NON-COAGULANT PRIMARY FIXATIVES 41 



Oxidized, however, by atmospheric oxygen to formic acid, and the 

 standard solution, prepared by the dilution of commercial 

 formalin with distilled water, has a pH of about 4. If calcium 

 carbonate is present in excess, the pH is 6-4.^^" 



Oxidation-potential. Formaldehyde can be reduced to methanol, 

 and thus may act as an oxidizing agent. Its oxidation potential, 

 however, is lower than that of any other fixative, so far as is 

 known (0-23 volt). 



Reactions with proteins. This subject is particularly well under- 

 stood, because it has been necessary for industrial chemists to 

 study it carefully in connexion with the so-called 'tanning' of 

 leather. It will therefore be treated here at some length. 



One of the most effective ways of studying additive fixation is 

 to prepare polypeptides consisting of a single amino-acid many 

 times repeated, and to find how much of the fixative they take up 

 from solution. It is only necessary to know how much polypep- 

 tide is present and the initial and final concentrations of the fixa- 

 tive substance. This method has been very successfully used in the 

 study of fixation by formaldehyde. Although no protein consists 

 of a chain composed entirely of similar links, yet some are made 

 up of very few amino-acids, and these also are useful in the study 

 of fixation. 



Polyglycine is an artificial polypeptide consisting of nothing 

 but a group of glycine molecules, joined by peptide bonds. Simi- 

 larly, polyglutamic acid consists of a group of glutamic acid 

 molecules, joined in the same way. The fibroin of silk consists 

 mostly of alanine and tyrosine. All these substances bind very 

 little formaldehyde."^ 



NH NH NH o 



I i I /• 



HCH HC.CHg HCCCHOaC 



I I I " \ 



C=0 C=0 C=0 NH, 



J. I I 



glycine alanine gliitamine 



Structural formulae of certain amino-acids, as components of 

 polypeptides and proteins 



