242 AGRICULTURAL ANALYSIS 



hydroxids. The mixed hydroxids are collected on a filter, washed 

 with water, the filtrate and washings being collected in a beaker. 



The precipitate is dissolved with a small quantity of a solution 

 of ammonium nitrate and nitric acid, again precipitated with am- 

 monia, filtered, washed, ignited, and weighed. This treatment 

 is for the purpose of excluding all possibility of error from the 

 presence of molybdic anhydrid. After weighing, the mixed oxids 

 are fused with sodium bisulfate, the magma dissolved in water, and 

 the iron determined volumetrically with potassium permanganate 

 after reduction to the ferrous state. 



McElroy has shown later that even the molybdate method of 

 separating the iron and alumina from phosphoric acid with the 

 improvements as first suggested by Krug and himself, may not 

 always give reliable results. 1 In a solution containing ferrous 

 iron equivalent to 56.4 milligrams of ferric oxid, was placed 

 enough of a solution of sodium phosphate to correspond to 100 

 milligrams of phosphorus pentoxid. The precipitate was dis- 

 solved by adding nitric acid, oxidized with bromin water, and the 

 phosphoric acid thrown out with ammonium molybdate. The pre- 

 cipitate was washed with weak nitric acid and the combined fil- 

 trate and washings neutralized with ammonia. The resultant pre- 

 cipitate was dissolved in a solution of ammonium nitrate and nitric 

 acid, filtered, and again precipitated with ammonia. In two in- 

 stances the quantities of material recovered after ignition were 

 56.9 and 57.3 milligrams, respectively, instead of the theoretical 

 amount, viz., 56.4 milligrams. 



When the work was repeated after the addition of 400 milli- 

 grams of calcium oxid the weight of the precipitate recovered 

 was 62.3 and 63.1 milligrams in duplicate determinations. Sim- 

 ilar determinations were made with a known weight, viz., 35.6 

 milligrams of alumina. The treatment of the mixture was pre- 

 cisely as indicated above for iron. The quantity of alumina fin- 

 ally obtained was 28.9 and 29.3 milligrams, respectively, in 

 duplicate determinations. When the lime was added, however, 

 the weights of alumina recovered fell to 19.8 and 20.6 milligrams 

 respectively. These results show that the molybdate method for 

 1 Journal of the American ChemicU Society, 1895, 17 : 260. 



