THE INDIRECT ATTACHMENT OF DYES TO TISSUES 211 



The formulae can also be written as AlK(S04)2*i2H20, etc., 

 but this method shows the composition less well. There is no 

 purpose in constructing the formula in a way that will make the 

 molecule appear as small as possible, since there are no molecules 

 in the crystal, and only ions in solution. 



Aluminium, ferric, and chromic sulphate act similarly to the 

 alums. There is no compelling reason why sulphates should be 

 used in preference to other salts. Ferric chloride, for instance, has 

 been used as a mordant for haematein.^^^ 



The cation of the mordant salt, then, is the part that interests 

 us in microtechnique. The metal is in each case tervalent in these 

 compounds, but the ions are complex. The water of crystallization 

 is partly bound up with the metal. For instance, the ferric cation 

 that gives the violet colour to a crystal of iron alum is 

 [Fe(OH2)6]^^^. The chromic cation makes a crystal of chrome 

 alum almost black. When a brilliant light is shone through a small 

 crystal, the colour is seen to be violet. The red component is 

 easily seen when a small crystal is held against an electric light 

 bulb. The cation responsible for the deep violet colour is 

 [Cr(OH2)6]^^^. The water molecules are co-ordinated to the iron 

 or chromium atoms by dative covalent bonds from their oxygen 

 atoms. The metal may be described as sexi-covalent. 



-HaO^ ^OHa 1 + + + 



HaOCr^OHs 



.HsO^ ^OH2_j 

 The chromic cation in a crystal of chrome alum 



When iron alum is dissolved in water, the solution is not violet 

 but yellow or brownish-yellow. When chrome alum is dissolved 

 in hot water, the solution is not violet but green. In both cases 

 the solutions are quite strongly acid. The changes that occur in 

 the cation have been carefully studied in the case of chromium. A 



-U.O^yOU -| + + 

 H2O— Cr— OH2 



The chromic cation in a solution of chroyne alum 



hydrogen ion is lost from the cationic complex and goes off with 

 one of the positive electric charges, thus acidifying the solution, 

 and a second may follow suit.^^^ Similar changes occur when iron 

 alum dissolves, though the yellowish cationic complexes produced 

 are usually represented as [FeOH]" ' and [Fe(0H)2]^. 



