Microdetermination of Primary Amines

Abstract

An abnormal reaction of aliphatic primary amines in the Van Slyke ethod is well known, and its mechanism has been clarified in some respects; but little is known with respect to the aromatic series. By the u e of the preceeding method (I), quantitative determinations of p-nitroaniline, aniline hydrochloride, and sulfanilic acid have been carried out with results of considerable (+) effect in all cases. This error was found to be due to the hydroxyl group produced by the decomposition of the diazonium salt. Moreover, it is affected by three factors : (1) the concentration of sobium nitrite in the solution, (2) the time of standing of the diazonium salt before heating, and (3) the time of heating of the solution. The cause may be attributed to the acceleration of nitrosoation and formation of oxime by (1) snd (2), snd the increased rate of decomposition of oxime by (3). The method of using nitrous acid in slight excess, for the above reason, was investigated, and was found that the determination of aniline hydrochloride can be made with fair accuracy. Thus a method of suppressing the abnormal reaction of aromatic amines is made possible.
Based on the knowledge obtained in the preceeding experiment (II), quantitative determinations of 16 kinds of aromatic primary amines have been carried out with the use of sodium nitrite in a very slight excess. The sample (520 mg) in 2 ml hydrochloric or acetic acid or a dilute alkali, 1 ml 4 M CuCl₂ as a decomposition catalyst, and 2 ml 3 M KBr as an accelerator for the reaction are placed in the apparatus. The air in the apparatus is replaced by CO₂ gas, and 0.5 ml 1 % NaNO₂ is added, the mixture allowed to stand for 2 min. at room temperature, and then heated for 45min at 145°C. The nitrogen gas evolved is collected in an azotometer by introducing CO₂ gas at a rate of 56 ml/min. The time required for the determination was 35 min. Using HNO₂ in an amount 1.41.6 times the theoretical amount gave the best result and it is desirable to be at least within the range of 1.22.0 times, while the change of other factors even to a considerable extent gave less effect. Results with relative error 0.51.0 % were obtained. Samples reacting slowly with HNO₂ can be analyzed by this method. Also, primary amines having hydroxyl groups can be determined with good accuracy.