Thiopentone pKa — FRCA Primary MCQ

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ModeratePharmacology - Intravenous AnaestheticsThiopentone pKaFRCA Primary

Thiopentone is a weak acid with a pKa of 7.6. At a physiological pH of 7.40, approximately what percentage of thiopentone is unionised?

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Correct answer: B — 60%

Explanation lettering: D = shown as B · B = shown as D

The correct answer is D, 60%. Thiopentone is a weak acid, so its unionised form is HA and its ionised form is A−. Applying the Henderson–Hasselbalch equation gives [A−]/[HA] = 10^(7.40−7.60) = 0.63. The unionised fraction is therefore 1/(1+0.63) = 0.61, approximately 60%. Option B, 40%, approximates the ionised rather than the unionised fraction. Option C may result from incorrectly treating the 0.2-unit pH–pKa difference as a simple percentage rather than a logarithmic relationship. Options A and E substantially overestimate the effect of this relatively small difference between pH and pKa.

Reference: Baum GA. From pKa to the pH of weak base solutions. Biochemistry and Molecular Biology Education. 2020;48:131–142. https://pubmed.ncbi.nlm.nih.gov/32040240/; Thiopental powder for solution for injection, Summary of Product Characteristics, section 6.2, updated 2023. https://www.medicines.org.uk/emc/product/665/smpc