43 AGRICULTURAL ANALYSIS 



pare it about four times the strength given above and then dilute 

 it as required. Four grams of sublimed indigotin will furnish 

 more than two liters of the color solution. 



The solution is prepared as follows : 



Four grams of indigotin are digested for a few hours with five 

 times that weight of Nordhausen sulfuric acid, diluted with water, 

 filtered and made up to a volume of two liters. The strength 

 of the indigotin solution is determined with a solution of potas- 

 sium nitrate of the strength mentioned above. The process is 

 performed as follows: 



From 10 to 20 cubic centimeters of the standard nitrate solution 

 are placed in a wide-mouthed flask of about 150 cubic centimeters 

 capacity. A portion of the indigotin solution is added, such as 

 will be deemed sufficient for the process, and the whole is well 

 mixed. Strong sulfuric acid is measured from a burette into a 

 test-tube, in volume equal to the united volumes of the nitrate solu- 

 tion and indigotin. The whole of the sulfuric acid is then poured, 

 as quickly as possible, into the solution in the flask and rapidly 

 mixed, and the flask transferred to a calcium chlorid bath, the 

 temperature of which should be maintained at 140. It is essen- 

 tial to the success of the operation that the sulfuric acid should 

 be mixed with the greatest rapidity. It is poured in at once and 

 the whole well shaken without waiting for the test-tube, contain- 

 ing the acid, to drain. The flask is covered with a watch-glass 

 while it is held in the bath. As soon as the larger part of the 

 indigotin is oxidized the flask in the bath is gently rotated. With 

 very weak solutions of nitrate it may be necessary sometimes to 

 keep the flask in the bath for five minutes. When the indigo 

 color is quickly discharged, it shows the presence of nitric acid 

 in considerable excess and a larger quantity of indigo must be 

 used in the next experiment. The experiments are continued un- 

 til just the quantity of indigotin necessary to consume the nitric 

 acid is found, the amount of indigotin being in very slight excess, 

 not exceeding one-tenth cubic centimeter of the indigotin solution 

 used. The tint produced by the small excess of indigotin re- 

 maining is best seen by filling the flask with water. On substances 

 of approximately known strength about four experiments are 



