Phelps — Titrimetric Estimation of Nitric Acid. 441 



account of the solubility of the nitrogen dioxide in the ferrous 

 solution used as a reducer and, in other cases, on account of 

 the solubility of that gas in the solution of sodium hydroxide 

 over which it is measured. Further there is the effect of 

 oxygen upon the nitrogen dioxide ; if present in the measur- 

 ing burette in the proportions found in air, it will not give 

 any appreciable effect in moderate amounts, as was observed 

 by Roberts ; but if it is present while the gas is in con- 

 tact with the ferrous solution, the nitrous or nitric acid formed 

 will be taken up by the solution, and, should the oxygen be 

 introduced continuously, as an impurity in carbon dioxide 

 furnished by a Kipp generator, the iron salts will never be free 

 from oxidized nitrogen. If, however, the oxygen is present 

 in some other proportion of dilution than that of the air — as, 

 for example, in the proportion in which it dissolves in aqueous 

 solutions — it will produce an error no matter whether it comes 

 in contact with the gases of the ferrous solution or those of the 

 collecting burette : in the example taken, the proportion of 

 oxygen being greater than that in the air, the error produced 

 is one of deficiency. The effect of oxygen has been recog- 

 nized before. For example, in his process of determining 

 nitrates by reducing them with ferrous salt and hydrochloric 

 acid in an atmosphere of carbon dioxide (furnished by the 

 action of hydrochloric acid on marble), boiling the ferrous solu- 

 tion completely to dryness, collecting the gases over mercury, 

 absorbing the carbon dioxide by sodium hydroxide solution 

 and the nitrogen dioxide by repeated treatment with a con- 

 centrated solution of ferrous salt, Warington found that, 

 when the carbon dioxide was made as free from air as possible 

 by using marble boiled in water and hydrochloric acid also 

 boiled and adding a little cuprous chloride to the acid, the 

 results were decidedly better. 



The estimation of the amount of oxidation of a ferrous salt 

 may be accomplished with a simpler apparatus than that required 

 when the nitrogen dioxide is measured, and the purpose of this 

 article is to put on record such a method, following most nearly 

 the procedure of Holland. The apparatus used consisted 

 of a 250^^™^ flask closed with a rubber stopper carrying in 

 two perforations the inlet and exit tubes. A stoppered funnel 

 of 50^™^ capacity with its tube constricted at its lower end was 

 used as the inlet tube ; and a glass tube of '8'^"' internal diam- 

 eter, enlarged just above the stopper to a small bulb (to pre- 

 vent mechanical loss of the solid contents of the flask during 

 the boiling) and bent twice at right angles, served as the exit 

 tube. 



The analysis was made as follows : A solution of ferrous 

 sulphate, mildly acidified with sulphuric acid, was made of 



