112 FIXATION 



The only disadvantage in doing so is that the concentration of the 

 formaHn from which solutions are made up is not always exactly 

 40% w/v, and the concentration of formaldehyde in diluted solu- 

 tions is therefore not exactly known. Any error from this source, 

 however, is likely to be small. 



The trade-term 'formol' is objectionable, for the ending in -ol is 

 unsuitable. 'Formal' is not allowable, because it is the accepted 

 name of another substance. 



Ionization. Formaldehyde ionizes to a minute extent to form 

 hydronium ions,^^^ but acidity from this cause is negligible. The 

 acidity of formaldehyde solutions is due to oxidation to formic 

 acid by atmospheric oxygen. The pH of formalin is said to vary 

 from 3-1 to 4'i.^"^ The pH of the 4% w/v solution, made by dilut- 

 ing formalin with 9 times its volume of distilled water, is given by 

 various authors at figures varying from 3-4 to 4-6.112,397,176,459 

 The addition of 5 ml of pyridine to 100 ml of 10% formaldehyde 

 brings the pH to 7-0.^^ The 4% solution 'neutralized' by excess 

 of calcium carbonate has a pH of 6-4; ^^^ if basic magnesium car- 

 bonate is used the pH is 7-6.^^^ In the presence of 1% of calcium 

 chloride and excess of calcium carbonate the pH is 9-0; ^^^ this 

 is the most alkaline fixative used in microtechnique. 



Oxidation-potential. Formaldehyde is capable of being reduced 

 to methanol according to the equation HCHO + 2H^ -\- 2e~ = 

 H3COH. It can thus act as an oxidizer, though a weak one. It was 

 claimed by Kingsbury -^^ long ago, in an important contribution 

 to the theory of fixation, that formaldehyde acts as an oxidizing 

 fixative. The oxidation-potential of the 4% solution is 0-23 volt. 

 This is the lowest of the oxidation-potentials of fixatives measured 

 by Casselman.ii^ 



Manufacture. Formaldehyde is made by passing a mixture of air 

 and vaporized methanol over a heated catalyst. Silver gauze at 

 635° C is suitable. Water formed by the reaction passes over and 

 dissolves the gas; also some unchanged methanol. The amounts of 

 these two substances are adjusted to give a solution at about 

 40% w/v, with as much methanol as is considered appropriate by 

 the manufacturer for protection against excessive polymerization. 



Introduction as fixative. Formaldehyde was introduced into 

 microtechnique in 1893, later than any other important fixative. 

 The discovery of its fixative properties was made accidentally by 

 Blum, ^2 who had previously introduced it as an antiseptic. One day 

 he slit up a mouse that was infected with anthrax and left it over- 



