426 AGRICULTURAL ANALYSIS 



color marks the end of the reaction. Indigo is the only coloring 

 matter that has been used to any extent in this process. 



368. Method of Marx. The indigo method as usually practiced 

 is conducted according to the principle described by Marx. 5 * 

 There are required for the process the following reagents and 

 apparatus : 



a. A solution of pure potassium nitrate containing i .8724 grams 

 per liter. One cubic centimeter of the solution is equivalent to one 

 milligram of nitric anhydrid (N 2 O 5 ). 



b. A solution of the best indigo carmine in water, which should 

 be approximately standardized by solution in the manner described 

 hereafter, and then diluted so that from six to eight cubic centi- 

 meters equal one milligram of nitric acid. 



c. Chemically pure sulfuric acid of specific gravity 1.842, per- 

 fectly free from sulfurous and arsenious acids and nitrogen oxids. 



d. Several thin flasks of about 200 cubic centimeters capacity. 



e. A small cylindrical measure holding 50 cubic centimeters 

 and divided into cubic centimeters. 



/. A Mohr's burette divided into tenths of a cubic centimeter. 



g. A 25 cubic centimeter pipette or another burette. 



h. A five cubic centimeter pipette divided into cubic centime- 

 ters or half cubic centimeters. 



i. A measuring flask of 250 cubic centimeters capacity. 



Preliminary Trial. Twenty-five cubic centimeters of the sam- 

 ple are transferred to a flask; the 50 cubic centimeter measure 

 is filled with sulfuric acid and the burette with indigo solution. 

 The sulfuric acid is added to the sample all at once, shaken for 

 a moment, and the indigo run in as quickly as possible with 

 shaking until a permanent greenish tint is produced. If the 

 sample does not require more than 20 cubic centimeters of indigo 

 solution of the above strength, it can be titrated directly, other- 

 wise it must be diluted with a proper quantity of pure water, 

 and subjected again to the preliminary trial. 



The Actual Titration. (i) Twenty-five cubic centimeters of 

 the sample, properly diluted if necessary, are poured into a flask, 

 and as much indigo as was used in the preliminary trial is added ; 

 54 Zeitschrift fur analytische Chemie, 1868, 7 : 412. 



