A Practical Example on Uncertainty Evaluation: Spectrophotometric Determination of pK a

As it can be read in EURACHEM/CITAC Guide for quantifying uncertainty in analytical measurements [1], many important decisions are based on the results of chemical quantitative analysis and, for this reason, is very important to ensure the quality of analytical methods. Nowadays, there has been a great emphasis on the precision of results obtained using a specified analytical method and in establishing their traceability to a defined standard. For this aim, internal quality control procedures, proficiency testing an accreditation have become very useful tools. Chemists are being asked to demonstrate the quality of their results, including their agreement with others results, being measurement uncertainty one useful measure of this. As a consequence of these requirements, our students are expected to be able to apply measurement uncertainty evaluation and propagation. In this work, we propose a training practical example that can be used by people learning about the evaluation of uncertainty in measurements and its propagation in different cases. The chosen example is the photometric determination of the ionization constant of acetaminophen by measuring the absorbance of an acetaminophen solution at different pH values. According to Hildebrand and Reilley method [2], the next relation can be established: