THE xHALOGENS 497 



being burnt (or subjected to dfy distillation) an ash is left which 

 chiefly contains salts of potassium, sodium, and calcium. The metals 

 occur in the sea-weed a~s salts of organic acids. On being burnt these 

 organic salts are decomposed, forming carbonates of potassium and 

 sodium. Hence, sodium carbonate is found in the ash of sea plants. 

 The ash is dissolved in hot water, and on evaporation sodium car- 

 bonate and other salts separate, but a portion of the substances 

 remains in solution. These mother liquors left after the separation of 

 the sodium carbonate contain chlorine, bromine, and iodine in combi- 

 nation with metals, the chlorine and iodine being in excess of the bromine 

 13,000 kilos of kelp give about 1,000 kilos of sodium carbonate and 

 15 kilos of iodine. 



The liberation of the iodine from the mother liquor is effected with 

 comparative ease, because chlorine disengages iodine from potassium 

 iodide and its other combinations with the metals. Not only chlorine, 

 but also sulphuric acid, liberates iodine from sodium iodide. Sulphuric 

 acid, in acting on an iodide, sets hydriodic acid free, but the latter 

 easily decomposes, especially in the presence of substances capable of 

 evolving oxygen, such as chromic acid, nitrous acid, and even ferric 

 salts. 61 Owing to its sparing solubility in water, the iodine liberated 

 separates as a precipitate. To obtain pure iodine it is sufficient to 

 distil it, and neglect the first and last portions of the distillate, the 

 middle portion only being collected. Iodine passes directly from a state 

 of vapour into a crystalline form, and settles on the cool portions of the 



61 In general, 2HI + O = I 2 + H 2 O, if the oxygen proceed from a substance from which 

 it is easily evolved. For this reason compounds corresponding with the higher stages of 

 oxidation or chlorination frequently give a lower stage when treated with hydriodic acid. 

 Ferric oxide, Fe^Gs, is a higher oxide, and ferrous oxide, FeO, a lower oxide ; the former 

 corresponds with FeXj, and the latter with FeX 2 , and this passage from the higher to the 

 lower takes place under the action of hydriodic acid. Thus hydrogen peroxide and 

 ozone (Chapter IV.) are able to liberate iodine from hydriodic acid. Compounds of copper 

 oxide, CnO or CuX 2 , give compounds of the suboxide Cu a O, or CuX,. Even sulphuric acid, 

 which corresponds to the higher stage SOs, is able to act thus, forming the lower oxide 

 80 2 . The liberation of iodine from hydriodic acid proceeds with still greater ease under 

 the action of substances capable of disengaging oxygen. In practice, many methods 

 are employed for liberating iodine from acid liquids containing, for example, sulphuric 

 acid and hydriodic acid. The higher oxides of nitrogen are most commonly used ; they 

 then pass into nitric oxide. Iodine may even be disengaged from hydriodic acid by the 

 action of iodic acid, &c. But there is a limit in these reactions of the oxidation of. hydri- 

 odic acid because, under certain conditions, especially in dilute solutions, the iodine 

 set free is itself able to act as an oxidising agent that is, it exhibits the character 

 of chlorine, and of the halogens in general, to which we shell again have occasion to 

 refer. In Chili, where a large quantity of iodine is extracted in the manufacture of Chili 

 nitre, which contains NaIO 3 , it is mixed with the acid and normal sulphites of sodium 

 In solution ; the iodine is then precipitated according to the equation 2NaIOs + SNa^SOs 

 -f 2NaHSO 3 = 5Na2SO 4 + 1 2 + H 2 O. The iodine thus obtained is puriBed by sublimation, 



