Acids derived from iodine have found the world-wide application either as reducing agents (hydroiodic acid HI) or oxidizing agents (iodic HIO3, periodic oxoacid HIO4). This diversity is a direct consequence of elemental iodine position in the periodic table that allows it to exist in several oxidation states from -I up to +VII. Basically, iodous acid HIO2 has been proven to exist too, but the compound is unstable and it quickly decomposes into elemental iodine and iodates. The acids of iodine can be generally used for fabrication of numerous important salts (metal iodides and iodates) used in organic synthesis or analytical chemistry. Hydroiodic and iodic acid can be prepared easily from elemental iodine as we demonstrate it below.
The hydroiodic acid is prepared either by dissolution of hydrogen iodide gas in water or by reduction of elemental iodine suspension with hydrogen sulfide according to the equation
I2(s) + H2S(aq) -> 2HI(aq) + S(s)
The solution of hydroiodic acid is slowly oxidized in air according to the equation 4HI + O2 -> 2H2O + I2 leaving elemental iodine that causes coloring of the solution. The example is given in the picture below. In an old bottle (over 30 years) of hydroiodic acid big crystals of elemental iodine were found after pouring out a dark brown solution. According to this, due to its instability the acid is usually used as a reducing agent.
Conversely, iodic acid is prepared upon addition of strong oxidizing agents into a suspension of elemental iodine. For example gaseous chlorine bubbled through iodine suspension gives iodic acid according to the equation
I2(s) + 6H2O(l) + 5Cl2(g) -> 2HIO3(aq)
It is a white hygroscopic crystalline substance that is consequently decomposed into iodine pentaoxide I2O5 when heated above ~160 degrees of celsius. If prepared from mixture of concentrated nitric acid and hydrogen peroxide, typically the product is off-white color. It can be further used for preparation of metal iodates used as oxidizing agents.
Hydroiodic acid synthesis
Iron sulfide preparation:
Powdered sulphur (6.413 g, 0.2 mol) and iron metal powder (11.169 g, 0.2 mol) were mixed together in stoichiometric (1:1) amount and the mixture was perfectly homogenized in a mortar with pestle. The solid mixture was then placed into a steel can and ignited by magnesium metal powder. After the reaction was over (no more visible flames), the mixture of the product – iron(II) sulfide – along with traces of unreacted elemental sulfur and iron was ground to a powder form. No more purification of the product was carried out. Iron(II) sulfide formed was used for hydrogen sulfide generation.
Iron(II) sulfide powder (5 g, 0.057 mol) was placed into a 250 mL round-bottom flask with a side arm and a separatory funnel containing diluted hydrochloric acid (1:1) was mounted to the flask. The side arm of the flask was connected to a gas-washing bottle with a suspension of unknown amount of iodine in water (the suspension formed by oxidation of hydroiodic acid was used, this can be exchange for elemental iodine suspended into water as well). Hydrochloric acid was added to the flask dropwise and the evolving hydrogen sulfide gas was directed into the gas-washing bottle with iodine/water mixture. After a few minutes a color change was observed. The gas was bubbled through the solution until the reaction was over leaving clear and colourless solution of hydroiodic acid. Concentration of this solution may be determined simply by acid-base titrimetry.
Iodic acid synthesis
Iodine (5 g, 0.0196 mol) was resublimed to acquire higher purity and larger surface for the reaction. Then it was added into a double necked round bottom flask along with concentrated HNO3 (5.72 g, 65%) and H2O2 (5.22 g, 35%). A smaller round bottom flask with ice and water was put on top of the bigger neck to resublime iodine escaping from the reaction flask. The smaller neck was capped but not clamped in case the neck with the ice flask on top gets clogged and pressure builds up. The reaction flask was heated to 70°C and kept so for an hour. When the mixture cleared up the volume of the solution was reduced via boiling. Then the solution was transferred to a beaker and rest of the solution was boiled off. The white precipitate was the recrystallized. Yield: 1.22 g (17.6%).