An Easy and Versatile Experiment to Demonstrate Solvent Polarity Using Solvatochromic Dyes

In this easy and versatile experiment, undergraduate students of experimental physical chemistry can discuss the concept of media polarity on the basis of the solvatochromic approach. Two solvatochromic dyes, Reichardt's betaine 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenoxide and Brooker's merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one were used to construct polarity scales for various solvents by means of visual observation and UV-vis spectroscopy. Changes in media polarity arising from mixed solvents or added electrolytes are also easily demonstrated. The data are used to justify kinetic and spectroscopic parameters.