Titrations (OCR GCSE Chemistry A (Gateway))

Titrations

• Titrations are a method of analysing the concentration of solutions

• Acid-base titrations are commonly used to determine exactly how much alkali is needed to neutralise a quantity of acid – and vice versa

• You may be asked to calculate the moles present in a given amount, the concentration or volume required to neutralise an acid or a base

• Titrations can also be used to prepare salts or other precipitates and in redox reactions

• Indicators are used to show the endpoint in a titration, with phenolphthalein being a popular choice

  • Wide range indicators such as litmus are not suitable for a titration as they do not give a sharp enough colour change at the end point

  • Universal indicator is also not suitable as it is actually a mix of indicators and has too many subtle colour changes

• Some of the most common indicators with their corresponding colours are shown below

Table to Show the Colours of Common Indicators

Titration Technique

The titration method can be used to calculate for many different types of analysis.

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

Here is an example of an acid-base titration being used to calculate the concentration of an acid:

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 indicator
Clamp stand, clamp & 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

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. Swirl after each addition and rinse the sides of the flask down with distilled water to make sure that all that was added has reacted

9. Finish at the first sign of a colour change that persists after swirling

10. Record the volume to the nearest 0.05 cm³

11. Repeat until you achieve two concordant results (two results that are within 0.1cm3 of each other) to increase accuracy

Results:

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

Titration Results Table

Evaluation: Only concordant results should be used to calculate a mean titre. This can include the rough if it was concordant.

Conclusion:

The mean titre is calculated and used to calculate the concentration of the acid in mol/dm³, ignoring any anomalous results. This is a common assessment question, so check results carefully.