114 CYTOLOGICAL TECHNIQUE 



one of the -OH groups is ionized. There are now two oxygen 

 atoms near one another, both capable of donating electrons to 

 suitable acceptors. If purpurine is added to a solution of an 

 aluminium salt, these two oxygen atoms will tend to replace two 

 of the water molecules that are combined with aluminium. Thus 

 the purpurine will form a complex or 'co-ordination compound' 

 with the aluminium. 1' 2 



The purpurine ion might attach itself either to an aluminium 

 ion that was associated with 6 water molecules, or else to one that 

 was associated with 5 water molecules and one -OH group 

 (fig. 6, right); but the resulting compound would probably be 

 the same in both cases, for the acceptance of purpurine by the 

 aluminium atom would be likely to be accompanied by the accep- 

 tance of a hydrogen ion from a hydronium ion in the solution, 

 which would attach itself to the -OH group of the hydrated 

 aluminium ion.^^^ 



Since the fully hydrated aluminium ion has a triple positive 

 charge, while the purpurine ion brings with it a single negative 

 charge, the complex ion as a whole has a positive charge of two. 



In the practical dyeing solution, standard aluminium purpurine 

 (p. Ill), there is only one molecule of purpurine to 8 atoms of 

 aluminium. The composition of the solution may be expressed in 

 terms of the mordant quotient ^"^^ that is to say, the number of 

 atoms of mordant metal in the solution, divided by the number of 

 dye molecules. In the standard aluminium purpurine solution, 

 then, the quotient is 8. It must therefore be supposed that the 

 majority of the hydrated aluminium ions in the solution that are 

 combined with purpurine at all are combined with only one ion 

 of it. 



When ionized purpurine has associated itself with aluminium 

 in the way indicated on p. 1 13, a new ring has been formed, con- 

 sisting of 6 atoms arranged in the following order: aluminium, 

 oxygen, carbon, carbon, carbon, oxygen. This ring is particularly 

 stable. Since the purpurine sends out two bonds that grip the 

 aluminium like the chela of a lobster, the compound is said to be 

 chelate. ^^2 



Since the aluminium-purpurine ion in the dye-solution is posi- 



