METHOD OF WARINGTON 429 



ply. After the second decantation the residue is treated with 

 750 cubic centimeters of equal parts of water and pure concen- 

 trated hydrochloric acid and boiled for an hour. After cooling, 

 the undissolved portion is collected on a filter and washed at first 

 with hot, and afterwards with cold water, until the filtrate is no 

 longer colored and is free of acid. The dried residue is treated 

 with ether under a bell-jar, or in a continuous extraction appara- 

 tus until the ether is only of a faint blue tint. The 50 grams of 

 indigo will yield about 25 grams of the purified article, which, 

 however, will still leave a little ash on combustion. 



Solution in Sulfuric Acid. Five grams of the purified indigo 

 are placed in a flask having a ground-glass stopper, treated with 

 25 grams of fuming sulfuric acid, and allowed to digest two or 

 three days at a temperature of from 50 to 60. From 70 to 

 200 drops of the solution thus made are placed in 100 cubic centi- 

 meters of water for use in the process. 



Standardization of the Indigo Solution, The solution as pre- 

 pared above is standardized by a solution of one gram of pure 

 potassium nitrate in 1000 cubic centimeters of distilled water. 

 The oxidation of the indigo solution is accomplished as described 

 above. Of this strength of standard nitrate solution two cubic 

 centimeters are used, corresponding to two milligrams of potas- 

 sium nitrate. The indigo solution for this strength potassium 

 nitrate solution should have only 20 drops of the sulfuric acid 

 solution of indigo to ioo cubic centimeters of water. If 20 grams 

 of potassium nitrate are used for 1000 cubic centimeters of the 

 standard solution, then 200 drops of the sulfindigotic acid should 

 be used to ioo cubic centimeters of water. 



370. Method of Warington. The modification of the indigo 

 method as used by Warington, applicable only in absence of 

 organic matter, is the one chiefly employed in England. 56 



Instead of the ordinary indigo of commerce, indigotin is used. 

 The normal solution of the coloring matter is made of such a 

 strength as to be equivalent to a solution of potassium nitrate 

 containing 0.14 gram of nitrogen per liter. When large quan- 

 tities of the coloring matter are to be used, it is advisable to pre- 

 M Journal of the Chemical Society, 1879, 36 '. 5?8. 



