OPTIMIZATION OF THE EXTRACTION OF ANTHOCYANINS PRESENT IN BLACK BEANS AND IMPREGNATION OF THE EXTRACT IN NATURAL POLYMERIC MATRIX FOR USE AS A pH INDICATOR. In this work, a method of extracting natural indicators, anthocyanins, present in black beans was optimized. The best extraction condition was obtained with 50 g of beans and 93 g of water, under agitation at 150 rpm, at a temperature of 40oC for 24 h. The wavelengths of greatest absorption for the indicator in its acidic and basic form were 523 nm and 620 nm respectively and these wavelengths were used to determine the pKa of the indicator. At least three pKa values were observed: 1.9; 7.2 and 10.7. A simplified condition for anthocyanins extraction, the same amount of water, tap water in these case, and beans kept together for 12 h without agitation and temperature control, was also evaluated. Both extracts were impregnated with qualitative filter paper, coffee filter paper and cotton buds and the materials impregnated with the indicator were tested with solutions of HCl, vinegar, NaOH and milk of magnesia. A comparison between the impregnated material and a universal pH indicator paper using buffer solutions pH 4.0, 7.0, 10.0 and 11.2 proved the applicability of the developed material since it presented adequate distinction in its color for the different pH conditions.