📚 Neutralisation Reactions: Experimental Procedures | 中和反应实验操作
Neutralisation reactions between an acid and a base are central to many analytical and industrial processes. In the school laboratory, careful experimental design allows us to determine concentrations, measure enthalpy changes, and observe indicator colour shifts. This article provides a detailed, step‑by‑step guide to the most common neutralisation experiments, including titration, pH monitoring, and calorimetry, with an emphasis on accurate technique and safety.
酸碱中和反应是许多分析与工业过程的核心。在学校实验室中，精心的实验设计使我们能够测定浓度、测量焓变并观察指示剂的颜色变化。本文详细介绍了最常见的中和实验——包括滴定、pH 监测和量热法——并强调了精准的操作技术与安全要求。
1. Safety Precautions | 安全注意事项
Always wear safety goggles and a laboratory coat. Acids such as hydrochloric acid, sulfuric acid, and bases like sodium hydroxide are corrosive. Handle all solutions with care, and in the event of skin contact, rinse immediately with plenty of water. Use a fume cupboard when working with concentrated reagents or volatile species.
务必佩戴护目镜和实验服。盐酸、硫酸等酸以及氢氧化钠等碱具有腐蚀性。小心处理所有溶液，若皮肤接触，立即用大量清水冲洗。使用浓试剂或挥发性物质时应在通风橱内操作。
Ensure that glassware, especially burettes and pipettes, is free from cracks. Clamp apparatus securely and keep the work area tidy. When heating solutions, use a water bath rather than a direct flame if flammable indicators are present.
确保玻璃仪器，尤其是滴定管和移液管，没有裂纹。将装置牢固夹紧，保持工作区域整洁。加热溶液时，如果存在易燃指示剂，应使用水浴而非明火。
2. Equipment and Materials | 仪器与材料
A typical neutralisation titration requires a 50 cm³ burette (class B accuracy or better), a 25 cm³ bulb pipette and pipette filler, a conical flask, a white tile, a wash bottle with distilled water, and a suitable indicator. For enthalpy measurements, a polystyrene cup, a lid, and a sensitive thermometer (0.1 °C divisions) are essential.
典型的中和滴定需要一支 50 cm³ 滴定管（B 级或更高精度）、一支 25 cm³ 球型移液管及洗耳球、一个锥形瓶、一块白色瓷砖、装有蒸馏水的洗瓶以及合适的指示剂。测量焓变时，必须使用聚苯乙烯杯、杯盖和分度为 0.1 °C 的灵敏温度计。
Common reagents include standard sodium hydroxide solution (e.g. 0.100 mol dm⁻³), hydrochloric acid of unknown concentration, phenolphthalein solution, and methyl orange. A pH meter or data logger with a pH probe can extend the investigation to titration curves.
常用试剂包括标准氢氧化钠溶液（如 0.100 mol dm⁻³）、未知浓度的盐酸、酚酞溶液和甲基橙溶液。pH 计或配有 pH 探头的数字采集器可将实验扩展为滴定曲线探究。
3. Choosing an Indicator | 选择合适的指示剂
The choice of indicator depends on the strength of the acid and base involved. Phenolphthalein, which changes from colourless to pink over pH 8.2–10.0, is ideal for strong acid–strong base and weak acid–strong base titrations. Methyl orange, changing from red to yellow between pH 3.1–4.4, suits strong acid–weak base combinations.
指示剂的选择取决于所涉及的酸和碱的强度。酚酞在 pH 8.2–10.0 范围内从无色变为粉红色，适用于强酸–强碱以及弱酸–强碱滴定。甲基橙在 pH 3.1–4.4 之间从红色变为黄色，适合强酸–弱碱的组合。
For the neutralisation of hydrochloric acid with sodium hydroxide, either indicator is acceptable because the equivalence point lies near pH 7, but phenolphthalein gives a sharper endpoint for student titrations. Avoid using universal indicator for quantitative work as it changes colour gradually over a wide pH range.
对于盐酸与氢氧化钠的中和，两种指示剂均可接受，因为等当点接近 pH 7，但在学生滴定中酚酞能给出更敏锐的终点。避免使用通用指示剂进行定量分析，因为它在较宽的 pH 范围内颜色逐渐变化。
4. Titration Procedure | 滴定操作步骤
Rinse the burette first with distilled water, then with a small portion of the standard sodium hydroxide solution. Fill the burette, ensuring the tip contains no air bubble, and record the initial reading to the nearest 0.05 cm³. Rinse the pipette with the acid solution, then transfer exactly 25.0 cm³ of the acid into a clean conical flask.
先用蒸馏水、再用少量标准氢氧化钠溶液润洗滴定管。装液时确保管尖无气泡，并记录初始读数至 0.05 cm³。用酸溶液润洗移液管后，准确移取 25.0 cm³ 酸液至洁净的锥形瓶中。
Add two or three drops of phenolphthalein to the flask and place it on a white tile. Swirl the flask continuously while adding alkali from the burette. When a faint pink colour persists for 30 seconds, the endpoint has been reached. Record the final burette reading.
向锥形瓶中加入两到三滴酚酞，并将其置于白色瓷砖上。不断摇动锥形瓶，同时从滴定管滴加碱液。当淡粉红色能持续 30 秒不褪去时，表示达到终点。记录滴定管最终读数。
Repeat the titration until three concordant results (volumes within 0.10 cm³ of each other) are obtained. Calculate the mean titre and discard any rough or anomalous values.
重复滴定，直至获得三次吻合的结果（各次体积相差不超过 0.10 cm³）。计算平均滴定体积，并舍弃粗略值或异常值。
5. Monitoring pH Changes | pH变化的监测
Instead of a visual indicator, a pH meter connected to a data logger can follow the neutralisation in real time. Add 25.0 cm³ of hydrochloric acid to a beaker and place the pH probe into the solution while gently stirring with a magnetic stirrer.
可使用连接至数据采集器的 pH 计代替目视指示剂，实时跟踪中和过程。取 25.0 cm³ 盐酸于烧杯中，将 pH 探头插入溶液，同时用磁力搅拌器轻轻搅拌。
Record the initial pH, then add sodium hydroxide solution in small increments. Near the neutralisation point, add 0.5 cm³ portions to capture the sharp pH jump. Record the volume and pH after each addition. Plotting pH against volume produces a classic sigmoid titration curve.
记录初始 pH，然后少量多次地加入氢氧化钠溶液。接近中和点时，每次加入 0.5 cm³，以捕捉 pH 的突跃。每次添加后记录体积和 pH 值。以 pH 对体积作图，可以得到经典的 S 形滴定曲线。
From the curve, identify the equivalence volume at the point of steepest slope. This method is especially valuable for polyprotic acids or buffers where multiple equivalence points appear.
从曲线上斜率最陡处确定等当点体积。此方法对于多元酸或缓冲体系尤为有用，因为这些体系中会出现多个等当点。
6. Temperature Measurement: Enthalpy of Neutralisation | 温度测量：中和焓变
The enthalpy change of neutralisation (ΔHneut) can be determined by mixing known volumes of acid and base in a polystyrene cup calorimeter. Measure 50.0 cm³ of 1.0 mol dm⁻³ HCl into the cup and record its temperature every 30 s for 3 minutes.
中和焓变（ΔHneut）可通过在聚苯乙烯杯量热器中混合已知体积的酸和碱来测定。量取 50.0 cm³ 1.0 mol dm⁻³ 盐酸于杯中，每 30 秒记录一次温度，持续 3 分钟。
At the fourth minute, quickly add 50.0 cm³ of 1.0 mol dm⁻³ NaOH (at room temperature), stir gently, and continue recording the temperature every 30 s for a further 10 minutes. Plot temperature against time, extrapolate the cooling curve, and find the maximum temperature rise ΔT.
在第 4 分钟时，迅速加入 50.0 cm³ 1.0 mol dm⁻³ 的 NaOH（室温），轻轻搅拌，并在接下来的 10 分钟内继续每 30 秒记录温度。绘制温度–时间曲线，外推冷却线，求出最大温升 ΔT。
ar enthalpy change per mole of water formed is ΔH = −q / n. Typical values for strong acid–strong base neutralisation are −57 kJ mol⁻¹.
计算放出的热量 q = mcΔT，其中 m 为溶液总质量（假设密度为 1.00 g cm⁻³），c 为比热容（4.18 J g⁻¹ K⁻¹）。每生成 1 mol 水的摩尔焓变为 ΔH = −q / n。强酸与强碱中和的典型值为 −57 kJ mol⁻¹。
7. Data Recording and Analysis | 数据记录与分析
Maintain a well‑organised results table. For a titration, columns should include rough trial, accurate titres 1, 2, and 3, initial and final burette readings, and the volume added. Concordant titres should be highlighted and the mean calculated to the appropriate number of significant figures.
保持清晰有序的结果表格。滴定表格应包含粗略试验、第 1、2、3 次精确滴定、滴定管初始与最终读数以及加入的体积。应标明吻合的滴定结果，并以适当的有效数字计算平均值。
For pH and temperature experiments, data tables should list time, volume of base added, temperature, and pH. Graphs should have clear axes with units, a descriptive title, and a smooth curve or line of best fit. Use a spreadsheet to calculate gradients and intercepts for accurate determination of equivalence points.
对于 pH 和温度实验，数据表应列出时间、加入的碱体积、温度和 pH。图表应有清晰的轴和单位、描述性标题以及平滑曲线或最佳拟合线。使用电子表格计算斜率和截距，以准确确定等当点。
8. Calculations of Concentration | 浓度计算
The balanced equation for the neutralisation of HCl with NaOH is:
HCl + NaOH → NaCl + H₂O
盐酸与氢氧化钠中和的化学方程式为：
HCl + NaOH → NaCl + H₂O
. Hence, the concentration of HCl = (2.45 × 10⁻³ mol / 25.0 cm³) × 1000 = 0.0980 mol dm⁻³.
物质的量之比为 1:1。若 0.100 mol dm⁻³ NaOH 的平均滴定体积为 24.50 cm³，则所用 NaOH 的物质的量为 (24.50 / 1000) × 0.100 = 2.45 × 10⁻³ mol。25.0 cm³ 试样中 HCl 的物质的量与之相同。因此，HCl 浓度 = (2.45 × 10⁻³ mol / 25.0 cm³) × 1000 = 0.0980 mol dm⁻³。
Always express the final answer with the appropriate units and to the correct precision based on the measuring instruments. Include a step for averaging multiple determinations and comment on the consistency of your results.
最终答案应始终注明适当单位，并根据测量仪器给出正确的精密度。要包含多次测定的平均步骤，并评价结果的一致性。
9. Sources of Error and Improvements | 误差来源与改进措施
Common sources of error in titration include parallax when reading the burette meniscus, incomplete mixing, and overshooting the endpoint. Use a burette card and ensure your eye is level with the meniscus. Practise swirling and add alkali drop‑by‑drop near the endpoint to minimise overshoot.
滴定中常见的误差来源包括读取滴定管弯液面时的视差、混合不充分以及超过终点。可使用滴定卡，并确保视线与弯液面持平。多加练习，临近终点时逐滴加入碱液，以减少过终点。
For calorimetry, heat loss to the surroundings is the principal error. Improve insulation by using a lid with holes for the thermometer and stirrer, and extrapolating the cooling curve. An electronic thermometer or data logger gives more reliable temperature readings than a standard mercury thermometer.
在量热法中，热量散失到周围环境是主要误差。可通过使用带孔盖子（用于插温度计和搅拌器）并外推冷却曲线来改善保温。电子温度计或数据采集器比普通水银温度计提供更可靠的温度读数。
| Error / 误差 | Improvement / 改进 |
|---|---|
| Bubbles in burette tip / 滴定管尖气泡 | Rapidly open tap to expel air before final filling / 装液前快速打开旋塞排尽空气 |
| Diluted sample by rinsing flask with water / 用水润洗锥形瓶稀释了样品 | Rinse conical flask with distilled water only; the number of moles remains unchanged / 锥形瓶只可用蒸馏水润洗，物质的量不变 |
10. Advanced Experiment: Conductometric Titration | 进阶实验：电导滴定
A conductometric titration uses the change in electrical conductivity during neutralisation to locate the endpoint. As dilute ammonia is added to ethanoic acid, the initial high conductance of free ions first drops, then rises as excess base is added. The intersection of the two linear segments gives the equivalence volume.
电导滴定利用中和过程中电导率的变化来确定终点。将稀氨水加入乙酸时，初始自由离子的高电导率先下降，然后随着过量碱的加入而上升。两条线性线段的交点即为等当点体积。
This method requires a conductivity probe and a magnetic stirrer. Record the conductance after each 0.5 cm³ addition of base, then plot conductance versus volume. It is particularly useful for titrating very weak acids or bases where colour indicators give no sharp endpoint.
该方法需要电导探头和磁力搅拌器。每次加入 0.5 cm³ 碱后记录电导值，然后绘制电导–体积曲线。对于颜色指示剂无法给出敏锐终点的极弱酸或极弱碱的滴定，此法特别有用。
11. Real‑World Applications | 实际应用
Neutralisation reactions are used to treat acid‑indigestion with antacid tablets containing magnesium hydroxide, to control soil pH in agriculture with lime (calcium carbonate), and to manage acidic effluents from factories before release into waterways.
中和反应的应用包括：用含氢氧化镁的抗酸片缓解胃酸过多，用石灰（碳酸钙）调节农业土壤的 pH，以及处理工厂的酸性废水，使其在排入水体前达到安全标准。
Understanding the experimental procedures behind these titrations equips students with the skills to monitor and control neutralisation processes in environmental science, pharmacy, and food technology.
掌握这些滴定背后的实验操作，使学生具备在环境科学、制药和食品技术中监测和控制中和过程的能力。
12. Conclusion and Exam Tips | 结论与考试提示
e repeat titrations until concordancy is achieved, and always check your calculations for mole ratios and unit conversions.
掌握中和实验需要细致的操作、仔细的记录，并理解其本质离子方程式：H⁺(aq) + OH⁻(aq) → H₂O(l)。反复练习滴定，直至获得吻合结果，并始终核验计算中的摩尔比例与单位换算。
When tackling exam questions, sketch the apparatus with labels, suggest realistic errors, and explain the choice of indicator with reference to the pH range at the equivalence point. Calorimetry questions demand accurate extrapolation and an appreciation of heat loss corrections.
解答考试题目时，应绘制并标注仪器图，提出切合实际的误差，并根据等当点的 pH 范围说明指示剂的选择。量热类题目要求准确外推并理解热量损失的校正方法。
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