THE INDIRECT ATTACHMENT OF DYES TO TISSUES 215 



-OH group is not clear,^^^ and indeed it is absent in lac-dye, 

 which works with mordants. 



In the absence of a nearby donor oxygen atom, two -OH groups 

 can form chelate compounds with iron. Certain derivatives of 

 catechol provide examples of this.^^^ It is therefore interesting to 



HO 



HO^I 



\/ 

 Catechol 



find two -OH groups in this position in haematein (p. 173), which, 

 however, also possesses a phenolic -OH in close proximity to a 

 donor oxygen, in another part of the molecule. 



The oxazine dyes that work with mordants also possess the 

 necessary groupings for the formation of chelate bonds with 



OH ^ 



(H3C)YY'/Y 



\0H 



Gallocyanine 



metals, though perhaps in a somewhat disguised form. The 

 formula for gallocyanine is printed here in such a way as to em- 

 phasize the relationship with other mordant dyes. 



Haematein, carminic acid, and the mordant oxazines are all 

 amphoteric dyes. Indeed, one would suspect this from a glance at 

 their chemical formulae. Gallamine blue, for instance, presents two 

 basic groups (-NHg and -N(CH3)2) and two acidic (-0H). The 

 iso-electric point of haematein is about pH 6-5, of carminic acid 

 about pH 4-2, of gallocyanine and gallamine blue about pH 4-1. 

 The lakes are in all cases basic throughout the pH range in which 

 they are used, and they act like basic dyes apart from their com- 

 plete insolubility in neutral fluids after they have once attached 

 themselves to the tissues. The positive charge on the lakes can be 

 proved by cataphoretic experiments.*^^' ^^^ 



Sometimes a dye-lake has a colour that is different from that of 

 its parent dye. Thus haematein is yellowish at about pH i and 

 changes its colour towards orange and dirty red as aciditv lessens 

 towards neutrality ; but the aluminium lake is blue and the charac- 

 teristic iron lake dark blue or blue-black. Carminic acid is some- 

 what less affected, for the dye by itself, whether in acid or alkaline 



