The operation of measuring the concentration of an acid or base using a solution with a known concentration of the other is called neutralization titration.
Basic Knowledge
The basic equation (quantitative relationship) for neutralization titration is as follows.
Acid valency × acid molar concentration × acid volume = base valency × base molar concentration × base volume
Using symbols: a・ca・Va = b・cb・Vb
(a, b: valency, c: mol/L, V: L)
Example: What volume of 0.10 mol/L NaOH is required to neutralize 20.0 mL of 0.10 mol/L HCl?
1×0.10×20 = 1×0.10×Vb → Vb = 20 mL.
Equipment used: volumetric pipette (to take a solution accurately), burette (to measure the volume added dropwise), conical beaker, and volumetric flask (for dilution). The color-change point is read using an indicator (near the equivalence point).
Neutralization titration（The operation of determining the concentration of a solution with an unknown concentration using a solution with a known concentration）
Burette（Equipment used to read the volume added dropwise to an accuracy of 0.05 mL）
Volumetric pipette（Equipment used to take a fixed volume of solution accurately）
Volumetric flask（Equipment used to prepare a solution of a fixed volume. Used for dilution）
Equivalence point（The point at which the amounts of acid and base react completely without either being in excess）
Titration curve（A graph of pH against the volume of base (acid) added. It changes rapidly near the equivalence point）
Further Study (Background・Significance)
The choice of indicator is determined by the pH at the equivalence point.
・Strong acid + strong base: Equivalence-point pH ≒ 7. BTB, methyl orange, and phenolphthalein can all be used.
・Weak acid + strong base (CH3COOH + NaOH): Equivalence-point pH > 7. Use phenolphthalein.
・Strong acid + weak base (HCl + NH3): Equivalence-point pH < 7. Use methyl orange.
・Weak acid + weak base: Because the titration curve does not change rapidly, this combination is not suitable for neutralization titration.
When handling equipment, the distinction between rinsing with the solution (rinsing the volumetric pipette and burette with the solution inside) and rinsing with pure water (using only water for the conical beaker and volumetric flask) is frequently tested.
- Quantitative relationship in neutralization: a・ca・Va = b・cb・Vb
- Valency × concentration × volume is equal on both sides
- Equivalence-point pH for a strong acid + strong base ≒ 7
- Equivalence-point pH for a weak acid + strong base > 7 → phenolphthalein
- Equivalence-point pH for a strong acid + weak base < 7 → methyl orange
- Rinsing with the solution: volumetric pipette・burette
- Rinsing with pure water: conical beaker・volumetric flask
Points to Note (Easily Confused)
① The equation for neutralization titration must always include valency. For H2SO4, a=2. ② The equivalence point and pH=7 are not the same. The equivalence point of a weak acid + strong base has pH > 7. ③ Do not reverse rinsing with the solution and rinsing with pure water. Rinsing a conical beaker with the solution changes the amount of solute. ④ The graduations on a burette increase downward. The difference between the initial and final readings is the volume added.
Practice
- How many mL of 0.10 mol/L NaOH are required to neutralize 10 mL of 0.10 mol/L H2SO4?
- 20 mL of HCl with an unknown concentration was exactly neutralized by 25 mL of 0.10 mol/L NaOH. Find the molar concentration of the HCl.
- When acetic acid is titrated with NaOH, name a suitable indicator.