Topic O: Analytical Chemistry (Elective)

Analytical Chemistry is about answering two questions: what is in a sample, and how much of it is there. This elective develops qualitative tests that identify the ions and gases present, the titrimetric methods that measure amounts precisely, and the instrumental techniques that extend analysis to tiny quantities and complex mixtures with great speed and accuracy.

Qualitative analysis: identifying cations

Qualitative analysis identifies which species are present in a sample. Cations are often identified by flame tests and by precipitation reactions. In a flame test, a clean wire dipped in the sample and held in a flame gives a characteristic colour: lithium red, sodium yellow, potassium lilac, calcium brick-red and copper green-blue. Adding sodium hydroxide solution produces metal hydroxide precipitates whose colours help identify the metal: iron(II) gives a green precipitate, iron(III) a red-brown one, and copper(II) a blue one. Some hydroxides, such as those of aluminium and zinc, dissolve in excess sodium hydroxide, providing a further distinguishing test. Aqueous ammonia gives similar precipitates and is used to confirm results, building a reliable scheme of identification.

Qualitative analysis: identifying anions and gases

Anions are identified by specific reactions that give observable products. Carbonates react with dilute acid to release carbon dioxide, which turns limewater milky. Adding dilute nitric acid then silver nitrate solution gives precipitates that identify halides: a white precipitate for chloride, cream for bromide and yellow for iodide. Sulfates give a white precipitate with acidified barium chloride solution. Gases are identified by their own tests: hydrogen gives a squeaky pop with a lighted splint, oxygen relights a glowing splint, carbon dioxide turns limewater milky, and ammonia turns damp red litmus blue. Combining cation, anion and gas tests in a systematic scheme allows an unknown compound to be identified with confidence from simple observations.

Titrimetric analysis: acid-base titrations

Titrimetric (volumetric) analysis measures concentration by reacting measured volumes to a known end point. In an acid-base titration, a pipette delivers a fixed volume of one solution into a flask, and a burette adds the other until an indicator just changes colour, marking the end point. The concentration is calculated from moles using n = c x V with the mole ratio from the balanced equation. The indicator must be chosen to change colour at the right pH for the acid and base used. Repeating titrations until concordant results (within 0.10 cm3) are obtained improves reliability, and the average of these titres is used in the calculation. Careful technique with the burette and indicator is essential for accurate results.

Titrimetric analysis: redox titrations

Redox titrations measure the amount of an oxidising or reducing agent by reacting it to an end point where electron transfer is just complete. Potassium permanganate is a common oxidising agent and acts as its own indicator: the purple solution is decolourised as it reacts, and the first permanent pink tinge marks the end point, so no separate indicator is needed. Iodine-thiosulfate titrations are used to determine oxidising agents, with starch added near the end point to give a sharp blue-black to colourless change. As in acid-base work, the calculation uses moles and the mole ratio from the balanced redox equation. Redox titrations extend volumetric analysis to substances that have no acid-base behaviour.

Instrumental methods: spectroscopy

Instrumental methods use machines to analyse samples quickly, accurately and in tiny amounts. Spectroscopic techniques study how substances interact with light and other radiation. Atomic absorption and emission spectroscopy measure the characteristic wavelengths absorbed or emitted by atoms, allowing metals to be identified and their concentrations measured at very low levels. Infrared spectroscopy detects the bonds present in a molecule, since each type of bond absorbs characteristic frequencies, helping to identify functional groups. Ultraviolet and visible spectroscopy measures the absorption of coloured solutions and is used to find concentrations. These techniques are far more sensitive than simple tests and can analyse mixtures, which is why they are used in environmental monitoring, medicine and forensic science.

Instrumental methods: chromatography and separation

Chromatography separates the components of a mixture by their different attractions to a stationary phase and a moving mobile phase. In paper or thin-layer chromatography, a spot of mixture is carried up a sheet by a solvent, and components that are more strongly attracted to the solvent travel further, separating into distinct spots. The distance each component moves relative to the solvent front gives an Rf value that helps identify it. Gas chromatography and high-performance liquid chromatography are sensitive instrumental versions used to separate and measure complex mixtures, such as drugs, pollutants or food additives. Often chromatography is combined with mass spectrometry, which identifies each separated component by its mass, giving a powerful tool for analysing real samples.