362 DETERMINATION OV PHOSPHORIC OXIDE. 



phomolybclate in sodium hydroxide as rapidly as possible, and 

 titrating back with acid without delay. It has also been pointed 

 out that a large excess of sodium hydroxide is to be avoided owing 

 to the danger of loss of ammonia, fhe large quantity of indicator 

 is necessitated by the obscuring of the end point in the presence 

 of ammonium salts. If the relatively large quantity of indicator 

 recom!nended is employed, and the back titration carried out as 

 rapidly as is consistent with accuracy, there is no difficulty in 

 determining the point at which the pink colour vanishes. If, on 

 the other hand, only a drop or so of indicatiM- solution be used, 

 and the back titration is performed by adding the standard acid 

 drop by drop, the results obtained may be incorrect. 



Soil Extracts and Plant .Ishes. — For the determination of 

 phosphoric oxide and potash in hydrochloric acid extracts of soils 

 and in plant ashes, the method of Neubauer* is very generally 

 used. In this method the soil extract, or the hydrochloric acid 

 extract of the ash, is evaporated to dryness with the addition of 

 about 0.5 gram of calcium carbonate, the residue hnely grovmd 

 with a glass pestle and gently ignited. The grinding is repeated 

 several times during the ignition. The residue is then extracted 

 with water, and the jX)tash determined in the aqueous solution. 

 The insoluble residue is extracted with dilute sulphuric acid, and 

 phosphoric oxide determined in the resulting solution. 



The volumetric method is admirably adapted for the deter- 

 mination of the small amounts of phos])horic oxide usually 

 present in such solutions, and is probably cai-»al)le of greater 

 accuracy than methods in which the ])hos]>horic oxide is weighed 

 as magnesium pyrophosphate. 



We append a note by R. J. Sniit, Division of Chemistry, on 

 the recovery of ammonium molybdate from the residues from 

 the above method. 



Notes on a Method for the Recovery of Ammonium 

 molvrdate from residues. 



Owing to high cost of ammonium molybdate at the present 

 time, and what is of even greater importance, the difficulty of 

 procuring supplies, the recovery of the molybdate residues is 

 desirable. Several methods for the recovery of ammonium 

 molybdate from these residues have lieen published.* and a 

 method may be found in almost any text-book. Since the von 

 Lorenz method of precipitation requires a special reagent contain- 

 ing no ammonium nitrate, the majority of the recovery processe'^ 

 hitherto published are inapplicable. 



Up to the present we have m.^t found it to- be economical to 

 recover ammonium molybdate from residues other than the 

 filtrates from the precipitates of ammonium phosphomolvbdate. 

 The filtrates are collected and evaporated to a small bulk. During 



* Landzv. Verss. Stat.. tqo6. 63. 141. 



fE.fi., Presoott, J. A.: .'hialyst (1915), 40, 390. 



