How do we use titration to find an unknown concentration, and what shape does a titration curve take?
Perform and interpret acid-base titrations, sketch titration curves, and calculate unknown concentrations from volumetric data
A focused answer to the WACE Year 12 Chemistry dot point on volumetric analysis, the technique and calculations of acid-base titration, the shape of titration curves for different acid-base combinations, and the equivalence point, with a worked example and common exam mistakes.
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What this dot point is asking
Volumetric analysis is a quantitative technique that uses an accurately measured volume of a solution of known concentration (a standard solution) to determine the concentration of another solution.
The technique
A pipette delivers a precise volume (an aliquot) of one solution into a conical flask. The other solution is added from a burette until the reaction is just complete, signalled by an indicator colour change (the end point). With careful technique the end point coincides with the equivalence point. Apparatus is rinsed appropriately: the burette and pipette with the solution they will deliver, the conical flask with distilled water only, to avoid changing the amount of substance being measured.
The titration calculation
The calculation uses three steps. First find the moles of the standard from . Second use the mole ratio from the balanced equation to find moles of the unknown. Third divide by the unknown's volume to get its concentration.
The shape of titration curves
A titration curve plots the pH of the flask against the volume of titrant added. Its key feature is a steep, near-vertical rise through the equivalence point. The shape depends on the strengths involved:
- Strong acid with strong base: curve starts low, rises gently, then jumps steeply through pH 7 (the equivalence point), then levels off high. The steep region is large (about pH 3 to 11).
- Weak acid with strong base: starts at a higher pH, has a buffer region where pH changes slowly, and the equivalence point lies above pH 7 (the salt is basic). The vertical region is shorter.
- Strong acid with weak base: the equivalence point lies below pH 7 (the salt is acidic).
The indicator must change colour within the steep region so the end point matches the equivalence point.
Standard solutions
A primary standard is a pure, stable substance of accurately known formula (such as anhydrous sodium carbonate) that can be weighed to make a solution of precisely known concentration. Solutions of substances that are not stable enough to be primary standards (such as sodium hydroxide, which absorbs moisture and carbon dioxide) must be standardised against a primary standard before use.
Why this matters
Volumetric analysis is the foundation of quantitative chemistry in the laboratory, from quality control to environmental monitoring. The titration curve links the calculation back to the equilibrium ideas of pH, and indicator choice, tying the acid-base section of Unit 3 together.