CHAPTER 4 



Non-Coagulant Primary Fixatives 



The non-coagulants form rather a heterogeneous group. Form- 

 aldehyde and osmium tetroxide resemble one another in being 

 additive non-coagulants that harden protein gels without separat- 

 ing the water from the protein in them, and fix protoplasm with- 

 out producing microscopical spongeworks. Potassium dichromate 

 and acetic acid are anomalous. They are not fixatives for simple 

 proteins. The former is used chiefly for its action on certain lipids, 

 the latter for its swelling effect, which counteracts the shrinkage 

 caused by other reagents used in fixation and embedding. 



Formaldehyde and osmium tetroxide are of outstanding impor- 

 tance in modern microtechnique. The former is much used in 

 histochemical studies, the latter in electron-microscopy. They will 

 therefore be treated at greater length than the other primary 

 fixatives selected for description in this book. 



FORMALDEHYDE 



Standard concentration. 4 % w/v aqueous solution. 



Formula. HoCO. 



Description. Colourless gas; it is very soluble in water as 

 HO(H2CO)nH. The monomer, HOCHoCO)!!, predominates at 

 the standard concentration. The commercial fluid, 'formalin', is 

 approximately at 40% w/v solution, containing some methanol. 

 Polymers predominate in this, and the high polymer, para- 

 formaldehyde (n = 100 or more), tends to be deposited as a white 

 precipitate. 



Ionization. Formaldehyde itself ionizes to a minute degree, with 

 production of a negligible amount of hydronium ions. It is easily 



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