Aspirin Titration – Determining the purity of aspirin

In this experiment you will determine the percentage purity of aspirin in two different strengths of over the counter aspirin tablets by performing a titration with sodium hydroxide.

The chemical name for aspirin is acetylsalicylic acid. The fact that it is an acid allows us to quantify the amount of aspirin in a solution by using an acid-base titration. In this experiment the base used is sodium hydroxide (NaOH). The chemical structureof aspirin is shown below:

Preparation

  • Make sure you wear gloves and safety goggles at all times.

Methodology

In all titration experiments careful measurements are essential for accurate results.

  1. Fill the burette with 0.1M NaOH solution using the funnel provided.
  2. Fill to the Zero mark.
  3. Ensure there are no bubbles by tapping the side of the burette.
  4. Grind up ONE aspirin tablet to a fine powder using the pestle and mortar.
  5. Place a clean, dry conical flask on a top balance.
  6. Zero the reading.
  7. Add all the powder to the conical flask using a spatula and record the weight of the powder on the table on page 4.
  8. Measure out accurately 10ml of 95% ethanol and transfer to the conical flask. Note: Ethanol is used in this case as aspirin does not easily dissolve in water.
  9. Add 25ml of distilled water to the conical flask.
  10. Add 4 drops of phenolphthalein indicator solution.
  11. Swirl the conical flask carefully until the powder is fully dissolved.
  12. Swirl for at least 3 minutes.
  13. Titrate carefully with sodium hydroxide.
  14. Swirl the flask continuously.
  15. The NaOH solution should be added very slowly.
  16. The end point is reached at the first instance of the pink colour persisting.
  17. Record the volume of the sodium hydroxide used on the table on page 4.
  18. Repeat this procedure twice for each tablet type.

To determine the percentage purity of aspirin in each tablet you will need the following information:

At room temperature 1 mole of aspirin reacts with 1 mole of sodium hydroxide.

Mass of 1 mole of aspirin = 180.2g

Formula 1:

% Aspirin in tablet =Mass of aspirin calculated by titrationX 100

Mass of weighed aspirin tablet

Formula 2:

% Aspirin in tablet=Mass of aspirin stated by manufacturerX 100

(Manufacturer)Mass of weighed aspirin tablet

Results

Aspirin Type / A / B
Measurement / 1 / 2 / 1 / 2
Mass of Powder (g)
Volume of NaOH added (ml) = Y
Moles of NaOH added to flask (mol) =Y x 0.1
1000
Moles of aspirin which reacted with NaOH (mol) = Z
(1:1 reaction so Z is the same as the number of moles of NaOH added to flask – above calculation).
Mass of aspirin which reacted with NaOH (g) = Z x Mass of 1 mole of aspirin (180.2g)
% aspirin in tablet calculated by titration = Formula 1 (on previous page)
% aspirin in tablet according to manufacturer = Formula 2 (on previous page)

Questions:

  1. What was the average percentage of aspirin within each tablet type?
  1. For each tablet type was the percentage aspirin the same as that stated by the manufacturer? Explain why your results may be different.

What do your results tell you? What factors may affect the purity of a product?

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Evaluate the accuracy of your results ensuring that you compare your results. Do you think that your results are accurate? Can you account for any differences because of your technique?

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Technician’s List

Apparatus: Each group will have

0.1M NaOH (300ml)

95% Ethanol (100ml bottle)

1 Burette & Stand

White Tile

Pipettes (1ml & 10ml)

Pipette filler (green)

6 Conical flasks

Pestle & Mortar

Measuring cylinder (10ml)

1 Filter funnel

Phenolphthalein solution (dropper bottle)

1 Wax pencil

Top pan balance

1 spatula

3 types of aspirin tablet (labelled A, B & C)

Practical 2

The students are supplied with 0.05M Potassium Permanganate but the bottles should be labelled as 0.005M Potassium Permanganate.

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