226 NON-METALS AND THEIR COMBINATIONS. 



These three acids show different reactions, act differently upon the 

 animal system, and form different salts. 



Metaphosphoric acid, HPO 3 , PO 2 OH (Glacial phosphoric, acid). 

 This acid is always formed when phosphoric oxide is dissolved in 

 water ; gradually, and more rapidly on heating with water, it absorbs 

 the latter, forming orthophosphoric acid; by heating the latter to 

 near a red heat metaphosphoric acid is re-formed. 



Metaphosphoric acid is a monobasic acid which forms colorless, 

 transparent, amorphous masses, readily soluble in water. It coagu- 

 lates albumin (pyro- and orthophosphoric acids do not) and gives a 

 white precipitate with ammonio-silver nitrate ; it is not precipitated 

 by magnesium sulphate in the presence of ammonia and ammonium 

 chloride. It acts as a poison, while common phosphoric acid is 

 comparatively harmless. 



Pyrophosphoric acid, H 4 P 2 O 7 , P 2 O 3 (OH) 4 . This is a tetra-basic acid 

 which gives a white precipitate with ammonio-silver nitrate, while orthophos- 

 phoric acid gives a yellowish precipitate ; it is not precipitated by ammonium 

 molybdate, and does not coagulate albumin. 



Phosphoric acid, Orthophosphoric acid, Acidum phosphoricum, 

 H 3 PO 4 , PO(OH) 3 = 97.29 (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 16. 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. 10 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 



