212 On Iodine. [March, 



as the iodine dissolves in the alkali, which it does with rapidity, we 

 obtain a granular white precipitate, which melts on burning coals 

 like nitre, and which is decomposed by heat, giving out oxygen 

 and ioduret of potassium. This precipitate is an alkaline iodate of 

 potash. The liquid contains hydriodate. Hence water must have 

 been decomposed ; * its hydrogen, uniting with iodine, has fofmed 

 hydriodic acid, and its oxygen iodic acid. 



The potash solution remains slightly coloured ; orange yellow, 

 when the alkali is in excess ; but it is of a deep reddish brdwn 

 when the alkali is saturated with iodine. This colour is owing 

 principally to the solution of iodine in the hydriodate of potash. I 

 found that when a solution of a determinate quantity of potash, so 

 weak as not to allow the iodate of potash to precipitate, is entirely 

 saturated with iodine, we must add a quantity of potash equal to 

 the original dose to bring it to the point when its colour is a light 

 orange-yellow. Even when the solution is saturated with iodine, it 

 is always alkaline ; though if we dissolve in water ioduret of 

 potassium or of zinc, we obtain neutral compounds. This differ- 

 ence, which we find in the analogous combinations of sulphur and 

 chlorine, is owing to this, that the forces which tend to decompose 

 water are smaller in the first case than in the second. 



Concentrated soda produces, like potash, a fulminating salt, 

 which is partly precipitated, and a hydriodate which remains in 

 solution. Barytes, lime, and strontian, present the same pheno- 

 mena ; with this difference only, that their iodates are very little 

 soluble. This last circumstance enables us to obtain these salts in 

 a state of purity. In order to obtain the iodates of potash and soda 

 free from hydriodates, and perfectly neutral, it is necessary to 

 crystallize them repeatedly, which greatly diminishes their quantity. 

 I shall therefore state the method which I employ to prepare them 

 in order to avoid that inconvenience. 



Upon a determinate quantity of iodine I pour solution of potash 

 or soda till the liquid ceases to be coloured. I then evaporate to 

 dryness, and digest the dry salt in alcohol of the specific gravity 

 0"8I or 0*82. As the iodate is not soluble in this liquid, while the 

 hydriodate is very soluble, the two salts easily separate from each 

 otlier. After having washed the iodate two or three times with 

 alcohol, I dissolve it in water, and neutralize it with acetic acid. 

 1 evaporate to dryness, and digest the dry salt in alcohol, to remove 

 the acetate. After two or three washings, tlie iodate is pure. As 

 for the alcohol coutainiug the hydriodate, we distil it off, and then 

 neutralize the potash by means of hydriodic acid. 



Here it may be asked whether, at the moment that the alkali acts 

 upon the iodine, iodate and hydriodate form and exist separately. 



♦ I reason on llic hypothesis of the existence of hydriodates. But if we do not 

 admit their existence, the oxygen which combines with the. iu(Jine, and coiivcriji U 

 iatu an acid, uiu^t have been furuijhed by a portion af the potash. 



