THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTTable 3: Physical Properties of HCN 322Molecular weight 27.026 g mol -1Boiling point (1 atm)25.7°CMelting point-13.24°CSpecific density of the gas at 31°C (air = 1) 0.947Explosion limits in air 6-41 Vol.% 323as a result that all the oxygen of the earth’s atmosphere would settle inthe lower fifth of the atmosphere, as a consequence of which the entiresurface of the earth would get oxidized, i.e., burn. This obviously doesnot happen. Thus, a spontaneous separation of hydrogen cyanide gaswould never take place in air.The 5% lesser density of pure hydrogen cyanide gas compared toair (this corresponds to a density difference of 35°C warm air as comparedto 20°C warm air) can however very well lead to a density convection,when pure gaseous hydrogen cyanide is released in a locationwith the same temperature as the ambient air. The gas would then riseslowly, but gradually mix with the ambient air. But to conclude fromthis that hydrogen cyanide vapors always rise, would be an incorrectconclusion. At 15°C, for example, on physicochemical grounds, noconcentrations higher than 65% of hydrogen cyanide can occur in air(see Graph 1); the density of such a mixture lies only approximately3% below that of air. Furthermore, a great deal of energy is withdrawnfrom the ambient air by the evaporating hydrogen cyanide. Consequently,the ambient temperature sinks until exactly as much energy istransported to the liquid (adsorbed) HCN as needed for the deceleratedevaporation at the corresponding lower temperature. It is thereforetheoretically possible for hydrogen cyanide vapors containing littleHCN, but which are cold, to be denser, that is, heavier than the ambientair.Graph 1 shows the equilibrium percentage of hydrogen cyanide inair as a function of temperature. Even at 0°C, the percentage still lies at322 W. Braker, A.L. Mossman, Matheson Gas Data Book, Matheson Gas Products, East Rutherford1971, p. 301. I have left out some of the less interesting dimensions in this connection:heat capacity (20.9°C): 2.625 J g -1 K -1 (Water=4.187 J g -1 K -1 ); dielectricity constant (20°C):114 (Water=78.5); evaporation heat: 28 kJ mol -1 ; evaporation entropy: 190 J mol -1 K -1 ; spontaneouscombustion temperature: 538°C; flash point: -17.8°C; regarding dielectricity constants,see: R.C. Weast (ed.), Handbook of Chemistry and Physics, 66th Ed., CRC Press, BocaRaton, Florida 1986, E 40. However, under normal conditions (1 atm, 25°C), hydrogen cyanideis not a gas.323 1 vol.% is 10,000 ppm (for HCN, roughly 12 g/m³)156