440 Phelps — Titrimetric Estimation of Nitric Acid. 



Art. XLII. — The Titrimetric Estimation of Nitric- Acid ; 

 by I. K. Phelps. 



[Contributions from the Kent Chemical Laboratory of Yale University— No. CXIII.] 



In the methods for the quantitative estimation of nitric acid 

 which depend upon its reduction with a ferrous salt and the 

 determination of the amount of oxidation produced, scrupu- 

 lous care is necessary that the atmosphere in contact with the 

 ferrous salt while the nitrogen dioxide is present shall be 

 free from oxygen. This fact was recognized by Fresenius,"^ 

 who modified the original process of Pelouzef by filling the 

 flask with carbon dioxide or hydrogen at the outset. Eder:(: 

 used carbon dioxide similarly. Holland's method, § roughly 

 described, consists in boiling, until the air is expelled, the solu- 

 tion of the nitrate in a flask provided with a doubly bent exit 

 tube, rubber-jointed and fitted with a pinch cock, then admit- 

 ting through the tube as the flask cools a mixture of ferrous 

 salt and strong hydrochloric acid, heating the mixture on a 

 water bath, and, finally, titrating the resulting ferric salt with 

 stannous chloride. 



All of these methods give results which are higher than the 

 theoretically expected — those which use carbon dioxide on 

 account of the oxygen invariably present in the gas as ordi- 

 narily produced in the laboratory, and Holland's process pos- 

 sibly because of the slow leakage through the rubber connections 

 during the long heating, or because the nitrogen dioxide, which 

 is not driven out completely from the solution of the iron salts, 

 acts somewhat with the atmospheric oxygen during the titra- 

 tion. 



Another series of methods has depended upon the deter- 

 mination of the nitrogen dioxide evolved, usually by direct 

 measurement as nitrogen dioxide. Among these may be men- 

 tioned the methods of Schloessing,] Reichart,Tf Schulze and 

 Wulfert,^* Tiemann,tt Wildt and Scheibe,:}::!: Warington,§§ 

 Boehmer,|||| Kratschmer,lt Wilfarth,'^-^* Morse and Linn.,ttt 

 Berger,J:j:j and Koberts.§§§ In general, these methods give 

 results lower than demanded by the theory — in many cases, on 



*Anii., cvi, 217; Zeit. anal. Chem., i, 32. 



fAnn. de Chim. Phys. [3J, xx, 129. ^ Zeit. anal. Chem., xvi. 267. 



§Chem. News, xvii, 219. || Ann. de Chim. Phys. [3], xl, 479. 



II Zeit. anal. Chem., ix, 26. ** Ibid., ix, 400. 



f f Anleituug zur Untersuchung von Wasser von W. Kubel. Zweite Auflage 

 von F. Tiemann, 55. 



:j::|: Zeit. anal. Chem., xxiii, 151. 



§§JoTir. Chem. Soc.,xxxvii, 468; xli, 345. 



III Zeit. anal. Chem., xxii, 20. ^[^Ibid., xxvi, 608. 

 ***Ibid., xxvii, 411. f ff Amer. Chem. Jour., viii, 274. 

 X\X Chem. Zeit., xix, 305. g§§This Journal, xlvi, 126 (1893). 



