HYDRIODIC ACID, or HYDROGEN IODIDE is an acid composed of hydrogen in combination with iodine, and having the formula HI. Hydriodic acid is analogous to the more familiar hydrochloric acid, both in its chemical structure and in its general prop erties. It may be prepared by the direct union of hydrogen and iodine at a red heat. A more convenient method, however, consists in passing sulphuretted hydrogen gas (HIS) into water in which a little pulverized iodine is suspended. The reaction is 1-12.5 + 21 .2HI + S. The ac tion is slow owing to the deposition of sulphur which encases the iodine particles and prevents their free solution. The larger the amount of hydriodic acid formed, the faster the operation progresses. Fresh supplies of iodine are added from time to time, and the liberated sulphur is finally removed by agitation and filtration, the sulphuretted hydrogen remaining in solution be ing also removed by the application of gentle heat. In another method the iodine is dis solved in carbon bisulphide, and the solution covered with a stratum of water. When sul phuretted hydrogen is passed into this mixture, the hydriodic acid as it forms is dissolved in the water, and the sulphur set free is dissolved in the carbon bisulphide. The aqueous solu tion is boiled to expel the sulphnretted hydro gen it may have taken up and the hydriodic acid remains in a pure state in the solution.
The aqueous solution so prepared possesses strongly acid properties, and combines with bases to form salts called iodides. The pure acid is a colorless gas, with an odor similar to that of hydrochloric acid gas. It has a density about 63 times as great as that of hydrogen, and at a temperature of 32° F. it condenses to a liquid when subjected to a pressure of four atmospheres; the liquid so obtained freezing to a solid mass at about 67° F. below zero. The specific heat of the gas, at ordinary tempera tures, is about 0.055 (at constant pressure), and the ratio of its specific heat at constant pressure to its specific heat at constant volume is 1.397. It dissolves freely in water, the spe cific gravity of a saturated aqueous solution, at 32° F., being about 2.00. The aqueous solution is colorless when pure, but it is slowly decom posed by sunlight, becoming dark from the liberation of iodine. It is also decomposed by sulphuric and nitric acids and by chlorine and bromine, which set the iodine free. In med icine hydriodic acid is administered as a syrup.