Acid-Base Titration (WJEC GCSE Chemistry: Combined Science)

Introduction

• A titration is carried out to determine the reacting volumes of a strong acid and a strong alkali by titration
• The titration method can be used to calculate the concentration of an acid

Apparatus

• 25 cm³ volumetric pipette
• Pipette filler
• 50 cm³ burette
• 250 cm³ conical flask
• Small funnel
• 0.1 mol dm^–3 sodium hydroxide solution
• Hydrochloric acid of unknown concentration
• Phenolphthalein indicator
• Clamp stand, clamp & white tile

Diagram

Apparatus used in a titration

Performing a titration requires a burette, conical flask and pipette

Method

1. Use the pipette and pipette filler and place exactly 25 cm³ sodium hydroxide solution into the conical flask
2. Fill the burette with hydrochloric acid , place an empty beaker underneath the tap. Run a small portion of acid through the burette to remove any air bubbles
3. Record the starting point on the burette to the nearest 0.05 cm³
4. Place the conical flask on a white tile so the tip of the burette is inside the flask
5. Add a few drops of a suitable indicator to the solution in the conical flask
6. Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm³ portions, while swirling the flask vigorously
7. Quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus
8. Now repeat the titration with a fresh batch of sodium hydroxide
9. As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour
10. Record the volume to the nearest 0.05 cm³
11. Repeat until you achieve two concordant results (two results that are within 0.1 cm³ of each other) to increase accuracy

Practical Tip

• Use a funnel to fill the burette but be sure to remove it before starting the practical as it can drip liquid into the burette, making the initial reading false

Analysis of Results

Results 

Table of results

Evaluation

• Calculate mean titre using concordant results 
• Mean titre =  = 14.95 cm³

Conclusion

• The mean titre is used to determine the relative concentrations of both solutions
• 14.95 cm³ of hydrochloric acid was required to neutralise 25 cm³ 0.1 mol dm^–3 sodium hydroxide therefore the acid is more concentrated than the alkali