The ferrous tartrate method (Japanese official
method) [20]
0.2mL of sample (50–300gmL−1), 0.5mL of ferrous
tartrate solution comprising 0.1 g of ammonium ferrous sulfate
hexahydrate plus 0.5 g of potassium sodium (+)-tartrate
tetrahydrate dissolved in 100mL of deionized water and 2.5mL
phosphate buffer (0.1M pH 8) were mixed sequentially. The
absorbance at 540 nm was measured after 20 min of mixing at
ambient temperature