Aspects of visible spectrophotometry can be presented to students using simple experiments in which the color of the crude extract of Macroptilium lathyroides (L.) Urb. is bleached in the presence of nitrite ions in acidic medium. The dependence of the absorption intensity with time, the reaction completeness and the Beer law can be demonstrated. Quantitative results for mineral water samples "contaminated" with nitrite ions were obtained from a method based on the Griess reaction and a procedure based on the bleaching reaction between the crude extract and NO2- ions. Both the Griess and the bleaching reactions were found to be time dependent. Recoveries of about 100 - 104% were obtained with these procedures. The use of natural dyes attracted students' interest enhancing the teaching process. Experiments performed by the teaching staff suggested that the proposed methodology can be performed in a 4 h class, with relative errors ranging from 0.19 to 1.86% in relation to the Griess method.
acid-base chemistry; instrumental methods; water
