Potassium ferricyanide, potassium dihydrogen phosphate,
disodium phosphate, sodium carbonate, and ammonium ferrous
sulfate hexahydrate were obtained from Nacalai Tesque, Inc.
(Kyoto, Japan). Moreover, tannic acid and potassium sodium(+)-
tartrate tetrahydrate were purchased from Wako Pure Chemical
Industries, Ltd. Finally, Folin–Ciocalteau reagent was obtained
from Sigma Chemical Co. (St. Louis, MO, USA). All chemicals
were of analytical-reagent grade and used as received without
further purification