Chemicals Commonly Used in Fixing Solutions 7 



Formaldehyde, HCHO 



Formaldehyde is a gas sold as a solution in water (approximately 40% 

 in content), in which form it is known as formalin. The use of the term 

 formol is incorrect, since a terminal -ol designates an alcohol or phenol. 

 Unless the author of a technic specifies that the dilution of his formalin 

 is in terms of actual formaldehyde content, dilutions are to be made 

 from the commercial product; i.e., a 10% soliuion would be 10 volumes 

 of formalin (40% formaldehyde saturated aqueous solution) to 90 vol- 

 umes of water. 



Formaldehyde on standing over long periods of time may either pol- 

 ymerize to form paraformaldehyde, or oxidize into formic acid. A white 

 precipitate in a stock solution indicates polymerization; the solution 

 has been Aveakened. Cares {1945) suggests: shake the solution to suspend 

 the sediment. Pour into Mason jars and seal them tightly. Autoclave 

 at 15 lbs. for thirty minutes. Cool. This should produce a clear solution. 

 Dilute solutions (such as 10%) tend to oxidize more readily than do 

 stock solutions (40%). Marble chips may be left in the bottom of diluted 

 stock jars to keep the solution neutralized. 



Formalin progressively hardens tissue, but is a mild coagulant of 

 proteins; in fact it is so Aveak that it might be considered a noncoagu- 

 lant. It neutralizes basic groups and increases the acidity of proteins. 

 In consequence, formalin-fixed proteins will stain well in basic dyes, 

 but less well in acidic dyes. Formalin has a moderate speed of penetra- 

 tion but its action is slow and somewhat incomplete unless tissues are 

 left in it for some time. It reacts most efficiently at pW 7.5-8. Although 

 formalin preserves the cells adeqtiately it may not protect them com- 

 pletely, because it requires a long time to harden the tissue. Shrinkage, 

 therefore, can take place if dehydration, clearing and infiltration are 

 initiated before the hardening action is complete. Since these processes 

 are not employed in the freezing technic, formalin is a satisfactory fixa- 

 tive for this method. 



Formalin is a good fixative for lipids; it does not dissolve lipoids or 

 fats. It does not fix soluble carbohydrates and it does dissolve glycogen 

 and urea. 



The so-called formalin pigment may appear in tissues rich in blood. 

 This pigment is formed when hematein of the hemoglobin has escaped 

 from red blood corpuscles before or during death and reacts with the 

 formalin. It may be prevented by a short period of fixation in formalin 

 followed by a prolonged soaking in 5% merciuic chloride. Once 



