METHOD OF LORENZ 123 



molybdate mixture. The precipitate obtained weighed 0.9182 

 gram, showing a content of 18.44 per cent, of phosphoric acid in 

 the original sample. A gravimetric determination yielded the same 

 figure. On the other hand a sample of soil which showed 0.14 

 per cent, of phosphoric acid by the gravimetric method, was ex- 

 tracted with HNO 3 and the acid extract after separation of the 

 silica, made up to 100 cubic centimeters and the whole poured 

 into the molybdate mixture, gave a content of 0.14082 per cent, 

 of phosphoric acid. 



in. Method of Lorenz. The various methods which have been 

 proposed for the direct weighing of the ammonium molybdate pre- 

 cipitate of phosphoric acid, have been made the subject of a 

 practical and critical study by Lorenz. 91 



The methods of Meineke, Hanamann, Woy and Hundes- 

 hagen are compared and the modifications thereof described. 



The reagents employed by Lorenz are : 



1. Ammonium Molybdate. This reagent is prepared by pour- 

 ing on TOO grams of pure dry ammonium sulfate in a glass cylin- 

 der of two liters capacity, one liter of nitric acid of 1.36 specific 

 gravity at 15 and stirring until the salt is dissolved. In a liter flask 

 300 grams of purest dry ammonium molybdate are dissolved in 

 hot water, cooled to about 20 and the flask filled to the mark. 

 The contents of the flask are poured in a thin stream with con- 

 stant stirring into the solution of ammonium sulfate in nitric acid. 

 After standing for at least 48 hours, the solution is filtered and 

 stored in a well stoppered bottle kept in the dark. 



2. Nitric acid of 1.20 specific gravity at 15. 



3. A mixture of sulfuric and nitric acids made by pouring 30 

 cubic centimeters of sulfuric acid of 1.84 specific gravity into a 

 liter of nitric acid of 1.20 specific gravity. 



4. A two per cent, aqueous solution of pure ammonium nitrate. 



5. Alcohol of from 90 to 95 per cent, strength. 



6. Pure ether. 



The solution of the phosphate is run into a measuring cylinder 

 in quantities of 10, 15, or 20 cubic centimeters, according to its 

 strength in phosphoric acid and the volume completed to 50 cubic 

 91 Die landwirtschaftlichen Versuchs-Stationen, 1901, 55 : 183. 



