34 CYTOLOGICAL TECHNIQUE 



cell, and may even eventually be dissolved out. Ethanol is there- 

 fore a very poor fixative for nuclei as well as for chromosomes. 

 Compatibility with other fixatives. Compatible with mercuric 

 chloride, formaldehyde, and acetic acid. Since it tends to be oxid- 

 ized through aldehyde to acetic acid, it should not ordinarily be 

 mixed with chromium trioxide, potassium dichromate, or osmium 

 tetroxide (though its reaction with the two latter is slow). 



MERCURIC CHLORIDE 



Standard concentration. Saturated aqueous solution. 



Formula. HgCL- 



Description. Colourless, needle-shaped crystals. A covalent 

 compound, with much lower melting-point than most salts: 

 sublimes easily. Soluble in water at about 7 % ; readily soluble in 

 ethanol and in benzene. Very poisonous. 



Ionization. Ionizes only partially, with hydrolysis, to give 

 [HgClJ^ (particularly abundant in the presence of other 

 chlorides), Hg++, hydronium ions (the pH of the standard solu- 

 tion is about 3-2), and other ions. 



Oxidation-potential. Rather a strong oxidizer (o.p. about 

 0-75 volt). 



Reactions with proteins. An additive, coagulant fixative. The 

 reactions depend on the pH at which the fixative is used.^^" 



If the pH is well below (more acid than) the iso-electric point 

 of the protein, the -NHg groups of the basic amino-acids will 



mostly be ionized as -NH3. There will therefore be electrostatic 



! 

 NH 



I + 



HC(CH2)4NH3 



c=o 



I 



Lysine forming part of a protein chain, in a solution 

 on the acid side of the iso-electric point 



attraction between this positively charged group and the nega- 

 tively charged ion, [HgClJ^^, which could associate itself with 



