In colorimetric estimation of a drug, the following sequence of reactions is carried out: treatment of the aqueous solution of the drug with sodium nitrite solution in acidic medium followed by addition of sulphamic acid and then treatment with N-(lnaphthyl) ethylene-diamine in slightly basic medium to obtain a pink colour; which is measured at a fixed wavelength to correlate the quantity of the drug with the optical density. Identify the drug under estimation.