Acid-Base Titrations

Understanding Acid–Base Titrations

• Definition:
  An acid–base titration is a quantitative analytical method used to determine the unknown concentration of an acid or base by reacting it with a solution of known concentration until the neutralisation point (end-point) is reached.
• Core Principle:
  The reaction is based on the neutralisation reaction:
  Acid + Base → Salt + Water
  Example:
  HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Essential Apparatus in Acid–Base Titrations

1. Burette

• Structure:
  Long, graduated glass tube with a tap at the bottom for controlled release of liquid.
  Graduations typically marked in 0.1 cm³ divisions for accurate readings.
• Use:
  • Rinse the burette with the solution it will contain (acid or base).
  • Fill using a small funnel (remove funnel before titration to avoid drips).
  • Ensure no air bubbles are in the tip.
  • Clamp vertically on a retort stand.
  • Deliver liquid dropwise for precise control, especially near the end-point.
• Accuracy: ±0.05 cm³ when read at eye level, avoiding parallax error.

2. Volumetric Pipette

• Structure:
  Narrow glass tube with a bulb in the middle, calibrated to deliver one fixed volume (commonly 25.0 cm³) with high accuracy.
• Use:
  • Rinse with the solution to be measured.
  • Use a pipette filler for safety (never mouth pipette).
  • Fill to the calibration mark, ensuring the bottom of the meniscus is level with the mark at eye level.
  • Empty into the conical flask, allowing the liquid to drain naturally (do not blow out the last drop; it is accounted for in calibration).

3. Conical Flask

• Purpose:
  Contains the solution from the pipette for the titration reaction.
• Shape Advantage:
  Narrow neck reduces splashing, wide base allows easy swirling to mix reactants.

4. Suitable Indicator

• Purpose:
  Shows a visible colour change near the equivalence point (when stoichiometric amounts of acid and base have reacted).
• Common Indicators for Strong Acid–Strong Base titrations:
  • Phenolphthalein: Colourless in acid, pink in alkali; end-point is the first permanent pale pink colour.
  • Methyl Orange: Red in acid, yellow in alkali; end-point is the first appearance of orange.
• Choice Criteria:
  Indicator must change colour sharply at the equivalence point pH of the reaction.

Procedure for an Acid–Base Titration

1. Preparation
   • Rinse all apparatus with the correct solutions to avoid contamination or dilution.
   • Burette → solution from burette (acid if acid–base titration is in this order).
   • Pipette → solution to be measured (usually base in acid–base titration).
   • Conical flask → distilled water (removes impurities).
2. Filling the Burette
   • Close tap before filling.
   • Use funnel to add solution above 0.00 cm³ mark.
   • Remove funnel after filling to avoid drips changing volume.
   • Record initial reading to ±0.05 cm³.
3. Measuring the Fixed Volume with Pipette
   • Attach pipette filler, draw solution above calibration mark.
   • Release until the bottom of the meniscus is exactly at the mark (eye level).
   • Deliver into conical flask.
4. Adding the Indicator
   • Add 2–3 drops of the chosen indicator into the conical flask.
5. Titration Process
   • Place flask on white tile for better visibility of colour change.
   • Open tap to allow solution from burette to flow into flask while swirling continuously.
   • As end-point nears, slow down to dropwise addition.
   • Stop at the exact moment of permanent colour change.
6. Repeat for Accuracy
   • Perform a rough titration to estimate the end-point.
   • Perform at least two concordant titres (values within 0.10 cm³ of each other).
   • Use average of concordant results for calculations.

Identifying the End-Point

• The end-point is when the indicator changes colour permanently (lasting for at least 30 seconds).
• Examples:
  • With phenolphthalein:
    Colourless → pale pink (stop immediately at first permanent pale pink).
  • With methyl orange:
    Red → orange (stop as soon as orange appears).
• Avoid overshooting — adding even one drop past the end-point can cause a large error in calculated concentration.

Calculations After Titration

• Formula:
  c₁V₁ / n₁ = c₂V₂ / n₂
  where:
  • c = concentration (mol/dm³)
  • V = volume (dm³)
  • n = mole ratio from balanced chemical equation
• Example:
  If 25.0 cm³ of NaOH (c = unknown) is neutralised by 23.45 cm³ of 0.100 mol/dm³ HCl:
  Reaction: HCl + NaOH → NaCl + H₂O (1:1)
  Moles HCl = 0.100 × (23.45 ÷ 1000) = 2.345 × 10⁻³ mol
  Moles NaOH = 2.345 × 10⁻³ mol
  Concentration NaOH = (2.345 × 10⁻³) ÷ (25.0 ÷ 1000) = 0.0938 mol/dm³