📚 Case Study: Determining Aspirin Purity – A Practical Exercise for Year 12 SQA Chemistry | SQA 12年级化学案例分析实战演练：阿司匹林纯度测定
This case study walks you through a common practical investigation in Scottish Higher Chemistry: assessing the purity of over‑the‑counter aspirin tablets using back titration and thin‑layer chromatography. You will see how theoretical concepts connect to hands‑on analysis and develop skills in data handling, error evaluation, and scientific reasoning – exactly what SQA examiners look for.
本案例分析带你走一遍苏格兰高等化学中常见的实践调查：用返滴定法和薄层色谱法评估市售阿司匹林药片的纯度。你将看到理论概念如何与实际分析相连，并培养数据处理、误差评估和科学推理的技能——这些正是 SQA 考官所看重的。
1. Background: Why Purity Matters | 背景：纯度为何重要
Acetylsalicylic acid (aspirin) is one of the most widely used analgesics. Commercial aspirin tablets must meet strict pharmacopoeial standards for active ingredient content. If the aspirin has partially hydrolysed back to salicylic acid during storage, its potency drops and unwanted side effects may increase. Quantitative analysis of tablet purity is therefore a key quality‑control procedure in the pharmaceutical industry.
乙酰水杨酸（阿司匹林）是使用最广泛的镇痛药之一。市售阿司匹林药片必须满足严格的药典活性成分含量标准。如果阿司匹林在储存过程中部分水解回水杨酸，其药效会下降，不必要的副作用也会增加。因此，对药片纯度进行定量分析是制药行业一项关键的质量控制程序。
2. Aim of the Investigation | 实验目的
The aim is to determine the mass of acetylsalicylic acid in a commercial aspirin tablet and compare it with the manufacturer’s stated value, using an acid–base back titration. A secondary aim is to confirm the presence of any salicylic acid impurity by thin‑layer chromatography (TLC).
实验目的是用酸碱返滴定法测定一片市售阿司匹林中乙酰水杨酸的质量，并与生产商标示值进行比较。次要目的是通过薄层色谱法（TLC）确认是否存在水杨酸杂质。
3. Sample and Reagent Preparation | 样品与试剂准备
One aspirin tablet (labelled 300 mg) was crushed and transferred into a 250 cm³ conical flask. 25.0 cm³ of 1.0 mol dm⁻³ sodium hydroxide solution was added using a volumetric pipette. The mixture was gently heated under reflux for 15 minutes to ensure complete hydrolysis of aspirin to sodium salicylate and sodium ethanoate. After cooling, the solution was quantitatively transferred to a 250 cm³ volumetric flask and made up to the mark with distilled water.
取一片标注 300 mg 的阿司匹林药片碾碎，转移至 250 cm³ 锥形瓶中。用移液管加入 25.0 cm³ 浓度为 1.0 mol dm⁻³ 的氢氧化钠溶液。混合物在回流下温和加热 15 分钟，确保阿司匹林完全水解为水杨酸钠和乙酸钠。冷却后，将溶液定量转移至 250 cm³ 容量瓶中，用蒸馏水定容至刻度。
4. Principle of the Back Titration | 返滴定原理
Aspirin reacts with excess sodium hydroxide in a 1:2 mole ratio: C₉H₈O₄ + 2 NaOH → C₇H₅O₃Na + CH₃COONa + H₂O. The unused NaOH is then titrated with standard hydrochloric acid. The difference between the initial amount of NaOH and the amount left unreacted gives the amount that reacted with aspirin. This indirect method avoids slow reaction kinetics and carbon dioxide interference that would occur with a direct titration of a weak acid.
阿司匹林与过量的氢氧化钠以 1:2 摩尔比反应：C₉H₈O₄ + 2 NaOH → C₇H₅O₃Na + CH₃COONa + H₂O。未反应的 NaOH 随后用标准盐酸滴定。初始 NaOH 的量减去剩余未反应的量，即为与阿司匹林反应了的 NaOH 的量。这种间接方法避免了直接滴定弱酸时反应动力学缓慢和二氧化碳干扰的问题。
5. Titration Procedure | 滴定步骤
25.0 cm³ aliquots of the diluted aspirin hydrolysate were titrated against 0.10 mol dm⁻³ HCl using phenolphthalein indicator. The end point was reached when the pink colour just disappeared. The titration was repeated until three concordant results were obtained (within 0.2 cm³). A blank titration using distilled water instead of sample was also carried out to correct for any alkali consumed by the solvent or glassware.
用移液管量取 25.0 cm³ 稀释后的阿司匹林水解液，以酚酞为指示剂，用 0.10 mol dm⁻³ 盐酸滴定。当粉红色刚好消失时即为终点。重复滴定直到获得三次一致的读数（相差不超过 0.2 cm³）。同时还用蒸馏水代替样品进行空白滴定，以校正溶剂或玻璃器皿消耗的碱量。
| Trial | 1 | 2 | 3 |
|---|---|---|---|
| Final burette reading (cm³) | 19.80 | 37.20 | 18.50 |
| Initial burette reading (cm³) | 0.00 | 17.40 | 0.00 |
| Titre (cm³) | 19.80 | 19.80 | 18.50 |
Blank titre = 0.10 cm³
Concordant results are 19.80, 19.80, so mean titre for sample = 19.80 cm³. Corrected titre = 19.80 – 0.10 = 19.70 cm³.
一致读数为 19.80、19.80，因此样品平均滴定体积 = 19.80 cm³。校正后体积 = 19.80 – 0.10 = 19.70 cm³。
6. Calculating the Mass of Aspirin | 计算阿司匹林质量
Moles of HCl used in titration = c × V = 0.10 × (19.70/1000) = 1.97 × 10⁻³ mol. This equals moles of NaOH left unreacted in the 25.0 cm³ aliquot. Total unreacted NaOH in the 250 cm³ flask = 1.97 × 10⁻³ × 10 = 1.97 × 10⁻² mol. Initial moles of NaOH added = 1.0 × (25.0/1000) = 2.50 × 10⁻² mol. Therefore moles of NaOH that reacted with aspirin = (2.50 × 10⁻²) – (1.97 × 10⁻²) = 5.30 × 10⁻³ mol. From the 1:2 stoichiometry, moles of aspirin = 5.30 × 10⁻³ / 2 = 2.65 × 10⁻³ mol. Mass of aspirin = n × M = 2.65 × 10⁻³ × 180.0 = 0.477 g = 477 mg per tablet.
滴定所用 HCl 物质的量 = c × V = 0.10 × (19.70/1000) = 1.97 × 10⁻³ mol。这等于 25.0 cm³ 分液中剩余 NaOH 的物质的量。250 cm³ 容量瓶中未反应的总 NaOH = 1.97 × 10⁻³ × 10 = 1.97 × 10⁻² mol。初始加入的 NaOH 物质的量 = 1.0 × (25.0/1000) = 2.50 × 10⁻² mol。因此与阿司匹林反应的 NaOH = (2.50 × 10⁻²) – (1.97 × 10⁻²) = 5.30 × 10⁻³ mol。根据 1:2 化学计量比，阿司匹林物质的量 = 5.30 × 10⁻³ / 2 = 2.65 × 10⁻³ mol。阿司匹林质量 = n × M = 2.65 × 10⁻³ × 180.0 = 0.477 g = 477 mg/片。
Percentage purity relative to label claim = (477 / 300) × 100% = 159%. This value greatly exceeds 100%, indicating a systematic error, likely due to incomplete hydrolysis or the presence of other acidic impurities.
相对于标示量的纯度百分比 = (477 / 300) × 100% = 159%。这个数值远超 100%，表明存在系统误差，很可能是因为水解不完全或存在其他酸性杂质。
7. Thin‑Layer Chromatography Check | 薄层色谱检测
A small portion of the crushed tablet was dissolved in ethanol and spotted onto a silica gel TLC plate alongside standard solutions of pure aspirin and salicylic acid. The plate was developed in a mobile phase of ethyl ethanoate/hexane (1:3) containing a few drops of ethanoic acid. After visualisation under UV light, the sample spot gave two distinct spots: one matching the Rf of aspirin (0.56) and a weaker spot matching salicylic acid (Rf 0.72). This confirms the presence of hydrolysed impurity, which would contribute extra acidic protons during titration, inflating the result.
取少量碾碎的药片溶解在乙醇中，与纯阿司匹林和水杨酸标准溶液一同点在硅胶 TLC 板上。流动相为乙酸乙酯/己烷（1:3），加入几滴乙酸。紫外灯下显色后，样品显示出两个清晰的斑点：一个与阿司匹林的 Rf（0.56）相匹配，另一个较弱的点与水杨酸（Rf 0.72）一致。这确认了水解杂质的存在，它会在滴定过程中贡献额外的酸性质子，从而使结果偏高。
8. Identifying Sources of Error | 误差来源分析
Several factors contributed to the overestimation. Incomplete hydrolysis of aspirin would leave unreacted ester groups, but the more significant error is the presence of salicylic acid — a stronger acid than aspirin — which consumes NaOH without representing active aspirin. Additionally, the reflux step might have caused some loss of volatile ethanoic acid if the condenser was not efficient, altering the acid/base balance. The phenolphthalein end point can also be subjective, leading to a slight overtitration of the strongly alkaline solution.
高估结果源于多个因素。阿司匹林水解不完全会残留未反应的酯基，但更重要的是水杨酸杂质的存在——它是一种比阿司匹林更强的酸，消耗 NaOH 但并不代表有效阿司匹林。此外，如果冷凝管效率不高，回流步骤可能造成部分挥发性乙酸损失，改变酸碱平衡。酚酞终点判断也带有主观性，在强碱溶液中可能导致轻微过滴定。
9. Evaluation and Proposed Improvements | 评价与改进建议
To obtain a more accurate purity value, the hydrolysis step should be performed under controlled reflux for a longer period (30 min) with a calibrated condenser. Using a pH meter instead of an indicator would give a sharper end point. It would also be beneficial to quantify the salicylic acid separately by UV‑Vis spectroscopy and subtract its contribution mathematically. Finally, repeating the analysis on a fresh, unexpired batch of tablets can exclude storage degradation.
为获得更准确的纯度值，水解步骤应在校准过的冷凝管下控制回流更长时间（30 分钟）。使用 pH 计代替指示剂可获得更敏锐的终点。另外，用紫外‑可见光谱单独定量水杨酸并从数学上扣除其贡献也十分有益。最后，对新鲜、未过期的药片批次重复分析可排除储存降解因素。
10. Comparison with Pharmacopoeia Standards | 与药典标准对比
The British Pharmacopoeia typically requires aspirin tablets to contain 95.0–105.0% of the stated dose. Had the calculation yielded a value within this range after deducting impurities, the batch would pass. In our case, even after allowing for salicylic acid, the raw result highlights how a single titrimetric method can be misleading when impurities are present. This reinforces the principle that analytical results must be cross‑checked with an independent technique such as chromatography or spectroscopy.
英国药典通常要求阿司匹林片剂的含量为标示量的 95.0–105.0%。如果在扣除杂质后计算值落在此范围内，该批次即为合格。在我们的实例中，即使已考虑到水杨酸，原始结果仍然突出显示了当存在杂质时，单一滴定法可能具有误导性。这印证了分析结果必须通过色谱或光谱等独立技术进行交叉检验的原则。
11. SQA Exam‑Style Questions to Practise | SQA 风格练习题
- Explain why a back titration is preferred over a direct titration for aspirin analysis. (2 marks)
- Calculate the percentage by mass of aspirin in a tablet given titre data, and comment on its purity. (4 marks)
- Suggest why the Rf value of salicylic acid is higher than that of aspirin on a normal‑phase silica plate. (2 marks)
- 解释为何阿司匹林分析中更倾向于使用返滴定而非直接滴定。（2 分）
- 根据滴定数据计算药片中阿司匹林的质量百分比，并对其纯度进行评述。（4 分）
- 说明为什么在正相硅胶板上水杨酸的 Rf 值比阿司匹林高。（2 分）
Practising these will sharpen your ability to link practical details with core chemical concepts under timed conditions.
练习这些题目将增强你在限时条件下将操作细节与核心化学概念联系起来的能力。
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