(i88 G. N. JENKINS AND C. DAWES 



platiiiicliloiides. In some cases, owing to the existence of two or 

 more groups in the acid molecule which from their nature and steric 

 position can combine with a metal, the latter becomes part of a ring 

 compound (Fig. 1). Such compounds are called "chelates," and the 



I Co 



'3 



H 



Fig. 1. The structural formula of one of the possible forms of calcium lac- 

 tate chelate. 



acids possessing the necessary groupings are called chelating or 

 sequestering agents. For example, acetic acid combines with cal- 

 cium to form the salt calcium acetate, but when an amino group 

 is introduced into the molecule ( thus converting it into glycine ) , 

 calcium can be attached both to the acid group (b\- ionic bonds) 

 and to the amino group (bv coordinate covalent l)onds), thus form- 

 ing the ring. Although the term "chelate" should be used onlv for 

 this special tvpe of complex ion in which the metal is part of a ring, 

 there has been some tendency to use it looselv for complex ions in 

 general. Chelates are characterized by great stability, especially if 

 the metal is attached to the molecule at several sites. Some acids 

 can combine with metals in two wavs: as an ordinary salt, and as 

 a chelate. The chelating properties of a substance are greath' in- 

 fluenced by pH and tend to increase as pH rises, because the metal 

 ions compete directly with hvdrogen ions. This is of great practical 

 importance because manv chelating agents are capable of dissolving 

 at neutral or alkaline pH values salts such as calcium phosphate 

 previously regarded as virtually insoluble except in acids. Chelators 

 may dissolve these salts even in solutions such as blood and saliva 

 which, from the ionic point of view, are already satmated. 



Many naturally occurring substances have a structure which en- 

 dhles them to form chelates, and this fact has suggested the possibil- 



