing colour in acids and alkalis
- 1909
索伦森提出 pH 概念
Sørensen introduces the pH scale
- 1894
奥斯特瓦尔德提出指示剂变色的电离理论
Ostwald explains indicator colour changes by ionisation