Isolation of the natural colorant from the roots of Arnebia nobilis and its use as a new neutralization indicator

Department of Chemistry, Gurukula Kangri University, Hardwar-249 404, India
E-mail: akindray@sancharnet.in          Fax: 91-1334-246366
Manuscript received 27 June 2002, revised 3 March 2003. accepted 17 March 2004

.Solution of natural dye from the roots of the plant Arnebia nobilis in methanol is an excellent acid-alkali indicator capable
of working in much wider range than the usual indicators phenolphthalein and methyl orange. Volumetric titrations of strong
acid-strong alkali, strong acid-weak alkali and weak acid-strong alkali are performed and very satisfactory results found in
the wider range of concentrations( 10^-3 to 4 N) of acid and alkali. The structures of the indicator in acid and basic mediums
are suggested. The pK of indicator is 5.21. The pH at the equivalence point in three types of titrations varies in the range
of 3.75 to 11.70; λ_max are found to be 235 and 250 nm. Active component, arnebin-7, is isolated pure and also used as indicator
to confirm the results.