FREE ACID IN PHOSPHATES l8/ 



0.00445=0.0645 gram P 2 O 5 . Consequently the superphosphate 

 contains 12.90 per cent, of water-soluble P 2 O 5 . 



Total Phosphoric Acid. Twenty grams of the superphosphate 

 are boiled with 50 cubic centimeters of hydrochloric acid of from 

 16 to 18 Beaume for about 10 minutes and, after cooling, made 

 up to one liter with water and filtered. Twenty-five cubic centi- 

 meters of the filtrate are treated with 10 cubic centimeters of 

 Joulie's citrate solution, a few pieces of filter paper thrown in. 

 15 cubic centimeters of magnesia mixture added, and the 

 whole thoroughly stirred. After standing two hours the contents 

 of the flask are filtered, the precipitate is washed with dilute 

 ammonia, and the filter and the precipitate are placed in a platinum 

 crucible. The crucible is heated slowly until the moisture is driv- 

 en off and the filter burned. Then the temperature is gradually 

 raised to a white heat. The residue is cooled and weighed. 



Example : The precipitate weighs, after the subtraction of the 

 filter ash, 0.1390 gram; then the superphosphate contains 13.90 

 X 2X0-64= 1 7. 79 per cent, phosphoric acid. 



160. Determination of Free Acid in Phosphates for Technical 

 Purposes. A speedy and approximately accurate method of 

 determining free phosphoric acid in superphosphates is useful in 

 technical work, and for this purpose Gerhardt has proposed the 

 following process. 30 There are valid objections to both the meth- 

 ods in common use. When the free acid is extracted with 

 water and the acidity of the extract determined by titration with 

 an alkali, the end reaction is obscured by the separation of acid 

 calcium phosphate, CaHPO 4 . On the other hand, when absolute 

 alcohol is used to dissolve the acid, the separation is not exact 

 because of the water content of the sample, and drying the sample 

 would cause a decomposition and the formation of new com- 

 pounds of the free phosphoric acid. The principle of the fol- 

 lowing process is based on the addition of an excess of calcium 

 carbonate to the sample and the subsequent determination of the 

 undecomposed portion. 



When pure phosphoric acid is shaken with calcium carbonate 

 the following reaction takes place : 



2H 3 PO 4 +CaCO 3 =CO 2 +H 2 O+CaH 4 (PO 4 ) 2 



* Chemiker-Zeitung, 1905, 29 : 178. 



