=113. The Silver Method.=—The separation of the phosphoric acid by silver according to the method of Perrot has been investigated by Spencer, who found the process unreliable.[96] By a modification of the process, however, Spencer obtained fairly satisfactory results. The principle of this method depends on the separation of the phosphoric acid by silver carbonate and the subsequent 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. An aliquot part is taken, after filtration, varying with the supposed strength of the solution so as to contain about 100 milligrams of phosphorus pentoxid. In the slightly nitric acid solution add freshly prepared silver carbonate 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 carbonate in nitric acid and remove the silver from the solution with sodium chlorid. The phosphoric acid is determined in the filtrate by means of a standard solution of uranium nitrate in the manner already described. Spencer found that the separation of the phosphoric acid by the silver method was more exact than by the Joulie magnesium citrate process. With practice on the part of the analyst in determining the end reaction the process is both rapid and accurate. The method is also inexpensive, as both the silver and uranium are easily recovered from the waste. =114. Volumetric Silver Method.=—Holleman has proposed a modification of the silver method for the volumetric determination of phosphoric acid, which is conducted in the following manner:[97]