Evaluating the measurement uncertainty in determination of the content of benzylpenicillin sodium for injection by acid–base titration

Abstract

This evaluation focused on measurement uncertainty in determining the content of benzylpenicillin sodium in injections using acid–base titration. The process involved β-lactam ring hydrolysis and back-titration of excess alkali with hydrochloric acid (HCl). The study identified and quantified uncertainty sources including repeatability, weighing, volumetric measurements (burette), and titrant concentration. The combined standard uncertainty was calculated using a spreadsheet approach adhering to GUM principles, and the impact of room temperature variations and triple determinations was assessed. The average benzylpenicillin sodium content was determined to be 96.9% with an expanded uncertainty (k = 2) of ± 2.2%. Triple determination reduced the contribution from repeatability, but volumetric uncertainties and titrant concentration uncertainty remained substantial contributors. Correcting for a mean room temperature of 25 °C did not markedly alter the result. Incorporating all quantified uncertainty components provided a reliable estimate of benzylpenicillin sodium content with defined confidence intervals. However, the inherent limitation of titration's specificity and the need for full metrological traceability of titrants are important considerations.