Application of anthocyanins as indicators for alkalimetric titration

Abstract

Along with synthetic substances, various naturally-occurring pigments can response to pH variations in solutions by changing their colour. Such substances include, for example, anthocyanins. In this study, the possibility of applying a set of anthocyanins obtained from berry extracts as a potential acid-base indicator. This indicator can be a good alternative to the widely-used phenolphthalein test. Aqueous and alcoholic extracts derived from the fruit of ashberry, sea-buckthorn, lingonberry, blueberry, currant, blackberry, cranberry, rose hip, cherry and hawthorn plants were studied. The presence of anthocyanins in the test samples was confirmed by chemical and physicochemical methods of analysis, including qualitative tests, ascending thin-layer chromatography and direct spectrophotometry. Quantification of anthocyanins was carried out by pH-differential spectrophotometry. Compared to alcoholic extracts, aqueous berry extracts were found to contain anthocyanins in far lesser amounts. In order to compare the pH indicator potential of phenolphthalein and the studied set of anthocyanins, the content of ascorbic acid in the test samples was assessed by direct alkalimetry. A control experiment was performed using a 5% ascorbic acid solution for injection. The pH value of aqueous and alcoholic solutions of all berry extracts was determined by an instrumental procedure. The pH of the extracts ranged from 2.95 to 5.04. The content of the total amount of anthocyanins in aqueous and alcoholic extracts of blackberries, cranberries, black currants, blueberries and lingonberries in terms of cyanidin-3-glucoside was determined both quantitatively and qualitatively. A good agreement was achieved between the results of alkalimetric titration performed using phenolphthalein and the set of anthocyanins extracted from blackcurrant berries with 95% ethanol.