PHOSPHOEIC ACID. 11 



Monocalcic phosphate or superphosphate is characterized by 

 its complete sohibihty (a) in water, (6) in ammoniacal citrate of 

 ammonia (aqueous solution of ammonium citrate with excess of 

 ammonia), and (c) in strong mineral acids. The sole object of 

 superphosphate manufacture is to convert the tricalcic phosphates 

 of bones and mineral phosphates into monocalcic or acid phosphate, 

 assimilable by plants. Dicalcic phosphate, or precipitated phos- 

 phate, is insoluble in water, but soluble in ammoniacal citrate of 

 ammonia, and strong mineral acids. It is obtained in the treating 

 of bones by hydrochloric acid, in the manufacture of gelatine (glue), 

 as will be seen in the sequel. Tricalcic phosphate forms the chief 

 mass of the substance of bones which resists combustion (bone ash)^ 

 of phosphorites, of apatite, and of a great number of guanos. In 

 the calcined state, or as apatite, it is insoluble in w^ater, slightly 

 soluble in water saturated with carbonic acid. It is completely 

 soluble in nitric and hydrochloric acids, which convert it into mono- 

 calcic phosphate, displacing two-thirds of the lime to form, w^ith it, 

 calcium nitrate or calcium chloride. Sulphuric acid completely 

 decomposes it into free phosphoric acid and sulphate of lime 

 (gypsum). Phosphoric acid is precipitated from acid solutions, by 

 excess of lime water, as a fine white powder, which is partly soluble 

 in water, especially when it contains potassium sulphate, common 

 salt, nitrate of soda, ammoniacal salts, or carbonic acid. 



PJiosjjhates of Aimnonium, Sodium, etc. — Let us examine a few 

 other phosphates of importance in manure manufacture : — 



1. Basic Phosphate of Ammonium, (NII^)3P04, is a slightly sol- 

 uble, very unstable salt, obtained by mixing syrupy phosphoric acid 

 with excess of ammonia. Its aqueous solution exposed to the air 

 loses one-third of its ammonia, and is converted into the monacid 

 phosphate ; heated, it loses a second third, and is then converted 

 into the acid phosphate. 



2. Monacid Phosphate of Sodium {ordinary commercial phos- 

 phate of soda, Na^HPO^ -f- ISH^O) is contained in urine. It forms 

 large crystals, which turn dull in the air, through loss of a portion of 

 their water. Heat converts it into pyrophosphate by expulsion of 

 water. It is prepared on the large scale by saturating crude phos- 

 phoric acid with soda to alkaline reaction and crystallizing. 



3. Monacid Phosjjhate of Ammonium and Sodium, NaNH^ 

 HPO4 + 4H^0. — This salt is prepared by dissolving ammonium 

 chloride in a solution of ordinary phosphate of soda. Heat converts 

 it into sodium metaphosphate, NaPOg, with liberation of ammonia 

 and water. Putrefying human urine yields a deposit of this salt. 

 It forms crystals as clear as water, and is found in that state in 



cipitate on warming gently, phosphoric acid may be taken as absent. Even a 

 precipitate is no sign of the presence ot phosphoric acid unless all traces of 

 arsenic have previously been removed. — Tr. 



