Ilydriodic gas is p irtly decomposed by a red heat. Ily the presence of oxygen. the decomposition is rendered more rapid and complete, and water and iodine are the products.
Hydriodic gas has a very strong attraction for water ; a very large quantity of the gas is absorbed by a very quantity of water, imparting to it a great increase of den sity, and the solution when strong is smoaking The most convenient way of obtaining this acid in a liquid state is, to dissolve ioduret of phosphorus in water. Part of the water is decomposed, the hydrogen forming hydriodic acid, and the oxygen a poi tion of phosphorous acid. When heat is applied, the two acids are separated. Another and an easier method is, to pass sulphureted hydrogen through water containing iodine. The hydrogen goes to form the acid required, and the sulphur is precipitated Heat is then applied to drive off the excess of sulphureted hydro gen, and a colourless hydriodic acid is procured, from which the sulphur precipitated to the bottom of the vessel may be decanted off. This acid, like the sulphuric, may be concentrated by evaporation, and, till it is raised to 257n, the vapour which it loses is almost pure water While under this temperature, therefore, it may be evaporated in an open vessel, or the liquid which distils over may be thrown away. When raised to this temperature, it gives over an abundant quantity of acid, and it does not admit of being heated above 262k under the common atmosphe ric pressure. This is the boiling point of the acid ; and, being comparatively high. it prevents it from being disen gaged from any neutralizing base by the volatile acids.
When distilled, or even simply exposed to the air, it assumes a brownish-red colour, from a partial decomposi tion by the contact of oxygenous gas, which forms water with its hydrogen ; and the iodine, which is then evolved, combines with the liquid acid. This change is assisted by light and heat. The property of absorbing and dis solving iodine is one of the most prominent that belong to this acid, and the iodine is not capable of being driven off from it by heat, as it may from a simple solution in water.
Ilydriodic acid is rapidly decomposed by sulphuric and nitric acid, and by chlorine, which seize on its hydrogen, and the iodine is either precipitated in brown crystals, or exhales in purple vapours. Chlorine is a delicate test of this acid, as the purple vapours instantly appear when it is dropped into any solution which contains it ; but they are speedily re-dissolved by the acid, and therefore the chlo rine must be cautiously added. When heated with black oxide of manganese it affords iodine, and an hydriodate of the metal. With red oxide of lead it affords iodine and an ioduret of the metal, the oxygen of the oxide going to form an additional quantity of water.
Before we proceed farther, it may be proper to state the meaning of some terms connected with this subject. Io dine and hydriodic acid have been explained. The com pounds of the acid, with neutralizing bases, are called hy driodatea. Iodine, as we shall find, may he combined with oxygen, and the compound possesses acid properties. This acid is called by Gay-Lussac the iodic, and the com pounds which it forms by uniting with neutralizing bases, lie calls iodates. Dr. Murray calls this acid the oxiodic, and its compounds oxiodates. NVe shall adopt these last terms, as well fitted to keep the facts in mind, and capa ble of being adapted to any theory.
The combination of iodine with azote cannot be produ ced by direct experiment. Iocluret of ammonia must be first formed. We shall therefore describe the phenomena which iodine exhibits with ammonia, previously to the consideration of the other alkalis. It differs from these others by not containing oxygen, a circumstance which materially affects the characteristic results of the chemical changes. \Vhen amtnoniacal gas is passed over iodine, a viscid shining liquid is immediately formed, of a brown ish-black colour, which, in proportion as it is saturated with ammonia, loses its lustre and viscosity. This is an ioduret of ammonia. When dissolved in water, the hydro gen of the ammonia unites with one portion of the iodine to form hydriodic acid, and its azote unites with another, to form an ioduret of azote. Or this substance may be obtained directly, by putting iodine in fine powder into an aqueous solution of ammonia. This ioduret is pulvcrulent, and of a brownish-black colour. It detonates from the snit.11est shock and from heat. The flame is of a feeble vio let colour. \Vhen delicately prepared. it sometimes de tonates spontaneously. When this compound is put in potash, the azote is disengaged, and the same products are obtained as when iodine is brought in contact with that alkali Iodine undergoes no change on the contact of gaseous sulphurous acid ; but, when the acid is dissolved in water, part of the water is decomposed Its oxygen converts the sulphurous into sulphuric acid, and its hydrogen the iodine into hydriodic. The se cannot be separated by heat, for the temperature which volatilizes hydriodic acid decom poses, under these circumstances, the sulphuric, repro ducing the original sulphurous acid and the iodine. While the liquid is distilled over, these are the two ingredients, and it is coloured by the iodine ; but, when cooled in the receiver, it resumes the state of sulphuric and hydriodic acids in limpid mixture.