A NOTE ON THE PREPARATION OF HYDRIODIC ACID T. H. W ~ E H E AUNIVERSIR' D,
OR
GEORGIA, ATHENS. GEORGIA
I n the preparation of hydriodic acid from hydrogen suljide and iodine, i t has been found that the formation of iodine on distilling the hydriodic solution can be prmented ,if the mixture i s dialyzed through a collodion membrane for forty-eight hours. This remwes the colloidal sulfur which seems to cause ozidation of the hydriodic acid.
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The instructor of inorganic chemistry is so often embarrassed by his inability to explain the results obtained in elementary laboratories that he welcomes help from any source. This note is offered in the hope that it will help clear up one of the many ditficnlties encountered in the laboratory. When hydriodic acid is prepared by passing hydrogen sulfide gas into a water solution of iodine, the sulfur formed filtered off, and the filtrate fractionally distilled, there is almost invariably a considerable amount of iodine liberated in the distilling flask and the student wants to know why. The instructor usually answers that hydrogen iodide is "unstable" and breaks up, when heated, into hydrogen and iodine. This sounds all right and many elementary texts invite this answer even though they do not say outright that hydriodic acid is thermally unstable below 127'C. The author investigated this reaction and thinks the evidence to be presented will show that there is a more valid explanation. Hydriodic acid was prepared as described above: HzS
+ 12 + 2HI + S(?)
This equation is the one usually given. After filtering off the sulfur, the filtrate was transferred to a sac prepared from U. S. P. collodion (nitrocellulose) and dialyzed against distilled water for forty-eight hours. Care was taken to protect the whole outfit from the atmosphere because hydrogen iodide reacts readily with oxygen to form water and iodine. The water containing hydrogen iodide which diffused through the membrane was then distilled and no iodine was formed although a t times the thermometer went as high as 130°C. This procedure was repeated three times and uniform results were obtained. The following explanation is offered. When sulfide ion is oxidized in water solution by iodine, it is probable that more than free sulfur is formed. Od6n ( I ) has reported the formation of colloidal sulfur. If a colloidal sulfur complex was formed, i t would run through ordinary filter paper but could not dialyze through a collodion membrane. The usual procedure leaves this colloidal sulfur in the solution to be distilled and it reacts with 541
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JOURNAL OF CHEMICAL EDUCATION
MARCH, 1931
hydrogen iodide to form iodine and hydrogen sulfide. Dialysis removed the colloidal sulfur and no such reaction took place when the free hydriodic acid solution was distilled. This offers an empirical explanation which the author thinks is preferable to the loose one of "instability" even though it be more complicated. Literahue Cited (1) ODBN,Z. physik. Chem.,80,709(1912).
