The mixed reagent contained sulfuric acid (6.6 mol L−1), ammonium molybdate tetrahydrate (0.018 mol L−1)
and potassium antimonyl tartrate (0.003 mol L−1) [13]. The solution was stored in an amber colored glass flask in a refrigerator at 4 °C. Ascorbic acid (0.5 mol L−1) in water was used as the reducing solution.