The laboratory experiment challenges students to determine 6-propyl-2-thiouracil (PTU) based on Prussian blue complex formation. Prussian blue is formed by ferricyanide and Fe(II) ions which are generated in situ from Fe(III) ions reduced by PTU. The absorbance of this product was measured at a wavelength of 840 nm, after a reaction time of 30 min. The range of determination was 0.34-3.4 µg mL[superscript -1] of PTU per sample. The method was successfully used in an upper-division instrumental analysis laboratory course to determine the concentration of PTU in commercially available pharmaceuticals. Additionally, students in introductory or high school chemistry classes by conducting this experiment will gain a greater understanding of the chemical analysis process, redox reactions, and the Beer-Lambert law.