Hydriodic acid

General
Surname	Hydriodic acid
other names	Hydroiodic acid Hydriodic acid Acidum hydrojodicum
Molecular formula	HI (aq)
Brief description	pale yellowish, aqueous, acidic solution of hydrogen iodide - turns dark in air and light when iodine is released
External identifiers / databases
CAS number 10034-85-2 (for hydrogen iodide) Wikidata Q908093
properties
Molar mass	127.91 g · mol -1
Physical state	aqueous solution
pK s value	−10 (at 25 ° C)
safety instructions
GHS labeling of hazardous substances danger H and P phrases H: 314 P: 280-305 + 351 + 338-310
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Hydroiodic acid is a solution of hydrogen iodide (HI) in water. Their salts are called iodides . After hydrofluoric acid , hydrochloric acid and hydrobromic acid, it is the penultimate link in the series of hydrohalic acids . The acid strength increases within the main group. However, hydriodic acid is not only the strongest stable hydrohalic acid (after the unstable hydrogen astat), but also the strongest known stable oxygen-free acid at all (see: acid strength ).

Hydroiodic acid is usually sold as a 57 percent acid.

synthesis

Hydroiodic acid can be made as follows:

One synthetic route takes place by means of a displacement reaction . The hydroiodic acid is liberated from its salt ( potassium iodide ) by means of a strong acid ( phosphoric acid ) and then distilled off. The result is hydrogen iodide and a phosphorus salt of potassium, potassium phosphate .

${\displaystyle \mathrm {3\ KI+H_{3}PO_{4}\longrightarrow 3\ HI+K_{3}PO_{4}} }$

Potassium iodide and phosphoric acid react to form hydrogen iodide and potassium phosphate.

Another synthesis is possible through the oxidation of hydrogen sulfide with iodine. Elemental sulfur flocculates while the hydrogen iodide remains dissolved. The sulfur can then be filtered off.

${\displaystyle \mathrm {I_{2}+H_{2}S_{aq}\longrightarrow 2\ HI_{aq}+S\downarrow } }$

Iodine and hydrogen sulfide react in aqueous solution to form hydriodic acid and sulfur, which flocculates in aqueous solution.

Another synthesis of hydrogen iodide takes place using a two-step process: First, iodine is burned with phosphorus in a low-oxygen environment. Then the resulting phosphorus triiodide is slowly added to water.

${\displaystyle \mathrm {2\ P+3\ I_{2}\longrightarrow 2\ PI_{3}} }$

Phosphorus and iodine react to form phosphorus triiodide.

${\displaystyle \mathrm {2\ PI_{3}+6\ H_{2}O\longrightarrow 6\ HI+2\ H_{3}PO_{3}} }$

Phosphorus triiodide and water react strongly exothermically with the formation of hydriodic acid and phosphonic acid .

proof

The detection of iodide ions can be carried out with silver nitrate solution. A yellow, extremely sparingly soluble precipitate of silver iodide is observed. This is less soluble than that of the other silver halides by powers of ten. Therefore, a precipitate is observed when a saturated silver chloride solution is added to a solution containing iodide.

safety instructions

Hydroiodic acid is very irritating to the skin, respiratory tract and mucous membranes. As a strong acid, it is able to destroy living tissue ( burns ). Serious injuries that heal poorly or not at all can be the result of hydriodic acid burns.

Individual evidence

1. ↑ ^{a b} Entry on hydriodic acid in the GESTIS substance database of the IFA , accessed on November 8, 2018(JavaScript required) .
2. ↑ chem.wisc.edu: pKa Data , Compiled by R. Williams (PDF, 78 kB).
3. ↑ Hydriodic acid data sheet from Sigma-Aldrich , accessed on May 13, 2017 ( PDF ).
4. ↑ G. Brauer (Ed.), Handbook of Preparative Inorganic Chemistry 2nd ed., Vol. 1, Academic Press 1963, pp. 286-289.