Sustainable pH sensing: repurposing the anthocyanin-rich extract from Dioscorea alata for green analytical chemistry

Over the years, synthetic indicators have been the choice for acid-base titrations. However, due to environmental and safety concerns, the search for natural compounds that can act as acid-base indicators is important. This study aims to identify an eco-friendly natural indicator from the Dioscorea alata L. yam species. Dried D. alata yam was ground into a coarse powder and extracted with an acidified methanol-water mixture for 15 minutes. The resulting extract was tested for colour changes in the pH range from 1 to 12. An acid-base titration was carried out using the D. alata extract as the indicator. The results were compared with synthetic standard indicators (phenolphthalein, methyl orange, and methyl red). An acid-base test paper strip was developed using the D. alata extract to determine the acidity or basicity of an aqueous solution. The results showed a clear color change in the extract from red to dark mustard when the pH changed from 1 to 12, and the UV-Vis spectrum showed a hypsochromic to bathochromic shift when the pH of the medium changed from acidic to basic. The D. alata L. yam extract performed well in strong acid-strong base, strong base-weak acid, and strong acid-weak base titrations, and it showed sharp color changes at the endpoints of the titrations, which coincided with the endpoints obtained from the standard synthetic indicators. The acid-base test paper strips developed were red in colour; they remained red in the acidic medium and changed to yellow in the basic medium. Thus, the D. alata yam extract has potential for application as a natural and safe alternative to the conventional acid-base indicators currently used in chemical laboratories.