Anthocyanins, pigments of flavonoids class, are the most important chromophors found in blue, red and purple vegetal tissues, whose color changes depending on the pH. The goal of this work was to propose a simple and cheap experiment to chemical education, using vegetal species easily found in Brazil. Such class of comounds presents a potential interest to students and teachers alike, to be useful in illustrating chemical principles such as acid-base equilibria, pKa, light absorption, effects of changes in conjugated double bounds, etc. The specie used was Solanum nigrum L (maria-preta) whose fruits crude extract was used to obtain molecular absorption spectra, to verify the Lambert-Beer law and as an indicator in acid-base titrations. The end point results obtained with the crude extracts when compared to the glass electrode ones showed relative errors between 0,1 and 1,0%. The color changed from red to yellow between pH 4 and 10. To determine the highest absorption wavelenght and demonstrate the shape changing of spectra in function of the acidity, the absorption spectra in UV and visible regions were obtained in different pH values.
Anthocyanins; Chemical education; Natural indicators
