PRIMARY fixatives: non- CO agulants 119 



Most compounds of osmium are dark or black. On reduction, 

 osmium tetroxide is converted to the grey or black anhydrous 

 dioxide, OsOg, or to the brown, gelatinous, hydrated dioxide, 

 OSO2.2H2O or Os(OH)4.^^^ Reduction takes place readily when 

 solutions of osmium tetroxide are exposed to light, though it is 

 claimed that this is prevented by the complete exclusion of dust.^^ 

 Reduction by light is prevented by strong oxidizers (p. 125), and 

 is also hindered, curiously enough, by sodium chloride. ^^ 



Ionization. Osmium tetroxide scarcely ionizes. It was recognized 

 long ago that the electrical conductivity of its solutions was ex- 

 tremely low. Hofmann and his colleagues ^^^ found that the 

 specific conductivity of a 1% solution in distilled water was 

 1-09 X 10"^; that of the water used was 0-5 X io~^. 'Osmium 

 tetroxide', thev remarked, 'is therefore chemicallv neutral and no 

 acid.' 



On solution, osmium tetroxide takes up a molecule of water 

 and becomes HoOsOj.^*^ This substance cannot be isolated. 

 A minute amount of ionization occurs, according to the 

 formula HoOsOj-*-^- H^ + HOsOj". The ionization constant is 

 8-0 X io~^^. ^^^ It follows from this that 100 ml of a i % solution of 

 osmium tetroxide contain 0-000000035 § ^^ hydrogen ion. To call 

 such a substance an acid seems rather far-fetched, though a 

 sodium salt, NaHOsOj, can in fact be obtained. Hydrogen 

 peroxide has an ionization constant of 1-78 X lO"^-, **^ more than 

 tw^ice that of HoOsOs, but it is not called hydroperoxidic acid. 

 The pH of a solution of osmium tetroxide is almost exactly that of 

 the distilled water used to make it. The values published in the 

 biological literature give us information about the distilling 

 apparatus or glassware used in various laboratories, but not about 

 osmium tetroxide. 



The proper name for HoOsOs is not easy to determine. It cannot 

 be osmic acid, for on the analogy of manganic acid (HgMnOJ, 

 osmic acid is H2OSO4; and this is a real entity, as we have seen 

 (p. 62), though it is not formed when osmium tetroxide is dis- 

 solved in water. Permanganic acid is HMn04, and HgMnOs 

 would be per-permanganic acid. On this analogy HgOsOs would 

 be per-perosmic acid,^"*^ if it were unequivocally an acid. It seems 

 best to call it hydrogen per-perosmate. 



Oxidation-potential. The 2% solution has an oxidation-potential 

 of 0-64 volt, somewhat less than that of a 5% solution of mercuric 

 chloride. 



