Natural dyes as neutralization indicators-a short review

Deparatment of Chemistry, Gurukula Kangri University, Hardwar-249 404, India E-mail: akindray@sancharnet.in Manuscript received 12 November 2002 Some of the naturally occurring coloured substances are pH-sensitive giving distinctly different colours in acid and basic mediums. Some of them, like the red cabbage aqueous extract, are colour-sensitive even to the same medium pH variation also and make a very good laboratory experiment for pH determination. Anthocyanidins are among the commonly occurring natural pigments. Natural colours giving a sharp, distinct and stable colour change on a change of acid to alkaline medium may be used as acid-alkali indicators in volumetric titrations. Extracts from brazilwood (Caesalpinia sappan), ratanjot (A.mebia nobilis) and heartwood of jack tree (A.rtocarpus heterophyllus) are found satisfactory natural indicators for much better range than what is provided by conventional acid-base indicators phenolphthalein and methyl orange. Active constituents and structural changes are also discussed