Acid-Base Titrations: A Complete Guide
Acid-base titrations are fundamental analytical procedures used to determine the concentration of an acid or base in a solution. This guide covers theory, calculations, and titration curves.
What is an Acid-Base Titration?
Definition: An acid-base titration is a technique used to measure the volume of an acid (or base) of known concentration required to completely neutralize a known volume of a base (or acid) of unknown concentration.
The Neutralization Reaction
The basic principle is:
H⁺(aq) + OH⁻(aq) → H₂O(l)
Or in molecular form:
Acid + Base → Salt + Water
Titration Procedure and Apparatus
Standard Equipment:
- Burette (containing titrant – the standard solution)
- Conical flask (containing the analyte – the solution being titrated)
- Pipette (for measuring precise volumes)
- Indicators (for end-point detection)
Types of Acid-Base Titrations
1. Strong Acid – Strong Base Titration
Example: HCl + NaOH
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Characteristics:
- Equivalence point pH = 7
- Sharp vertical section in titration curve at equivalence point
- Any indicator can be used
- pH range at equivalence point: 4-10
Titration Curve Analysis:
- Before equivalence point: pH rises gradually (buffer region)
- At equivalence point: Steep vertical rise
- After equivalence point: pH rises gradually
2. Strong Acid – Weak Base Titration
Example: HCl + NH₃
HCl(aq) + NH₃(aq) → NH₄Cl(aq)
Characteristics:
- Equivalence point pH < 7 (around pH 5-6)
- Steep vertical section slightly below pH 7
- Suitable indicators: Methyl orange or methyl red
- NOT suitable: Phenolphthalein
Why pH < 7? The salt formed (NH₄Cl) hydrolyzes:
NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq)
This produces H₃O⁺ ions, making the solution acidic.
3. Weak Acid – Strong Base Titration
Example: CH₃COOH + NaOH
CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)
Characteristics:
- Equivalence point pH > 7 (around pH 8-10)
- Steep vertical section slightly above pH 7
- Suitable indicator: Phenolphthalein
- NOT suitable: Methyl orange or methyl red
- Buffer region exists before equivalence point
Why pH > 7? The salt formed (CH₃COONa) hydrolyzes:
CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)
This produces OH⁻ ions, making the solution alkaline.
4. Weak Acid – Weak Base Titration
Characteristics:
- No sharp vertical section at equivalence point
- No suitable indicator with sharp color change
- These titrations are not recommended
- Conductivity measurements or potentiometric titration preferred
Worked Examples – Titration Calculations
Example 1: Strong Acid-Strong Base
Problem: 25 cm³ of 0.1M HCl is titrated with 0.1M NaOH. Calculate the volume of NaOH required for neutralization.
Solution:
HCl + NaOH → NaCl + H₂O 1:1 ratio n(HCl) = Molarity × Volume = 0.1 × 25/1000 = 0.0025 mol From stoichiometry, n(NaOH) = n(HCl) = 0.0025 mol Volume of NaOH = n / Molarity = 0.0025 / 0.1 = 0.025 dm³ = 25 cm³
Example 2: Weak Acid-Strong Base
Problem: 25 cm³ of 0.1M acetic acid is titrated with 0.1M NaOH. What is the pH at the equivalence point? (Ka for acetic acid = 1.8 × 10⁻⁵)
Solution:
At equivalence point: All acetic acid is converted to sodium acetate.
n(CH₃COOH) = 0.1 × 25/1000 = 0.0025 mol Volume of NaOH needed = 25 cm³ Total volume = 25 + 25 = 50 cm³ [CH₃COO⁻] = 0.0025 / (50/1000) = 0.05 M
The acetate ion undergoes hydrolysis:
CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq) Kb = Kw / Ka = 1×10⁻¹⁴ / 1.8×10⁻⁵ = 5.6 × 10⁻¹⁰ Using the weak base equation: [OH⁻] = √(Kb × [CH₃COO⁻]) = √(5.6×10⁻¹⁰ × 0.05) = 5.3 × 10⁻⁶ pOH = -log(5.3 × 10⁻⁶) = 5.28 pH = 14 - 5.28 = 8.72
Summary Table: Titration Types and Indicators
| Titration Type | Acid | Base | Equiv. Point pH | Indicator | pH Range |
|---|---|---|---|---|---|
| Strong-Strong | HCl, HNO₃ | NaOH, KOH | 7 | Any indicator | 4-10 |
| Strong-Weak | HCl | NH₃ | 5-6 | Methyl orange | 3.5-6.5 |
| Weak-Strong | CH₃COOH | NaOH | 8-10 | Phenolphthalein | 7.5-10.5 |
| Weak-Weak | CH₃COOH | NH₃ | Variable | None | Variable |
Key Points in Titration Curves
- Initial pH: Determined by the acid or base being titrated
- Buffer Region: Before equivalence point, where pH changes gradually
- Equivalence Point: Where moles of acid equal moles of base
- Steep Vertical Section: Where the indicator changes color sharply
- End Point: Where the indicator changes color (should coincide with equivalence point)
Practical Tips for Titrations
- Always start with burette below the zero mark
- Use only a few drops of indicator
- Perform at least 2-3 titrations for accuracy
- Discard the first (rough) titration result
- Use the average of concordant results
- Choose indicator based on equivalence point pH, not just color
