In the first case, the legally prescribed maximum value is presently 50 mg NO3 /l in Germany and 40 mg/l in Switzerland. The continuous measurement is primarily used when the nitrate content is reduced to a certain amount by means of biological or physical-chemical processes. The blending with nitrate-free water also requires a permanent control.
In waste water treatment, nitrate measurement monitors the second stage of the biological elimination of nitrogen, in which the nitrates (resulting during the first stage by the oxidation of ammonia) are reduced to form gaseous nitrogen.
Among the known analytical methods (ion-sensitive electrodes, amperometry, colorimetry), the direct photometrical determination is the only one not requiring reagents. It is therefore best suited for industrial application as a result of its low maintenance requirements.