114 NON-METALS AND THEIR COMBINATIONS. 



Phosphoric acid, Orthophosphoric acid, Acidum phosphoricum 

 H 3 PO 4 =98 (Trihydric phosphate, Common or tribasic phosphoric acid). 

 Nearly all phosphates found in nature are orthophosphates. 



Phosphoric acid may be made by burning phosphorus, dissolving 

 the phosphoric oxide in water, and boiling for a sufficient length of 

 time to convert the meta- into orthophosphoric acid. 



Experiment 12. Place a piece of phosphorus (about 0.5 gramme), after having 

 dried it quickly between filter paper, in a small porcelain dish, standing upon 

 a glass plate ; ignite the phosphorus by touching it with a heated wire, and 

 place over the dish an inverted large beaker. The white vapors of phosphoric 

 oxide soon condense into flakes, which fall on the glass plate. Collect the 

 white mass with a glass rod, and dissolve in a few c.c. of water. Use a portion 

 of the solution for tests of metaphosphoric acid; evaporate the remaining 

 quantity in a porcelain dish until it becomes syrupy, dilute with water and use 

 it for making tests for orthophosphoric acid, either as such or after having 

 neutralized with sodium carbonate. How much phosphorus is needed to make 

 490 grammes of the U. S. P. 50 per cent, phosphoric acid ? 



Phosphoric acid is also made by gently heating pieces of phos- 

 phorus with diluted nitric acid, when the phosphorus is oxidized, 

 red fumes of nitrogen tetroxide escaping : 



3P + 5HNO 3 + 2H 2 O = 3H 3 PO 4 + 5NO. 



The liquid is evaporated until the excess of nitric acid has been 

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

 tains 85 per cent, of the pure H 3 PO 4 . Specific gravity 1.710. 



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

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

 absolute orthophosphoric acid. 



Phosphoric acid is a colorless, odorless, strongly acid liquid, which, 

 on heating, loses water, and finally is 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. 

 Ca.,H 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. 



