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. @JASEM In spite of the numerous instrumental techniques currently available for the chemical analyses of various samples, conventional methods of analyses are still relevant and find application in many situations. Some conventional analytical techniques that are still popular include gravimetry and titrimetry. In titrimetry, the equivalence point is usually determined by the end point in the titration. The end point in traditional titrimetry is usually indicated by some substances added into the analyte solution, which change colour immediately after the equivalence point has been attained. These substances are generally referred to as indicators. Several types of indicators are available for different types of titrimetric analyses. For acid-base titrations, organic dyes, which are either weak acids or bases, serve excellently as indicators. A large number of dyes are obtainable as natural products. In Nigeria, several workers have extracted a number of dyes from a variety of local plants. According to Akpuaka et al. (1998), Osabohien et al. (2002), the local plants Camwood, Redwood, Henna, Annato, Rothmania, Terminalia, Indiqovine, Kola, Banana, Tumeric, Roselle and Ginger all contain different types of dyes which are used for various purposes. The suitability of some of these dyes for dyeing purposes has been investigated on different types of fabrics. An evaluation of other properties of a number of dyes including synthetic dyes have also been reported by a number of workers (Obanda et al., 1997; Dambata et al.,1997). Ekandem et al. (1997) and Eze et al. (2002) have also reported their findings on the use of some natural dye extracts as indicators in acid-base titrimetry. Other than these few reported cases, very little attention has been paid to the use of local dye extracts as indicators in acidbase titrimetry. Basella alba (Indian spinach), called Ogborogi by the Izon people of the Niger Delta, is a very popular vegetable in many coastal communities of southern Nigeria. Recently, Odilora et al. (2002) extracted a purple dye from the ripe fruits of B. alba with several solvents including methanol and ethanol. The purple dye was applied on 100% cotton and polyester fibres and the wash fastness, heat fastness and light fastness properties of the dye were evaluated, but the acidbase indicator properties of this purple dye were, however, not reported. Ekandem et al. (1997) reported the acid-base indicator properties of ethanol extracts from flowers of Hibiscus rosasinensis, a Roselle plant. The acid-base indicator properties and other industrial applications of hexane extracts of Telfaira occidentalis (Fluted pumpkin) have also been recently reported (Eze et al., 2002). The acidbase properties of H. sabdariffa (also a Roselle plant) have, however, not been reported. The calyx of H. sabdariffa (commonly known as Zobo in the northern communities of Nigeria) is popular, mostly as a beverage drink. The acid-base indicator properties of the calyx of this variety of the Hibiscus have also not been reported. In this paper, we report the findings of our investigations on the acid-base indicator properties of dyes obtained from these two plants, Basella alba and Hibiscus sabdariffa, the Roselle plant (Zobo).