758 



phosphoric acid in the solution remained practically unchanged, indi- 

 cating that the dicalcic phosphate had caused no decomposition of the 

 monophosphate. 



Action of tricalcic phosphate. — To study the action of tricalcic on 

 monocalcic phosphate, 3.46 grams of the former and 2.52 grams of 

 the latter were placed in each of eight 2 litre flasks, and the flasks were 

 then filled to the mouth with water. The soluble phosphoric acid was 

 determined in the solution in each flask after 2 hours, 1, 2, 5, 14, and 30 

 days, and 6 and 12 mouths, respectively. The results showed that the 

 tricalcic phosphate acted but slowly on the soluble monocalcic phos- 

 phate. The percentage of soluble acid, which after 2 days' standing 

 was 56.63, was reduced to 0.59 per cent after 8 mouths' standing. When 

 2.52 grams of monocalcic and 3.46 grams of tricalcic phosphate were 

 dried in a platinum dish at 100° G. for 6 hours a loss equal to 2 per cent 

 of the soluble phosphoric acid was noticed. According to these trials 

 the decomposition is much more rapid with the phosphates in a dry 

 state than in solution. 



In a series of experiments 2.52 grams of monocalcic, 3.46 grams of 

 tricalcic phosphate, and 0.98 gram of chemically pure orthophosphoric 

 acid were mixed in a dry state, and in separate trials determinations 

 made of the free and soluble phosphoric acid in the mixtures after a 

 half hour, 2 hours, 5 days, and 10 days. In these trials, as indicated 

 by the results, the free i)hosphoric acid and the tricalcic phosphate 

 formed a dicalcic phosphate, the monocalcic phosphate remaining 

 unchanged. 



In similar exi)eriments in which 0.098 gram of orthophosphoric acid, 

 0.756 gram of monocalcic, and 1.384 grams of tricalcic phosphate were 

 used, it was found that the free phosphoric acid combined with 1 mole- 

 cule of the tricalcic phosphate forming diphosphate, the remaining phos- 

 phate then forming dicalcic j)hosphate with the monophos{!hate. 



These reactions may take place in the preparation and keeping of 

 superphosphates, and the author believes they clear up certain points 

 regarding the changes which superphosphates may undergo. They 

 teach that the content of free phosphoric acid in superphosphates is a 

 very important factor, as this acid plays a part second to none in the 

 changes. If the superphosphate contains 15.4 per cent of water-soluble 

 phosphoric acid including 8.2 i)er cent of P2O5 as free acid, it can con- 

 tain 4 per cent of P2O5 as tricalcic phosphate and no reverted acid 

 be formed; but the monophosphate, on the contrary, will increase until 

 at last all the P2O5 of the triphosphate is changed to the soluble form. 



When suli^huric acid weak or slightly in excess is used in the man- 

 ufacture of superphosphates free P2O5 will be formed ; when concentrated 

 acid of 55-60° B(5. or no excess of acid of 50° Be. is used, small amounts 

 of free P2O5 are formed and reactions resulting in the formation of dicalcic 

 phosphate may occur. 



