A DETERMINATION OF TOTAL NITROGEN BY SPECTROPHOTOMETRY

1 The determination is based on the Berthelot reaction, in which a phenol derivative (here salicylate) forms an azo dye in the presence of ammonia and hypochlorite. In alkaline medium, the indophenol thus formed has a green-blue colour, of which the absorbance is measured at a wavelength of 660 nm. This is a measure for the concentration of ammonium, formed by the nitrogen compounds in the sample. 

2 This determination may be carried out on digest 2.1 (digestion with H SO., - salicylic acid - HjOj Se) and digest 2.2 (digestion with H SO. - salicylic acid - Hj,OJ. 

To prevent precipitation of hydroxides, EDTA should be added prbr to raising the pH. 

The pH should be adjusted with a buffer solution, since the digests contain variable ameunts 

1 This procedure yields a standard curve that is linear up to approximately 15 mg/L N. 

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