THE ACTION OF MORDANTS 119 



than the critical figure, the solution acts as a dye; if higher, as 

 a differentiating agent. In the case of aluminium purpurine, with 

 the aluminium salt dissolved at M/40 in 60*?o ethanol, the critical 

 quotient is 16. 



If an iindyed section be placed in an aluminium purpurine solu- 

 tion in which the mordanting quotient is at the critical figure, dye- 

 ing will take place very slowly until chromatin is strongly and 

 cytoplasm feebly coloured; no further increase of colour will 

 occur, however long the section may remain in the dye.^^ 



Solutions in which the mordanting quotient is not very many 

 times less than the critical figure are particularly easy to use, 

 because there is not much tendency to over-dye. Standard alu- 

 minium purpurine ('purpural'), with a mordanting quotient of 

 8, is an example. It can be recommended to beginners as a 

 routine dye for chromatin and other negatively charged tissue 

 constituents. It is to be used progressively. The period of dyeing 

 varies according to the fixative used, but half-an-hour or an hour 

 usually suffices. The section is then simply washed in 50% or 

 70% ethanol. 



Haematein reacts with aluminium salts in much the same way as 

 purpurine. As the formula on p. 94 shows, it possesses an =0 

 and a phenolic -OH, so situated in relation to one another as to 

 provide means of chelation with mordants. 



The mordants commonly used with haematein in micro- 

 technique are potassium alum, Al2(S04)3.K2S02.24H20, and 

 ammonium alum, Al2(S04)3.(NH4)2S04.24H20. The colour of 

 aluminium haematein is always blue, whatever salt of aluminium 

 is used. The potassium and ammonium sulphates contained in 

 these alums play no part in lake-formation, but they may aff"ect 

 the pH of dye solutions and the solubility of dyes. The double 

 salts are easily crystallized and therefore easily purified. It was 

 probably for this reason that potassium alum was used when 

 mordant dyeing was invented in Turkey many centuries ago. The 

 traditional use of alums persists in microtechnique to the present 

 day, but there is no rigorous proof that alums are preferable to 

 simple salts of aluminium. 



