Analytical Techniques in A-Level Chemistry | 化学分析技术

📚 Analytical Techniques in A-Level Chemistry | 化学分析技术

Analytical chemistry is the science of identifying what substances are present in a sample and determining how much of each component exists. At A-Level, you need to master both qualitative tests, which reveal the identity of ions or functional groups, and quantitative methods, which measure exact quantities or concentrations.

分析化学是识别样品中存在哪些物质并确定每种组分含量的科学。在A-Level阶段，你需要掌握既能揭示离子或官能团身份的定性检验，又能测量精确含量或浓度的定量方法。

1. Qualitative vs Quantitative Analysis | 定性分析与定量分析

Qualitative analysis answers the question ‘What is present?’ It uses characteristic chemical reactions, flame colours, or spectroscopy to detect specific ions or compounds. Quantitative analysis answers ‘How much is present?’ and relies on measurements such as titres, absorbances, or peak areas.

定性分析回答”存在什么？”的问题。它使用特征化学反应、焰色或光谱技术来检测特定离子或化合物。定量分析回答”含量是多少？”的问题，依赖滴定体积、吸光度或峰面积等测量值。

Qualitative: observation of colour change, precipitate, or gas evolution.
定量: numerical data processed through calculations or calibration curves.

2. Test Tube Reactions for Cations | 阳离子的试管检验

Addition of sodium hydroxide solution to a metal cation often produces a characteristic hydroxide precipitate. Excess NaOH can then distinguish amphoteric hydroxides, such as Al(OH)₃, from insoluble ones like Cu(OH)₂.

向金属阳离子中加入氢氧化钠溶液通常会产生特征的氢氧化物沉淀。继续加入过量NaOH可以区分两性氢氧化物（如Al(OH)₃）与不溶性氢氧化物（如Cu(OH)₂）。

Cation	NaOH result	Excess NaOH
Cu²⁺	Blue precipitate	Insoluble
Al³⁺	White precipitate	Dissolves, colourless
Fe³⁺	Rust-brown precipitate	Insoluble

Ammonia solution can be used similarly; Cu²⁺ forms a deep blue complex ion, [Cu(NH₃)₄(H₂O)₂]²⁺, when excess ammonia is added.

氨溶液也有类似用途；加入过量氨水时，Cu²⁺会形成深蓝色配离子[Cu(NH₃)₄(H₂O)₂]²⁺。

3. Test Tube Reactions for Anions | 阴离子的试管检验

Common anion tests rely on specific precipitation reactions. Carbonate ions release CO₂ with dilute acid, which turns limewater milky. Sulfate ions give a white precipitate with barium chloride, and halide ions produce silver halide precipitates of different colours.

常见阴离子检验依赖于特殊的沉淀反应。碳酸根与稀酸反应释放CO₂，使石灰水变浑浊。硫酸根与氯化钡作用产生白色沉淀，而卤离子则生成不同颜色的卤化银沉淀。

CO₃²⁻: acid + limewater → white precipitate of CaCO₃.
SO₄²⁻: BaCl₂(aq) → white BaSO₄ precipitate.
Cl⁻: AgNO₃ → white AgCl precipitate (soluble in dilute NH₃).
Br⁻: AgNO₃ → cream AgBr (soluble in concentrated NH₃).
I⁻: AgNO₃ → yellow AgI (insoluble in NH₃).

4. Flame Tests | 焰色反应

Heating a metal salt in a non-luminous Bunsen flame excites electrons to higher energy levels. When they fall back, characteristic wavelengths of visible light are emitted, producing distinct flame colours.

在无光焰中加热金属盐会使电子激发到较高能级。当电子回落时，发射出特定波长的可见光，产生不同的焰色。

Ion	Flame colour
Li⁺	Crimson red
Na⁺	Yellow/orange
K⁺	Lilac
Ca²⁺	Brick red
Cu²⁺	Blue-green

Flame tests are only qualitative and are best used for group 1 and group 2 metal ions. Interference can occur if sodium impurities are present; a cobalt-blue glass filters out the yellow sodium emission.

焰色反应仅用于定性分析，最适合第一族和第二族金属离子。钠杂质会干扰观察，可使用钴蓝玻璃滤去黄色钠光。

5. Acid–Base Titration | 酸碱滴定

Titration is a core quantitative technique. A solution of known concentration (the titrant) is added from a burette to a measured volume of analyte until the equivalence point is reached. An appropriate indicator changes colour at this endpoint.

滴定是核心定量技术。将已知浓度的溶液（滴定剂）从滴定管加入量取的待测液，直到达到等量点。合适的指示剂会在终点变色。

moles = concentration (mol dm⁻³) × volume (dm³)

At the equivalence point, the number of moles of H⁺ equals the number of moles of OH⁻ reacted. For example, in the titration of hydrochloric acid with sodium hydroxide:

在等量点，H⁺的物质的量等于已反应的OH⁻的物质的量。例如，盐酸与氢氧化钠的滴定中：

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Titration calculations often require converting cm³ to dm³ by dividing by 1000, and using the mole ratio from the balanced equation.

滴定计算通常需要将cm³除以1000换算为dm³，并使用化学方程式中的摩尔比。

6. Redox Titrations | 氧化还原滴定

Redox titrations involve the transfer of electrons between oxidising and reducing agents. A common A-Level example is the titration of iron(II) ions with potassium manganate(VII) in acidic solution.

氧化还原滴定涉及氧化剂与还原剂之间的电子转移。A-Level常见例子是在酸性溶液中用高锰酸钾滴定铁(II)离子。

MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺

The manganate(VII) ion is purple, while the Mn²⁺ product is pale pink, so no separate indicator is needed. The endpoint is detected when a faint permanent pink colour appears. Another common oxidising agent is iodine, which requires starch indicator.

高锰酸根呈紫色，产物Mn²⁺为淡粉色，因此无需额外指示剂。当出现淡粉红色且不褪去时即为终点。另一种常见氧化剂是碘，需要使用淀粉指示剂。

Preparation of standard Fe²⁺ solution requires acidified water to prevent hydrolysis.
制备Fe²⁺标准溶液时需使用酸化水以防止水解。
Always rinse burette with the solution it will contain.
滴定管必须用待装溶液润洗。

7. Colorimetry | 比色法

Colorimetry measures the absorbance of visible light by a coloured solution. A colorimeter passes light of a specific wavelength through the sample; the amount absorbed depends on the concentration of the coloured species, following the Beer–Lambert law.

比色法测量有色溶液对可见光的吸收。比色计使特定波长的光通过样品；吸收量取决于有色物种的浓度，遵循比尔-朗伯定律。

A = εcl

Here A is absorbance, ε is molar absorptivity, c is concentration, and l is path length. In practice, a calibration curve is plotted using standard solutions of known concentration, and the unknown concentration is read from the graph.

其中A为吸光度，ε为摩尔吸光系数，c为浓度，l为光程。实际中，使用已知浓度的标准溶液绘制校准曲线，再从曲线上读出未知浓度。

Colorimetry is especially useful for determining transition metal ions or coloured organic compounds, provided the solution obeys the linear relationship at the chosen wavelength.

比色法特别适用于测定过渡金属离子或有色有机化合物，前提是溶液在所选波长下满足线性关系。

8. Infrared Spectroscopy | 红外光谱

Infrared (IR) spectroscopy identifies functional groups in organic molecules. Molecules absorb infrared radiation at specific wavenumbers corresponding to bond vibrations. The absorptions are recorded on a spectrum, with wavenumber on the x-axis and transmittance on the y-axis.

红外光谱可鉴定有机分子中的官能团。分子在特定波数处吸收红外辐射，对应化学键的振动。光谱图以波数为横坐标、透射率为纵坐标记录吸收情况。

Bond	Wavenumber range (cm⁻¹)
O–H (alcohol)	3200–3600 (broad)
O–H (carboxylic acid)	2500–3300 (very broad)
C=O	1630–1750
C–H	2850–3100

For example, an aldehyde shows a strong C=O stretch near 1725 cm⁻¹ and a C–H aldehyde peak near 2720 cm⁻¹. A carboxylic acid shows a very broad O–H absorption overlapping the C–H region.

例如，醛在1725 cm⁻¹附近有强C=O伸缩峰，在2720 cm⁻¹附近有醛基C–H峰。羧酸则显示一个覆盖C–H区域的极宽O–H吸收峰。

9. Mass Spectrometry | 质谱法

Mass spectrometry measures the mass-to-charge ratio (m/z) of gaseous ions. In organic analysis, a molecule is ionised and may fragment; the resulting mass spectrum shows the molecular ion peak (M⁺) and fragment peaks. The m/z of the M⁺ peak gives the relative molecular mass.

质谱法测量气态离子的质荷比(m/z)。在有机分析中，分子被电离并可能碎裂；所得质谱显示分子离子峰(M⁺)和碎片峰。M⁺峰的m/z给出相对分子质量。

Molecular ion peak: highest m/z (apart from isotope or M+1 peaks), gives Mr.
M+1 peak: caused by the presence of ¹³C; height indicates carbon number.
Fragment peaks: help identify structural groups.

分子离子峰：最高m/z（除同位素峰或M+1峰外），给出Mr。M+1峰：由¹³C引起，高度可指示碳原子数。碎片峰：有助于推断结构基团。

In inorganic analysis, mass spectrometry is used to determine relative atomic masses and isotopic abundances, yielding accurate atomic weights.

在无机分析中，质谱用于测定相对原子质量和同位素丰度，从而获得精确的原子量。

10. Chromatography | 色谱法

Chromatography separates the components of a mixture based on their different affinities for a stationary phase (e.g. paper or silica) and a mobile phase (e.g. solvent or gas). Common techniques at A-Level include thin-layer chromatography (TLC) and gas chromatography (GC).

色谱法根据各组分对固定相（如纸或硅胶）和流动相（如溶剂或气体）的不同亲和力来分离混合物。A-Level常见技术包括薄层色谱(TLC)和气相色谱(GC)。

For TLC, the retention factor Rf is calculated using:

对于TLC，比较值Rf的计算公式为：

Rf = distance moved by component / distance moved by solvent front

Compounds are identified by comparing Rf values with known standards under identical conditions. In GC, the retention time is characteristic of each compound, and the peak area is proportional to concentration, enabling quantitative analysis.

通过比较相同条件下Rf值与已知标准物来鉴定化合物。在气相色谱中，保留时间对每种化合物是特征的，峰面积与浓度成正比，可用于定量分析。

11. Choosing the Right Technique | 选择合适的技术

Exam questions often ask you to justify a particular analytical method. Consider what information is needed: identity, structure, concentration, or purity. Also consider the state of the sample and the required sensitivity.

考试问题常要求你说明选择某一分析方法的理由。考虑需要什么信息：身份、结构、浓度或纯度，同时也要考虑样品状态和所需灵敏度。

Identify a metal cation → flame test or NaOH precipitation.
鉴定金属阳离子 → 焰色反应或NaOH沉淀法。
Confirm functional group → infrared spectroscopy.
确认官能团 → 红外光谱。
Determine exact concentration of a coloured solution → colorimetry or titration.
测定有色溶液精确浓度 → 比色法或滴定法。
Separate and quantify mixture components → GC or HPLC.
分离并定量混合物组分 → 气相色谱或高效液相色谱。

12. Common Pitfalls in Analytical Questions | 分析类题目的常见误区

Students frequently lose marks by forgetting to state observation colours precisely, by using wrong units in calculations, or by confusing qualitative and quantitative evidence. Always specify the exact precipitate colour, solubility behaviour in excess reagent, and the units for concentration and absorbance.

学生常因未准确描述观察到的颜色、计算中单位错误或混淆定性与定量证据而失分。务必指明沉淀的确切颜色、在过量试剂中的溶解行为，以及浓度和吸光度的单位。

For titration, remember to quote volumes to two decimal places if using a burette reading (e.g. 23.45 cm³), and to calculate the mean titre only from concordant results (within 0.10 cm³).

对于滴定，如果使用滴定管读数，需保留两位小数（如23.45 cm³），并且只对符合要求的平行结果（相差不超过0.10 cm³）计算平均滴定体积。

Practice past paper questions that combine IR, mass spec, and chemical tests to deduce unknown organic structures.

练习结合红外、质谱和化学检验推断未知有机结构的历年真题。

Published by TutorHao | Chemistry Revision Series | aleveler.com

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Analytical Techniques in A-Level Chemistry | 化学分析技术