METHOD OF WARINGTON 



431 



usually necessary to determine the proper amount of indigotin, 

 but with unknown substances a larger number may be necessary. 



Usually in determinations of this kind it is directed to use 

 double the volume of sulfuric acid mentioned above. In this 

 case not only is the quantity of indigotin oxidized much greater 

 than with a smaller portion of acid, but the prejudicial effect of 

 organic matter is also greater when the smaller quantity of acid 

 is employed. 



An indigotin solution standardized as above is strictly to be used 

 for a solution of nitrate of the strength employed during the stan- 

 dardization. The quantity of indigotin oxidized in proportion 

 to the nitric acid present diminishes as the nitrate solution 

 becomes more dilute. Instead of determining this during each 

 series of experiments it may be estimated once for all and a table 

 of corrections used. 



The following table is based upon experimental determinations : 



The table is used as follows: 



Suppose that 20 cubic centimeters of water under examination 

 have required 5.36 cubic centimeters of indigotin solution for 

 the oxidation of the nitric acid contained therein. By inspec- 

 tion of the table it is seen that this number is five-tenths cubic 

 centimeter above the nearest quantity given, viz., 4.86 cubic centi- 

 meters. From the last column in the tabie it is found that the 

 correction for five-tenths cubic centimeter of indigotin solution 

 is 0.000000149 cubic centimeter, being half that for the one cubic 

 centimeter given in the table. This is to be subtracted from the 



