📚 A-Level Chemistry Jun 18 Insert 2: Essential Titration Experimental Skills | A-Level 化学 2018年6月插入材料2：核心滴定实验技能
In the A-Level Chemistry June 2018 Insert 2, you are presented with the procedure and data for determining the concentration of ethanoic acid in vinegar using a standard sodium hydroxide solution. This article breaks down every essential experimental technique required to perform the titration accurately, avoid common mistakes, and achieve reliable results for your practical assessment. We cover standard solution preparation, burette and pipette setup, indicator choice, step-by-step titration, data handling, calculations, error analysis, and safety – all tailored to the Jun 18 insert and beyond.
在A-Level化学2018年6月的插入材料2中，给出了用标准氢氧化钠溶液测定食醋中乙酸浓度的实验步骤与数据。本文将拆解每一个关键的实验操作技巧，帮助你准确完成滴定、避开常见错误，获得可靠的实验结果以应对实践评估。内容涵盖标准溶液配制、滴定管与移液管安装、指示剂选择、滴定分步操作、数据处理、浓度计算、误差分析和安全注意事项等，紧扣Jun 18插入材料并拓展通用技能。
1. Overview of the Jun 18 Insert 2 Titration Experiment | Jun 18 Insert 2 滴定实验概述
The June 2018 Insert 2 requires you to carry out an acid-base titration between a standardised solution of NaOH (usually about 0.10 mol dm-3) and a diluted sample of vinegar containing ethanoic acid (CH3COOH). The goal is to find the exact concentration of the acid in the original vinegar. You will use phenolphthalein, a suitable indicator that changes from colourless to pink at the endpoint of this weak acid–strong base titration.
2018年6月的插入材料2要求你进行氢氧化钠标准溶液（通常约0.10 mol dm-3）与含乙酸（CH3COOH）的稀释食醋样品之间的酸碱滴定，目标是求出原始食醋中酸的准确浓度。实验中使用酚酞作为指示剂，在这种弱酸–强碱滴定的终点由无色变为粉红色。
2. Preparing and Standardising the Sodium Hydroxide Solution | 氢氧化钠溶液的配制与标定
Solid NaOH is hygroscopic and absorbs CO2 from the air, so a directly weighed mass cannot give a primary standard. Instead, you prepare an approximately 0.1 mol dm-3 solution and then standardise it using a primary standard like potassium hydrogen phthalate (KHC8H4O4, often called KHP). Accurately weigh about 2.0 g of KHP, dissolve in distilled water, add phenolphthalein, and titrate with your NaOH solution. The exact concentration of NaOH is then calculated from the titration data.
固体NaOH易潮解且吸收空气中的CO2，因此直接称量无法获得基准物质。你需要先配制大约0.1 mol dm-3的溶液，然后用基准物质如邻苯二甲酸氢钾（KHC8H4O4，常称KHP）进行标定。准确称取约2.0 g KHP，用蒸馏水溶解，加入酚酞，再用NaOH溶液滴定。根据滴定数据即可计算出NaOH的精确浓度。
3. Diluting the Vinegar Sample Accurately | 准确稀释食醋样品
The vinegar is too concentrated for direct titration, so you must dilute it quantitatively. Use a volumetric pipette (e.g., 25.0 cm3) to transfer the original vinegar into a 250.0 cm3 volumetric flask, then make up to the mark with distilled water. Stopper, invert and shake thoroughly to obtain a homogeneous diluted solution. Every drop counts – rinse the pipette with the vinegar first, and allow drainage time.
食醋浓度较高，不宜直接滴定，因此需要定量稀释。用移液管（如25.0 cm3）吸取原食醋样品，转移至250.0 cm3容量瓶中，加蒸馏水定容至刻度。塞好瓶塞，上下颠倒充分摇匀，得到均匀的稀释溶液。每一步操作都需精确——移液管要用食醋润洗，并且放液后保持滴落时间。
4. Rinsing and Filling the Burette and Pipette | 润洗和装液：滴定管与移液管
Before use, the burette must be rinsed with the NaOH standard solution (not with water, which would dilute the titrant). Fill the burette using a small funnel, ensuring the tip is free of air bubbles. Open the tap to flush out bubbles. The pipette should be rinsed with the diluted vinegar solution, then filled to just above the mark using a pipette filler – never your mouth. Allow the liquid to drain until the bottom of the meniscus rests exactly on the mark.
使用前，滴定管须用NaOH标准溶液润洗（不可用水，否则会稀释滴定剂）。用小漏斗装液至零刻度以上，确保尖嘴处无气泡，打开活塞冲出气泡。移液管用稀释后的食醋溶液润洗，使用洗耳球吸取液体至略高于刻度线，缓慢放液至弯月面下缘恰好与刻度线相切。绝对禁止用嘴吸液。
5. Choosing the Indicator and Understanding the Endpoint | 选择指示剂并理解终点
Phenolphthalein is ideal for this weak acid–strong base titration because its pH transition range (8.2–10.0) lies entirely within the steep portion of the pH curve. It appears colourless in the acidic vinegar/NaOH mixture and turns a permanent pale pink at the endpoint. Avoid a deep magenta colour, which indicates overtitration. A single drop makes the difference near the endpoint.
酚酞用于该弱酸–强碱滴定非常合适，其变色范围（pH 8.2–10.0）完全落在滴定曲线的陡峭部分。在酸性的食醋与NaOH混合液中呈无色，到达终点时变为持久不褪的淡粉红色。如果变为深紫红色，说明已经过量。终点附近往往一滴之差就决定颜色变化。
6. Step-by-Step Titration Technique | 滴定分步操作技巧
Place a white tile under the conical flask to see the colour change clearly. Run the NaOH solution from the burette into 25.0 cm3 of diluted vinegar in the flask, swirling continuously. Initially, you can add the solution rapidly. When the pink colour begins to persist longer, slow to dropwise addition. Near the endpoint, add only half-drops: open the tap just enough to form a hanging drop, then wash it into the flask with distilled water. Record the final burette reading to the nearest 0.05 cm3.
在锥形瓶下方垫一张白瓷砖，以便清晰观察颜色变化。将25.0 cm3的稀释食醋注入锥形瓶，边摇动边从滴定管加入NaOH溶液。开始时可以稍快，当粉红色褪去变慢时改为逐滴加入。接近终点时只加半滴：轻轻旋转活塞形成悬挂液滴，用洗瓶蒸馏水冲下瓶壁并入溶液中。读取滴定管终点读数，估读至0.05 cm3。
7. Recording and Processing Titration Data | 记录和处理滴定数据
Perform at least two concordant titres – readings that agree within 0.10 cm3. Record initial and final burette readings for each trial in a well-organised table. Calculate the titre volume (final minus initial). Use only concordant results to calculate the mean titre, which will be used in subsequent calculations. An example table from Insert 2 might look like this:
需获得至少两次符合要求的有效滴定——即两次滴定剂体积相差不超过0.10 cm3。将每次试验的初读数和终读数记录在清晰的表格中。计算滴定体积（终读数 – 初读数）。仅用符合要求的滴定值计算平均体积，用于后续计算。插入材料2中的示例数据表可能如下：
| Trial | Initial burette reading (cm3) | Final burette reading (cm3) | Titre (cm3) |
|---|---|---|---|
| 1 | 0.00 | 24.50 | 24.50 |
| 2 | 0.10 | 24.65 | 24.55 |
| 3 | 0.20 | 28.40 | 28.20 (reject) |
Only trials 1 and 2 are concordant; trial 3 is an outlier and should be discarded. The mean titre is (24.50 + 24.55)/2 = 24.53 cm3 (to 2 decimal places).
只有试验1和2符合要求；试验3为异常值应舍去。平均滴定体积为(24.50 + 24.55)/2 = 24.53 cm3（保留两位小数）。
8. Calculating the Concentration of Ethanoic Acid | 计算乙酸浓度
The balanced equation is:
NaOH + CH3COOH → CH3COONa + H2O
Mole ratio NaOH : CH3COOH = 1:1. First, calculate moles of NaOH used: n(NaOH) = c(NaOH) × V(NaOH in dm3). For example, if c(NaOH) = 0.100 mol dm-3 and mean titre = 24.53 cm3 = 0.02453 dm3, then n(NaOH) = 0.100 × 0.02453 = 2.453×10-3 mol. This equals moles of CH3COOH in the 25.0 cm3 aliquot. Therefore, concentration of diluted acid cdil = n / V = 2.453×10-3 / 0.0250 = 0.0981 mol dm-3. Finally, scale back to original vinegar: the dilution factor is 250/25 = 10, so original [CH3COOH] = 0.0981 × 10 = 0.981 mol dm-3. You can also express it in g dm-3 using molar mass of ethanoic acid (60.0 g mol-1): 0.981 × 60.0 = 58.9 g dm-3.
反应方程式为：NaOH + CH3COOH → CH3COONa + H2O。摩尔比1:1。先计算所用NaOH的物质的量：n(NaOH) = c(NaOH) × V(NaOH，以dm3计)。若c(NaOH) = 0.100 mol dm-3，平均滴定体积24.53 cm3 = 0.02453 dm3，则n(NaOH) = 0.100 × 0.02453 = 2.453×10-3 mol。这也就是25.0 cm3等分稀释液中CH3COOH的物质的量。稀释液浓度cdil = n / V = 2.453×10-3 / 0.0250 = 0.0981 mol dm-3。最后折算回原食醋：稀释因子为250/25 = 10，因此原乙酸浓度 = 0.0981 × 10 = 0.981 mol dm-3。也可利用乙酸摩尔质量（60.0 g mol-1）换算为质量浓度：0.981 × 60.0 = 58.9 g dm-3。
9. Common Sources of Error and Their Minimisation | 常见误差来源与减小方法
Systematic errors may arise from using a wrongly calibrated balance, a pipette with a chipped tip, or a burette that leaks. Random errors include misreading the meniscus or inconsistent swirling. Minimise these by calibrating equipment, always reading the bottom of the meniscus at eye level, using a white tile, and rinsing all glassware with the solution it will contain. Air bubbles in the burette tip lead to erroneously high titre readings; always flush them before starting. Over-titration is caught by adding half-drops and washing them in.
系统误差可能源于天平未经校准、移液管尖嘴破损或滴定管漏液。随机误差包括读数时视差、摇动不均匀等。减小误差的措施包括：校准仪器；始终保持视线与弯月面水平，使用白瓷砖便于观察；所有玻璃器皿需用所盛溶液润洗。滴定管尖嘴存在气泡会导致滴定体积读数偏高，必须在开始前清除气泡。过量滴定可通过半滴操作和冲洗瓶壁加以控制。
10. Safety Precautions in This Titration | 本滴定实验安全注意事项
Sodium hydroxide solution (0.1 mol dm-3) is corrosive and can cause skin burns and eye damage. Wear safety goggles and a lab coat at all times. If any spills on skin, wash immediately with plenty of water. Phenolphthalein solution in ethanol is flammable; keep away from flames. Vinegar is dilute ethanoic acid and is considered low hazard, but still handle with care. All glassware should be inspected for cracks before use. Always use a pipette filler, never mouth-pipette.
0.1 mol dm-3的氢氧化钠溶液具有腐蚀性，可灼伤皮肤和损伤眼睛。必须始终佩戴护目镜和实验服。若皮肤接触，立即用大量清水冲洗。酚酞的乙醇溶液易燃，应远离明火。食醋为稀乙酸溶液，虽属低危害物质，但仍需小心操作。所有玻璃器皿使用前必须检查无裂纹。移液时务必使用洗耳球，严禁口吸。
11. Frequently Asked Questions and Misconceptions | 常见问题与误区澄清
“Why can’t I use universal indicator?” Universal indicator changes gradually through many colours, so its endpoint is not sharp. Phenolphthalein gives a sudden colour change at a precise pH, which is essential for a sharp endpoint.
“Can I just use the first titre as my trial?” The first trial is typically a rough titration to find the approximate endpoint. It is usually discarded because you may overshoot. Subsequent careful titrations give concordant results.
“Does the conical flask need to be dry?” No, it can be wet with distilled water because the number of moles of acid in the flask does not change. However, the pipette and burette must be rinsed with the solution they will deliver.
“为什么不能用通用指示剂？”通用指示剂在变色过程中呈现多种颜色过渡，变化不敏锐。酚酞在精确pH处发生突跃，确保获得清晰的终点判断。
“可以直接用第一次滴定值吗？”第一次滴定通常用作粗测，找到大致终点，之后往往舍去，因为容易过量。后续精细滴定才能获得相符的结果。
“锥形瓶需要干燥吗？”不需要，可以用蒸馏水润洗，因为瓶内酸的物质的量并未改变。但移液管和滴定管必须用待装溶液润洗，以免浓度被稀释。
12. Key Takeaways and Exam Tips | 核心要点与考试提示
The Jun 18 Insert 2 experiment epitomises the precision and rigour expected in A-Level practical chemistry. Always remember: rinse correctly, eliminate bubbles, use a white tile, add half-drops near the endpoint, record readings to two decimal places, and only concordant titres count. In exams, you may be asked to identify errors, calculate concentration with appropriate significant figures, or suggest improvements. Consistent practice of these techniques will secure high marks in the practical endorsement and written papers.
Jun 18 Insert 2 的实验浓缩了A-Level化学实验所要求的准确与严谨。请始终牢记：正确润洗、排除气泡、使用白瓷砖、近终点加半滴、读数至小数点后两位、只取符合要求的数据。考试中可能要求你辨别误差、用合适的有效数字进行计算或提出改进方案。反复练习这些技能，将使你在实验考核和笔试中稳取高分。
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