130 Roberts — Reduction of Nitric Acid by Ferrous Salts. 



the gas from the generator is never so pure that a hundred 

 cubic centimeters of it will not leave a tiny bubble when 

 shaken with a solution of caustic soda. 



With the apparatus described the following experiments 

 were made. Potassium nitrate (0*1 grm.) was introduced into 

 the retort, generally in the dry condition, carbon dioxide was 

 passed through the apparatus until the gas collected over 

 sodium hydroxide left only the minute bubble which the gas 

 from the generator alone had been found to give, and 40 cubic 

 centimeters of a boiled solution of ferrous chloride in hydro- 

 chloric acid was admitted through the funnel tube, after shut- 

 ting off the carbon dioxide and lowering the leveling tube of 

 the Hempel burette. With the stop cocks arranged as in 

 sketch, the liquid was then slowly heated to boiling and the 

 process continued until the reaction of the ferrous salt upon 

 the nitrate was apparently complete, when the carbon dioxide 

 was again passed through the apparatus to secure complete re- 

 moval of the nitric oxide, the absorption of the carbon dioxide 

 being hastened by inclining and shaking the burette at inter- 

 vals. The volume of the gas under existing barometric and 

 thermometric conditions was noted. To discover how much 

 of the gas collected was actually nitric oxide, I have found it 

 best to absorb this gas by potassium permanganate. This is 

 very easily and quickly accomplished by attaching a funnel to 

 the Hempel burette through which a strong solution of potas- 

 sium permanganate can be introduced. I have generally taken 

 the precaution of adding water first in order to prevent the 

 possibility of any local action between the potassium perman- 

 ganate and caustic soda by means of which oxygen might be 

 produced. 



The following tables give the results of experiments per- 

 formed in this way. In the first column is given the absolute 

 volume of the gas unabsorbed by KMn0 4 , in the second the 

 per cent KN0 3 reckoned as if the total volume were nitric 

 oxide, and in the third the percentage reckoned from the 

 absorbed nitric oxide. In cases where potassium iodide was 

 used in the Will and Varrentrapp tube to break up any higher 

 oxides of nitrogen that might be formed, the amount of iodine 

 thus set free is given. 



In experiment (3) of Table I, the ferrous chloride was made 

 by dissolving iron wire in boiled hydrochloric acid, and the 

 potassium nitrate was dissolved in boiled water and added to 

 the ferrous chloride. With this dilution, the reaction pro- 

 ceeded very slowly and it was found necessary to add several 

 successive portions of hydrochloric acid before the reaction 

 was complete, but the results as finally obtained are quite com- 

 parable with those obtained by a more rapid process. 



