Understanding Acid-Base Titration Curves

Expert reviewed • 22 November 2024 • 6 minute read

Introduction

Titration curves are graphical representations of pH changes during acid-base titrations. These curves provide crucial information about the neutralization process and help in selecting appropriate indicators for accurate endpoint detection.

Key Concepts

Equivalence Point vs. Endpoint

  • The equivalence point occurs when acids and bases react in their exact stoichiometric ratios
  • The endpoint is the point where an indicator changes color
  • For accurate titrations, these points should coincide as closely as possible

Common stoichiometric ratios in acid-base reactions:

1:1 ratio: NaOH+HCl→NaCl+H2O\text{1:1 ratio: NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} 2:1 ratio: 2NaOH+H2SO4→Na2SO4+2H2O\text{2:1 ratio: 2NaOH} + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{SO}_4 + \text{2H}_2\text{O} 3:1 ratio: 3NaOH+H3PO4→Na3PO4+3H2O\text{3:1 ratio: 3NaOH} + \text{H}_3\text{PO}_4 \rightarrow \text{Na}_3\text{PO}_4 + \text{3H}_2\text{O}

Types of Titration Curves

1. Strong Acid-Strong Base Titration

  • Produces a neutral salt (pH = 7 at equivalence point)
  • Shows a sharp vertical change at equivalence point
  • Example reaction: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)\text{NaOH}_{(aq)} + \text{HCl}_{(aq)} \rightarrow \text{NaCl}_{(aq)} + \text{H}_2\text{O}_{(l)}

Suitable indicators:

  • Bromothymol blue (pH 6.0-7.4)
  • Phenolphthalein (pH 8.3-10.0)
  • Methyl orange (pH 3.2-4.4)

2. Strong Acid-Weak Base Titration

  • Produces an acidic salt (pH < 7 at equivalence point)
  • Example reaction: HCl(aq)+NH3(aq)→NH4Cl(aq)\text{HCl}_{(aq)} + \text{NH}_3{(aq)} \rightarrow \text{NH}_4\text{Cl}_{(aq)}

The acidic nature results from the hydrolysis of ammonium ions:

NH4(aq)++H2O(l)⇌NH3(aq)+H3O(aq)+\text{NH}_{4(aq)}^{+} + \text{H}_{2}\text{O}_{(l)} \rightleftharpoons \text{NH}_{3(aq)} + \text{H}_{3}\text{O}_{(aq)}^{+}

Best indicator: Methyl orange (pH 3.2-4.4)

3. Weak Acid-Strong Base Titration

  • Produces a basic salt (pH > 7 at equivalence point)
  • Example reaction: CH3COOH(aq)+NaOH(aq)→NaCH3COO(aq)+H2O(l)\text{CH}_3\text{COOH}_{(aq)} + \text{NaOH}_{(aq)} \rightarrow \text{NaCH}_3\text{COO}_{(aq)} + \text{H}_2\text{O}_{(l)}

The basic nature results from acetate ion hydrolysis: CH3COO(aq)−+H2O(l)⇌CH3COOH(aq)+OH(aq)−\text{CH}_3\text{COO}^-_{(aq)} + \text{H}_2\text{O}_{(l)} \rightleftharpoons \text{CH}_3\text{COOH}_{(aq)} + \text{OH}^-_{(aq)}

Best indicator: Phenolphthalein (pH 8.3-10.0)

4. Weak Acid-Weak Base Titration

  • Final pH depends on relative Ka and Kb values:
    • If Ka > Kb: pH < 7
    • If Ka < Kb: pH > 7
  • Generally avoided due to unreliable endpoints

Buffer Regions in Titration Curves

Buffer regions appear when the solution contains significant amounts of both a weak acid/base and its conjugate. These regions resist pH changes and appear as flattened sections on titration curves.

Key Characteristics of Buffer Regions:

  • Form during titrations involving weak acids or bases
  • Most prominent at half-equivalence point
  • Buffer capacity depends on relative concentrations

Polyprotic Acid Titrations

Polyprotic acids show multiple equivalence points corresponding to each ionizable hydrogen.

Example: Phosphoric acid (H₃PO₄) shows three distinct equivalence points during titration.

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