NON-COAGULANT PRIMARY FIXATIVES 45 



mercuric chloride, and acetic acid. Reduces osmium tetroxide and 

 potassium dichromate slowly, chromium trioxide quickly. 



OSMIUM TETROXIDE 



Standard concentration. 1 % w/v aqueous solution. 



Formula. OSO4. 



Description. Pale yellow molecular (non-ionic) crystals, soluble 

 in water at about 7% w/W (in carbon tetrachloride at about 

 375 % w/W). The crystals melt at 4P C. They begin to sublime at 

 a much lower temperature than this; the gas given off is damaging 

 to the epithelium of the eyes, nose, and mouth. This is by far the 

 most expensive of all primary fixatives (1 g costs £3. 10^.). 



Ionization. On solution, it takes up a molecule of water to 

 become hydrogen per-perosmate, HoOs05.^^^" This ionizes to 

 a minute extent to produce hydronium ions and HOSO5. The 

 substance can, however, scarcely be called an acid. Acetic is a very 

 weak acid, yet its ionization constant is well over 20 million times 

 as great as that of hydrogen per-perosmate. 



Osmic acid, H0OSO4, is not used in fixation. 



Oxidation-potential. A moderately strong oxidizer (the 2% 

 solution has o.p. 0-64 volt). On reduction, the brown or blackish 

 hydrated dioxide, OSO2.2H0O, is left. 



Reactions with proteins. Osmium tetroxide gives no coagulum 

 with albumin solution, but on the contrary renders the protein no 

 longer coagulable by ethanol or heat.^^ It sets strong protein 

 solutions into gels, and stabilizes gelatine gels against solution by 

 warm water. It does not coagulate nucleoproteins. 



Osmium tetroxide is an additive fixative, but the exact site of 

 its attachment to proteins is not known. It is capable, in certain 



Action of osmium tetroxide at the ends of a double bond 



