Titration
Also known as: volumetric analysis
Titration is a laboratory technique for finding an unknown concentration by gradually adding a solution of known concentration (the titrant) until the reaction is complete. The endpoint is signaled by an indicator or a pH meter.
Titration is a quantitative technique for determining the concentration of a solution. A titrant of precisely known concentration is added gradually from a burette to a measured volume of the unknown (the analyte) until the reaction between them is just complete. From the volume of titrant used and the reaction's stoichiometry, the unknown concentration can be calculated.
The moment when exactly enough titrant has been added to react with all the analyte is the equivalence point. In an acid-base titration, moles of acid equal moles of base there (adjusted for stoichiometry), which gives the working formula for monoprotic reactions: M₁V₁ = M₂V₂. For example, if 25.0 mL of HCl is neutralized by 30.0 mL of 0.100 M NaOH, the acid's concentration is (0.100 × 30.0) / 25.0 = 0.120 M. The experimentally observed signal — an indicator's color change — is called the endpoint, and a good indicator is chosen so the endpoint falls close to the equivalence point.
Plotting pH against titrant volume produces a titration curve. Strong acid-strong base titrations have an equivalence point at pH 7; a weak acid titrated with a strong base finishes above pH 7, and its curve shows a buffer region where pH = pKa at the half-equivalence point. That half-equivalence relationship is a favorite exam detail because it lets you read pKa straight off the curve.
The MCAT tests titration within its chemistry foundations — calculating concentrations, interpreting titration curves, matching indicators to equivalence points, and recognizing the half-equivalence point where pH equals pKa. The same stoichiometric reasoning appears in its coverage of solution chemistry and stoichiometry.
Key takeaways
- Titration finds an unknown concentration by adding a known titrant until the reaction is complete.
- At the equivalence point, moles of titrant match moles of analyte by stoichiometry; M₁V₁ = M₂V₂ handles simple acid-base cases.
- Titration curves reveal chemistry: weak acid-strong base curves have a buffer region, and pH = pKa at the half-equivalence point.
- Indicators are chosen so the visible endpoint falls near the true equivalence point.
- The MCAT tests titration calculations, curves, and indicator selection.