THE MANUFACTURE OF ACID PHOSPHATE. 11 



The advantages of having small quantities (and in some cases 

 large quantities) of carbonate of lime present in phosphate rock are 

 threefold: First, the heat evolved in the reaction between carbonates 

 and sulphuric acid is sufficient to warm the pasty mass of acid and 

 phosphate rock and thus promote chemical action between these 

 more slowing reacting substances; second, the escape of carbon 

 dioxide from the mass renders the acid phosphate porous and more 

 readily dried; and third, the gypsum formed prevents the formation 

 of the gelatinous iron and aluminum compounds and thus helps 

 render the product dry and in good condition for distributing or 

 mixing with other fertilizing ingredients. 



REVERSION OF SUPERPHOSPHATE. 



The reversion of superphosphate, as the term implies, originally 

 meant the return of the phosphoric acid to a condition insoluble (or 

 nearly so) in water. In reality the expression "reverted" phosphoric 

 acid is now wrongly used in a much broader sense and includes all of 

 the phosphoric acid of superphosphate which is soluble in certain 

 citrate solutions. In this paper, however, reverted phosphoric acid 

 is used in the strict sense of the word. 



When a superphosphate is allowed to stand and take up water 

 from moist air, as it sometimes does while in storage ; or is diluted by 

 the soil water when it is applied to the soil; or is added to an excess 

 of water, as is done in the laboratory before commencing analytical 

 operations, then, in any one and in all of these cases, less soluble 

 compounds of phosphoric acid are formed. If compounds of iron 

 and aluminum are present the formation of phosphates insoluble in 

 water is much more marked. This general process is known as 

 reversion, and the superphosphate is said to have reverted, and the 

 product is called reverted phosphate. The theory of this reversion is 

 now clearly understood, owing to the investigations in this country 

 of Cameron and Bell x and Seidell, 2 and of Bassett 3 in England, who 

 have shown that certain concentrations of phosphoric acid or of other 

 acids must exist in the water in contact with a calcium or iron phos- 

 phate for the solid definite ' ' acid ' ' compounds to be stable. Dilution 

 of the acid liquor causes the solids to decompose into more basic and 

 less soluble compounds. While the theory of these phenomena has 

 been made clear only recently, the main facts have long been known, 

 and as is so commonly the case, certain popular misconceptions have 

 held sway long enough to become regarded as facts even by many well- 

 trained chemists. Thus, it is popularly held that monocalcium phos- 

 phate is soluble in water, but dicalcium phosphate is not; dicalcium 



i Bui. 41, Bureau of Soils, U. S. Dept. Agr. (1907). 



2 Jour. Am. Chem. Soc, 27, 1503 (1905). 



3 Chem. News, 95, 21 (1907); Zeit, anorg. Chem., 53, 34 (1907). 



