Analytical Chemistry
Acid-Base Titration
Determining concentration through controlled neutralization — equivalence and endpoint
Acid-base titration is a technique to determine the concentration of an acid or base by reacting it with a solution of known concentration. Add titrant slowly until reaction complete (equivalence point — moles of acid = moles of base). Detected by indicator color change (endpoint) or pH meter. Key relations: M_a × V_a = M_b × V_b (for monoprotic, 1:1). Titration curves show pH vs volume — characteristic shapes for strong-strong, weak-strong, etc. Methods: weighed standards, primary standards (KHP for bases, sodium carbonate for acids).
- PrincipleM_a × V_a = M_b × V_b (1:1 stoichiometry)
- Equivalence pointMoles acid = moles base
- EndpointIndicator color change
- Common indicatorsPhenolphthalein (8-10), methyl orange (3.1-4.4), bromothymol blue (6-7.6)
- Strong-strong pH at eq7
- Weak acid + strong base pH> 7 (basic salt)
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Why titration matters
- Quantitative analysis. Determine concentrations precisely.
- Quality control. Pharmaceuticals, food.
- Environmental. Water analysis, acidity.
- Research. Determining Ka, Kb values.
- Industrial. Process monitoring.
- Education. Foundational lab technique.
- Forensics. Drug analysis, evidence.
Common misconceptions
- Equivalence pH is always 7. Only for strong-strong.
- Endpoint = equivalence point. Endpoint slightly off; choose indicator carefully.
- Stronger acid means lower pH at start. Concentration matters too.
- Indicators always work. Must match equivalence pH.
- NaOH solutions are stable. Absorb CO₂ over time.
- Titration is just neutralization. Includes redox, complexometric, precipitation.
Frequently asked questions
How does titration work?
Add standard solution (titrant) of known concentration via burette to unknown sample. Reaction proceeds. Equivalence point: stoichiometrically equivalent. Detected by: (1) Indicator color change. (2) pH meter reading. (3) Conductivity change. From volume of titrant used + concentration: calculate moles → original concentration.
What's the equivalence point?
When stoichiometry is satisfied — equal moles of acid and base for 1:1 reaction (HCl + NaOH). For polyprotic acids, multiple equivalence points (e.g., H₂CO₃: at OH⁻ neutralizing first H⁺, then second). pH at equivalence depends on salt formed. Strong acid + strong base: pH = 7. Weak acid + strong base: pH > 7 (conjugate base hydrolyzes).
How do indicators work?
Weak acid/base. Different color in conjugate forms. HIn (color 1) ⇌ H⁺ + In⁻ (color 2). pH change shifts equilibrium → color change. Each indicator has working pH range (~2 pH units). Choose indicator whose range matches equivalence pH. Phenolphthalein: colorless → pink at pH ~9; for weak acid + strong base titration.
What's a titration curve?
Plot pH vs volume of titrant. Strong acid + strong base: gentle curve, sharp jump at equivalence (pH 7). Weak acid + strong base: starts higher pH; "buffer region" before equivalence (Henderson-Hasselbalch); equivalence pH > 7. Weak base + strong acid: mirror image. Polyprotic: multiple equivalence points.
What's the half-equivalence point?
For weak acid + strong base titration. Halfway to equivalence: half the weak acid converted to conjugate base. At this point: [HA] = [A⁻]; pH = pKa (Henderson-Hasselbalch). Useful for: determining pKa of weak acid by reading pH at half-equivalence on titration curve.
What's a primary standard?
Substance used to calibrate other solutions. Properties: high purity, stable, non-hygroscopic, accurate molar mass. Common: (1) KHP (potassium hydrogen phthalate) — for bases (acidic). (2) Sodium carbonate (or sodium oxalate) — for acids. Why needed: NaOH solution absorbs CO₂ over time; can't trust labeled concentration. Standardize against KHP.
What about polyprotic acids?
Acids with multiple ionizable H (H₂SO₄, H₃PO₄). Each H has different Ka. H₂SO₄ very strong (1st H), strong (2nd H). H₃PO₄: pKa1 ≈ 2.1, pKa2 ≈ 7.2, pKa3 ≈ 12.4. Titration curve: multiple steps, multiple equivalence points. Can determine each Ka if curves separated enough.