442 Phelps — TitrimetriG Estimation of Nitric Acid. 



about one-fifth normal strength and standardized against stand- 

 ard decinormal arsenious acid sokition by treating portions of 

 it, measured from a burette (and weighed as a check on the 

 burette readings) with an excess of decinormal iodine solution, 

 adding about 3 grms. of Rochelle salt in solution, neutralizing 

 with acid potassium carbonate and adding in succession 15^""^ 

 of a saturated solution of acid potassium carbonate, starch 

 paste and then standard arsenious acid solution to the bleaching 

 of the starch blue, and, finally, titrating to color with the 

 iodine solution. For the determination of nitric acid, the pure 

 potassium nitrate of commerce was used. In the smaller 

 amounts, the nitrate was taken in a solution of known strength 

 measured from a burette and in the larger amounts as the dry 

 ^alt. Where the solution of nitrate was used, it was measured 

 into the flask, the stem of the separating funnel being com- 

 pletely tilled with water, and the exit tube of the flask reach- 

 ing to the surface of mercury which was placed in a test tube to 

 the depth of about three centimeters. The nitrate solution 

 was then boiled to small volume, an amount of the standardized 

 ferrous sulphate solution known to be in excess introduced into 

 the separating funnel, the exit tube plunged a centimeter or 

 two deep into the mercury (which is readily accomplished by 

 changing the position of the flask on the wire gauze provided 

 that the gauze is depressed well at the center and the flask is 

 set well up on the higher part at the beginning of the opera- 

 tion), and then the flame withdrawn until diminution of pres- 

 sure sufiicient to draw the ferrous solution into the flask is 

 made evident by the rise of the mercury in the exit tube, ^j 

 applying and withdrawing the flame and by regulating the 

 rate of inflow of the solution, the ferrous salt may be intro- 

 duced without admitting air and the funnel washed carefully 

 with an amount of concentrated hydrochloric acid nearly 

 enough to equal the total volume of the liquid in the flask. 

 After the pressure has been restored in the apparatus by heat- 

 ing the flask, the exit tube is again raised to the surface of the 

 mercury and the solution in the flask boiled to a volume of 

 lO-lS^""". The excess of acid is then nearly neutralized with 

 sodium carbonate solution, the carbon dioxide evolved assisting 

 in maintaining the pressure in the apparatus so that the con- 

 densed liquid in the test tube which may contain oxidized 

 nitrogen dioxide is not returned to the iron solution ; the flask 

 is cooled and the ferrous salt remaining determined by iodine 

 as previously described or by a standard solution of potassium 

 permanganate. In case permanganate is used, the contents of 

 the flask are diluted with 600°"^^ of water, 2-3 grms. of crystal- 

 lized manganous chloride,* and titrated to color with potassium 



* Gooch and Peters, this Journal, vii, 461 (1899). 



