📚 Reactions 1.2.1: Acids, Alkalis and pH – Experimental Techniques | 反应1.2.1：酸、碱和pH值 – 实验操作
Understanding acids, alkalis and the pH scale is fundamental in chemistry, but practical skill is where theory meets reality. This guide covers the core experimental techniques you need to master, from measuring pH with different tools to performing accurate titrations, making standard solutions, and drawing titration curves. Every method is described step by step so that you can safely and precisely investigate acid–base behaviour in the laboratory.
理解酸、碱和pH标度是化学的基础，但实验技能才是理论联系实际的关键。本指南涵盖你需要掌握的核心实验技术，从使用不同工具测量pH值，到精确进行滴定操作、配制标准溶液和绘制滴定曲线。每一步方法都逐步描述，让你能够安全、精准地在实验室中研究酸碱行为。
1. Measuring pH with Indicators and pH Paper | 用指示剂和pH试纸测量pH值
Litmus paper only tells you whether a solution is acidic or alkaline, but universal indicator and narrow-range pH papers give a more precise reading. Tear off a small strip of pH paper, dip one end into the test solution, and immediately compare the colour with the chart on the container. Do not leave the paper soaking, because dyes can leach out and distort the result. For universal indicator solution, add 2–3 drops to about 5 cm³ of sample, swirl gently, and match the colour to the standard chart within 30 seconds.
石蕊试纸只能告诉你溶液是酸性还是碱性，但通用指示剂和精密pH试纸能给出更精确的读数。撕下一小条pH试纸，将一端浸入待测溶液中，立即与包装上的比色卡对照。不要把试纸长时间浸泡，因为染料会渗出，使结果失真。对于通用指示剂溶液，向约5 cm³样品中加入2–3滴，轻轻摇匀，在30秒内与标准比色卡对比颜色。
Indicator choice depends on the expected pH range. For instance, phenolphthalein is colourless below pH 8.3 and pink above pH 10.0, making it ideal for strong-acid/strong-base titrations. Methyl orange changes from red (pH < 3.1) to yellow (pH > 4.4), suited for strong-acid/weak-base reactions. Always record the colour transition observed, not the colour you expect.
指示剂的选择取决于预期的pH范围。例如，酚酞在pH 8.3以下无色，在pH 10.0以上变为粉红色，因此非常适合强酸强碱滴定。甲基橙由pH低于3.1时的红色变为pH高于4.4时的黄色，适用于强酸弱碱反应。始终记录观察到的颜色变化，而非你预期的颜色。
2. Using a pH Meter and Calibration | 使用pH计及校准
A pH meter gives a direct, digital reading and is the most accurate tool for measuring pH, provided it is properly calibrated. Before measuring any unknown solution, rinse the electrode with distilled water and gently blot it with lint-free tissue. Immerse the electrode in a standard buffer solution of known pH (e.g. pH 4.00, 7.00, and 10.00) and adjust the meter until it displays the correct value. Always perform a two-point or three-point calibration for best accuracy; a single-point calibration is acceptable only for rough checks.
pH计能给出直接的数字读数，如果校准得当，它是测量pH值最准确的工具。在测量任何未知溶液前，用蒸馏水冲洗电极，并用无绒纸巾轻轻吸干。将电极浸入已知pH值的标准缓冲溶液（如pH 4.00、7.00和10.00）中，调整仪器直至显示正确的数值。为了获得最佳准确度，始终进行两点或三点校准；单点校准仅适用于粗略检测。
After calibration, rinse the electrode again and place it in the sample. Wait until the reading stabilises; this may take 20–60 seconds. Record the temperature, because pH is temperature-dependent. Never allow the electrode to dry out; store it in a storage solution recommended by the manufacturer, usually 3 mol dm⁻³ KCl solution.
校准后，再次冲洗电极并将其放入样品中。等待读数稳定，这可能需要20–60秒。记录温度，因为pH值依赖于温度。切勿让电极干燥；应将其保存在制造商推荐的储存液中，通常是3 mol dm⁻³ KCl溶液。
3. Preparing a Standard Solution from a Solid Acid or Base | 用固体酸或碱配制标准溶液
Many titrations require a standard solution whose concentration is accurately known. To prepare 250 cm³ of 0.100 mol dm⁻³ sodium carbonate solution, accurately weigh out the required mass (e.g. 2.65 g of anhydrous Na₂CO₃) on a balance reading to at least ±0.01 g. Transfer the solid to a beaker, add about 100 cm³ of distilled water, and stir until completely dissolved. Pour the solution through a funnel into a 250 cm³ volumetric flask. Rinse the beaker and funnel several times with distilled water and transfer the washings to the flask. Fill the flask to just below the graduation mark, then add water dropwise until the bottom of the meniscus touches the line. Stopper and invert the flask 10–15 times to mix thoroughly.
许多滴定需要浓度精确已知的标准溶液。要配制250 cm³、0.100 mol dm⁻³的碳酸钠溶液，用精度至少为±0.01 g的天平准确称量所需质量（如2.65 g无水Na₂CO₃）。将固体转移到烧杯中，加入约100 cm³蒸馏水，搅拌至完全溶解。将溶液通过漏斗转移到250 cm³容量瓶中。用蒸馏水冲洗烧杯和漏斗数次，并将洗涤液转入容量瓶。加水至略低于刻度线，然后逐滴加水直到弯月面底部与刻度线相切。盖上瓶塞，将容量瓶倒转10–15次以充分混匀。
Label the flask immediately with the name of the substance, concentration, and date. If the solid is hydroscopic (e.g. NaOH), do not attempt to weigh it accurately in open air; instead, prepare an approximately concentrated solution and standardise it against a primary standard like potassium hydrogen phthalate (KHP). This two-step approach gives a reliable concentration.
立即在瓶身贴上标签，注明物质名称、浓度和日期。如果固体易吸潮（如NaOH），不要在开放空气中精确称量；而是先配制一个近似浓度的溶液，然后用邻苯二甲酸氢钾（KHP）等基准物质进行标定。这种两步法可以得到可靠的浓度。
4. The Burette, Pipette and Conical Flask Setup | 滴定管、移液管和锥形瓶的组装
A titration’s success starts with clean glassware. Rinse a 25.0 cm³ pipette first with distilled water, then with a small portion of the solution it will deliver. Use a pipette filler – never mouth-pipette. Draw the solution above the mark, then let it drain until the meniscus rests on the line. Touch the tip to the inside of a clean conical flask to release the set volume. Add 2–3 drops of a suitable indicator.
滴定成功始于洁净的玻璃器皿。先用蒸馏水冲洗25.0 cm³移液管，再用它将移取的溶液润洗2–3次。使用洗耳球——绝不能用嘴吸液。将溶液吸至刻度线上方，然后放出液体直到弯月面与刻度线相切。将移液管尖端接触干净锥形瓶内壁，以释放设定体积。加入2–3滴合适的指示剂。
Set up a burette clamped vertically. Rinse the burette with the titrant, then fill it above the zero mark. Open the tap briefly to fill the tip, ensuring no air bubbles remain. Record the initial burette reading to the nearest 0.05 cm³ (or better, 0.01 cm³). Place a white tile under the flask to see the colour change clearly. Swirl the flask continuously while adding titrant, and slow to dropwise additions as the endpoint approaches.
将滴定管垂直固定在铁架台上。用滴定剂润洗，然后加液至零刻度以上。短暂打开活塞，充满管尖，确保无气泡残留。记录初始读数，读数精确到0.05 cm³（最好0.01 cm³）。在锥形瓶下放一块白色瓷砖，以便清晰观察颜色变化。持续旋摇锥形瓶，同时加入滴定剂，接近终点时改为逐滴加入。
5. Performing an Acid–Base Titration | 进行酸碱滴定
Carry out a rough titration first to estimate the endpoint volume. Add the titrant fairly quickly until a permanent colour change flashes, then note the volume. For the accurate titrations, repeat the process more carefully: run the titrant to within ~2 cm³ of the expected endpoint, then add drop by drop until a single drop causes a permanent colour change. Record the final burette reading immediately. The difference between final and initial readings is the titre volume.
先进行一次粗滴定，估算终点体积。较快地加入滴定剂，直到出现瞬间持久颜色变化，记录体积。进行精确滴定时，更仔细地重复操作：滴定至预期终点前约2 cm³的位置，然后逐滴加入，直到一滴溶液引起永久颜色变化。立即记录滴定管最终读数。最终与初始读数之差即为滴定体积。
Refill the burette between titrations and always read from the bottom of the meniscus. Repeat until you obtain two concordant titres (within 0.10 cm³ of each other). Average the concordant results; do not include the rough titre. This average is used for concentration calculations. Common student errors include forgetting to remove the funnel from the burette (which may drip), misreading the meniscus, and rinsing the conical flask with the solution it will contain instead of with water.
每次滴定前重新加满滴定管，始终读取弯月面底部。重复操作，直至获得两个相互吻合的滴定值（彼此相差在0.10 cm³以内）。取吻合结果的平均值；不要包含粗滴定值。该平均值用于浓度计算。学生常见错误包括：忘记从滴定管上取下漏斗（可能滴液）、读错弯月面、以及用待盛装溶液而非蒸馏水润洗锥形瓶。
6. Choosing the Right Indicator and Identifying Endpoints | 选择合适的指示剂及终点判断
The equivalence point of a titration is not always at pH 7. A strong-acid/strong-base titration has an equivalence point near pH 7, where both phenolphthalein and methyl orange work well. In a strong-acid/weak-base titration, the equivalence point is acidic (pH ~5), so methyl orange is suitable. For a weak-acid/strong-base titration, the solution is alkaline at equivalence (pH ~9), making phenolphthalein the correct choice. Weak-acid/weak-base titrations rarely give a sharp pH jump and are usually avoided; reliable indicators do not exist for these.
滴定等当点并不总是在pH 7。强酸强碱滴定的等当点接近pH 7，酚酞和甲基橙都适用。强酸弱碱滴定的等当点为酸性（pH ~5），因此甲基橙适合。弱酸强碱滴定在等当点溶液呈碱性（pH ~9），故酚酞是正确的选择。弱酸弱碱滴定的pH突跃不明显，通常被避免使用；这类滴定没有可靠的指示剂。
Observe the endpoint when the faintest permanent colour appears, not an intense shade; overtitrating leads to systematic error. If using a pH meter, plot the pH against volume of titrant added to produce a titration curve, and locate the midpoint of the steepest vertical section – this is the equivalence point. The indicator’s pKₐ should lie within the pH jump region.
当最浅的持久颜色出现时即判定为终点，而非浓烈的颜色；过量滴定会导致系统误差。若使用pH计，绘制pH值对滴定剂加入量的曲线，找到斜率最陡峭部分的中间点——这就是等当点。指示剂的pKₐ应位于pH突跃区间内。
7. Drawing and Interpreting Titration Curves | 绘制和解读滴定曲线
Using a data-logging device or manual pH meter, add titrant in small increments (0.5–1.0 cm³ initially, then 0.1–0.2 cm³ near the endpoint) and record the pH after each addition. Plot a graph of pH on the y-axis and volume of titrant added on the x-axis. The curve starts with the initial pH of the analyte, rises or falls slowly during the buffer region, and then undergoes a sharp near-vertical jump at the equivalence point. The equivalence point is the volume at the midpoint of this steep rise, where the gradient is maximum.
使用数据采集设备或手动pH计，以小增量加入滴定剂（起始0.5–1.0 cm³，接近终点时0.1–0.2 cm³），每次加入后记录pH值。以pH为y轴，滴定剂加入体积为x轴作图。曲线从待测物初始pH开始，在缓冲区域内缓慢上升或下降，然后在等当点经历急剧的近乎垂直的突跃。等当点即此陡峭上升中点处的体积，该点斜率最大。
From the curve you can read the equivalence volume and also estimate the pKₐ of a weak acid – it equals the pH at half-neutralisation (when the volume of base added is half the equivalence volume). For a strong-acid/strong-base curve, the equivalence point is at pH 7; for a weak-acid/strong-base curve, it lies above 7; for a strong-acid/weak-base curve, it lies below 7. Knowing these shapes helps you select the correct indicator for future titrations.
从曲线上可以读出等当体积，还可以估算弱酸的pKₐ——它等于半中和点（即加入碱的体积为等当体积一半时）的pH值。对于强酸强碱曲线，等当点pH为7；弱酸强碱曲线等当点高于7；强酸弱碱曲线等当点低于7。了解这些形状有助于为将来的滴定选择正确的指示剂。
8. Investigating Buffer Action Experimentally | 实验探究缓冲作用
A buffer solution resists changes in pH when small amounts of acid or alkali are added. To demonstrate this, prepare a buffer by mixing equal volumes of 1.0 mol dm⁻³ ethanoic acid and 1.0 mol dm⁻³ sodium ethanoate in a beaker. Measure its initial pH with a pH meter. Then add 1 cm³ of 0.1 mol dm⁻³ HCl dropwise with stirring and record the pH. Repeat the test with distilled water instead of the buffer, and compare the pH change – water shows a dramatic drop, while the buffer’s pH changes by less than 0.2 units. Repeat the same experiment using 0.1 mol dm⁻³ NaOH to show resistance to alkali addition.
缓冲溶液能在少量酸或碱加入时抵抗pH变化。为验证这一点，在烧杯中混合等体积的1.0 mol dm⁻³乙酸和1.0 mol dm⁻³乙酸钠，用pH计测量其初始pH。然后逐滴加入1 cm³ 0.1 mol dm⁻³ HCl并搅拌，记录pH。用蒸馏水代替缓冲液重复测试，比较pH变化——水的pH会发生巨大下降，而缓冲液的pH变化小于0.2个单位。用0.1 mol dm⁻³ NaOH重复相同实验，以展示其对碱的抵抗能力。
This experiment can be extended to measure buffer capacity by adding acid or base in 0.5 cm³ portions until the buffer ‘breaks’. Biological buffers such as hydrogencarbonate ions in blood can be modelled with a carbonate/hydrogencarbonate mixture. Always wear eye protection and handle acids and alkalis with care.
此实验可通过以0.5 cm³增量加入酸或碱直至缓冲液“失效”来扩展，以测量缓冲容量。血液中的碳酸氢根离子等生物缓冲液可用碳酸盐/碳酸氢盐混合物模拟。实验时始终佩戴护目镜，小心处理酸和碱。
9. Distinguishing Strong and Weak Acids Experimentally | 实验区分强酸与弱酸
Measure the pH of 0.1 mol dm⁻³ solutions of hydrochloric acid (a strong acid) and ethanoic acid (a weak acid) under identical conditions. HCl gives a pH close to 1.0, indicating complete dissociation, while CH₃COOH shows a pH around 2.9, characteristic of partial dissociation. Alternatively, measure the rate of reaction with a metal like magnesium ribbon: the strong acid produces hydrogen gas bubbles much more vigorously than the weak acid of the same concentration, because [H⁺] is higher.
在相同条件下测量0.1 mol dm⁻³盐酸（强酸）和0.1 mol dm⁻³乙酸（弱酸）溶液的pH值。HCl的pH值接近1.0，表明完全电离；CH₃COOH的pH值约2.9，为部分电离的特征。另一种方法是测量与金属（如镁条）反应的速率：相同浓度的强酸产生氢气气泡的剧烈程度远高于弱酸，因为强酸的[H⁺]更高。
Conductivity measurements also reveal the difference. Using a simple circuit with a bulb or ammeter, a strong acid solution lights the bulb brightly, while a weak acid solution of the same concentration glows dimly. This confirms that strong acids contain a greater number of mobile ions. Always use fresh solutions and clean electrodes to avoid contamination.
电导率测量也能揭示差异。使用带灯泡或电流表的简单电路，同浓度的强酸溶液使灯泡明亮，而弱酸溶液则发光昏暗。这证实了强酸中含有更多可自由移动的离子。务必使用新鲜溶液和清洁电极，以避免污染。
10. Safety and Risk Assessment in Acid–Base Experiments | 酸碱实验的安全与风险评估
Always wear a lab coat, safety goggles and nitrile gloves when handling concentrated acids or alkalis. Work in a fume hood if the acid is volatile (e.g. concentrated HCl or HNO₃) to avoid inhaling corrosive fumes. In the event of a skin splash, rinse immediately with copious water for at least 15 minutes and seek medical attention. Never add water to concentrated acid; always add acid slowly to water while stirring to dissipate heat.
处理浓酸或浓碱时，务必穿着实验服、佩戴安全护目镜和丁腈手套。如果酸具有挥发性（如浓盐酸或浓硝酸），需在通风橱中操作，避免吸入腐蚀性蒸气。若皮肤溅到酸碱，立即用大量水冲洗至少15分钟，并就医。绝不可将水加入浓酸中；始终在搅拌下将酸缓慢加入水中以散热。
When heating solutions, use a water bath or hot plate rather than a direct flame if flammable organic acids are present. Dispose of waste solutions in designated containers – acids and alkalis must be neutralised before being poured down the sink, following local regulations. Always complete a risk assessment before starting any experiment and know the location of the nearest eyewash station and safety shower.
加热溶液时，如果存在易燃有机酸，请使用水浴或电热板而非明火。将废液倒入指定的容器中——酸碱必须中和后才可按照当地规定倒入水槽。开始任何实验前务必完成风险评估，并知道最近的洗眼器和安全淋浴的位置。
11. Recording, Processing and Presenting Data | 记录、处理和呈现数据
Record all raw data in a bound laboratory notebook, using ink, with a single line drawn through any mistake – never use correction fluid. Prepare a results table before the experiment with columns for rough, run 1, run 2 and run 3 titres. Record burette readings to the nearest 0.05 cm³ (or the precision of your burette). Indicate concordant values clearly and show the calculation of the mean titre.
使用钢笔将所有原始数据记录在装订好的实验记录本中，错误以单线划掉——切勿使用涂改液。实验前准备结果表格，包括粗滴定、第1次、第2次和第3次滴定体积列。滴定管读数精确到0.05 cm³（或滴定管的精度）。明确标示吻合值，并展示平均滴定体积的计算过程。
Calculate the concentration of the unknown solution stepwise: use the balanced equation to find the mole ratio, determine moles of known substance, find moles of unknown, and finally its concentration in mol dm⁻³. Express the final answer with appropriate significant figures and units. In practical assessments, marks are awarded for recording data with consistent precision and for correct calculation, so do not round intermediate values.
逐步计算未知溶液的浓度：利用配平方程式找出摩尔比，确定已知物质的摩尔数，求出未知物的摩尔数，最后得出其浓度（mol dm⁻³）。最终答案用适当的有效数字和单位表示。在实验考核中，记录数据时保持精度一致和正确计算均可得分，因此不要对中间数据进行四舍五入。
12. Common Pitfalls and How to Avoid Them | 常见错误及解决方法
One frequent mistake is rinsing the conical flask with the solution it will receive rather than with distilled water. This adds extra moles of the substance and leads to a titre that is too high. The flask should only be rinsed with water. Another error is leaving a funnel in the top of the burette during titration; liquid may drip and alter the reading. Always remove the funnel after filling.
一个常见错误是用待盛装溶液而不是蒸馏水润洗锥形瓶。这会增加额外的物质的量，导致滴定体积偏高。锥形瓶只应用水润洗。另一个错误是在滴定过程中将漏斗留在滴定管顶部；液体可能滴落并改变读数。加液后务必取下漏斗。
Air bubbles trapped below the burette tap are another source of error, because they initially occupy volume but are dispelled during titration, making the titre appear larger than it truly is. Always flush the tip after filling and before taking the initial reading. Finally, never re-use a pasteur pipette or dropper for different reagents without thorough cleaning, as cross-contamination will ruin results.
滴定管活塞下方截留的气泡是另一个误差来源，因为它们最初占据体积，但在滴定过程中被排出，使滴定体积看起来比实际大。加液后、记录初始读数前务必冲洗管尖。最后，切勿在不同试剂间重复使用未经彻底清洗的滴管，因为交叉污染会毁掉实验结果。
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