Here, students determine aspirin (acetylsalicylic acid) mass in pharmaceutical tablets using a colorimetric method. Aspirin, salicylate, and salicylic acid do not absorb visible light. Thus, in alkaline medium, acetylsalicylic acid was hydrolyzed to salicylate; then, it was reacted with an acidic Fe(III) solution, and a violet complex was formed.…

Here, students determine the total phenolic content in beers using the Folin-Ciocalteu assay. The Folin-Ciocalteu reagent is a yellow complex, in an alkaline medium; it reacts with phenols and non-phenolic reducing substances to form a blue complex. Quantitative analysis was carried out using absorbance measured at 765 nm (standard method) and…

The Griess assay is widely used by regulation agencies as an official method for nitrite quantification in water and food samples. In Brazil, the official method, which has been used to determine nitrite in food, was described by Instituto Adolfo Lutz (283/IV) in 1984. It uses 8 mL of reactants and provides 50 mL (reactants plus sample) of waste…

This experiment describes a simple protocol for teaching acid-base titrations using potentiometry, conductivity, and/or photometry to determine end points without an added indicator. The chosen example examines the titratable acidity of a red wine with NaOH. Wines contain anthocyanins, the colors of which change with pH. Importantly, at the…