Anthocyanin pigments of rose flowers whose color changes depending on the pH.
The goal of this work was to propose a simple and cheap indicator. Among various pigments
in rose petals, anthocyanins, a class of flavonoids, the chemical structures are primarily
determined by their color, i.e. the number of hydroxy groups on the B-ring and/or aromatic
acyl moieties modifying anthocyanins increase, causing a bathochromic shift (Honda and
Saito 2002). Anthocyanins change their color depending on the pH of the vacuole in which
anthocyanins localize; their color is bluer in weakly acidic or neutral pH, and redder in acidic
pH. Co-pigments, usually flavones and flavonols, cause a bathochromic shift of
anthocyanins when they stack with anthocyanins (Goto and Kondo 1991). To be useful in
illustrating chemical principles such as acid –base equilibria, pKa, light absorption, effects
of changes in conjugated double bounds, etc. The specie used was Rosa rosa whose flowers
crude extract was used to obtain molecular absorption spectra, to verify the Lambert-Beer
law and as an indicator in acid- base titrations. The end point results obtained with the
acidified methanolic extracts when compared to the conventional ones showed relative errors
between 0.1% and 1.0%. The color changed from dark pink to mehndi green between pH 2
and 9. To determine the highest absorption wavelength and demonstrate the shape changing
of spectra in function of the acidity, the absorption spectra in UV and visible regions were
obtained in different pH values. Addition of Stannous salt to the extract further enhanced the
changes in the pH and the absorption wavelength.