📚 OxfordAQA 9620/CH05 Experiment: Mastering the Titration Technique | 牛津AQA 9620/CH05 实验：精通滴定操作技巧
The OxfordAQA International A‑level Chemistry 9620/CH05/WRE paper from June 2023 is a written examination that assesses students’ ability to design, interpret and evaluate experimental procedures. One of the most frequently examined practical skills in this component is the acid–base titration, including the use of a pH meter to produce a titration curve and the subsequent handling of quantitative data.
2023年6月的牛津AQA国际A‑level化学9620/CH05/WRE试卷是一份书面考试，重点评估学生设计、解释与评价实验步骤的能力。该试卷中最常出现的实验技能之一就是酸碱滴定，包括运用pH计绘制滴定曲线以及对定量数据的处理。
1. Specification Context for 9620/CH05 | 9620/CH05的考纲背景
Paper CH05 is the written response to experimental work. It evaluates AO3 skills: planning, implementing, analysing results and drawing conclusions. In the June 2023 series, many questions were anchored around a core acid–base titration, challenging learners to identify errors, suggest improvements and perform structured calculations using concordant titre volumes.
试卷CH05是对实验工作的书面回应，考核AO3能力：计划、实施、分析结果并得出结论。在2023年6月的考试中，许多题目都围绕一个核心的酸碱滴定实验展开，要求学生识别误差、提出改进方案并使用一致滴定体积进行条理分明的计算。
2. Objective of the Core Practical | 核心实验目标
The experiment aims to determine the exact concentration of a sodium hydroxide (NaOH) solution by titrating it against a standardised hydrochloric acid (HCl) solution. An optional extension uses a pH meter to monitor the pH changes during the titration and plot the neutralisation curve.
该实验的目标是用已标定的盐酸(HCl)溶液滴定氢氧化钠(NaOH)溶液，从而测定氢氧化钠的准确浓度。选做延伸部分使用pH计监测滴定过程中的pH变化，并绘制中和曲线。
3. Principal Equipment and Chemicals | 主要仪器与试剂
You will need a 50 cm³ burette (class B is acceptable for school labs), a 25 cm³ volumetric pipette, a 250 cm³ conical flask, a white tile, a wash bottle with distilled water, a funnel, a pH meter with standard buffer solutions (pH 4, 7 and 10), and a magnetic stirrer if available. Chemicals include 0.1 mol dm⁻³ HCl (initially approximate), solid anhydrous sodium carbonate (Na₂CO₃) as a primary standard, methyl orange indicator and phenolphthalein indicator, plus the NaOH solution of unknown concentration.
你需要一支50 cm³滴定管（学校实验室用B级即可）、一支25 cm³移液管、一个250 cm³锥形瓶、一块白陶瓷板、装有蒸馏水的洗瓶、漏斗、用标准缓冲溶液（pH 4、7和10）校准的pH计，以及磁力搅拌器（如有）。试剂包括约0.1 mol dm⁻³的盐酸溶液、作为基准物质的无水碳酸钠(Na₂CO₃)固体、甲基橙指示剂和酚酞指示剂，以及未知浓度的NaOH溶液。
4. Standardising the Hydrochloric Acid | 标定盐酸溶液
The HCl solution must be standardised because commercial concentrated HCl is not a primary standard. Weigh accurately about 0.15–0.20 g of anhydrous Na₂CO₃ on a balance with 0.001 g resolution. Dissolve it in deionised water, transfer to a conical flask and add a few drops of methyl orange. Titrate with the HCl until the colour changes from yellow to a permanent orange‑pink. This reaction uses two moles of HCl per mole of Na₂CO₃, so the concentration of HCl is calculated from the mass of Na₂CO₃ and the titre volume.
盐酸溶液必须标定，因为市售浓盐酸不是基准试剂。用精度0.001 g的天平准确称取0.15–0.20 g无水碳酸钠，溶于去离子水中，转移至锥形瓶并加入几滴甲基橙。用盐酸滴定至指示剂由黄色变为永久的橙色‑粉红色。该反应中每摩尔碳酸钠消耗两摩尔HCl，因此根据碳酸钠质量和滴定体积即可计算盐酸的准确浓度。
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂
5. Preparing Burette and Pipette | 滴定管与移液管的准备
Rinse the burette first with distilled water and then with a small volume of the standardised HCl solution. Ensure the tap is closed and use a funnel to fill the burette; remove the funnel before titrating. Open the tap briefly to fill the tip completely, expelling all air bubbles. Record the initial reading to the nearest 0.05 cm³. For the pipette, rinse it with the NaOH solution after washing with distilled water, then use a pipette filler to deliver exactly 25.0 cm³ of NaOH into a clean conical flask.
先用水再用少量已标定的HCl溶液润洗滴定管。确保活塞关闭，用漏斗装液，滴定前取下漏斗。短暂开启活塞，使管尖完全充满液体并排出所有气泡。记录初始读数，精确至0.05 cm³。移液管经水洗后，用NaOH溶液润洗，然后使用吸耳球准确移取25.0 cm³ NaOH转移至洁净锥形瓶中。
6. Titration Procedure with Indicator | 使用指示剂的滴定步骤
Add 2–3 drops of phenolphthalein indicator to the NaOH aliquot; it will turn pink. Place the conical flask on a white tile and swirl continuously while adding HCl from the burette. As the endpoint approaches, pink swirls will decolourise more slowly, so add the acid drop‑wise. Stop when the pink colour just disappears and remains colourless for 30 seconds. Record the final burette reading. Repeat the titration until you obtain two concordant titres differing by no more than 0.10 cm³.
向NaOH待测液中加入2–3滴酚酞指示剂，溶液呈粉红色。将锥形瓶置于白瓷砖上，边摇动边从滴定管中加入HCl。临近终点时，粉红色褪去变慢，需逐滴加入酸。当粉红色恰好消失且30秒内不变色时停止，记录终读数。重复滴定，直至获得两次相差不超过0.10 cm³的一致滴定体积。
7. Using a pH Meter to Build the Titration Curve | 使用pH计构建滴定曲线
Instead of an indicator, a calibrated pH meter can be immersed in the NaOH solution during titration. Record the pH after each 1.0 cm³ addition of HCl initially, then after 0.2 cm³ increments near the equivalence region (pH 3.5–10.5). The steepest part of the curve gives the equivalence volume. Plot pH (y‑axis) against volume of HCl added (x‑axis). This technique is often evaluated in CH05 to test understanding of buffer regions and the choice of indicator.
可不使用指示剂，而是将校准后的pH计电极插入NaOH溶液中，在滴定过程中记录pH。起始阶段每加入1.0 cm³ HCl记录一次pH，在突跃范围附近（pH 3.5–10.5）以0.2 cm³为增量记录。曲线斜率最大处即为等当点体积。以加入的HCl体积为横轴、pH为纵轴作图。CH05常考查这一操作，以检验对缓冲区域和指示剂选择的理解。
8. Data Recording and Concentration Calculation | 数据记录与浓度计算
Prepare a results table showing the initial and final burette readings, the titre volume for each trial, and the concordant tick. Use only the concordant titres to calculate the mean titre. Apply the stoichiometric ratio: 1 mol HCl reacts with 1 mol NaOH. Therefore, the unknown NaOH concentration, c(NaOH), is found using c(NaOH) = (c(HCl) × mean titre) ÷ 25.0, where all volumes are in dm³. Express the final answer to an appropriate number of significant figures, typically 3.
准备一个结果表格，显示每次滴定的初读数、终读数、滴定体积及是否一致。仅使用一致滴定值计算平均滴定体积。运用化学计量比：1 mol HCl与1 mol NaOH反应。因此，未知NaOH浓度c(NaOH) = (c(HCl) × 平均滴定体积) ÷ 25.0，所有体积需换算成dm³。最终答案通常保留三位有效数字。
n(H⁺) = n(OH⁻) ⇒ c₁V₁ = c₂V₂
n(H⁺) = n(OH⁻) ⇒ c₁V₁ = c₂V₂
9. Identifying Sources of Error | 识别误差来源
Systematic errors include using a pipette without temperature calibration, an incorrectly standardised acid, or a parallax error from reading the meniscus above eye level. Random errors arise from the difficulty of judging the exact colour change, variations in swirling, or slight overshooting of the endpoint. Both types of error are frequently tested in the CH05 written paper through data‑response questions.
系统误差包括使用未经温度校准的移液管、标定不正确的酸液，或视线高于弯月面造成的读数视差。随机误差来源于判断确切颜色变化的困难、摇动方式差异或滴定终点的轻微过量。这两类误差经常通过数据响应题在CH05书面考试中进行考查。
10. Evaluating Reliability and Improving Accuracy | 评估可靠性并提高准确度
Reliability is assessed by the spread of concordant titres. To improve accuracy, always rinse glassware with the solution it will contain, avoid splashing, re‑weigh the primary standard carefully, and perform a blank titration if necessary. Using a pH meter reduces indicator‑related uncertainties and allows the identification of the endpoint even in deeply coloured or turbid solutions.
可靠性通过一致滴定体积的偏差来评估。为提高准确度，务必将玻璃器皿用即将盛装的溶液润洗、避免飞溅、仔细重称基准试剂，并在必要时进行空白滴定。使用pH计能降低与指示剂相关的不确定度，甚至能在深色或浑浊溶液中准确判定终点。
11. Common Mistakes in School Laboratories | 学校实验室常见错误
Students often forget to remove the funnel after filling the burette, leaving drops that increase the titre. Air bubbles trapped below the burette tap cause an overestimation of the volume delivered. Adding too much indicator can alter the pH at the endpoint. Not swirling the flask adequately delays colour mixing, leading to premature endpoint detection.
学生们常忘记在装液后取下漏斗，致使液滴落入滴定管而增大滴定体积。活塞下方残留的气泡会导致流出体积偏大。加入过量指示剂会改变终点pH。摇动不充分则延缓颜色混合，造成过早判定终点。
12. Summary and Exam Tips for CH05 Success | 总结与CH05高分技巧
The OxfordAQA 9620/CH05 paper values clarity in method description, critical evaluation of data, and the ability to link procedure flaws to their numerical impact. Practice constructing a results table with consistent headings and units, and learn to justify why concordant titres are needed. When drawing a titration curve, label the equivalence point, buffer region and explain why phenolphthalein is a suitable indicator for a strong acid–strong base reaction.
牛津AQA 9620/CH05考试重视实验方法的清晰描述、数据的批判性评价，以及将操作瑕疵与数值影响联系起来的能力。练习设计带一致标题和单位的结果表格，并学会解释为何需要一致滴定体积。绘制滴定曲线时，务必标出等当点、缓冲区域，并阐述为何酚酞是强酸‑强碱反应的适宜指示剂。
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