Avaliação da utilização do extrato da palha de espigas do milho da variedade palha roxa como indicador natural ácido-base

Abstract

Through history, different theories have been developed in order to explain acid base concepts. In chemistry education, it is attributed to Arrhenius (1859-1927) the most widespread theory. In this theory, substances which, in aqueous medium, produce cations H+ are classified as acids, whereas bases are substances which, in same condition, produce anions OH-. In aqueous medium, H+ concentration can be expressed as units of pH (hydrogenic potential), with values ranging from 0 to 14. Besides equipment uses, there are substances capable of evaluate, by chemical reactions, if a compound shows acid or base character. These substances, natural or synthetic, are known as pH indicators. Among the natural compounds whose colors changes depending on pH values, anthocyanins are highlighted. This metabolite class is mainly found in flowers, leaves, fruits, barks, and seeds, usually, present the colors pink, red, purple, blue, and black. The use of corn cob straw of the “Palha Roxa” variety to produce natural pH indicator is in accordance with green chemistry principles since it represents the use of an agricultural residue and the decrease of non-renewable feedstocks dependence and its environmental impacts. Three different solvents were used for extract production: distilled water, ethanol 50 % (acidified with HCl 1 %), and ethanol 96 o GL. Qualitatively, aqueous extract, obtained by decoction, presented better results once it presented sharper colors in the pH scale. In this context, the use of water, a renewable, nontoxic, and accessible solvent, is recommend for educational purposes. Therefore, this work contributed to the advance in knowledges related to natural pH indicators area outstanding the need of future investigations concerning the use of corn cob straw of the “Palha Roxa” variety as sustainable resource