New York Agricultural Experiment Station. 293 



II. THE USE OF SODIUM CITRATE FOR THE 

 DETERMINATION OF REVERTED PHOS- 

 PHORIC ACID. 1 * 



ALFRED W. BOSWORTH. 



In 1871, Fresenius, Neubauer and Luck 2 published a method for 

 the determination of reverted phosphoric acid in phosphates which 

 involves the use of a solution of neutral ammonium citrate, specific 

 gravity 1.09. This method, with a change in the temperature of 

 the solvent, has been in constant use since that time 3 with no attempt 

 by any one to give an explanation of the chemical reaction involved. 

 It has been quite generally believed that the neutral ammonium 

 citrate solution possesses a selective power which enables it to separate 

 dicalcic-phosphate from tricalcic-phosphate. This is not true, for 

 it has been found in this laboratory that 100 c.c. of the official ammon- 

 ium citrate solution 3 is capable of dissolving 1.3 grams of precipitated 

 tricalcic-phosphate in one-half hour at a temperature of 65° C. 

 This dissolving of the tricalcic-phosphate is accompanied by a 

 precipitation of calcium citrate. 



This separation of calcium citrate led to the belief that the solvent 

 action of the citrate solution was the result, of a double decomposition 

 started by the free phosphoric acid always present in an aqueous 

 solution which is in contact with a solid phase composed of a phos- 

 phate. 4 This double decomposition might be indicated by the 

 following : 



CaHP0 4 +2C 6 H 5 6 (NH 4 ) 3 > (NH 4 ) 2 HP0 4 + [C 6 H 5 6 (NH 4 ) 2 J 2 Ca 



Ca 3 P 2 8 + 6C 6 H 5 6 (NH 4 ) 3 > 2(NH 4 ) 3 P0 4 + 3[C 6 H 5 6 (NH 4 ) 2 ] 2 Ca 



If appreciable amounts of calcium are taken into solution, calcium 

 citrate will separate out. 



3[C 6 H 5 6 (NH 4 ) 2 ] 2 Ca > 4C 6 H 5 6 (NH 4 ) 3 + (C e H60 fl )«Caa 



A great deal of work has been done upon methods of making neutral 

 ammonium citrate solutions and several such methods have been 

 published. The fact that neutral ammonium citrate is very unstable 

 and easily loses ammonia has not been sufficiently considered in 

 this connection, however. Why should extreme care be taken to 

 secure an absolutely neutral solution, if this solution is to lose 

 ammonia when heated a few degrees above the room temperature? 

 Most chemists who have used the neutral ammonium citrate solution 

 know that ammonia is constantly given off during the half hour 

 allowed for the solvent action to take place. The final result then, 



1 Read before the Association of Official Agricultural Chemists, Washington, 

 D. C, Nov. 17, 1913, and published in the Jour. Indust. Eng. Chem., 6 : 227. 

 *Ztschr. Analyt. Chem., 10 : 133. 



3 U. S. Dept. Agr., Bur. of Chem., Bull. 107 (revised). 



4 Cameron and Hurst, Jour. Amer. Chem. Soc, 26, 905. 



* Reprint of part of Technical Bulletin No. 34, May; see p. 286. 



