126 Roberts — Reduction of Nitric Acid by Ferrous Salts. 



Akt. XYIII. — On the Reduction of Nitric Acid by Ferrous 

 Salts; by Chablotte F. Robebts. 



[Contributions from the Kent Chemical Laboratory of Yale College — XXV.] 



Among the most common methods for the estimation of 

 nitric acid are those which depend upon its reduction by a 

 ferrous salt in acid solution. The amount of the nitrate acted 

 upon is reckoned either from the oxidation of this ferrous com- 

 pound or from the nitric oxide set free and subsequently deter- 

 mined. The first named method was first put in practical 

 form by Pelouze, and in its present modified form,* is capable 

 of yielding accurate results in the analysis of pure nitrates. 

 Organic matter interferes, however, with the accuracy of the 

 indications of Pelouze's process. Schloesingf was the first to 

 use the nitric oxide evolved as a measure of the original nitrate, 

 and he was able to show that the presence of organic matter 

 does not influence the yield of nitric oxide in the decomposi- 

 tion. Schloesing originally estimated the nitric* oxide by col- 

 lection over mercury, re-conversion by the action of oxygen to 

 nitric acid, and titration of the last with standard alkali, but 

 Reichardt4 Wildt and Scheibe§ and others have very much 

 simplified the apparatus of the original process, and shown 

 that the gas may be collected over caustic soda or potash with- 

 out materially diminishing the accuracy of the analytical 

 process. Schulze and Wulfert|] determined by direct measure- 

 ment the nitric oxide collected over mercury, and Tiemann 

 showed that the determination loses little in exactness when 

 the gas is measured over an aqueous solution of caustic soda. 



Many modifications and extensions of these processes have 

 appeared from time to time, the detailed consideration of which 

 is not essential to the purpose of the present discussion, but 

 all, whether dependent upon the determination of the change 

 of condition of the iron salt in the residue, or upon the esti- 

 mation of the nitric oxide evolved, rest upon the assumption 

 that under the conditions of action nitric oxide is the sole 

 reduction product in the complete destruction of the* nitric 

 acid by the ferrous salt. That this assumption is true in the 

 main is evidenced by the analytical results of the methods. 

 Thus, in his careful and critical examination of methods, 

 Eder^f found that the indications given by titration of the 

 residue of treatment with ferrous chloride and hydrochloric 

 acid by means of potassium permanganate showed errors of 

 excessive oxidation amounting only to 0*3 per cent to 1 per 



*Fresenius, Quant. Anal., 6th Aufl., p. 519. 



f Ann. de Chemie, III, xl, 479. JZeitschr. fur Anal. Chem., ix, 26. 



gZeitschr. fur Anal. Chem., xxiii, 151. jj Zeitschr. fur Anal. Chem., ix, 400. 



*[Zeitschr. fur Anal. Chem., xvi, 267. 



