Hydriodic acid

This acid, which is exactly similar in properties to hydrochloric and hydrobromic acids, is best obtained, in the form of gas, by gently heating a mixture of one part of phosphorus and 16 of iodine, stratified with moistened sand or powdered glass, in a small tube. The gas may be collected over mercury, or absorbed by water, if the liquid acid is required. As iodine and phosphorus act very violently on each other, and indeed take fire in the open air, they ought to be brought in contact in the retort. 

Hydiiodic add gas is transparent, colourless, acid to the taste, and suffocating to the smell. It has a Sp. G. of 4 385, fumes strongly in the air, and is absorbed by water to ets great an extent as hydrochloric acid gas, from which it is not to be easily distinguished, except by the action of chlorine or bromine, which seizing the hydrogen, disengage the purple vapours of iodine. 

The aqueous solution is prepared by passing a current of the gas through water, by passing a current of sulphuretted hydrogen through water in wmch iodine is suspended, or by adding diluted sulphuric add to a solution of iodide of barium. The two latter methods are thus expressed — 

With metallic oxides, this acid forms water and iodides. Thus, with solution of silver, it forms a yellowish-white precipitate of iodide of silver, with solution of lead, a fine yellow precipitate of iodide of lead and with solution of peroxide or bichloride of mercury, a beautiful scarlet precipitate of biniodide of mercury. With chlorine and starch, it of course gives the blue iodide of starch. 

Analogous to chloric and bromic acids. To obtain it, iodine is boiled in a long-necked flask with the strongest nitric acid, when it is gradumly oxidised, and the excess of nitric acid is expelled by a moderate heat. Or 9 parts of iodate of baryta are boiled with a mixture of 2 parts of oil of vitriol and 10 or 12 of 

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The base pyridine removes the hydriodic acid formed. The endpoint occurs when the brown colour of free iodine is seen, i.e. when all the water has been used up. This method is widely used. 

In the first reaction, sulphur may be filtered off, leaving only hydriodic acid. 

Properties—Hydrogen iodide is a colourless gas. It is very soluble in water and fumes in moist air (cf. hydrogen chloride), to give hydriodic acid. Its solution forms a constant boiling mixture (cf. hydrochloric and hydrobromic acids). Because it attacks mercury so readily, hydrogen iodide is difficult to study as a gas, but the dissociation equilibrium has been investigated. 

Hence hydrogen iodide cannot be produced by the reaction of sulphuric acid with an iodide. Hydriodic acid is slowly oxidised by air (more rapidly in light) liberating iodine ... 

E values. Nitric acid attacks mercury, oxidising it to Hg (aq) when the acid is concentrated and in excess, and to Hgf (aq) when mercury is in excess and the acid dilute. Hydriodic acid Hl(aq) attacks mercury, because mercury(Il) readily forms iodo-complexes (see below, p. 438). 

Principle. A definite amount of the substance ca. 25 mg.) is weighed out and transferred to the reaction bulb of the apparatus where it is decomposed by refluxing with constant-boiling hydriodic acid, the essential reaction being ... 

Hydrogen iodide. This gas may be conveniently prepared by allowing a solution of two parts of iodine in one part of hydriodic acid, sp. gr. 1 7 (for preparation, see Section 11,49,2), to drop on to excess of red phosphorus. The evolution of hydrogen iodide takes place in the cold when the evolution of gas slackens considerably, the mixture should be gently warmed. 

Hydriodic acid. A 1-5 litre three-necked flask is charged with a mixture of 480 g. of iodine and 600 ml. of water. The central aperture is fitted with a stopper carrying an efficient mechanical stirrer leading... 

By the slow distillation of the alcohol with constant boiling point (57 per cent) hydriodic acid, for example ... 

Mix 30 g. (38 ml.) of iaopropyl alcohol with 450 g. (265 ml.) of constant boiling point hydriodic acid (57 per cent.) (Section 11,49,2) in a 500 ml. distilling flask, attach a condenser for downward distillation, and distil slowly (1-2 drops per second) from an air bath (compare Fig. II, 5, 3). When about half the liquid has passed over, stop the distillation. Separate the lower layer of crude iodide (80 g.). Redistil the aqueous layer and thus recover a further 5 g. of iodide from the flrst quarter of the distillate (1). Wash the combined iodides with an equal volume of concentrated hydrochloric acid, then, successively, with water, 5 per cent, sodium carbonate solution, and water. Dry with anhydrous calcium chloride and distil. The isopropyl iodide distils constantly at 89°. 

Allyl Iodide. Use 29 g. (34 ml.) of allyl alcohol and 340 g. (200 ml.) of 57 per cent, hydriodic acid 84 g. of crude iodide are obtained. Upon adding 29 g. (34 ml.) of allyl alcohol to the combined residue in the flask and the aqueous layer and distilling as before, a further 72 g. of crude allyl iodide may be isolated. B.p. 99-101° (mainly 100°). The compound is very sensitive to light the distillation should therefore be conducted in a darkened room and preferably in the presence of a little silver powder. 

Cleavage of ethers with hydriodic acid. Aliphatic ethers suflFer fission when boiled with constant boiling point hydriodic acid ... 

Reflux 1 ml. of the ether with 5 ml. of freshly distilled, constant boiling point hydriodic acid (Section 11,49,2), b.p. 126-128°, for 2-3 hours. Add 10 ml. of water, distil and collect about 7 ml. of liquid. Decolourise the distillate by the addition of a httle sodium bisulphite, and separate the two layers by means of a dropper pipette (Fig. 11,27,1). Determine the b.p. of the resulting iodide by the Siwoloboff method (Section 11,12) and prepare a crystalline derivative (Section 111,42). 

A good 3ueld of 5-iodo-2-aminotoluene may be obtained by intimately mixing o-toluidine hydrochloride, iodine and calcium carbonate, and then adding water to the mixture. The liberated hydriodic acid reacts at once with the Calcium carbonate and the lij driodide of the base is not formed. 

It is believed that the red phosphorus is the true reducing agent and the iodine (or iodide) functions as a hydrogen carrier. This proc ure replaces the obsolete method of heating with red phosphorus and concentrated hydriodic acid in a sealed tube. 

Hydrolysis of the azlactone leads to the acylaminooinnamic acid the latter may be be reduced catal3rtlcally (Adams PtOj catalyst 40 lb. p.s.i.) and then hydrolysed by hydrochloric acid to the amino acid. Alternatively, the azlactone (say, of a-benzylaminocinnamic acid) may undergo reduction and cleavage with phosphorus, hydriodic acid and acetic anhydride directly to the a-amino acid (d/ p phenylalanine). 

Tartaric acid is reduced stepwise with concentrated hydriodic acid, first to R(+)-mahc acid (2) and then to succinic acid [110-15-6]. 

Dialkyl peroxides can be reduced to the corresponding alcohols and/or ethers using a variety of reducing agents, some of which, eg, hydriodic acid, have been used for analysis. 

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