📚 A-Level Chemistry Insert 3 June 2022: Practical Operations | A-Level 化学：2022年6月 插入材料3 实验操作
Insert 3 in the June 2022 A-level Chemistry Paper 3 presents a detailed experimental procedure for determining the concentration of sodium hypochlorite (NaClO) in household bleach by redox titration. This technique, based on the reaction of hypochlorite ions with iodide and subsequent titration of iodine with standardised thiosulfate, tests essential practical skills required at A-level. This article unpacks every operational step, calculates the concentration from the provided data, and highlights common errors and safety measures, helping students master both the theoretical and hands-on aspects of the insert experiment.
2022年6月 A-Level 化学试卷3 的插入材料3 给出了一种通过氧化还原滴定测定家用漂白剂中次氯酸钠（NaClO）浓度的详细实验操作。这种方法利用次氯酸根离子与碘离子反应，再用已标定的硫代硫酸钠滴定生成的碘，全面考查了 A-level 阶段必需的操作技能。本文拆解每一个实验步骤，根据提供的数据计算浓度，并强调常见误差和安全措施，帮助学生既掌握该插入实验的理论，又扎实应对实际操作考试。
1. Overview of the Insert Experiment | 实验概况
The insert describes a two‑stage redox titration. In the first stage, an acidified bleach sample is treated with excess potassium iodide, producing iodine according to ClO⁻ + 2I⁻ + 2H⁺ → Cl⁻ + I₂ + H₂O. The liberated iodine is then titrated against a standardised sodium thiosulfate solution using the reaction I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. The procedure provides exact masses, volumes and burette readings that students must use to calculate the concentration of the original bleach.
插入材料描述了一种两步氧化还原滴定。第一步，酸化的漂白剂样品与过量碘化钾反应，生成碘：ClO⁻ + 2I⁻ + 2H⁺ → Cl⁻ + I₂ + H₂O。释放出的碘随即用已标定的硫代硫酸钠标准溶液滴定，反应为 I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻。该步骤给出了精确的质量、体积和滴定管读数，学生需要用这些数据计算原始漂白剂的浓度。
2. Preparation of Standard Sodium Thiosulfate Solution | 标准硫代硫酸钠溶液的配制
The insert instructs to dissolve approximately 6.2 g of sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O) in distilled water and make up to 250 cm³ in a volumetric flask. Because thiosulfate is not a primary standard, its exact concentration is determined by titrating against a known amount of potassium iodate(V) under acidic conditions. The balanced equations for the standardisation are provided, ensuring students understand the need for a secondary standard.
插入材料要求称取约 6.2 g 五水合硫代硫酸钠（Na₂S₂O₃·5H₂O），用蒸馏水溶解并在 250 cm³ 容量瓶中定容。由于硫代硫酸钠不是基准物质，其准确浓度需通过在酸性条件下与已知量的碘酸钾(V) 滴定来确定。材料给出了标定反应的配平方程式，确保学生理解使用第二基准的必要性。
3. Pipetting the Bleach Sample | 移取漂白剂样品
A 25.0 cm³ pipette is used to transfer the diluted bleach solution into a conical flask. The insert emphasises the use of a pipette filler, rinsing the pipette first with distilled water and then with the diluted bleach. Students should note that the bottom of the meniscus must align with the calibration mark when the pipette is held vertically. This ensures that the volume delivered is exactly 25.0 cm³.
使用 25.0 cm³ 移液管将稀释漂白剂溶液转移至锥形瓶中。插入材料强调要使用洗耳球，并先用蒸馏水润洗移液管，再用稀释漂白剂润洗。学生应注意，当移液管竖直放置时，弯月面底部必须与刻度线齐平，这样才能保证转移的体积恰好为 25.0 cm³。
4. Acidification and Addition of Potassium Iodide | 酸化与加入碘化钾
The procedure states that 10 cm³ of 1 mol dm⁻³ sulfuric acid is added to the flask, followed by about 1 g of solid potassium iodide. The solution instantly turns brown as iodine is formed. The flask must be swirled gently and immediately covered with a watch glass to minimise the loss of volatile iodine. The insert reminds students to work efficiently at this stage because iodine can escape or react with oxygen in the air.
步骤要求在锥形瓶中加入 10 cm³ 1 mol dm⁻³ 硫酸，然后加入约 1 g 固体碘化钾。溶液立即变成棕色，表明有碘生成。必须轻轻摇动锥形瓶并立即盖上表面皿，以减少挥发性碘的损失。插入材料提醒学生这一阶段操作要迅速，因为碘会逸散或被空气中氧气氧化。
5. Titration Procedure and Starch Indicator | 滴定操作与淀粉指示剂
With a magnetic stirrer or careful swirling, the iodine solution is titrated with the standardised thiosulfate until the brown colour fades to pale yellow. At this point, a few drops of freshly prepared starch solution are added; the solution turns deep blue‑black. The titration is then continued dropwise until the blue colour just disappears, leaving a colourless solution. The insert emphasises that starch must be added only when the iodine concentration is low, otherwise an irreversible blue‑starch‑iodine complex forms, which would give a diffuse end point.
在磁力搅拌或小心摇动下，用已标定的硫代硫酸钠溶液滴定碘液，直至棕色退至浅黄。此时加入几滴新配制的淀粉溶液，溶液变为深蓝黑色。然后继续逐滴滴定，直到蓝色恰好消失，溶液变为无色。插入材料强调，淀粉只能在碘浓度较低时加入，否则会生成不可逆的淀粉-碘络合物，导致终点拖尾不敏锐。
6. Recording Accurate Burette Readings | 记录准确的滴定管读数
The insert provides a table of burette readings for the titration. Students must record initial and final readings to the nearest 0.05 cm³. The table below is similar to that in the insert, showing concordant titres. Concordant results are those within 0.10 cm³ of each other, and only these should be used to calculate the mean volume.
插入材料给出了滴定管读数表格。学生需要记录初始和最终读数，精确到 0.05 cm³。下表与插入材料中类似，展示了平行结果。平行结果要求彼此相差在 0.10 cm³ 以内，只有这些数据才能用于计算平均体积。
| Titration | Initial reading / cm³ | Final reading / cm³ | Titre / cm³ |
|---|---|---|---|
| 1 | 0.00 | 24.20 | 24.20 |
| 2 | 0.10 | 24.25 | 24.15 |
| 3 | 0.20 | 24.30 | 24.10 |
| 4 | 0.15 | 24.30 | 24.15 |
Titres 2, 3 and 4 are concordant; titre 1 is discarded. The mean titre is (24.15 + 24.10 + 24.15) / 3 = 24.13 cm³.
第 2、3、4 次滴定结果平行，第 1 次舍去。平均滴定体积为 (24.15 + 24.10 + 24.15) / 3 = 24.13 cm³。
7. Calculating the Concentration of Hypochlorite | 计算次氯酸根浓度
The insert supplies the concentration of the standardised thiosulfate, e.g. 0.100 mol dm⁻³. Using the stoichiometry of the two reactions, 2 mol S₂O₃²⁻ react with 1 mol I₂ which was produced from 1 mol ClO⁻. Therefore, the amount of ClO⁻ in the 25.0 cm³ sample is half the amount of S₂O₃²⁻. The step‑by‑step calculation is
插入材料提供了已标定硫代硫酸钠的浓度，如 0.100 mol dm⁻³。由两个反应的化学计量关系可知，2 mol S₂O₃²⁻ 与 1 mol I₂ 反应，而 1 mol I₂ 由 1 mol ClO⁻ 产生。因此，25.0 cm³ 样品中 ClO⁻ 的物质的量是 S₂O₃²⁻ 物质的量的一半。逐步计算如下：
n(S₂O₃²⁻) = c × V = 0.100 mol dm⁻³ × 0.02413 dm³ = 2.413 × 10⁻³ mol
n(ClO⁻) in 25.0 cm³ = ½ × 2.413 × 10⁻³ = 1.207 × 10⁻³ mol
c(ClO⁻) in diluted bleach = (1.207 × 10⁻³ mol) / 0.0250 dm³ = 0.0483 mol dm⁻³
If the original bleach was diluted by a factor of 10, the concentration of NaClO in the undiluted product is 0.483 mol dm⁻³. Converted to mass concentration: (0.483 mol dm⁻³) × (74.44 g mol⁻¹) = 36.0 g dm⁻³, which can be expressed as 3.60% w/v.
若原始漂白剂被稀释了 10 倍，则未稀释产品中 NaClO 的浓度为 0.483 mol dm⁻³。换算成质量浓度为 (0.483 mol dm⁻³) × (74.44 g mol⁻¹) = 36.0 g dm⁻³，可表示为 3.60% w/v。
8. Identifying Sources of Uncertainty | 识别不确定度来源
The insert requires students to evaluate the percentage uncertainty from measuring instruments. A typical 25.0 cm³ pipette has a tolerance of ±0.06 cm³; the burette has ±0.05 cm³ per reading. The total burette uncertainty for a titre is ±0.10 cm³ (two readings). The overall uncertainty can be expressed as a percentage and compared with the experimental error. Students should also consider the uncertainty in the balance used for weighing KI and the possible incomplete reaction if the mixture is not swirled effectively.
插入材料要求学生评估测量仪器带来的百分不确定度。典型的 25.0 cm³ 移液管允差为 ±0.06 cm³；滴定管每次读数的允差为 ±0.05 cm³，一次滴定需要两次读数，因此滴定管的不确定度为 ±0.10 cm³。总不确定度可用百分数表示，并与实验误差进行对比。学生还应考虑称量 KI 所用天平的不确定度，以及若混合不充分可能导致反应不完全。
9. Safety Considerations and Waste Disposal | 安全注意事项与废液处理
The insert reminds candidates that sulfuric acid is corrosive and potassium iodide is harmful if swallowed. Iodine vapour is toxic and irritating to the respiratory system, so the titration must be performed in a well‑ventilated laboratory. Eye protection must be worn at all times. After the experiment, the waste mixture contains unreacted iodide and thiosulfate; it should be neutralised with sodium hydrogencarbonate and flushed with excess water before disposal, according to local safety regulations.
插入材料提醒考生硫酸具有腐蚀性，碘化钾吞食有害。碘蒸气有毒且刺激呼吸系统，滴定必须在通风良好的实验室中进行。全程必须佩戴护目镜。实验结束后，废液中含有未反应的碘化物和硫代硫酸盐，应先用碳酸氢钠中和，再用大量水冲稀，按当地安全规定弃置。
10. Common Pitfalls and How to Avoid Them | 常见问题及避免方法
One frequent mistake is adding starch too early, which traps iodine and leads to a sluggish, indistinct end point. Another is forgetting to rinse the pipette with the bleach solution, causing dilution and a low titre. Burette taps must be checked for leaks before use, and the jet must be filled with no air bubbles. Finally, students sometimes misread the burette by not aligning their eye with the meniscus; using a white card behind the burette improves accuracy.
常见的错误之一是过早加入淀粉，这会导致碘被包裹，终点迟钝且不清晰。另一个错误是忘记用漂白剂溶液润洗移液管，造成样品稀释而使滴定体积偏低。使用前必须检查滴定管活塞是否漏水，且喷头需充满溶液、无气泡。最后，学生有时因眼睛未与弯月面齐平而读错滴定管刻度，在滴定管背衬一张白卡可以提高读数准确性。
11. Evaluating the Method and Improvement | 方法评价与改进
Although this redox titration gives precise results, the procedure could be improved by using a balance with a higher resolution for weighing KI, or by performing the titration under an inert atmosphere to prevent oxidation of iodide by air. The insert may ask students to suggest an alternative method, such as colorimetry, to determine the concentration of iodine. However, titration remains the standard practical skill assessed because it directly tests volumetric technique and understanding of stoichiometry.
该氧化还原滴定虽然能给出精确的结果，但可通过使用更高分辨率的天平称量 KI 来改进，或在惰性气氛下滴定以防止空气中的氧气氧化碘离子。插入材料可能会要求学生提出替代方法，例如利用比色法测定碘的浓度。然而，滴定依然是考察的核心操作技能，因为它直接检验了容量分析技术和学生对化学计量关系的理解。
12. Conclusion and Exam Tips | 结论与应试技巧
Insert 3 of June 2022 is a classic example of how A-level Chemistry integrates practical technique with data handling. To excel, candidates should be able to write balanced equations, identify the stoichiometric mole ratio, select concordant titres, and calculate both the concentration and the overall measurement uncertainty. Under time pressure, it is vital to follow the procedure exactly, note the colour changes at the correct stages, and present calculations in a logical sequence with the correct units. Mastering the operations in this insert provides a strong foundation for tackling any titration‑based practical question in the A-level examination.
2022年6月插入材料3 是 A-level 化学将实践操作与数据处理相结合的经典案例。要想取得好成绩，考生必须能够写出配平的化学方程式、确定化学计量摩尔比、选择平行滴定的数据，并计算浓度及总体测量不确定度。在时间紧迫的考试中，严格遵循步骤、在正确阶段记录颜色变化、以逻辑顺序呈现带单位的计算至关重要。掌握这份插入材料中的各项操作，能够为应对 A-level 考试中任何基于滴定的实验题打下坚实基础。
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