174 AGRICULTURAL ANALYSIS 



adhering ammonium phosphomolybdate is dissolved with ammo- 

 nia and added to the precipitate in the filter, and the filtrate is 

 collected in a 250 cubic centimeter griffin beaker. The flask is 

 washed with water which is poured upon the filter and the filter 

 is thoroughly washed until the solution measures about 60 cubic 

 centimeters. To this are added 10 cubic centimeters of strong 

 sulfuric acid and the solution is passed through the reductor, 

 similar in construction to the jones reductor already described. 

 Care should be exercised to keep the end of the small tube of the 

 reductor just below the surface of the liquid in the flask. The 

 heat which is caused by mixing the strong sulfuric acid with 

 the ammoniacal solution immediately before passing it through 

 the reductor is sufficient to insure a complete reduction. The 

 other precautions which have already been described to prevent 

 the access of air should be observed, and the operations should 

 be so continued that the whole reduction occupies about three 

 or four minutes. The liquid as it passes through the reductor 

 should be bright green in color. Permanganate solution is added 

 and the green color disappears. The solution becomes first, 

 brown, then pinkish yellow, and ultimately colorless. The ad- 

 dition of permanganate is continued drop by drop until the solu- 

 tion assumes a faint pink coloration, which remains at least one 

 minute. From the reading of the burette the amount of per- 

 manganate consumed in the blank determination, obtained as 

 described under the method for standardizing permanganate 

 solution, is subtracted and the number of cubic centimeters thus 

 obtained is multiplied by the value of one cubic centimeter in 

 terms of phosphorus. This product is multiplied by 100 and di 

 vided by the weight of the sample used and the resulting quotient 

 is the percentage of phosphorus in the steel. 



151. Standardization of the Solution of Potassium Permangan- 

 ate. This solution is prepared by dissolving two grams of crys- 

 tallized potassium permanganate in one liter of distilled water 

 and filtering through asbestos. For determining its value from 

 0.15 to 0.25 gram of clean soft steel wire, in which the content 

 of iron has been carefully determined, is put in an erlenmeyer 

 flask of 125 cubic centimeters capacity and covered with 30 cubic 



