Required quantity of Ascorbic acid (standard) and test samples were dissolved in methanol.
The reducing power of methanolic, water extract and ethyl acetate fraction of methanol extracts of the roots of R. tuberosa was determined according to the method of Oyaizu (1986).
Sample solutions at different amounts were mixed with 2.5 mL of 0.2 M phosphate buffer (pH 6.6) and 2.5 mL of potassium ferricyanide (1%).
After the mixture was incubated at 50°C for 20 min, 2.5 mL of TCA (10%) were added and the mixture was centrifuged at 3000 rpm for 10 min.
Supernatant (2.5 mL) was mixed with distilled water (2.5 mL) and 0.5 mL of ferric chloride (0.1%), kept for 10 min.
Control was prepared in similar manner excluding samples and the absorbance was measured at 700 nm and compared with standard.
Higher absorbance of the reaction mixture indicates greater reducing power.