194 



IODINE. 



Iodine. 



On potasi. 



Oxiodic 



rid. 



\Vhcn iodine and oxides are made to act on one ano- 

 ther in contact with water, the water is decomposed, 

 its hydrogen forms hydriodic acid with a portion of the 

 iodine, and its oxygen combines with another portion to 

 form oxiodic acid. This takes place with pot-.ss, soda, 

 barytt-8, strontites, lime, and magnesia. 



Iodine, when made to act on a concentrated solu- 

 tion of potass, is rapidly dissolved, -and forms, as one 

 of the results, a granular white precipitate, which is 

 shown to consist of oxiodic acid, combining to form an 

 oxiodate with potass ; for this substance deflagrates on 

 burning coals like nitre, giving out oxygen and be- 

 coming ioduret of potass. The solution consists ofhy- 

 driodate of potass, containing iodine. It is of an orange 

 yellow, but, when saturated with iodine, it is of a deep 

 reddish-brown. When it is evaporated, and heated to 

 redness, an ioduret of potassium is formed. In the first 

 part of this process, the oxygen, according to Sir H. 

 Davy, is expelled from one part of the potass, to form 

 the ioduret of potassium, and is* united to another to 

 form the oxiodate of potass. But Gay-Lussac's view 

 of it, that water is decomposed, its oxygen serving to 

 form the oxiodate, and its hydrogen the hydriodate, 

 seems more completely conformable to the facts. Sir 

 H. Davy, from his first experiments, conceived that io- 

 dine formed substances analogous to alkalis, by com- 

 bining with the alkaline metalloids ; for the compound 

 produced by its action on solution of potass, even when 

 it was in great excess, reddened turmeric paper, and 

 rendered the colouring matter of violets green ; but he 

 afterwards found that this was owing to a small quan- 

 tity of supercarbonate of potass which existed in the 

 hydrate ; and when the compound is treated with hy- 

 uriodic acid, and heated to redness, so as to produce an 

 ioduret, it loses this property, and acquires the taste of 

 a neutral rather than an alkali. Concentrated soda 

 produces similar phenomena; a deflagrating oxiodate is 

 precipitated, and a hydriodate retained in solution. The 

 case is the same with barytes, lime, and strontites, 

 and their oxiodates have little solubility. Hence these 

 salts are obtained in a state of considerable purity. 



When peroxide of mercury is exposed to a heat be- 

 tween 140 and 212 in water and iodine, a super-oxi- 

 odate of mercury is formed, which is held in solution, 

 and an oxiodate which is insoluble, and remains mixed 

 with the red ioduret, which is at the same time formed. 



The oxiodic acid, in order that its properties may be 

 ascertained, ought to be obtained in a state of separation 

 from the bases with which it is combined in its forma- 

 tion. For this purpose Gay-Lussac adopted the follow- 

 ing process : Upon oxiodate of barytes, he poured- sul- 

 phuric acid, diluted with twice its own quantity of wa- 

 ter, and heated the mixture. The oxiodic acid in this 

 case, according to him, abandoned its earthy base, 

 and combined with the water. A small quantity of the 

 sulphuric acid continued mixed with it, in consequence of 

 the strong affinity of oxiodie acid for barytes. Sir H. 

 Davy, however, found reason to be dissatisfied with these 

 results, as disguising the real properties of this com- 

 pound : anil he succeeded in producing one in a pure 

 state, by bringing together iodine and euchlorine, (hy- 

 peroxy muriatic acid.) in the form of gas, at the ordina- 

 ry temperature of the atmosphere. The resulting com- 

 pound, when the other ingredients were driven off by 

 heat, was a white semitranspurent solid, of an astrin- 

 gent and acid taste, and sufficiently heavy to sink in 

 sulphuric acid. It is deliquescent : its aqueous solu- 

 tion may be evaporated to the consistence of a syrup, 

 and, by the continuance of heat, may be reduced to the 

 original solid compound. It detonates when heated in 



mixture with inflammable substances. The oxiodic Iodine. 

 acid obtained by Gay-Lussac differed from this, by S "Y"*"' 

 containing a portion of sulphuric acid, which prevent- 

 ed that chemist from being able to procure it in a solid 

 form, and led him to believe that water was essential 

 to its constitution. This acid cannot be procured by 

 the direct union of iodine with oxygenous gas. 



Dry iodine, when presented to chlorine gas, rapidly Chlororct of 

 absorbs it in a quantity less than one-third by weight, iodine. 

 producing a compound which in some parts has a 

 fine orange yellow colour ; in others an orange red. 

 The yellow parts contain the largest proportion of 

 chlorine, and are the most volatile. The yellow com- 

 pound is called, by Gay-Lussac, the chloruret, and 

 the red the sub-chloruret of iodine. Both of them speed- 

 ily deliquesce in the air. The solution of the chloru- 

 ret is colourless, when the excess of chlorine is driven 

 off, and then the mutual saturation of the two consti- 

 tuents appears to be" complete. These solutions are very 

 acid, and destroy the colour of a solution of indigo in 

 sulphuric acid. The chloruret receives from Sir H. Da- 

 vy the name of chlorionic acid. When a solution of it is 

 saturated with an alkali, the chlorine acquires hydrogen, 

 and the iodine oxygen, so that a muriate and an oxio- 

 date of the alkali are obtained. Such are the relative 

 tendencies of these two substances. That of combining 

 with hydrogen is greatest in chlorine, and that of com- 

 bining with oxygen is greatest in iodine. Heat disenga- 

 ges part of the chlorine, and reduces the solution to a 

 sub-chloruret. The solution of the sub-chloruret is more 

 stable, and capable of being volatilized without decom- 

 position. 



We shall now describe the properties of the neutral salts 

 formed by combinations of the acids of iodine with salifi- 

 able bases. First the hydriodates, and then the oxiodates. 



The hydriodates may, in general, be obtained by Hydri*. 

 combining hydriodic acid with the bases. But those dates, 

 of potass, soda, barytes, strontian, and lime, may also 

 be prepared, by treating iodine with these bases, and 

 employing the methods already described for separating 

 these salts from the oxiodates, which are formed at the 

 same time. The hydriodates of zinc and iron, and in 

 general of all the metals that decompose water, are ob- 

 tained by dissolving the iodurets of those metals in wa- 

 ter, and applying heat. 



The hydriodates are not changed by sulphurous or 

 muriatic acid, or sulphureted hydrogen. Chlorine, ni- 

 tric acid, and concentrated sulphuric acid, constantly 

 decompose them, and separate the iodine. 



When a solution of hydriodate of potass is made to Hydrio- 

 crystallize, the oxygen of the potass and the hydrogen d;utofpot- 

 of the acid unite, according to Gay Lussac, to form wa- ass> 

 ter, and crystals of ioduret of potassium are formed ; 

 these are considered by that chemist as completely ana- 

 logous to the compound of chlorine and sodium, his 

 chloruret of sodium, the sodane of Sir H.Davy, and the 

 muriate of soda of former chemists. That there is no al- 

 kali in these compounds, and that the appearance of al- 

 kaline properties, neutralized by an acid, arises from 

 the chlorine supplying the place both of oxygen and 

 acid, though it contains no oxygen and possesses no 

 acid qualities, is one of the striking paradoxes which 

 the adherents of the old school rind most difficulty in 

 admitting, and which, in determining the plausibility 

 of the two theories, seems to form a sufficient counter- 

 poise to the curious coincidences in some other parti- 

 culars by which the new doctrines are supported. Hy- 

 driodate of potass is composed of 



Hydriodic acid 100. 



Potass . 37.426 



