Acid-base indicator properties of dyes from local plants I: Dyes from Basella alba (Indian spinach) and Hibiscus sabdariffa (Zobo)

The acid-base indicator properties of aqueous and ethanol extracts from calyces of H. sabdariffa (Zobo) and a dye obtained from the ripe fruits of Basella alba (Indian spinach), two local plants, were investigated. A purple coloured dye obtained from the ripe fruits of Basella alba showed a λmax at 580nm, absorptivity of 0.2269 and was found to be photochemically unstable. A deep red coloured dye obtained from aqueous and ethanol extracts of H. sabdariffa had λmax of 520nm and 540nm and absorptivities of 0.1909 and 0.1187 respectively. The peaks are associated with n→Л* transitions. In strong acid/strong base titrations using the dyes as indicators, the end-points obtained agreed well with those obtained using conventional indicators. The dyes were found not suitable for weak acid/weak base titrations. The Ka of the purple dye from Basella alba was of the order of 10^-5 while that of the red dye from H. sabdariffa was of the order of 10^-6.