In brief, to 0.1 mL of samples was added 1 mL of an acidic solution of ferrous sulfate [7.7 mg of FeSO4.7H2O dissolved in 50 mL of conc. HCl- n-butanol (2:3, v/v)]. The mixtures, loosely sealed in test tubes, were placed in a water bath 95C for 15 min.