HYDRIODIC ACID. 



and the mixture covered by pounded glass, and combination 

 determined by a gentle heat ; and afterwards decomposing this 

 iodide of phosphorus by a few drops of water. Hydriodic acid 

 instantly comes off as gas, and hydrated phosphorous acid remains 

 in the tube. Or PI 3 and 6H O = 3H I and 3H O + PO 3 . A 

 slight heat may be applied to the tube, when the action abates, to 

 expel the last portions of hyclriodic acid ; but if the temperature be 

 elevated, the residuary hydrated phosphorous acid is decomposed 

 with the evolution of phosphuretted hydrogen gas, which may, 

 therefore, be obtained by the same operation. This gas is very 

 soluble in water, and soon decomposed over mercury, which 

 combines with its iodine and liberates hydrogen, so that it 

 ought to be collected by the method of displacement. The 

 gas is conducted by a tube to the bottom of a dry bottle, 

 the air of which it displaces, as in the experiment with 

 hydrochloric acid (figure, page 355), and the bottle is closed 

 with a glass stopper when full of gas. Hydriodic gas is colour- 

 less, of density 4443 by experiment and 4385 by theory, and 

 consists of 2 volumes of iodine vapour and 2 volumes of hydro- 

 gen gas united without condensation, or forming 4 volumes, 

 which are, therefore, the combining measure of the gas. In the 

 combination of its constituents by volume, hydriodic acid re- 

 sembles hydrochloric gas and all the other hydrogen acids. Hy- 

 driodic gas is gradually decomposed by oxygen, with the forma- 

 tion of water; iodine is liberated. 



The solution of this acid in water may be obtained by trans- 

 mitting sulphuretted hydrogen gas through water in which 

 iodine is suspended ; the iodine combines with the hydrogen of 

 that compound and liberates the sulphur. The liquid may after- 

 wards be warmed to expel the excess of sulphuretted hydrogen, 

 and filtered. It is colourless at first, but in a few hours becomes 

 red, owing to the decomposition of hydriodic acid by the oxygen 

 of the air, and the solution of the iodine in the acid. The solu- 

 tion has its maximum boiling point, which lies between 257 and 

 262, when of sp. gr. 1.7, according to Gay-Lussac. Nitric and 

 sulphuric acids decompose it, and are decomposed themselves, 

 with the formation of water ; the starch test then indicates free 

 iodine. 



lodic acid, IO 5 . Iodine does not afford a peculiar acid com- 

 pound, with red oxide of mercury and those metallic oxides 

 which yield hypochlorous acid with chlorine. Nor is it absorbed, 



