THE SILVER METHOD 177 



wire is 49.9 cubic centimeters permanganate. The wire con- 

 tains 99.87 per cent, of iron. We have then the following 

 equation; namely, 0.1745X0.9987-^-49.9=0.0034923. This shows 

 that one cubic centimeter of permanganate is equivalent to that 

 quantity expressed as 0.0034923 gram metallic iron. Multiply- 

 ing the value in iron by the ratio of molybdic acid to iron, 

 namely, 0.88163, and the product by the ratio of phosphorus to 

 molybdic acid, namely, 0.01794, the product is found to be 

 0.000055238. This indicates that one cubic centimeter of per- 

 manganate is equivalent to 0.000055238 gram of phosphorus. If 

 the precipitated ammonium phosphomolybdate from two grams 

 of steel require 35.5 corrected cubic centimeters, then the per- 

 centage of phosphorus in steel is obtained by the following for- 

 mula: 35.5Xo.oooo55238Xioo-:-2=:o.098, which is equivalent 

 to the percentage of the phosphorus of the steel. For further 

 details of the process the work of Blair, already cited, should 

 be consulted. 



154. The Silver Method. The separation of the phosphoric 

 acid by silver according to the method of Perrot has been inves- 

 tigated by Spencer, who found the process unreliable. 31 By a 

 modification of the process, however, Spencer obtained fairly sat- 

 isfactory results. The principle of this method depends on the 

 separation of the phosphoric acid by silver carbonate and the sub- 

 sequent titration thereof with standard uranium solution after 

 the removal of the excess of silver. The operation is conducted 

 as follows : The fertilizer is first ignited until all organic matter 

 and residual carbon are destroyed. Solution is then accomplished 

 by .means of nitric acid and the volume completed to a definite 

 quantity. To an aliquot part of the slightly acid (nitric) solu- 

 tion, after filtration, varying with the supposed strength of the 

 solution so as to contain about 100 milligrams of phosphorus 

 pentoxid, freshly prepared silver carbonate is added in excess, 

 that is, sufficient to saturate any free acid present and also to 

 combine with all the phosphoric acid. Wash thoroughly with 

 hot water and then dissolve the mixed phosphate and silver car- 

 bonate in nitric acid, and remove the silver from the solution with 

 31 Eighth Annual Report of Purdue University, 1882 : 240. 



