After incubation at rt for 125 min, a 2%
(w/v) solution of sulfanilamide in 4% (v/v) H3PO4 (0.3 ml) was
added, and the mixture was incubated at rt for further 5 min. After
that, a 0.2% (w/v) solution of N-(1-naphthy)lethylenediamine dihydrochloride
in water (0.3 ml) was added, and the mixture was
incubated at rt for additional 30 min. Finally, the absorbance was
measured at 540 nm against the corresponding blank