How to measure Nitrate Nitrogen in water or wastewater?
There are three ways for determining nitrate nitrogen in waters and wastewaters:
a) Cadmium reduction method,
b) Chromotropic acid method, and
c) Devarda’s alloy reduction method.
The cadmium reduction technique is appropriate for concentrations less than 0.1 mg per litre of nitrate nitrogen,
The chromotropic acid technique may be used for concentrations ranging from 0.1 to 5.0 mg/l,
Whereas, Devarda's alloy reduction method can be utilized for quantities more than 2 mg/l of total nitrogen.
1: Cadmium Reduction Method
Principle of Cadmium Reduction Method - In the presence of cadmium, nitrate is converted to nitrite. The nitrite generated is quantified calorimetrically after di-azotizing with sulphanilamide and combining with N-1 naphthyl ethylenediamine to make a strongly coloured azo dye.
Interference - High concentrations of copper, iron, and other metals reduce reduction efficiency. To eliminate this interference, use EDTA.Oil and grease can also interfere, as might leftover chlorine. Remove oil and grease with organic solvents, and residual chlorine with sodium thiosulphate.
2: Method of Chromotropic Acid
Two moles of nitrate nitrogen react with one mole of chromotropic acid to produce a yellow reaction product with maximum absorbance at 410 nm.
Interferences - With chromotropic acid, residual chlorine, some oxidants, and nitrites produce a yellow colour. Sulphite is used to reduce interference from residual chlorine and oxidants. Nitrites are converted to nitrogen gas by urea.
3: Devarda’s Alloy Reduction Method
Principle - Nitrate and nitrite are reduced to ammonia in the presence of a reducing agent (Devarda's alloy) under hot alkaline circumstances. The generated ammonia distils and is caught in a boric acid-containing receiving flask. Direct nesslerization or acidimetric analysis can be used to determine ammonia. This approach is suggested for nitrate and nitrite nitrogen.
Preliminary distillation will be used to eliminate ammonia from the sample. This process also converts nitrite to ammonia. As a consequence, nitrite is determined separately and the result is subtracted. This approach is not suggested for nitrate nitrogen levels less than 2 mg/l.
How to measure Nitrite Nitrogen in India?
A technique for determining nitrite nitrogen using Nessler's cylinders or a spectrophotometer is prescribed. The referee method should be the spectrophotometric method.
Indian Standard Specification for Nitrogen
Under IS Code No. 3025: Part 34, nitrogen and other associate compounds are calculated for wastewater.
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