1 68 AGRICULTURAL ANALYSIS 



cubic centimeters of nitric acid (of 1.20 specific gravity) ; close 

 the flask with a rubber stopper, wrap it in a thick cloth, and shake 

 violently for five minutes. Collect the precipitate on a filter, using a 

 pump, and wash with dilute nitric acid (iHNO 3 : 5oH,O). If a 

 thin film of the precipitate should adhere to the flask it can be re- 

 moved by the ammonia in the next operation. Wash the molyb- 

 date precipitate into a 500 cubic centimeter flask with dilute am- 

 monia (iH 3 N:4H 2 O), using about 30 cubic centimeters. Add 

 80 cubic centimeters of hot dilute sulfuric acid (iH,SO 4 :4H 2 O) 

 and cover the flask with a small funnel. Add 10 grams of 

 granulated zinc and heat until rapid action begins, and then 

 heat gently for five minutes. The reduction is then complete. 

 During the reduction the colors, pink, plum, pale green and dark 

 green, are seen in the molybdate solution, the latter color mark- 

 ing the end of the reaction. 



To remove the zinc, pour through a large folded filter, wash 

 with cold water, and fill up the filter once with cold water. But 

 little oxidation takes place in this way. A port wine color is 

 seen on the filter, but this does not indicate a sufficient oxidation 

 to make an error. 



In titrating, the color becomes fainter and finally the solu- 

 tion is perfectly colorless and shows a single drop in excess 

 of the permanganate. The permanganate solution, for conven- 

 ience, is made so that one cubic centimeter is equal to o.oooi 

 gram of phosphorus. With iron its value is one cubic centime- 

 ter equals 0.006141 gram of iron; and one cubic centimeter equals 

 0.005574 gram of molybdenum trioxid. 



In the case of iron ores 10 grams are dissolved in hydrochloric 

 acid, evaporated to dryness, taken up with hydrochloric acid, 

 evaporated to a small bulk and the residual hydrochloric acid ex- 

 pelled by heating with nitric acid. The insoluble residue is re- 

 moved by filtration and the rest of the process conducted as 

 above described. This method of determination is advisable in 

 the solution of ores rich in phosphorus, intended for the manu- 

 facture of iron or steel where basic phosphoric slag is to be util- 

 ized as a by-product. 



149. Variation of Dudley and Noyes. The method of Em- 



