Ammonia compounds combine with chloride to from monochhloramine . MONOCHLORAMINE reacts with salicylate to 5-aminosalicylate. The 5-aminosalicylate is oxidzes in the presence of a sodium nitroprusside cstalyst to from a blue colored compound.The blue color is masked by the yellow color from the excess reagent present to givv a green colored solution. The measurement wavelength is 655 nm for spectrophotometeers