METHOD OF BOUSSINGAULT 427 



a quantity of sulfuric acid, equal in volume to the liquid in the 

 flask, is added all at once, the mixture shaken, and indigo solu- 

 tion run in quickly out of the burette until the liquid remains per- 

 manently of a greenish tint. 



(2) The last experiment is repeated as often as may be neces- 

 sary, adding to the water at first half a cubic centimeter less 

 indigo than the total quantity used previously, afterwards pro- 

 ceeding as in ( i ) until the final test shows too little indigo used. 



(3) From the rough titration of the indigo, calculate the 

 amount of potassium nitrate solution corresponding with the in- 

 digo solution used in (2), multiply the result by 10, transfer this 

 quantity of the standard nitrate solution to a 250 cubic centime- 

 ter flask, fill with pure water to the mark, and titrate 25 cubic 

 centimeters of this fluid with indigo as in (2). If the quantity 

 of indigo solution used is nearly the same as that required in (2), 

 its exact value may be calculated, but if it is not, another nitrate 

 solution may be made up in the 250 cubic centimeter flask, more 

 closely resembling the sample in strength, and the titration with 

 the indigo solution must be repeated. 



(4) If the water contains any considerable amount of organic 

 matter, it must first be destroyed by potassium permanganate. 

 In this case, the estimation of the organic matter and nitric acid 

 may be conveniently combined. 



The use of permanganate in the above case is likely to intro- 

 duce an error as has been shown by Warington. The method, 

 therefore, can not be recommended in the presence of organic 

 matter. 



369. Method of Boussingault. The process for the estima- 

 tion of nitric acid by the decoloration of a solution of indigo is 

 due originally to Boussingault. 55 In this method the extract, ob- 

 tained by washing slowly 200 grams of soil until the filtrate 

 amounts to 300 cubic centimeters, is evaporated until its volume 

 is no greater than two or three cubic centimeters, and it is trans- 

 ferred to a test-tube, with washings, and again evaporated in the 

 tube until the volume is not greater than that last mentioned. A 

 few drops of solution of indigo are added, and then two cubic 

 55 Encyclopedic chimique, 1888, 4 : 154- 



