Roberts — Reduction of Nitric Acid by Ferrous Salts. 127 



cent upon amounts approximating 0*4 grms. of potassium 

 nitrate, although the process involved the uncertainties of 

 titrating, without the now used precautions, with permanga- 

 nate in hydrochloric acid. The similar process in which use 

 is made of ferrous sulphate in sulphuric acid, instead of the 

 chloride in hydrochloric acid, gave excellent results and showed 

 errors of loss of the nitrate ranging from 0*05 per cent to 04 

 per cent upon similar amounts of the nitrate, while Braun's 

 process of determining the oxidized iron by stannous chloride 

 gave upon amounts of the nitrate varying from 0'16 grms. to 

 0'32 grms. errors of 0'3 per cent to 1*5 per cent. 



In' the processes involving the treatment of the nitric oxide, 

 the errors are usually greater and more variable. Thus, 

 Schloesing's process gave Eder errors of 0*2 per cent to 2 per 

 cent upon approximately 0'15 grms. of the nitrate, and 10 per 

 cent and 12*5 per cent respectively upon 0*0279 grms. and 

 0*0249 grms. of the nitrate. In Reichardt's modification the 

 errors are slightly larger on the average but vary similarly, 

 and Tiemann's method of measuring the gas over caustic soda 

 yields results closely comparable with those of the Reichardt 

 process and but little inferior to those of Sehloesing. 



The way in which the results vary in all the processes in 

 which, the nitric oxide is estimated has very generally been 

 regarded as suggestive of an oxidizing action upon the nitric 

 oxide, nearly constant in amount, and productive of increasing 

 percentage errors as the amount of nitrate decreases. The 

 intermixture of ordinary air would naturally produce diminu- 

 tion of the volume of the nitric oxide, but the oxidation of 

 this by free air of normal composition should produce no effect 

 upon the total volume of the gas in cases where this is meas- 

 ured, if the oxidation goes, as is generally supposed, to the 

 point of forming nitrous acid ; for the introduction of a given 

 volume of oxygen (in air) means the simultaneous introduction 

 of four volumes of nitrogen, and the single volume of oxygen 

 is capable of causing the disappearance of exactly four vol- 

 umes of nitric oxide converting it into nitrous acid, so that 

 four volumes of nitrogen would be left to replace the four 

 volumes of nitric oxide removed. If the oxidation be sup- 

 posed to go to the point of forming some nitric acid there 

 should be left three volumes of nitrogen to replace every 

 volume of nitric oxide converted to nitric acid, and the total 

 volume should be greater after oxidation than before. 



In order to account for the low results, therefore, on the 

 assumption of an oxidizing action due to free air, we must 

 conclude that such air is derived principally from solution in 

 the reagents, which would naturally hold oxygen in excess of 

 the proportion in which it exists in atmospheric air. Air in 

 solution in the receivers would most readily produce the ob- 



Am. Jour. Sci.— Third Series, Vol. XLVI, No. 272. — August, 1893. 

 9 



