The principles of chromatography for separating mixtures, the use of volumetric (titration) analysis to find an unknown concentration, the idea of standard solutions, and how to interpret analytical data to identify and quantify substances.

What this key area is asking

The SQA wants you to explain how chromatography separates the components of a mixture, describe volumetric (titration) analysis to find an unknown concentration, explain the role of a standard solution, and interpret analytical data to identify and quantify substances. The titration calculation is one of the most reliable sources of marks in the whole course, so the $n = CV$ method here must be secure.

Chromatography

The distance each component moves can be used to identify it by comparison with known substances run under the same conditions. If a single spot of a sample separates into several spots, the sample is a mixture.

Volumetric analysis

A standard solution is one of accurately known concentration, prepared by dissolving a known mass of a pure solute and making it up to a precise volume in a volumetric flask.

The key relationship

Worked example: a standardised titration

Reliability of results

To be reliable, repeated titres should be concordant (within about $0.2 \text{ cm}^3$ of each other), and the average of the concordant titres is used. A rough first titration is ignored when averaging. Accurate technique, reading the burette to the bottom of the meniscus, and swirling near the end point all improve reliability.

Examples in context

Volumetric analysis is the backbone of routine quality control. A brewery uses an acid-base titration to check the acidity of beer, and a swimming-pool technician titrates to monitor chlorine levels. In environmental monitoring, the concentration of dissolved oxygen in a river is found by a redox titration, where the same $n = CV$ logic applied above turns a titre volume into a concentration that tells ecologists whether fish can survive. Chromatography, meanwhile, is used by forensic scientists to separate the dyes in an ink left at a crime scene and match them to a suspect's pen, and by anti-doping laboratories to separate and identify banned substances in an athlete's sample.

Try this

Q1. Explain how chromatography separates the components of a mixture. [2 marks]

• Cue. Components are separated by how strongly each is attracted to the stationary phase relative to the mobile phase; those held less strongly travel further.

Q2. What is a standard solution? [1 mark]

• Cue. A solution of accurately known concentration, made from a known mass of pure solute and a precise volume.

Q3. $25.0 \text{ cm}^3$ of $NaOH$ needs $22.0 \text{ cm}^3$ of $0.100 \text{ mol l}^{-1}$ $HCl$ ($1 : 1$ ratio). Calculate the concentration of the $NaOH$. [3 marks]

• Cue. $n(HCl) = 0.100 \times 0.0220 = 2.20 \times 10^{-3}$ mol $= n(NaOH)$; $C = 2.20 \times 10^{-3} / 0.0250 = 0.0880 \text{ mol l}^{-1}$.