HI Hydriodic acid

Technical grade hydriodic acid is a 47% HI solution and usually has a brown color due to the presence of free iodine, produced by air oxidation of HI. Hydriodic acid should be stored in the dark to prevent photochemical decomposition, and free from air to prevent oxidation. The addition of 1.5% hypophosphorus acid (H3PO2) prevents oxidative decomposition. 

HI or HI( ) hydrogen moniodide or hydrogen iodide HI( ) hydriodic acid... 

HI hydriodic acid H2SO4 sulfuric acid HCIO4 perchloric acid... 

It is common practice to refer to the molecular species HX and also the pure (anhydrous) compounds as hydrogen halides, and to call their aqueous solutions hydrohalic acids. Both the anhydrous compounds and their aqueous solutions will be considered in this section. HCl and hydrochloric acid are major industrial chemicals and there is also a substantial production of HF and hydrofluoric acid. HBr and hydrobromic acid are made on a much smaller scale and there seems to be little industrial demand for HI and hydriodic acid. It will be convenient to discuss first the preparation and industrial uses of the compounds and then to consider their molecular and bulk physical properties. The chemical reactivity of the anhydrous compounds and their acidic aqueous solutions will then be reviewed, and the section concludes with a discussion of the anhydrous compounds as nonaqueous solvents. 

There are two main issues concerning the chemistry of the reaction and the separation. One is how to separate the hydriodic acid and sulfuric acid produced by the Bunsen reaction. The other is how to carry out the hydrogen iodide (HI) decomposition section, where the presence of azeotrope in the vapor-liquid equilibrium of the hydriodic acid makes the energy-efficient separation of HI from its aqueous solution difficult, and also, the unfavorable reaction equilibrium limits the attainable conversion ratio of HI to a low level, around 20%. 

As for the former problem, the researchers of GA found that the mixed acid solution produced by the Bunsen reaction separates spontaneously into two liquid phases in the presence of excess amount of iodine [17]. The heavier phase is mainly composed of HI, I2, and H20, and is called "Hix" solution. The main components of the lighter phase are H2S04 and H20. The phenomenon (liquid-liquid (LL)-phase separation) offered an easy way of separating the hydriodic acid and the sulfuric acid. As for the HI processing, some ideas have been proposed by GA [17], RWTH Aachen [18], and JAEA. JAEA studied the utilization of membrane technologies for concentrating the Hix solution to facilitate the HI separation and also for enhancing the one-pass conversion of HI decomposition [19,20]. 

Hence mercury is a poor reducing agent it is unlikely to be attacked by acids unless these have oxidising properties (for example nitric acid), or unless the acid anion has the power to form complexes with one or both mercury cations Hg2 + or Hg2+, so altering the E values. Nitric acid attacks mercury, oxidising it to Hg2+(aq) when the acid is concentrated and in excess, and to Hgf+(aq) when mercury is in excess and the acid dilute. Hydriodic acid HI(aq) attacks mercury, because mercury(II) readily forms iodo-complexes (see below, p. 438). 

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