Abstract:
Acid-base indicators are widely used for visual detection of the endpoint of an acid-base
titration. Most of these indicators are organic dyes of synthetic origin. These are
hazardous, expensive, and harmful to the environment. Natural pigments in plant extracts
such as Anthocyanin are highly colored substances that can change color at certain
intervals of pH. This study was designed to examine the indicator activity of methanolic
fruit extract rich in Anthocyanin as an alternative to synthetic indicators. Extracts from
the selected plants prepared using a sonication technique were screened for their use in
acid-base titrations. They performed promisingly in the strong acid-strong base, strong
acid-Weak base, and strong base-weak acid titrations. Sharp and clear color change from
reddish-brown to yellow for the Nephelium lappaceum peel extract and blue to red for
the Melastoma malabathricum extract were observed. The indicator dissociation constant
values that were determined using a spectrophotometric method for Nephelium
lappaceum and Melastoma malabathricum were about 6.22 ± 0.06 and 4.22 ± 0.05,
respectively. These values indicate that the pH ranges for the pigments investigated in this
study are comparable to that of the synthetic indicators. These extracts were costeffective, easy to extract, and eco-friendly yet performed exceptionally in acid-base
titrations. Thus, studied natural pigments would be a potential candidate to replace the
commercial synthetic indicators that are used in laboratories. Further, there is no
commercial value for the peel of Nephelium lappaceum and Melastoma
malabathricum seeds. This study exhibits the potential of converting these readily
available materials into many useful value-added products.
Keywords: Natural pigments, Indicator, Low-cost, Value-addition, Titrimetry
