📚 A-Level Chemistry Jun 18 Insert 5 Practical: Redox Titration for Iron(II) Determination | A-Level 化学 2018年6月 Insert 5 实验：氧化还原滴定法测定铁(II)
This article unpacks the practical task from the AQA A-level Chemistry June 2018 Paper 3 Insert (widely known as Insert 5), which centres on the redox titration of ammonium iron(II) sulfate with potassium manganate(VII). Mastering this experiment builds essential skills in preparing standard solutions, performing titrations, recording concordant results, and using stoichiometric calculations to determine the percentage by mass of iron in a sample.
本文详解 AQA A-level 化学 2018年6月试卷3插入页（常称作 Insert 5）中的实验任务，核心内容是用高锰酸钾氧化还原滴定法测定硫酸亚铁铵中的铁含量。掌握该实验可以夯实配制标准溶液、进行滴定、记录平行结果以及运用化学计量关系计算样品铁质量分数等关键技能。
1. Understanding the Task: Iron Determination | 理解任务：铁的测定
The objective of the Insert 5 practical is to determine the percentage by mass of iron in ammonium iron(II) sulfate, (NH₄)₂SO₄·FeSO₄·6H₂O. You will weigh out about 5 g of the solid, dissolve it, prepare a 250 cm³ solution, acidify an aliquot, and titrate it with standard potassium manganate(VII). The manganate(VII) ions are reduced by Fe²⁺ under acidic conditions, allowing the concentration of iron(II) to be found from the titre.
Insert 5 实验的目标是测定硫酸亚铁铵 (NH₄)₂SO₄·FeSO₄·6H₂O 中铁的质量分数。你需要称取约 5 g 固体，溶解并配制成 250 cm³ 溶液，取出一部分加酸酸化后，用标准高锰酸钾溶液滴定。在酸性条件下，MnO₄⁻ 被 Fe²⁺ 还原，通过滴定体积可以推算出铁(II)的浓度。
2. Principle of Redox Titration | 氧化还原滴定原理
The reaction is an oxidation-reduction process. Iron(II) ions are oxidised to iron(III), while manganate(VII) ions are reduced to manganese(II). The balanced equation shows a 5:1 mole ratio between Fe²⁺ and MnO₄⁻.
该反应为氧化还原过程。铁(II)离子被氧化成铁(III)，高锰酸根离子被还原成二价锰离子。配平后的方程式显示 Fe²⁺ 与 MnO₄⁻ 的物质的量之比为 5:1。
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
Manganate(VII) acts as its own indicator: the end point is marked by the first permanent pale pink colour that persists on swirling. No other indicator is required.
高锰酸钾自身可作指示剂：终点为摇匀后第一次出现且不褪色的浅粉红色。不需要额外加入指示剂。
3. Preparing the Standard Solution of Ammonium Iron(II) Sulfate | 配制硫酸亚铁铵标准溶液
Weigh approximately 5 g of ammonium iron(II) sulfate accurately in a clean, dry beaker. Record the mass to at least two decimal places. Dissolve the solid in about 100 cm³ of distilled water, stirring with a glass rod. Transfer the solution into a 250 cm³ volumetric flask using a funnel, rinse the beaker and rod several times, and add the washings to the flask. Make up to the graduation mark with distilled water, stopper the flask, and invert it repeatedly to ensure thorough mixing.
用干净干燥的烧杯准确称取约 5 g 硫酸亚铁铵，质量至少记录至小数点后两位。将固体溶于约 100 cm³ 蒸馏水中，用玻璃棒搅拌。用漏斗将溶液转移至 250 cm³ 容量瓶中，多次洗涤烧杯和玻璃棒，洗涤液一并转入。加蒸馏水定容至刻度线，塞好瓶塞，反复倒转容量瓶以确保溶液均匀。
- Weighing by difference minimises transfer losses.
- 差量称量法可减少转移损失。
- Ensure all solid is dissolved before transfer.
- 确保所有固体完全溶解后再转移。
4. Acidification and Its Importance | 酸化操作及其重要性
Pipette 25.0 cm³ of the iron(II) solution into a clean conical flask. Add approximately 25 cm³ of 1 mol dm⁻³ sulfuric acid. The acid provides the H⁺ ions required for the reduction of MnO₄⁻ and prevents the formation of a brown precipitate of manganese(IV) oxide, which would obscure the end point. Hydrochloric acid must not be used because chloride ions would be oxidised by manganate(VII), giving a false titre.
用移液管量取 25.0 cm³ 铁(II)溶液放入干净的锥形瓶。加入约 25 cm³ 1 mol dm⁻³ 硫酸。酸提供 MnO₄⁻ 还原所需的 H⁺，同时避免生成棕色的二氧化锰沉淀遮盖终点。不可使用盐酸，因为 Cl⁻ 会被高锰酸钾氧化，导致滴定体积偏高。
5. Titration Setup and Technique | 滴定装置与技巧
Fill a burette with the standard 0.0200 mol dm⁻³ potassium manganate(VII) solution. Run a small volume through the jet to remove air bubbles. Record the initial burette reading to the nearest 0.05 cm³. Place the conical flask on a white tile to help detect the colour change. Swirl the flask continuously while adding the titrant slowly until a persistent faint pink appears. The purple colour of MnO₄⁻ decolorises rapidly as it reacts with Fe²⁺; near the end point, add the solution dropwise.
用 0.0200 mol dm⁻³ 标准高锰酸钾溶液润洗并装满滴定管，排去管尖气泡。记录初读数，估读至 0.05 cm³。将锥形瓶置于白色瓷砖上以便观察颜色变化。边滴定边持续摇动锥形瓶，直到出现持久的淡粉色。紫色 MnO₄⁻ 遇 Fe²⁺ 迅速褪色；接近终点时应逐滴加入。
6. Performing the Titration and Recording Data | 进行滴定和数据记录
Carry out a rough titration first to gauge the approximate titre. Then perform a series of accurate titrations, repeating until at least two concordant results (within 0.10 cm³ of each other) are obtained. Discard any outliers and calculate the mean titre from the concordant values. Record all readings in a clearly labelled results table showing initial burette reading, final reading, and titre volume for each trial.
先进行初滴定，估算大致用量。然后进行多次精确滴定，直至获得至少两次平行数据（彼此相差不超过 0.10 cm³）。舍去异常值，取平行结果的平均值。将所有读数记录在设计清晰的表格中，包括每次的初读数、末读数和滴定体积。
Example Table Format
| Trial | Initial / cm³ | Final / cm³ | Titre / cm³ |
|---|---|---|---|
| Rough | 0.00 | 24.10 | 24.10 |
| 1 | 0.00 | 23.95 | 23.95 |
| 2 | 0.00 | 23.90 | 23.90 |
Mean concordant titre = (23.95 + 23.90) / 2 = 23.93 cm³ (to 2 d.p.).
7. Calculating the Percentage of Iron | 计算铁的百分含量
Use the titration result and the balanced equation to find the mass of iron in the sample.
利用滴定结果和配平方程式计算样品中铁的质量。
Step 1: Moles of MnO₄⁻ = c × V = 0.0200 mol dm⁻³ × (23.93 / 1000) dm³ = 4.786 × 10⁻⁴ mol.
第一步：MnO₄⁻ 的物质的量 = 浓度 × 体积 = 0.0200 mol dm⁻³ × (23.93/1000) dm³ = 4.786 × 10⁻⁴ mol。
Step 2: From the 5:1 ratio, moles of Fe²⁺ in the 25.0 cm³ aliquot = 5 × 4.786 × 10⁻⁴ = 2.393 × 10⁻³ mol.
第二步：依据 5:1 比，25.0 cm³ 取用液中 Fe²⁺ 的物质的量 = 5 × 4.786 × 10⁻⁴ = 2.393 × 10⁻³ mol。
Step 3: Moles of Fe²⁺ in the original 250 cm³ solution = 2.393 × 10⁻³ × (250 / 25.0) = 2.393 × 10⁻² mol.
第三步：原始 250 cm³ 溶液中 Fe²⁺ 的物质的量 = 2.393 × 10⁻³ × (250/25.0) = 2.393 × 10⁻² mol。
Step 4: Mass of iron = moles × Mᵣ(Fe) = 2.393 × 10⁻² × 55.8 g = 1.335 g.
第四步：铁的质量 = 物质的量 × 铁的摩尔质量 = 2.393 × 10⁻² × 55.8 g = 1.335 g。
Step 5: Percentage by mass of iron = (mass of Fe / mass of sample) × 100 = (1.335 / 5.12) × 100 ≈ 26.1% (assuming 5.12 g weighed).
第五步：铁的质量分数 = (铁的质量 / 样品质量) × 100 = (1.335 / 5.12) × 100 ≈ 26.1%（假设称样量为 5.12 g）。
8. Common Errors and Accuracy | 常见误差与准确度
- Failure to rinse the burette with manganate(VII) solution leads to a diluted titrant and a higher titre.
- 未用高锰酸钾溶液润洗滴定管会导致滴定剂被稀释，滴定体积偏高。
- Air bubbles in the burette jet give an overestimate of the volume delivered.
- 滴定管尖存在气泡会使测得的加入体积偏大。
- Incomplete dissolution or transfer losses reduce the amount of Fe²⁺ and lower the final percentage.
- 溶解不充分或转移损失减少 Fe²⁺ 的量，导致结果偏低。
- Using hydrochloric acid instead of sulfuric acid introduces extra reducing species, producing a false titre.
- 用盐酸代替硫酸会引入还原性物质，产生虚假滴定值。
- The end point is overrun if titrant is added too quickly near the finish; a light pink should persist for about 30 seconds.
- 接近终点时滴定过快易导致过量；浅粉色应维持约30秒才算终点。
9. Safety Considerations | 安全注意事项
Potassium manganate(VII) is a strong oxidising agent and can irritate the skin and eyes; wear safety goggles and a lab coat. Sulfuric acid is corrosive; handle with care and wash any spills immediately. Glassware should be handled carefully to avoid breakage. Dispose of reaction mixtures as directed by local regulations, as manganese residues should not be poured down the sink untreated.
高锰酸钾是强氧化剂，会刺激皮肤和眼睛，需佩戴护目镜和实验服。硫酸具有腐蚀性，操作应谨慎，溅到皮肤上立即冲洗。玻璃仪器轻拿轻放，避免破损。废液按当地规定处理，含锰残余物不可直接倒入下水道。
10. Evaluation and Improvements | 评估与改进
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