108 AGRICULTURAL ANALYSIS 



from the flame, 15 cubic centimeters more of sulfuric acid are 

 added, and the flask and contents are heated at a moderate tem- 

 perature for two or three minutes; the mercuric oxid is then 

 added and the operation completed as before described. 



Following are some of the advantages offered by the method 

 described : 



(i) It dispenses with the necessity of the frequently tedious 

 operation of bringing upon the filter and washing the residue 

 from the ammonium citrate digestion, while the ignition of this 

 residue together with the subsequent digestion with acid and 

 filtration are also avoided. 



(2) It affords a means for the direct estimation of that form of 

 phosphoric acid which, together with the water-soluble, consti- 

 tutes the available phosphoric acid, thus enabling the latter to 

 be determined by making only two estimations. 



(3) In connection with the advantages above mentioned it 

 permits of a considerable saving of time as well as of labor 

 required in manipulation. 



In addition to the tests with mercuric oxid, both potassium 

 nitrate and potassium sulfate are used in the digestion to facil- 

 itate oxidation. With the former, several additions of the salt 

 are necessary to secure a satisfactory digestion, and even then 

 the time required is longer than with the mercury or mercuric 

 oxid digestion. With potassium sulfate, the excessive foaming 

 which takes place interferes greatly with the execution of the 

 digestion process. 



99. Determination of Phosphoric Acid with Preliminary Pre- 

 cipitation as Stannic Phosphate. This method once much in use 

 and highly recommended, is now almost unknown among the pro- 

 cesses of fertilizer control. It was first proposed by Reynoso and 

 modified by Girard, and rests on the precipitation of the phos- 

 phoric acid in a nitric acid solution by means of metallic tin. 77 

 The stannic acid formed by the oxidation of the tin unites with 

 the phosphoric acid held in a free state by the nitric acid. The 

 precipitation of the phosphoric acid is said to be complete, and 

 considerable quantities of any iron or alumina which may be pre- 

 77 Comptes rendus, 1862, 54 : 468. 



