Acid-base titration
In a titration, a solution of known concentration (the titrant) is added slowly to a measured sample until the reaction is just complete. This point is the equivalence point. An indicator or pH meter shows when it is reached.
Last updated
Analogy
Filling a glass to the brim with a dropper: you stop at the exact drop that tips it over.
Example
25.0 mL HCl needs 20.0 mL of 0.100 M NaOHmol NaOH = 0.0200 × 0.100 = 2.00×10⁻³mol HCl = 2.00×10⁻³[HCl] = 2.00×10⁻³ / 0.0250 = 0.0800 M
In practice
Food chemists titrate vinegar with sodium hydroxide to check its acidity matches the label.
Practice
Flashcards
- What is the equivalence point?When moles of acid and base added are exactly equal.
Questions people ask
What is the difference between the equivalence point and the endpoint?
The equivalence point is when moles of acid and base match exactly. The endpoint is when the indicator changes colour. A good indicator is chosen so the two are very close. For a strong acid with a strong base, the equivalence point is at pH 7.
Why is the equivalence point above pH 7 for a weak acid with a strong base?
At equivalence, all the weak acid has become its conjugate base. This base reacts slightly with water, producing OH⁻, so the solution is basic. The pH is above 7, so an indicator like phenolphthalein, which changes around pH 8 to 10, is used.