PHOSPHORUS. 227 



expelled, and enough of water added to obtain an acid which contains 

 85 per cent, of the pure H 3 PO 4 . Specific gravity 1.707 at 25 C. 



Diluted phosphoric acid, U. S. P., is made by mixing 100 Gm. of 

 the 85 per cent, acid with 750 Gm. of water. It contains 10 per cent, 

 of absolute orthophosphoric acid. 



Phosphoric acid, U. S. P., is a colorless, odorless, strongly acid 

 liquid, which, on evaporation, forms a thick syrupy liquid. This, on 

 cooling, slowly solidifies in the form of large crystals, which are 

 highly deliquescent. Heated to a sufficiently high temperature the 

 acid loses water, being converted successively into pyrophosphoric 

 and metaphosphoric acid, which is finally volatilized at a low red 

 heat. It is a tribasic acid, forming three series of salts, namely : 



Na 3 PO 4 = Trisodium phosphate. 



Na 2 HPO 4 = Disodium hydrogen phosphate. 

 NaH 2 PO 4 = Sodium dihydrogen phosphate. 



If the metal be bivalent, the formulas are thus : 



Ca 3 (PO 4 ) 2 = Tricalcium phosphate. 

 Ca2H 2 (PO 4 ) 2 = Dicalcium orthophosphate. 

 CaH 4 (PO 4 ) 2 = Monocalcium orthophosphate. 



According to the number of hydrogen atoms replaced in the acid, 

 the salts formed are also termed primary, secondary, and tertiary 

 phosphates ; KH 2 PO 4 being, for instance, primary potassium phos- 

 phate ; Na 2 HPO 4 secondary sodium phosphate ; Ag 3 PO 4 tertiary sil- 

 ver phosphate. All the alkali phosphates, but only primary phos- 

 phates of the other metals, are soluble in water. 



All phosphates insoluble in water are dissolved by nitric, hydrochloric, or 

 sulphuric acid ; also by acetic acid, except those of lead, aluminum, and ferric 

 iron. All are soluble in phosphoric acid (forming acid phosphates), except those 

 of lead, tin, mercury, and bismuth. Primary alkali phosphates are acid to 

 litmus, but secondary alkali phosphates, although they are acid salts, are alka- 

 line to litmus because of partial hydrolysis by water into primary phosphate and 

 free alkali. Tertiary alkali phosphates are decomposed by water into the second- 

 ary salt and free alkali. 



Phosphoric acid belongs to the class of weak acids, and the three hydrogen 

 atoms in the molecule show very different degrees of dissociation. It dissoci- 

 ates chiefly according to this equation, H 3 PO 4 ~. H* + H 2 PO/. The dihy- 

 drophosphate ion, H 2 PO/, dissociates to a small degree into H* and HPO/' 

 ions, as is shown by the fact that monosodium phosphate has a slightly acid 

 reaction to litmus, thus : 



NaH 2 PO 4 ^ Na* + H 2 PO/ 

 H 2 PO 4 ' ^1 H' + HP0 4 /X . 



The ion HPO/' is practically not dissociated into H' and PO/' ions, as is 



