THE ACTION OF MORDANTS 



113 



4" ^' 



♦^ 



O Q 



HO-@CoH. 



% 



♦ 



'Q 



'^ 



//^. 5. Structural formulae for hydrated aluminium ions, drawn in 

 perspective. The ion on the left carries three positive charges. That 

 on the right has lost a hydrogen ion and carries two positive charges. 



nuclei (aluminium, oxygen, and hydrogen, together), may be re- 

 garded as holding 3 positive charges at the moment of its forma- 

 tion. The 6 water molecules, however, do not all remain intact. 

 One of them, at least, tends to lose a hydrogen ion,^^ which joins 

 a molecule of the solvent water to form a hydronium ion, HgO^, 

 and thus departs. Since it carries off a positive charge, the com- 

 plex ion (fig. 6, right) is only doubly charged; but the solution, 

 having gained a hydronium ion, has become acid. Thus the pH of 

 an M/40 solution of aluminium sulphate in 60% ethanol is 3-08. 

 When purpurine is dissolved in water or in solutions of ethanol 

 in water, it behaves like a phenol ; that is to say, like a weak acid. 

 A saturated solution in 60 % ethanol has a pH of 5-20. Some of the 

 hydrogens of the -OH groups are ionized and lost, and the dye is 

 now negatively charged. In the structural formula shown here, 



O 1^- 



OH 



OH 



O O 



Ionized purpurine 



OH 



OHo 



H,0 OH, J 

 77?^ aluminium purpurine ion 



