214 DYEING 



It is fitting that the man who was afterwards to receive the Nobel 

 Prize for his co-ordination theory of valency was the first person to 

 grasp the essentials of lake-formation. 



The chemistry of this process was subsequently studied by two 

 British chemists, Morgan and Smith, ^^^ to whom we owe the 

 expressive term 'chelate' for the pincer-like grip in which a dye 

 holds the mordant metal. They took the word from the analogy 

 with the chela of a lobster. 



It now remains to look for the phenolic -OH group and nearby 

 donor oxygen in the dyes that form lakes. Alizarine provides a very 

 simple example. A phenolic -OH is close beside a suitable oxygen 

 atom, and a six-membered ring (aluminium, oxygen, carbon, car- 



OH 



Alizarine Alizarine aluminium lake 



bon, carbon, oxygen) is readily formed. (The aluminium atom is 

 capable of linking with three alizarine molecules.) One has only 

 to look at the formula for carminic acid on p. 177 to see that it is 

 capable of acting in exactly the same way. 



It is important to notice that the metal atom (whether alumin- 

 ium, chromium, or ferric iron) makes two different kinds of links 

 with the dye. These links may for convenience be called primary 

 and secondary. The primary link is made by the substitution of 

 the metal for a hydrogen atom in an acidic -OH group. It is 

 reasonable to suppose that this primary linkage is initially electro- 

 valent, though it may be replaced by a somewhat polar covalency. 

 The secondary link is the dative covalency with the electron- 

 donor oxygen atom. The cation could make room for such a 

 covalency by losing one of the -OHg or -OH groups held to the 

 metal by 'subsidiary' valencies. (See the formulae for chromic 

 cations on p. 211.) 



It was pointed out nearly 70 years ago^^* that many anthra- 

 quinone dyes that can be used with mordants have two phenolic 

 -OH groups in or//zo-relationship to one another, and it was at first 

 thought that these gripped the metal. The action of the second 



