Sodium nitroprusside generates nitric oxide in aqueous solutions, at physiological pH, which is converted into nitric and nitrous acids on contact with dissolved oxygen and water. This method is based on
the inhibition of nitric oxide radical generation from sodium nitroprusside. The liberated nitrous acid is estimated using a modified Griess-Illosvoy method, where nitrous acid reacts with Griess reagent, to form a purple azo dye. In the presence of antioxidants the amount of nitrous acid will decrease and the degree of this reduction in formation of purple azo dye will reflect the extent of scavenging. The absorbance of the chromophore formed is measured at 540 nm [25, 26].% inhibition=(Absorbance of control–Absorbance of sample) / Absorbance of control ×100