Core Practical 2: Acid–Base Titration

Chemical Reaction

Equation:
NaHCO₃(aq) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(l)

1:1 ratio of NaHCO₃ with HCl

Note from Matt – you may have carried out a different practical to the example here or you have used different reagents. That is fine, don’t worry – the key thing is that you know how to accurately carry out a titration and make the required calculations.

Safety and Disposal

• Wear eye protection.
• Hydrochloric acid causes severe skin burns and eye damage.
• Sodium hydrogencarbonate is low hazard.

Apparatus and Chemicals

Equipment

• Burette (50 cm³), pipette (25.0 cm³), pipette filler
• Conical flask (250 cm³)
• Volumetric flask (250 cm³)
• Funnel
• White tile
• Wash bottle with distilled water
• Clamp stand and burette clamp
• Glass beaker, stirring rod

Chemicals

• Sodium hydrogencarbonate (NaHCO₃) — solid
• Hydrochloric acid (HCl), approximately 0.1 mol dm⁻³ (standard solution)
• Methyl Orange indicator

Method

A. Preparing the Standard Solution of Sodium Hydrogencarbonate

1. Weighing: Accurately weigh ~2.10 g of NaHCO₃ using a weighing boat and 2 dp balance. Transfer to a 250 cm³ beaker. Rinse any remaining solid into the beaker with distilled water.
2. Dissolving: Add about 100 cm³ distilled water and stir to dissolve.
3. Transferring and Diluting: Transfer solution to a 250 cm³ volumetric flask using a funnel.

4. Rinse the beaker and funnel with distilled water and add washings to the flask.
5. Fill up to the mark with distilled water using a dropping pipette for the final few drops.
6. Stopper the flask and invert several times to ensure uniform concentration.

B. Titration Procedure

1. Setup: Rinse burette with HCl and pipette with NaHCO₃ solution. Fill the burette with HCl and record initial volume.

2. Use pipette to transfer 25.0 cm³ of NaHCO₃ solution into a conical flask.
3. Add 2–3 drops of Methyl Orange indicator.

4. Titrating: Place flask on a white tile. Perform a rough titration first until colour changes from yellow to red.
5. Repeat, adding HCl dropwise near the end point until permanent colour change from yellow to red.
6. Record final burette reading and calculate titre.
7. Repeat until you have two concordant results within 0.1 cm³.

Example Calculations

Equation: NaHCO₃(aq) + HCl(aq) → NaCl(aq) + CO₂(g) + H₂O(l)

Suppose average HCl titre = 24.50 cm³
Mass of NaHCO₃ used = 2.10 g

1. Moles of NaHCO₃ in 250.0 cm³:
   Molar mass of NaHCO₃ = 84.0 g mol⁻¹
   Moles = 2.10 ÷ 84.0 = 0.02500 mol
2. Concentration of NaHCO₃:
   Volume = 0.2500 dm³
   Concentration = 0.02500 ÷ 0.2500 = 0.100 mol dm⁻³
3. Moles of NaHCO₃ in 25.0 cm³ sample:
   Volume = 0.0250 dm³
   Moles = 0.100 × 0.0250 = 2.50 × 10⁻³ mol
4. Moles of HCl (1:1 ratio):
   = 2.50 × 10⁻³ mol
5. Average titre = 24.50 cm³ = 0.02450 dm³
   Concentration of HCl = 2.50 × 10⁻³ ÷ 0.02450 = 0.102 mol dm⁻³

Sources of Error and Improvements

• Apparatus error: Burette and pipette tolerances ±0.05 cm³.
• Indicator: End point can be subjective. Use a white tile for clearer view.
• Standard solution: Inaccurate mass or incomplete transfer affects concentration.
• Meniscus error: Always read from the bottom at eye level.

Improvements

• Use more precise balances for weighing.
• Perform multiple repeats for reliability.
• Ensure thorough mixing and accurate rinsing of apparatus.