Nitrous acid reacts rapidly to conve ? the amino group of amino acids to a hydroxyl group: the volume of nitrogen liberated is used in the Van Slyke method [l] to estimate the number of primary amino groups present_ This reaction was extended t.o the determination of primary amino groups in aromatic amines, amides and sulphonamides f 21. Nitrous acid also reacts with secondary amines and N-monosubstituted amides to give N-nitroso derivatives, but without evolution of nitrogen.
By treating the amino compound with a much smaller escess of nitrous acid than that normally used, the unreacted nitrite can be determined calorimetrically and hence the amino compound determined by difference. The two major potential sources of error are loss of nitrogen oxides to the atmosphere, and disproportionation of nitrous acid to nitric oxide and nitric acid with further aerial oxidation of nitric oxide to nitrogen dioxide. Both these sources of error may be eliminated by conducting the reaction in a sealed system under reduced pressure and absorbing the evolved fumes in sodium hydroxide solution: the apparatus involved has been described [3]. Any nitrate ion formed by aerial oxidation is reduced to nitrite ion with copperized cadmium and the total excess of nitrite ion is determined colorimetricaily by reaction with SulphaniIamide and N-(l-naphthyl)-ethylenediamine [4].