📚 9620-CH02 Mark Scheme: Essential Experimental Techniques | 9620-CH02评分方案：核心实验操作技巧
Understanding the mark scheme for practical-based questions is key to achieving high marks in the 9620-CH02 unit. This article analyses the experimental techniques assessed in the 2016 mark scheme, with a focus on titrations, standard solution preparation, and precision in measurements. By learning exactly what examiners look for, you can avoid common mistakes and confidently describe correct experimental procedures.
理解基于实验操作的评分方案对于在9620-CH02单元获取高分至关重要。本文以2016年评分方案为蓝本，深入剖析滴定操作、标准溶液配制和测量精度等实验技术中常见的评分点。通过确切掌握考官的关注点，你能避免典型错误，并自信地描述正确的实验步骤。
1. Preparing and Rinsing the Burette | 滴定管的准备与润洗
Before beginning a titration, the burette should first be rinsed with distilled water to remove any contaminants. Immediately after, it must be rinsed with a small volume of the solution it will deliver (the titrant). This ensures that residual water does not dilute the titrant, which would lower its effective concentration and lead to a systematic error in the titre value.
滴定开始前，应先用蒸馏水冲洗滴定管以去除杂质，随后立即用少量待装溶液（滴定剂）润洗。这能确保管内残留的水不稀释滴定剂，否则会降低其有效浓度，导致滴定体积出现系统误差。
While filling the burette, it is essential to remove any air bubbles trapped below the tap or in the jet. Opening the tap and allowing a rapid stream of solution to flow through the tip displaces these bubbles. An air bubble expelled during the titration adds to the apparent titre volume, making it larger than the true value.
在向滴定管注入溶液时，必须排除旋塞下方及尖嘴中的气泡。打开旋塞让溶液快速冲出可以赶走这些气泡。若气泡在滴定过程中被排出，会增大表观滴定体积，使读数大于真实值。
Check that the burette tap does not leak; any leakage during the titration invalidates the readings. After use, thoroughly clean the burette and store it filled with distilled water to prevent deposit formation.
确保滴定管旋塞无渗漏，滴定过程中任何渗漏都会使读数失效。使用后应彻底清洗滴定管，并注满蒸馏水存放以防止沉积物形成。
2. Accurate Pipetting Technique | 准确移液技术
The pipette must be rinsed with distilled water and then with the solution to be measured. Using a pipette that has only been water-rinsed causes dilution of the transferred aliquot, introducing error. Always use a pipette filler (safety bulb) to draw liquid into the pipette; mouth pipetting is strictly forbidden.
移液管必须先用蒸馏水冲洗，再以待量取溶液润洗。仅用水洗过的移液管会使移取的液体被稀释，引入误差。务必使用洗耳球吸取液体，严禁用嘴吸取。
When measuring, the bottom of the meniscus should be aligned exactly with the calibration line while holding the pipette vertically. Release the liquid by touching the pipette tip against the inner wall of the receiving container and allow it to drain; do not blow out the last drop unless the pipette is specifically designed as a 'blow-out' type.
量取时，应竖直持握移液管，使弯月面底部恰好与刻度线齐平。放液时将管尖抵靠接收容器内壁，让液体自然流出；除非该移液管为吹出式设计，否则不得吹出最后一滴液体。
3. Reading the Meniscus and Avoiding Parallax | 读取弯月面与避免视差
All burette readings must be taken from the bottom of the meniscus at eye level. Viewing above or below the meniscus introduces parallax errors, causing the reading to be lower or higher than the true value. Using a white tile behind the burette enhances the visibility of the meniscus for colourless solutions.
所有滴定管读数必须在视线水平高度，以弯月面底部为准。从上方或下方观察会导致视差，使读数偏低或偏高。在滴定管后方放置白色瓷砖（白板）可提高无色溶液弯月面的清晰度。
Readings should be recorded to the nearest 0.05 cm³, which is half of the smallest graduation, and always written with two decimal places, such as 0.00, 23.45, or 25.20 cm³. This level of precision reflects the apparatus sensitivity and is explicitly required on the mark scheme.
读数应记录至最接近的0.05 cm³（最小分度的一半），并始终保留两位小数，如0.00、23.45或25.20 cm³。这一精度体现了仪器灵敏度，也是评分方案中明确要求的书写格式。
4. Selecting a Suitable Indicator | 选择合适指示剂
The choice of indicator depends on the strength of the acid and base involved to ensure the end-point lies within the steepest part of the pH curve. For a strong acid versus strong base titration, either methyl orange (pH 3.1–4.4) or phenolphthalein (pH 8.2–10.0) is acceptable, as the rapid pH change covers both ranges.
指示剂的选择取决于酸和碱的强度，以确保其变色范围落在pH曲线突跃范围内。强酸与强碱滴定时，甲基橙（pH 3.1–4.4）或酚酞（pH 8.2–10.0）皆可，因为快速的pH变化同时涵盖了这两个区间。
For titrations involving a weak acid and strong base, phenolphthalein is preferred because the equivalence point is alkaline. Methyl orange would change colour too early, well before equivalence. Conversely, for a weak base titrated with a strong acid, methyl orange is the correct choice.
若为弱酸与强碱滴定，应首选酚酞，因为等当点呈碱性。甲基橙会过早变色，远在等当点之前。反之，弱碱与强酸滴定时，甲基橙才是正确选择。
| Indicator | pH range | Colour change | Suitable for |
| Methyl orange | 3.1 – 4.4 | Red to yellow | Strong acid – weak base |
| Phenolphthalein | 8.2 – 10.0 | Colourless to pink | Strong base – weak acid |
5. Adding the Indicator and Recognising the End-Point | 指示剂的添加与终点识别
Only 2–3 drops of indicator should be added; using too much indicator can itself consume titrant and shift the apparent end-point. The flask should be swirled continuously during addition of the titrant, especially near the end-point, to ensure uniform mixing.
仅需加入2–3滴指示剂，过量指示剂本身会消耗滴定剂并改变表观终点。滴定过程中（尤其是接近终点时）应持续摇匀锥形瓶，保证混合均匀。
The end-point is indicated by a permanent colour change that persists for at least 30 seconds upon swirling. For methyl orange, the colour changes from red (acidic) to a peach-like/orange final colour, and eventually to a definite yellow if overtitrated. A white tile placed under the flask helps detect the exact colour change.
终点以摇匀后持续30秒以上的永久性颜色变化为准。使用甲基橙时，颜色由红（酸性）变为桃色/橙色终点色，过量滴定时转为明显黄色。瓶底垫白瓷砖有助于准确观察颜色变化。
6. Performing a Rough Titration | 进行粗略滴定
A rough titration is the first trial carried out quickly by adding titrant in 1–2 cm³ portions to estimate the endpoint volume. This value is not used in the final mean calculation but provides a guide for subsequent accurate titrations. Skilled students slow down the addition when they approach the rough titre volume to avoid overshooting.
粗略滴定是第一次快速试验，以每次1–2 cm³的速度加入滴定剂来预估终点体积。该数值不用于最终平均值计算，但为后续精确滴定提供参考。有经验的学生会在接近粗滴体积时放慢滴加速度，避免过量。
7. Obtaining Concordant Titre Values | 获取一致的滴定结果
After the rough run, repeated accurate titrations must be carried out until at least two results agree within 0.10 cm³ of each other. Only these concordant values are used to calculate the mean titre. The mark scheme penalises the use of non-concordant or rough values in the average.
粗略滴定之后，必须进行多次精确滴定，直到至少有两个测量值相差不超过0.10 cm³。仅将这些一致的结果用于计算平均滴定体积。评分方案将对将非一致值或粗滴值纳入平均值的做法扣分。
If three results are 24.10, 24.15 and 24.20 cm³, the closest pair (24.15 and 24.10, difference 0.05) are concordant and used for the mean. Spreadsheets or manual checks should always confirm this selection.
若三次结果为24.10、24.15和24.20 cm³，则最接近的两组（24.15和24.10，差值0.05）视为一致，用于计算平均值。应通过电子表格或人工核对确认这一选择。
8. Calculating the Mean Titre and Recording Data | 计算平均滴定体积与数据记录
age is rounded appropriately, e.g., (24.10+24.15)/2 = 24.125, rounded to 24.13 cm³ according to significant figure rules expected at A-level.
平均滴定体积仅由一致读数计算得出，并保留两位小数。当一致值有微小差异时，应合理修约平均值，例如 (24.10+24.15)/2 = 24.125，依据A-level有效数字规则修约为24.13 cm³。
All raw data must be clearly presented in a table with headings including units, separated from the body of the table. The mark scheme expects consistent decimal places and rejects the use of trailing zeros incorrectly. Never alter data retroactively; if a suspected outlier occurs, simply record it and mark it as not used in the calculation.
所有原始数据必须以带表头（含单位）的表格清晰呈现，表头与表内数据分开。评分方案要求小数位数一致，并避免误用末尾“0”。不得事后篡改数据；若出现可疑离群值，仅如实记录并标明不用于计算即可。
9. Preparing a Standard Solution | 配制标准溶液
To prepare a standard solution from a solid primary standard, accurately weigh a clean, dry weighing bottle or boat. Transfer the solid to a beaker, reweigh the empty bottle, and calculate the mass by difference. This method eliminates the uncertainty of ensuring complete transfer of powder from the container.
用固体基准物质配制标准溶液时，先准确称量洁净干燥的称量瓶或称量舟的质量。将固体转移至烧杯中后，再次称量空瓶，用差量法计算固体质量。该方法排除了容器中粉末是否完全转移的不确定度。
Dissolve the solid in distilled water, then transfer all the solution into a volumetric flask via a funnel. Rinse the beaker and rod with distilled water several times and add these washings to the flask to ensure quantitative transfer. Finally, fill the flask to the graduation mark with distilled water, stopper, and invert several times to mix.
将固体溶解于蒸馏水中，然后通过漏斗将全部溶液转移至容量瓶中。用蒸馏水淋洗烧杯和玻棒数次，并将洗涤液也转入容量瓶，确保定量转移。最后加蒸馏水至刻度线，塞好瓶塞，反复倒转摇匀。
10. Weighing and Mass Accuracy | 称量与质量准确度
ce must be tared or zeroed before each measurement. Weighing by difference is a critical skill: the mass of the solid transferred equals the initial mass of container + solid minus the final mass of container after transfer.
使用读数精度至少为±0.01 g的分析天平或电子天平，±0.001 g更佳。每次测量前必须归零或扣重。差量称量是核心技能：转移固体的质量等于初始（容器+固体）质量减去转移后容器质量。
Mark schemes often ask why the weighing bottle must be dry. Residual moisture adds mass and leads to overestimation of the solid's weight, directly affecting the calculated concentration of the standard solution.
评分方案常要求解释为何称量瓶必须干燥。残留水分增加质量，导致高估固体重量，直接影响标准溶液的计算浓度。
11. Common Errors and Their Impact on Results | 常见误差及其影响
Misreading the burette scale upside down (e.g., reading 24.50 cm³ as 25.50 cm³) is a gross error that yields inconsistent titres. Leaving the funnel in the burette during titration allows drops of solution to fall and alter the internal volume, potentially increasing the apparent titre.
颠倒读取滴定管刻度（如将24.50 cm³读成25.50 cm³）属于严重错误，会导致结果不一致。滴定过程中将漏斗遗留在滴定管上，漏斗中残留液滴可能落入管内，改变溶液体积，可能使表观滴定体积增大。
Rinsing the conical flask with the analyte solution rather than water increases the amount of substance to be titrated, leading to a titre value that is too high. Using distilled water to rinse the flask is acceptable because the number of moles remains unchanged.
用待测液而非蒸馏水润洗锥形瓶会使待滴物质量增加，导致滴定体积偏高。相反，用蒸馏水润洗锥形瓶则不会改变物质的量，因而不会引入误差。
Not removing the titration flask from beneath the burette when refilling the burette can cause solution drops to mistakenly fall into the flask, ruining the run. Always lower the clamp and fill away from the work area.
补充滴定管溶液时未将锥形瓶移开，可能导致溶液滴入瓶中，破坏本次滴定。因此，始终应降低滴定管夹并在远离工作区域处加液。
12. Exam Technique: Describing Experimental Procedures | 应试技巧：描述实验步骤
When a question asks you to 'describe how you would carry out an accurate titration', the mark scheme looks for a logical sequence: rinse apparatus, fill burette correctly, use pipette filler, add indicator, place white tile, run titrant dropwise near end-point, swirl, note colour change, repeat to concordant results, and calculate appropriately.
当题目要求“描述如何完成一次准确滴定”时，评分方案期望出现以下逻辑顺序：润洗仪器、正确注入滴定管、使用洗耳球、加入指示剂、放置白瓷砖、接近终点时逐滴加入、摇匀、记录颜色变化、重复至结果一致、并恰当计算。
Use command words such as 'rinse with distilled water followed by the solution', 'record to 2 decimal places', and 'repeat until two titres are within 0.10 cm³' to hit marking points. Always justify the choice of indicator and mention precautions against parallax and air bubbles.
善用指令关键词，如“先用蒸馏水冲洗，再用溶液润洗”“记录至两位小数”“重复滴定直至两次体积相差0.10 cm³以内”即可命中得分点。同时，务必说明选择指示剂的理由，并提及避免视差和气泡的预防措施。
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