Nitrate ion can be determined as ammonia by widely different reductive methods. Generally alkaline rccluction is employed, the DEWARDA-method being most commonly used. A general disadvantage of these methods is that the overdistilling of the hydroxide may often cause significant errors. COTTE AND KAEEANE~ developed a method attempting to eliminate this disadvantage by the application of ferrous hydroxide as reducing agent. BAUDISCH AND MAYER~ were the first to employ ferrous hydroxide for the reduction of nitrates. According to their investigations an alkali concentration exceeding 28 per cent. is needed to obtain quantitative reduction of the nitrate. Even so, the reduction iasts a long time extending over several hours. The expcriments of COTTE AND KAHANG (Z.C.) have shown that on the action of silver sulphate the rate of the reduction increases appreciably.
The authors succeeded in elaborating an exceedingly rapid and exact method for the determination of the nitrate ion by virtue of this catalytical reduction, this was, however, not carried out by distillation of the ammonia, but by the permanganometric back-titration of the non-oxidized part of a measured ferrous salt. This method was described in a previous communicationa.
The investigation of the interference of individual foreign ions revealed that copper is an cquaZ& cfficied catalyst as silver. Nevertheless, the application of copper as catalyst did not furnish satisfactory results, inasmuch as the ferrous hydroxide reduces the copper from divatent to monovalent state. This would, however, not yet exert a disturbing effect as it results in the formation of cuprous ions equivalent to the ferrous salt and the former is in turn oxidized by KMnO, to cupric state. The error is due to the fact that on acidification the excess of the sulphuric acid partly oxidizes the cuprous-ions, being itself reduced to sulphurous acid. The latter decomposes in the strongly acid medium to sulphur dioxide and water and on boiling the sulphur dioxide is expelled.
The oxidizing effect of sulphuric acid rcndercd the use of a copper catalyst inapplicable for the permanganomctric procedure. However, considering tCre References p. 419.