Describe the procedure for finding the percentage purity of aspirin in shop-bought tablets by back titration.

Weighed, crushed tablets are warmed with a measured excess of standard sodium hydroxide until the aspirin is fully hydrolysed. The cooled mixture is titrated with standard hydrochloric acid using phenolphthalein indicator.

Crushed tablets of mass 1.50 g are heated with 50.0 cm³ of 1.00 mol dm⁻³ NaOH, and the excess needs 34.0 cm³ of 1.00 mol dm⁻³ HCl. What is the percentage purity of the aspirin (Mr 180)?

The aspirin is 96.0% pure. NaOH reacting with aspirin = 0.0500 − 0.0340 = 0.0160 mol, so aspirin = 0.00800 mol. Mass = 0.00800 × 180 = 1.44 g, and 1.44 ÷ 1.50 × 100 = 96.0%.

Why are several aspirin tablets crushed together for analysis rather than one tablet used?

Individual tablets vary slightly in their aspirin content. A mixed sample of several tablets gives a representative average for the batch.

How is the iron content of an iron tablet found by titration with potassium permanganate?

The tablet is dissolved in dilute sulfuric acid and titrated with standard potassium permanganate solution. The end point is the first permanent pale pink colour, so no indicator is needed.

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Medicine, Drugs and Clinical Trials

Quantitative analysis of medicines and antibacterial bioassays

3 things to know
AS 6

Overview

Back titration of aspirin with a worked purity, permanganate titration of iron tablets, and ampicillin bioassays with dilutions, controls, reliability and risk assessment.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how the effect of ampicillin on bacterial growth can be investigated.

Key term

Why is the aspirin content of a tablet found by back titration rather than direct titration?

Key term

Write the equation for the reaction of aspirin with excess sodium hydroxide in a back titration.

Questions from this subtopic

Each one with the model answer underneath.

1
Why is the aspirin content of a tablet found by back titration rather than direct titration?
Model answer
Aspirin dissolves poorly in water and its ester group reacts only slowly with alkali at room temperature. A known excess of sodium hydroxide is heated with the aspirin, and the alkali left over is titrated.
2
Write the equation for the reaction of aspirin with excess sodium hydroxide in a back titration.
Model answer
The equation is C₉H₈O₄ + 2NaOH → CH₃COONa + HOC₆H₄COONa + H₂O. One mole of aspirin reacts with two moles of sodium hydroxide.
3
Describe the procedure for finding the percentage purity of aspirin in shop-bought tablets by back titration.
Model answer
Weighed, crushed tablets are warmed with a measured excess of standard sodium hydroxide until the aspirin is fully hydrolysed. The cooled mixture is titrated with standard hydrochloric acid using phenolphthalein indicator.
4
Crushed tablets of mass 1.50 g are heated with 50.0 cm³ of 1.00 mol dm⁻³ NaOH, and the excess needs 34.0 cm³ of 1.00 mol dm⁻³ HCl. What is the percentage purity of the aspirin (Mr 180)?
Model answer
The aspirin is 96.0% pure. NaOH reacting with aspirin = 0.0500 − 0.0340 = 0.0160 mol, so aspirin = 0.00800 mol. Mass = 0.00800 × 180 = 1.44 g, and 1.44 ÷ 1.50 × 100 = 96.0%.
5
Why are several aspirin tablets crushed together for analysis rather than one tablet used?
Model answer
Individual tablets vary slightly in their aspirin content. A mixed sample of several tablets gives a representative average for the batch.
6
How is the iron content of an iron tablet found by titration with potassium permanganate?
Model answer
The tablet is dissolved in dilute sulfuric acid and titrated with standard potassium permanganate solution. The end point is the first permanent pale pink colour, so no indicator is needed.

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