PRIMARY fixatives: COAGULANTS 97 



Oxidation-potential. The saturated solution shows an oxidation- 

 potential of 0-82 volt/^^ a little more than that shown by a 5% 

 solution of mercuric chloride. 



Ma?mfacture. Picric acid may be prepared by the direct action 

 of nitric acid on phenol, but this is wasteful because part of the 

 phenol is lost by oxidation. In industry the ortho- and para- 

 sulphonic acids are first formed, by treating melted phenol with 

 concentrated sulphuric acid. The resultant phenolsulphonic acids 

 are dissolved in water and treated with nitric acid; NO 2 groups 

 now occupy the ortho- and para-positions. 



Introduction as fixative. Picric acid was mentioned as a hardening 

 agent by Ranvier ^^^ in 1875, but it is not known who first intro- 

 duced it into microtechnique. It was much used by Flemming ^"^' ^^^ 

 from 1879 onwards in his pioneer w^ork on chromosomes. 



Reactions with proteins. Picric acid is a coagulant fixative. It is a 

 particularly strong coagulant of the histone of the nucleus. ^^^ It 

 does not stabilize gelatine gels. It forms crystalline compounds 

 with amino-acids and is probably to be regarded as an additive 

 fixative of proteins, though its precise points of attachment have 

 not been disclosed. It is an alkaloidal reagent, and the claim is 

 made that it fixes basic proteins while allowing acidic ones to 

 escape by solution. *^^ Coagulated protein is digestible by pepsin 

 and trypsin. 



With nucleoprotein solution picric acid produces fine flocculi, 

 which are eventually precipitated. DNA is either left in solution, 

 or precipitated ^^^ in a water-soluble form.^^^ The fact that picric 

 acid may precipitate the protein of nucleoprotein while leaving 

 nucleic acid in solution has been used in the preparation of DNA.^^^ 



Reactions with lipids. None has been recorded. Lipids are not 

 dissolved by aqueous solutions of picric acid. 



Reactions zvith carbohydrates. Picric acid is not a fixative of 

 carbohydrates, but is strongly recommended by Lison ^^^ as a 

 constituent of fixatives for glycogen. The latter substance is 

 apparently bound to protein (sometimes quite firmly as 'desmo- 

 glycogen'), and picric acid not only does not set it free, but acts 

 on the protein in such a way as to cause the glycogen to resist 

 solution. 



Rate of penetration. Picric acid penetrates into gelatine/albumin 

 gel more slowly than any other fixative that has been tried (K = 

 0-8). It also goes rather slowly into liver (K = 0-5) ^^ (see fig. 5, A, 

 opposite p. 67). Tellyesniczky ^^^ and Underbill ^^^ i^Q^h tested 



G 



