PHOSPHORUS COMPOUNDS 61 



Other elements may take part in the construction 

 of molecules from ions consisting of radicals or rests. 

 Among the most important substances absorbed by 

 the roots of plants are phosphates, sulphates, chlor- 

 ides, and nitrates. These salts exist in the sap of 

 plants chiefly in the form of ions ; e.g. the phosphoric 

 acid of the phosphates of potash ammonia or soda 

 absorbed is in the form of a trivalent anion, P0 4 . 

 An important organic compound is glycero-phos- 

 phoric acid, C 3 H 9 P0 6 , formed by the acid ion joining 

 to the tri-acid-alcohol called glycerine in ether 

 fashion, i.e. by the acid ion displacing one OH from 

 the glycerine molecule. Lecithin, another important 

 organic phosphorus compound, is formed from 

 glycero-phosphoric acid by fatty acids replacing the 

 other two hydroxyls in the glycerine of the com- 

 pound. The fatty acids may be stearic, palmitic, or 

 oleic, one or two kinds. Lecithin originates in 

 plants ; enters the animal organism, and is found in 

 nearly all cellular structures, being the chief link 

 between inorganic and organised phosphorus in the 

 living body. Phosphorus is pentad or pentavalent ; 

 phosphoric acid is trivalent. It forms three kinds of 

 salts, thus : normal sodium phosphate, which is 

 Na 3 P0 4 , may have one of its Na atoms replaced by 

 H as in hydrogen-disodium phosphate, HNa 2 P0 4 , or 

 by 2H as in H 2 NaP0 4 . Again, one hydrogen atom 

 may be displaced by another ion as NH 4 in hydrogen- 

 sodium-ammonium phosphate, HNa(NH 4 )P0 4 ; and 

 the other Na atom may be displaced by another 

 metallic ion, e.g. Mg, in the compound ammonium- 

 magnesium-phosphate, (NH 4 )MgP0 4 . 



The halogens Cl, Br and I, form monovalent ions 



