This is because the probability is high that interfering compounds might exist. There are many methods for determining nitrate simply because they only work within quite narrow ranges. These means that you must be careful in your selection of wavelength on the spectrophotometer. By the cadmium reduction method, nitrate is reduced almost quantitatively to nitrite in the presence of cadmium. Nitrite is determined by diazotizing with sulfanilimide and coupling with an EDTA-type complex to form the purple azo dye.
Determination is by colorimetry.
Phosphorus occurs in natural waters and in wastewaters almost solely in the form of various types of phosphate. These forms are typically classified as orthophosphates, condensed phosphates and organically bound phosphates. They may be solubilized, bound to detritus, or in the bodies of aquatic organisms. Phosphates are dumped into receiving streams as part of wastewater treatment, as detergents or as fertilizers in agricultural or aquacultural use. In this method, ammonium molybdate and potassium antimonyl tartrate react in an acid medium with dilute solution of orthophosphate to form acid complexes.