CHAPTER 6 



Vrimaiy Fixatives Considered 

 Separately 2. Non-coagulants 



FORMALDEHYDE (methanal) 

 Standard concentration for fixation. 4% w/v aqueous solution. 



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Formula and formula-weight. ^C — O. 30-0. 



W 



Description. Formaldehyde is a colourless gas. If liquefied it boils 

 at —19° C. It is very soluble in water and is commonly sold in 

 aqueous solution as 'formalin'. This is usually a 37% w/w solution 

 or thereabouts. The specific gravity of formalin is about i-o8, and 

 100 ml therefore contain almost exactly 40 g of formaldehyde. 

 Thus formalin is approximately a 40% w/v solution. Commercial 

 formalin contains a little formic acid (generally less than 0-05%) 

 and a considerable amount of methanol (6 to 15%). 



Monomeric formaldehyde probably exists in water as 

 HOCHgOH. It has a strong tendency to polymerize as dimer, 

 trimer, etc., having the general formula H0(CH20)nH. In the 

 40% solution, only a small part (11%) of the formaldehyde is 

 monomeric, but when this is diluted to 4%, the monomer pre- 

 dominates. Paraformaldehyde tends to be deposited from concen- 

 trated solutions of formaldehyde as a white powder. This is a 

 highly polymeric form (w = 100 or more). The methanol in com- 

 mercial formalin hinders polymerization. 



As Bethe '^^ pointed out 60 years ago, a considerable amount of 

 confusion is caused by the loose usage of the trade-name 'formalin'. 

 When authors say '10% formalin', they presumably mean form- 

 alin diluted with 9 times its volume of water or salt-solution to 

 give 4% w/v formaldehyde, but there is always the possibility that 

 they mean 10% formaldehyde. It is best to restrict the name forma- 

 lin to the commercial product at 40% and to express the concen- 

 tration of diluted fluids in terms of their formaldehyde content. 



Ill 



