Titration (OCR Gateway GCSE Chemistry)

Titration of a Strong Acid & Strong Alkali

Practical Skills Development

• Use of appropriate apparatus and techniques for conducting and monitoring chemical reactions, including appropriate reagents and/or techniques for the measurement of pH in different situations
• Use of appropriate qualitative reagents and techniques to analyse and identify unknown samples or products including gas tests, flame tests, precipitation reactions, and the determination of concentrations of strong acids and strong alkalis
• Use of appropriate apparatus to make and record a range of measurements accurately, including mass, time, temperature, and volume of liquids and gases

Aim

• To titrate a strong acid against a strong alkali and find the concentration of the acid using an appropriate indicator

Health & Safety Aspects

Hazard symbols to show substances that are corrosive and harmful to health

• Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
• Acids and alkalis are corrosive and should be handled with care
• For both substances, avoid contact with the skin and use safety goggles
• A pipette should always be used with a safety filler to avoid contact with corrosive liquids

Materials

• 25 cm³ volumetric pipette
• Pipette filler
• 50 cm³ burette
• 250 cm³ conical flask
• Small funnel
• 0.1 mol/dm³ sodium hydroxide solution
• Sulfuric acid – concentration unknown
• Phenolphthalein or methyl orange indicator 
• Retort stand, clamp, boss & white tile

Performing a titration

Practical Tip:

• Make sure you remove the funnel after filling the burette as if left it can drop solution into the burette, leading to error

Method:

1. Use the pipette and pipette filler and place exactly 25 cm³ sodium hydroxide solution into the conical flask
2. Place the conical flask on a white tile so the tip of the burette is inside the flask
3. Add a few drops of a suitable indicator to the solution in the conical flask
4. Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm³ portions, while swirling the flask vigorously, rinsing down the sides with distilled water after each addition
5. Quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus
6. Now repeat the titration with a fresh batch of sodium hydroxide
7. 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
8. Record the volume to the nearest 0.05 cm³ and read from the bottom of the meniscus where possible
9. Repeat until you achieve two concordant results (two results that are within 0.1 cm³ of each other) to increase accuracy

Results

• Record your results in a suitable table, e.g.:

Titration Results Table

(Only concordant results should be used to calculate a mean titre)

Conclusion:

• The mean titre is calculated and used to calculate the concentration of the acid in mol/dm³, ignoring any anomalous results