Aspirin – acetylsalicylic acid – is used to treat pain and to reduce fever, at low doses, to prevent — Physical Chemistry — Kinetics Chemistry Question
Determination of acetylsalicylic acid purity
Aspirin – acetylsalicylic acid – is used to treat pain and to reduce fever, at low doses, to prevent heart attacks, strokes, and blood clot formation. It is one of the most widely used medications in the world, with an estimated 80 billion of tablets of it being consumed each year. Aspirin tablets can be contaminated with salicylic acid – the reagent for synthesis of acetylsalicylic acid. The content of salicylic acid is controlled in accordance with national pharmacopoeias. It must not exceed 0.1 – 0.3 %. Redox titration using bromine as an oxidation agent is one of the possible techniques to determine salicylic acid in aspirin.
A portion of 4.4035 g of powdered aspirin tablets was dissolved in 250.0 cm3 volumetric flask. For redox titration, reagent (bromine) was generated in situ (in the same titration flask). Bromide-bromate mixture was made of potassium bromide (0.5950 g) and potassium bromate (0.1670 g) in 100 cm3 of water. 25.00 cm3 aliquot was transferred to a titration flask, also sulfuric acid and 20.00 cm3 of the above mixture was added. The volume was made up to 50.0 cm3 with water. Flask was corked up and stirred for 10 min.
Write the balanced equations of the reactions described.
Model Answer
5 KBr + KBrO3 + 3 H2SO4 → 3 Br2 + 3 H2O + 3 K2SO4
Acetylsalicylic acid does not react with bromine, but salicylic acid reacts as follows:
[VISUAL]
At what maximal pH bromide will react with bromate? The standard redox potentials are: E0(BrO3-,H+/Br2) = 1.52 V, E0(Br2/2Br-) = 1.09 V.
Model Answer
To predict the direction of a redox reaction, equilibrium constant must be calculated. As the reaction depends on the [H+] concentration, conditional equilibrium constant must be used. If it is larger than 1, then reaction proceeds and bromine forms.
log K’ = (10 * E'Δ) / 0.059 where 10 is the number of electrons.
E'Δ = E0'(BrO3-/Br2) - E0(Br2/2Br-) = (1.52 - 0.059 * 1.2 * pH) - 1.09
log K’ = 10 × (1.52 – 0.059 × 1.2 × pH – 1.09) / 0.059; pH = 6.07.
Solution obtained after filtration of the reaction mixture was titrated against a solution of NaAsO2 (c = 0.02015 mol dm-3) and a mean titre of 9.93 cm3 was obtained. Write the balanced equations of the reactions described.
Model Answer
Br2 + NaAsO2 + 2 H2O → 2 HBr + NaH2AsO4
Calculate the percentage of salicylic acid in tablets by mass.
Model Answer
n(KBr) = 0.5950 g / 119.00 g mol-1 = 5.00 mmol
n(KBrO3) = 0.1670 g / 167.00 g mol–1 = 1.00 mmol → n (Br2)total = 3.00 mmol in 100.0 cm3 and n(Br2)total = 0.600 mmol in 20.00 cm3.
n(NaAsO2) = n(Br2) = 0.02015 mol dm-3 × 9.93 cm3 = 0.200 mmol = excess of bromine
n(Br2) = 0.600 – 0.200 = 0.400 mmol (bromination of salicylic acid)
n(salicylic acid) = 0.400 / 3 in aliquot (25 cm3)
n(salicylic acid) = 0.400 mmol × 10 / 3 in 250 cm3
m(salicylic acid) = 0.400 mmol × 10 × 138.12 g mol–1 / 1000 / 3 = 0.1842 g
w(salicylic acid) = 0.1842 g / 4.4035 g = 0.0418 (4.18 %).
Do the tablets meet the regulatory requirements?
Model Answer
Impurity is present at a level greater than allowed.