g6 FIXATION 



the considerable reduction in volume that takes place when 

 ethanol is mixed with water. 



Compatibility with other fixatives. Ethanol is compatible with 

 picric acid, mercuric chloride, formaldehyde, and acetic acid, but 

 it reacts with the latter substance to produce ethyl acetate if 

 sulphuric acid be added to the mixture. Since ethanol tends to be 

 oxidized through aldehyde to acetic acid, it is best not to mix it 

 with chromium trioxide, potassium dichromate, or osmium 

 tetroxide. It does not react quickly with potassium dichromate, 

 however, unless the mixture be acidified. The reaction with 

 osmium tetroxide to produce a black precipitate is also slow at 

 room-temperature. 



Unclassified remarks. So far as is known, ethanol is the only 

 non-aqueous fluid, other than methanol and acetone, that gives 

 reasonably good results when used undiluted as a fixative, ^^^ 

 though some might make a claim for dioxane. 



PICRIC ACID (trinitrophenol) 



Standard concentration for fixation. Saturated aqueous solution. 



OH 



Formula and formula-weight. ^11 ^. 229-1 



NO2 



Description. Picric acid consists of yellow leaflets or prisms, 

 melting at 122° C. It is slightly soluble in water (1-4% w/w), more 

 so in ethanol (4-9%), stiU more in benzene (10%). Its solubility 

 in ethanol and ether often results in the yellow coloration of col- 

 lodion blocks containing tissues fixed in picric acid. 



Picric acid decomposes explosively when heated suddenly or 

 detonated, and is therefore usually damped for storage. Until 

 displaced by the related trinitrotoluene, it was the chief high 

 explosive used for military purposes. It was manufactured at 

 Lydd in Kent and received the name of lyddite. At the present 

 day it is still used as an explosive in the form of its potassium, 

 sodium, and ammonium salts. 



This is the only substance that is used in microtechnique both 

 as a fixative and as a dye (p. 185). 



The name is derived from the Greek work meaning bitter. 



Ionization. This is a much stronger acid than phenol. The pH of 

 the saturated solution has been given as 1-33 ^^^ and i-6.*^^ 



