酸碱指示剂Acid–base indicators
酸碱指示剂是本身为弱酸或弱碱的有机染料，其酸式和碱式颜色不同。溶液 pH 在变色范围内时呈现过渡色。滴定时应选择变色范围落在滴定突跃（等当点附近 pH 急剧变化区间）之内的指示剂。
An acid–base indicator is an organic dye that is itself a weak acid or base, with different colours in its acid and base forms. Within its transition range the solution shows an intermediate colour. For a titration, choose an indicator whose range lies within the steep part of the curve around the equivalence point.
变色范围Transition ranges
01234567891011121314
百里酚蓝（第一变色）thymol blue (first change)
1.22.8
溴酚蓝bromophenol blue
34.6
甲基橙methyl orange
3.14.4
溴甲酚绿bromocresol green
3.85.4
甲基红methyl red
4.46.2
石蕊litmus
58
溴百里酚蓝bromothymol blue
67.6
酚红phenol red
6.88.4
百里酚蓝（第二变色）thymol blue (second change)
89.6
酚酞phenolphthalein
8.210
百里酚酞thymolphthalein
9.310.5
横轴为 pH；色带左侧为酸式颜色，右侧为碱式颜色，中间渐变段为变色范围。The axis is pH; each band shows the acid colour on the left, the base colour on the right and the transition range between.
数据Data
| 指示剂 Indicator | 变色范围 pH range | 酸式色 Acid colour | 碱式色 Base colour |
|---|---|---|---|
| 百里酚蓝（第一变色）thymol blue (first change) | 1.2 – 2.8 | 红red | 黄yellow |
| 溴酚蓝bromophenol blue | 3 – 4.6 | 黄yellow | 蓝紫blue-violet |
| 甲基橙methyl orange | 3.1 – 4.4 | 红red | 黄yellow |
| 溴甲酚绿bromocresol green | 3.8 – 5.4 | 黄yellow | 蓝blue |
| 甲基红methyl red | 4.4 – 6.2 | 红red | 黄yellow |
| 石蕊litmus | 5 – 8 | 红red | 蓝blue |
| 溴百里酚蓝bromothymol blue | 6 – 7.6 | 黄yellow | 蓝blue |
| 酚红phenol red | 6.8 – 8.4 | 黄yellow | 红red |
| 百里酚蓝（第二变色）thymol blue (second change) | 8 – 9.6 | 黄yellow | 蓝blue |
| 酚酞phenolphthalein | 8.2 – 10 | 无色colourless | 粉红pink |
| 百里酚酞thymolphthalein | 9.3 – 10.5 | 无色colourless | 蓝blue |
变色范围为常见文献值，不同资料可能相差 0.1–0.2 个 pH 单位。Ranges are typical literature values and may differ by 0.1–0.2 pH units between sources.
滴定中指示剂的选择Choosing an indicator for a titration
| 滴定类型 Titration | 等当点 pH pH at equivalence | 适用指示剂 Suitable indicator |
|---|---|---|
| 强酸 – 强碱strong acid – strong base | 约 7about 7 | 甲基橙或酚酞均可（0.1 mol dm⁻³ 时突跃约为 pH 4–10）methyl orange or phenolphthalein (at 0.1 mol dm⁻³ the steep part spans about pH 4–10) |
| 弱酸 – 强碱weak acid – strong base | 大于 7above 7 | 酚酞phenolphthalein |
| 强酸 – 弱碱strong acid – weak base | 小于 7below 7 | 甲基橙methyl orange |
| 弱酸 – 弱碱weak acid – weak base | 取决于 Ka 与 Kbdepends on Ka and Kb | 无明显突跃，一般不用指示剂，改用 pH 计no sharp change; use a pH meter instead of an indicator |