THE ACTION OF MORDANTS 111 



been carefully worked out by the textile chemists. ^^' ^^^' ^^' ^^ 

 Unfortunately it is radically different from the mordanting of 

 carminic acid or haematein by aluminium or ferric salts. 



One of the simplest dyes that can be mordanted is purpurine, 

 which, like carminic acid, is an anthraquinone dye. Alizarine is 

 even simpler, but its low solubility in suitable solvents makes it 

 less convenient in practical use. The third -OH group, which 



occurs in purpurine but not in alizarine, is rather unreactive, and 

 the two dyes tend to behave similarly with mordants. ^^^ Both dyes 

 occur naturally in the form of glucosides in the root of the madder 

 plant, Rubia tinctonim (Rubiaceae), but the synthetic products 

 are almost invariably used. Purpurine is unfortunately named, 

 for it is a red dye. 



If purpurine is dissolved at saturation in 60 °o ethanol, it acts as 

 a typical though very weak acid dye. This can be shown by carry- 

 ing out the tests described on p. 101. If, however, the dye is dis- 

 solved in a solution of aluminium sulphate, the result is entirely 

 different. The following is a convenient solution. 



Take 0-8 g of purpurine and 7-88 g of aluminium sulphate crystals 

 (I6H2O); add 450 ml of 60°o ethanol; boil with reflux condenser until 

 the solids have dissolved; cool; make up to 500 ml with 60° ethanol.-^ 



The purpurine in this solution, if pure, is at M/160, the 

 aluminium sulphate at M/40. The solution may be called stand- 

 ard aluminium purpurine, or 'purpural' for short. 



The ethanol in this solution increases the solubility of the dye 

 and thus makes the solution last longer when used repeatedly; it 

 slows down or prevents the growth of bacteria and moulds ; and 

 it decreases ionization and thus 'equalizes' the action on tissues 

 (that is to say, prevents the solution from over-dyeing locally). 



