56 FIXATION 



to form picrates ; each amino-acid gives a crystal of characteristic 

 form. For a plate showing the crystals of eleven picrates of amino- 

 acids, see Schmidt. ^*^ Picric acid resembles other alkaloidal 

 reagents in coagulating soluble proteins, but the chemistry of 

 the process has not been worked out. The obvious point of 

 attack would be the amino side-groups of lysine and arginine, 

 but if all these were blocked, the protein would lose most of 

 its affinity for acid dyes. In fact, however, picric acid gives 

 egg-white a strong affinity for acid dyes, but scarcely any for 

 basic ones.*^^ It seems probable that acid dyes are able to re- 

 place picric acid at its points of association with amino-groups. 

 It is to be recollected that picric acid is not only a fixative but 

 also a dye (p. 185), and that dyes can replace one another in 

 this way. The reduction in affinity for basic dyes has not been 

 explained. Picric acid also forms additive compounds with 

 phenols, and combination with the side-group of tyrosine is not 

 excluded. 



Chloroplatinic acid is another alkaloidal reagent. The incorrect 

 name of platinum chloride disguises the fact that the metal forms 

 part of a complex anion, [PtClg]^. 



The mode of action of chromium trioxide on proteins in the 

 process of fixation is not well understood. We have a considerable 

 amount of knowledge about the reactions at high temperatures, 

 because they have been studied by the textile chemists. ^^^' ^^^' ^^^ 

 These reactions must be briefly mentioned here, though their 

 relevance is doubtful. 



Compounds of chromium are used in the pre-treatment of wool 

 before the application of dyes. The chemistry of the process of 

 'mordanting' will be dealt with in chapter 11. In industry, sodium 

 dichromate is commonly used for the purpose, often with the 

 addition of acid. When a dichromate is dissolved in water, the 

 anions produced are essentially the same as those produced by 

 chromium trioxide (see pp. 105 and 126). The chromium is anionic, 

 and it makes little or no difference whether chromium trioxide or 

 acidified dichromate is used. Industrial mordanting is carried out 

 at boiling point, and the fibre is often treated afterwards with a 

 reducing agent. 



Anionic chromium appears to associate itself chiefly with those 

 side-groups of the wool proteins that contain -NHg and -OH, 

 and with the -CO.NH- group of the protein backbone. Accord- 

 ing to the textile authorities, the metal is taken up partly as 



