Titration — Chemistry, 14–17 years
Finding the concentration of an unknown solution by adding a solution of known concentration until the reaction is exactly complete.
A measured chemical chase
In a titration, a solution with known concentration is added slowly to a measured volume of an unknown solution. An indicator or a pH sensor shows when the reacting amounts are just in the correct ratio; this is the endpoint. The volume used, together with the known concentration and equation, reveals the unknown concentration.
Why was titration developed?
A solution can look clear and still contain a great deal or very little dissolved chemical, so appearance cannot measure its concentration. Titration solves this by turning an invisible amount into a visible volume reading. It was developed as a careful way to analyse substances and check the strength of products such as acids, medicines and cleaning solutions.
Worked example: hydrochloric acid
A 25.0 mL sample of HCl is neutralised by 20.0 mL of 0.100 mol/L NaOH. The equation is HCl + NaOH → NaCl + H₂O, a 1:1 ratio. Moles of NaOH = 0.0200 × 0.100 = 0.00200 mol, so HCl also has 0.00200 mol. Its concentration is 0.00200 ÷ 0.0250 = 0.0800 mol/L.
The first colour change
A common mistake is to stop as soon as a drop creates a brief flash of colour. That colour may disappear because the drop has not mixed through the whole flask. The endpoint is usually a colour change that stays after gentle swirling, and the indicator must be chosen to change close to the reaction’s true equivalence point. Stopping late makes the calculated concentration wrong.
Checking what is really there
Titration is used to check the acidity of vinegar, the amount of active ingredient in a medicine and the chemicals in drinking or wastewater. It is useful when the substance is dissolved and cannot be judged reliably by colour or appearance. Modern instruments may automate the endpoint, but the chemical reasoning is still the same.
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