CLASSIFICATION OF METHODS 13! 



of color in the whole mass. Free tribasic phosphoric acid can not 

 be conveniently titrated directly with a standard alkali, because of 

 the development of an amphoteric action near the point of neu- 

 trality. When, for instance, an alkali is added to the acid, a com- 

 bination is formed of such a character that it will affect an indi- 

 cator both as an acid and alkali. All the volumetric processes 

 now in general use may be divided into two classes, viz., (i) the 

 direct titration of phosphoric acid and the determination of the 

 end reaction by any appropriate means, and (2) the previous 

 separation of the phosphoric acid, usually by means of a citro- 

 magnesium or molybdenum mixture, and in the latter case the 

 subsequent titration of the yellow ammonium phosphomolybdate 

 either directly or after reduction to a lower form of oxidation. 

 In respect of age and extent of application, by far the most im- 

 portant volumetric method is the one depending on titration by a 

 uranium salt after previous separation by ammoniacal magnesium 

 citrate. A promising method, after previous separation by 

 molybdenum, is the one proposed by Thilo and developed by Pem- 

 berton, and modified by Kilgore and other members of the Asso- 

 ciation of Official Agricultural Chemists, and it has now come into 

 quite general use in this country. For small quantities of phos- 

 phoric acid or of phosphorus, such as are found in steels and irons, 

 the method of Emmerton either as originally proposed or as modi- 

 fied by Dudley and Moves, has been frequently used. Where volu- 

 metric methods are applied to products separated by molybdic solu- 

 tion, the essential feature of the analytical work is to secure a yel- 

 low precipitate of constant composition. If this could be uniform- 

 ly done in such methods they would rival the gravimetric pro- 

 cesses in accuracy. Hence it is highly important in these methods 

 that the yellow precipitate should be secured as far as possible 

 under constant conditions of strength of solution, duration of 

 time, and manner of precipitation. In these cases, and in such 

 only, can the quicker volumetric methods be depended on for 

 accurate results. 



The direct volumetric titration of the phosphoric acid by 

 a uranium salt or otherwise is practised only when the acid is 

 combined with the alkalies and when iron and alumina are 

 absent and only small quantities of lime present. This method 



