IODINE. 



193 



Iodine, the hydriodic. The specific gravity of hydriodic gas is to 

 ""Y"^*' that of air as 4443 to 1000. Iodine and hydrogeti com- 

 bine in equal volumes to produce this gas, and, as the va- 

 pour of iodine is the heaviest we are acquainted with, be- 

 ing to that of oxygen as 15. (nil to 1, and to that of hydro- 

 gen a- 1 17.7 1 to I, it does not take one hundredth part 

 of its weight of the latter gas to be con verted into an acid. 

 On this part of the subject, Gay-Lussac take, occasion 

 to illustrate his principle on the ratio of saturation, as 

 being determined by the relative densities of bodies in 

 the state of vapour. The facts, as applied to the 

 well-known gaseous substances, were first pointed out 

 by that philosopher, and the doctrine extended by him 

 to all other bodies, though of course it can only be de- 

 termined with regard to those which we find capable 

 of being raised to the state of vapour in our experi- 

 ments, and not to all of these, as the high temperature at 

 which many of them are volatilized renders all mani- 

 pulations on their vapour, for the purpose of ascertain- 

 ing their specific gravity, unmanageable. 



Hydriodic gas is partly decomposed by a red heat 

 By the presence of oxygen, the decomposition is ren- 

 dered more rapid and complete, and water and iodine 

 are the products. 



jqoid hjr- Hydriodic gas has a very strong attraction for water ; 

 nodjc a very large quantity of the gas is absorbed by a very 

 ** small quantity of water, imparting to it a great in- 



crease of density, and the solution when strong is 

 smoaking. The most convenient way of obtaining this 

 acid in a liquid state is, to dissolve ioduret of phos- 

 phorus in water. Part of the water is decomposed, the 

 hydrogen forming hydriodic arid, and the oxygen a 

 portion of phosphorous acid. When beat is applied, 

 the two arids are separated. Another and an easier 

 method i, to pass sulphureted hydrogen through wa- 

 ter containing iodine. The hydrogen goes to form tin- 

 acid required, and the sulphur is precipitated. Heat 

 is then applied to drive off the excess of sulphuretrd In - 

 dragen, and a colourless hydriodic acid is procured, 

 from which the sulphur precipitated to the bottom of 

 wpmia. the vessel may be decanted off. This acid, like the sul- 

 phuric, may be concentrated by evaporation, and, till 

 it is raised to 257". the vapour which it loses is al- 

 most pure water. While under this temperature, there- 

 fore, it may be evaporated in an open vessel, or the 

 liquid which distils over may be thrown away. When 

 med to this temperature, it gives over an abundant 

 quantity of acid, and it does not admit of being heated 

 above t2| under the common atmospheric pressure, 

 i the boiling point of the acid; and, being com. 

 paratively high, it prevents it from being disengaged 

 trow any neutralizing base by the volatile acids. 



n distilled, or even simply exposed to the air, 

 it assumes brownish-red colour, from a partial decom- 

 position by the contact of oxygenous gas, which forms 

 water with its hydrogen, ami the iodine, which is then 

 evolved, combines with the liquid aci-l >iange 



is assisted by hght and beat The property of absorb- 

 ing and dissolving 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 



e solution in water. 



Hydriodic acid is rapidly decomposed by sulphuric 

 and nitric acid, and by chlorine, which seiae on its 

 hydrogen, and the iodine is either precipitated in 

 brown crystals, or exhales in purple vapours. Chlo- 

 rine is a delicate tct of this acid, as the purple va- 

 pour inrtanjUv appear when it is dropped into aay 

 solution which containi it ; but they are speedily re- 

 vet. xii. pAr i. 



dissolved by the acid, and therefore the chlorine must inline. 

 be cautiously added. When heated with black ox- S> "Y~ ^ 

 ide of manganese it affords iodine, and an hydrio- 

 date 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. 

 Iodine and hydriodic acid have been explained. The 

 compounds of the acid, with neutralizing bases, are 

 called fiydriodalet. Iodine, as we shall find, may be 

 combined with oxygen, and the compound possesses acid 

 properties. This acid is called by Gay-Lussac the io- 

 die, and the compounds which it forms by uniting 

 with neutralizing bases, he calls iodates. Dr. Murray 

 calls this acid the oiiodic, and its compounds oxiodates. 

 We shall adopt these last terms, as well fitted to keep the 

 facts in mind, and capable of being adapted to any theory. 



The combination of iodine with azote cannot be pro- Action of 

 duced by direct experiment. Ioduret of ammonia must iodine on 

 be first formed. We shall therefore describe the phe- tmmoni - 

 nomena which iodine exhibits with ammonia, previous- 

 ly to the consideration of the other alkalis. It differs 

 from these others by not containing oxygen, a circum- 

 stance which materially affects the characteristic results 

 of the chemical 'changes. \V hen ammoniacal gas is pas- 

 sed over iodine, a viscid shining liquid is immediately 

 formed, of a brownish- black colour, which, in propor- 

 tion as it is saturated with ammonia, loses its lustre and 

 viscosity. This is an ioduret of ammonia When dis- 

 solved in water, the hydrogen 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 mar be obtained directly, by lodunt of 

 putting iodine in fine powder into an aqueous solu- "ote. 

 tion uf ammonia. This ioduret is pulverulent, and <>f 

 a brownish-black colour. It detonates from the small- 

 est shock, and from heat. The flame is of a feeble vio- 

 let colour. When delicately prepared, it sometimes de- 

 tonates spontaneously. When this compound is put in 

 potash, the aaote 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 ga- Sulphurou* 

 aeons sulphurous acid ; but, when the acid is dissolved acid. 

 in water, part of the water is decomposed. Its oxygen 

 converts the sulphurous into sulphuric acid, and its 

 hydrogen the iodine into hydriodic. These cannot be 

 separated by heat, for the temperature which vola- 

 tilises hydriodic acid decomposes, under these circum- 

 stances, the sulphuric, reproducing the original sul- 

 phurous acid and the iodine. While the liquid is dis- 

 tilled over, these are the two ingredients, and it is co- 

 loured by the iodine ; but, when cooled in the receiver, 

 it resumes the state of sulphuric and hydriodic acids in 

 limpid mixture. 



The action of iodine on the oxides takes place both On the ox- 

 with and without the presence of water, but exhibits "In- 

 different phenomena according to this difference of cir- 

 cumstances. When brought in contact with the ox- 

 ide of potassium produced by combustion, it combines 

 with the potassium to form an ioduret, and the oxygen 



Barytes, strontites, and lime, unite with iodine, with- 

 out the evolution of any gas. The oxides of zinc and 

 iron undergo no change. The iodurets of barytes, 

 stror.tites, and lime, show strong alkaline properties 

 when dissolved in water, and are considered by Gay- 

 Lussac as subiodurets. 



SB 



