RECENT WORK l\ AGRICULTURAL SCIENCE. 



AGRICULTURAL CHEMISTRY. 



The determination of phosphoric acid by means of ammonium phospho- 

 molybdate, <i. P. Baxter and R. C. Griffin {Amer. ('htm. Jour., 84 (1905), No. 8, 

 pp. 204 211 1. This is a second paper on this subject and deals especially with the 

 nature and amount of the occluded substances in the ammonium phosphomolybdate 

 precipitate. 



The results of the investigation confirm the conclusion reached in the previous 

 paper "that it is possible to obtain ammonium phosphomolybdate constant in com- 

 position and in a state suitable for weighing, so that it may be used for the accurate 

 estimation of phosphoric acid." The precipitation should be performed at room 

 temperature by pouring the phosphate solution into the molybdic acid. "If the 

 precipitation is performed in the reverse manner the composition of the precipitate 

 varies considerably." 



The precipitate is the diammonium salt which when heated to a high temperature 

 with ammonium nitrate is converted almost completely into the triammonium phos- 

 phomolybdate. "The ammonium in the precipitate is readily replaced by potassium, 

 in the presence of a high concentration of potassium salts, but the reverse change 

 takes place if the potassium phosphomolybdate is then treated with concentrated 

 ammonium salts." It was found that the precipitate occludes varying amounts of 

 both molybdic acid and ammounium molybdate, according to the conditions of 

 precipitation. 



The Pemberton method of determining phosphoric acid by titration of ammonium 

 phosphomolybdate with standard potassium hydroxid is condemned as inaccurate 

 because ( 1 ) the precipitate occludes varying amounts of molybdic acid; (2 | 24 mole- 

 cules of alkali are required for the neutralization of 1 molecule of ammonium phospho- 

 molybdate and not 23 as commonly assumed; and (3) titration with phenolphthalein 

 in the presence of ammonia always yields uncertain results. 



" No method for estimating phosphoric acid, which depends upon the determina- 

 tion of the molybdic acid in ammonium phosphomolybdate, is accurate unless allow- 

 ance is made for the occluded ammonium molybdate and molybdic acid." 



Determination of citrate-soluble phosphoric acid in superphosphate, < ). 

 Beib {Ztschr. Analyt. Chem., 44 1 1905), No. 6-7, />/>. S97, 898).— The following quick 

 method is proposed as a substitute for the official Petermann method used in Belgium: 



To 2.1 gm. of the superphosphate in a small mortar add in 10 cc. portions while 

 still warm a mixture of L'Occ of concentrated sulphuric acid and 80 d-. of water, grind- 

 ing the superphosphate with the acid mixture and pouring off the solution into a 

 1 liter !la>k. The operation is repeated '■'< time-, when the residue is washed into the 

 Mask with t he remaining acid solution and the lla>k is -ha ken for- half an hour in a 

 rotary apparatus. The solution is cooled and the flask tilled to the mark with water. 

 The solution is filtered and •"><> CC. of the tilt rate i- used for the determination of phos- 

 phoric acid by means of magnesia mixture. Comparative tests by the author and 

 others of this method and IVtennann's method show closely agreeing results. 



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