Detection of nitrite in water using minoxidil as a reagent

Water analysis is one of the most important issues in environmental chemistry. The increasing scale of water contamination owing to the presence of nitrate and nitrite in the environment has converted it into one of the most serious public health problems in modern society. Here an easy colorimetric method for determining nitrite in water is reported. The method is based on the reaction of nitrite with minoxidil in acidic media, which gives nitrosominoxidil as a product that shows an absorption band in the λ = 315-330 nm range. Minoxidil was the first drug approved by the U.S. Food and Drug Administration as a treatment against alopecia (hair loss), thus leading minoxidil to be one of the most popular and commonly used drugs that can be purchased freely at low cost on the market in the form of topical solutions. The experiment can be completed over 3.5 h, and it can be extended to include a kinetic mechanistic study of the nitrosation reaction. The nitrite detection range makes the method suitable for environmental, food, and physiological analytical applications. By using a brand-name product, student curiosity and interest is kept high throughout the experiment. Finally, questions are provided in the student handout, requiring the students to engage further in topics associated with the context of this practical work.