178 AGRICULTURAL ANALYSIS 



sodium chlorid. The phosphoric acid is determined in the filtrate 

 by means of a standard solution of uranium nitrate in the man- 

 ner 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. 



155. Volumetric Silver Method. Holleman has proposed a 

 modification of the silver method for the volumetric determina- 

 tion of phosphoric acid, having for its chief purpose the more ac- 

 curate and easy determination of the end reaction, which is con- 

 ducted as described below. 32 The reaction which takes place is 

 represented by the equation, Na 2 HPO 4 -f3AgNO 3 =:Ag3PO 4 -|- 

 2NaNO 3 -(-HNO3. Although silver phosphate is insoluble in 

 water, the nitric acid formed holds some of it in solution. To 

 prevent this, acetate of soda is added in excess, and thus the whole 

 of the phosphoric acid is obtained as a silver salt. The light is 

 to be excluded during the determination by wrapping the flask in 

 a black cloth to avoid a discoloration of the silver compound. 

 Yolhard's reaction for silver is based on the fact that when solu- 

 tions of silver and an alkaline thiocyanate are mixed in the pres- 

 ence of a ferric salt, silver is precipitated as thiocyanate. 33 As soon 

 as the least excess of thiocyanate is added, brown ferric thio- 

 cyanate is formed, and this marks the end point of the solution. 



In a flask of 200 cubic centimeters capacity are placed 50 cubic 

 centimeters of the liquid to be analyzed, which should not contain 

 more than two-tenths gram of phosphoric acid. The solution is 

 treated with 10 cubic centimeters of a normal solution of sodium 

 acetate and afterwards with a slight excess of decinormal silver 

 solution, four and five-tenths cubic centimeters for each o.oi 

 gram of phosphoric acid. The solution is neutralized with tenth- 

 normal sodium hydroxid, the amount required having been pre- 

 viously determined by titrating 10 cubic centimeters of the liquid 

 to be analyzed, using phenolphthalein as an indicator. Five times 



" Recueil des Travaux chimiques des Pays-Bas, 1893, 12 : I. 



M Liebig's Annalen der Chemie, 1877, 190 : I. 



