IODINE 917 



slum. The mother-liquor still contains the iodide of sodium, sulphide of sodium, 

 and some carbonate of soda. This liquor is then mixed with about one -eighth of its 

 bulk of sulphuric acid, and allowed to stand for twenty-four hours ; carbonic and 

 sulphurous acid, and sulphuretted hydrogen gases escape, a fresh qxiantity of sulphate 

 of soda crystallising out, mixed with a precipitate of sulphur. 



The supernatant acid liquor is then transferred to a leaden still, to which is adapted 

 a double tubulated leaden head luted on with pipe-clay ; it is then heated to 140 

 Fahr., when binoxide of manganese is added. 



The temperature may be gently raised to 212 Fahr., but not higher, as some 

 chlorine would come over, and combine with some of the iodine, forming chloride of 

 iodine. 



This iodine is condensed in spherical glass condensers, each having two mouths 

 opposite to each other, and inserted the one into the other, the end one being fitted to 

 the neck of the leaden head. 



The iodine is purified by resublimation. 



The following formula represents the reaction : 



Iodide of Oxide of Sulphuric Sulphate of Sulphate of -jvviinp Water 

 Sodium Manganese Acid Soda Manganese locl] 



Nal + MnO 2 + 2HS0 4 = NaSO 4 + MnSO 1 + I + 2HO. 



2WaI + MnO 2 + 2H 2 SO l = XTa-'SO 4 + MnSO 4 + I 2 + 2H 2 O. 



The British iodine is largely manufactured at Glasgow, from the kelp of the west 

 coast of Ireland and the western islands of Scotland. 



In Mr. E. C. O. Stanford's method of preparing kelp, the dried and compressed sea- 

 weed, instead of being burnt in the open air, is distilled in retorts with superheated 

 steam, and thus all loss of iodine is prevented. 



The methods of preparing iodine from the mother-liquor of the kelp have been 

 variously modified. Thus, the use of oxide of manganese may be dispensed with ; or 

 chlorine of gas may be transmitted through the mother-liquor ; or the liquor may 

 be distilled with chloride of iron. 



Another source of commercial iodine is to be found in Chili saltpetre, or native 

 nitrate of soda. This salt in its raw state always contains iodine, though in only 

 very small proportion. This iodine may be extracted from the mother-liquor left in the 

 process of refining the salt, by treating the liquor with sulphurous or with nitrous acid. 



Iodine is a crystallisable solid, its primary form being a rhombic octohedron. It is 

 however usually met with in micaceous, soft, friable scales, having a greyish-black 

 colour, a metallic lustre, and an acrid hot taste. Even at ordinary temperatures, and 

 more especially when moist, it is sensibly volatile, emitting an odour like that of 

 chlorine, only much weaker. 



At 225 F. it fuses, and at 347 F. boils, and is converted into a magnificent violet 

 vapour. It may nevertheless be distilled, in the presence of steam, at a temperature 

 of 212, as is seen in the process of manufacture. 



Iodine, in the solid state, has a specific gravity of 4'947, the specific gravity of the 

 vapour being, according to Dumas, 8'716. Iodine is only very slightly soluble in 

 water, it requiring 7,000 parts of water to dissolve it; even then it imparts a yellow 

 colour to the solution, and is used in that state as a test for starch, with which it forms 

 a beautiful blue compound, which is, however, destroyed by heat. 



Alcohol and ether dissolve it more readily; but the most powerful solvents of 

 iodine are the solutions of the iodides. Iodine stains the skin, and most organic 

 substances, of a brown colour; it attacks the metals rapidly; iron or zinc being 

 readily dissolved by it if placed in water with it, an iodide of the metal being 

 formed. 



All the compounds of iodine with the metals and with hydrogen are decomposed 

 by chlorine, and even by bromine, iodine being set free. Advantage is taken of this 

 f.-ict in detecting the presence of iodine. If the iodine exists in combination with a 

 metal, or as hydriodic acid, its solution will not form the characteristic intense blue 

 compound with starch, but on the addition of a little chlorine, or solution of bleaching 

 powder, the iodine is set free and forms the blue compound with the starch. If, how- 

 ever, the iodine exists as iodic acid, it will not act upon starch until reduced by some 

 reducing agent, as sulphurous acid. In using the chlorine care must be taken not to 

 use too much, as it would unite with the iodine and prevent it acting on the starch. 



Iodine is used to a considerable extent in medicine ; particularly for glandular 

 swellings, and goitre. It is also much used in photography, and in the preparation of 

 certain coal-tar colours. The chemical symbol for iodine is I; its atomic weight 

 126'88 ; and the combining volume of its vapour 2. 



The price of iodine has suffered recently (1873-4) some remarkable fluctuations. 



