A NOTE ON THE NORMAL MELTING AND BOILING POINTS OF IODINE JOSEPH D. PARENT Loyola University of Chicago, Chicago, Illinois THERE seems to be an unduly wideqread opinion that iodine cannot be melted or boiled a t atmospheric pressure, or a t least in a vessel open to the atmosphere. For example, Marsh states, "The vapor Pressure of solids at ordinary temperatures is not always small. Iodine, for example, attains a vapor Pressure equal to atmospheric before i t melts and can, therefore, be distilled directly from the solid."' This prevalent misconception is probably due to the fact that when solid iodine is heated in an open vessel, vaporization without fusion or boiling is ordinarily observed. An examination of sublimation and vapor pressure data2reveals that the above quoted statement is incorrect, and one should expect to be able to realize the normal melting and boiling points, for the sublimation pressure reaches a maximum value of only approximately 90 mm. of mercury a t its melting point. The I M.,R~E, "principles of phase diagrams,,* ill ~,,,,k Co., Inc., New York City, 1935, P. 35. See, for example, the "International Critical Tables," McGrsw-Hill B w k Co.. Inc., New York City. 1938, Vol. 111, p. 201.
,.
.,
subsequent simple procedure will serve to clarify the situation. EXPERIMENTAL
Place a sample of iodine in an ordinary test-tube and draw out the end of the tube until the opening does not exceed a few mm. in diameter. 3f heat is now applied, fusion and boiling can be seen to take place. CONCLUSION
It seems that the reason for the usual non-observance of these phenomena lies in the fact that although the solid, when heated in an open container, exerts a sublimation pressure much less than atmospheric, it, nevertheless, attains such a high value that the solid may be completely vaporized into the surroundings before the melting or boiling points are reached, if the vessel is too open. If vapor losses are reduced by restricting the opening of the container, less of the solid phase will be vaporized in the tendency to attain equilibrium between the solid and vapor phases. By such means, the solid can be retained until fusion and boiling are realized.
