HYDROCYANIC ACID

(HCN HCy.) Syn. Pru ic acid , Hydric cyanide , Cyan hydric acid . Hydrocyanic acid was discovered by Scheele; but its nature and chemical properties were first investigated by Gay-Lu ac. Sources. This acid is found in water distilled from the kernels of the apricot, the peach, the plum, and cherry, the leaves of the laurel, and some other shrubs. The kernels of the bitter almond also yield it by distillation, mixed with an e ential oil. The juice of the tapioca plant (the Jatropha manihot ) likewise contains it. Many nitrogenous substances, when submitted to destructive distillation, also evolve hydrocyanic acid. Crystallised ammonic formiate subjected to heat in a retort yields a vapour which, pa ed through a red-hot tube, decomposes into this acid and water. Another method by which it may be obtained, consists in sending a current of dry sulphuretted hydrogen gas through a long tube filled with cyanide of mercury; and very recently it has been obtained by the direct combination of nitrogen and acetylene gas, by adding one volume of the former to two of the latter, and pa ing a series of electric sparks through the mixture, the gases combining without condensation. Lastly, it is yielded when a metallic cyanide or ferrocyanide is decomposed by an acid, this latter being the means by which it is invariably procured. 1. Anhydrous hydrocyanic acid may be prepared by Wöhler’s plan, which is as follows:—A crude pota ium cyanide is prepared by fusing eight parts of the dried pota ium ferrocyanide with three of pota ium carbonate and one of charcoal. The fused ma is treated with six times its weight of water in a well-closed ve el; the clear liquid is decanted from the iron, which it is the object of this operation to separate, and is poured into a retort: sulphuric acid, diluted with an equal weight of water, is gradually added in the proportion of one part of oil of vitriol to two parts of the cyanide. At first the distillation proceeds spontaneously from the heat developed by the admixture of sulphuric acid with the water. In order to condense the acid, the products are made to pa through a long U-shaped tube, immersed in cold water and filled with calcic chloride, with the exception of the first fourth of the tube, which contains fragments of the crude pota ium cyanide; to the bent tube is attached a second delivery tube, which pa es to the bottom of a bottle cooled with ice and salt. The calcic chloride in the syphon tube retains the moisture, and the pota ic cyanide any sulphuric acid that might chance to pa over, whilst the hydrocyanic acid collects in the anhydrous state in the cooled receiver. Trautwein recommends it to be prepared by the dehydration of the strong aqueous acid, by means of fused and pulverised chloride of calcium. [350] [350] The details of this proce are given in ‘Watt’s Chemical Dictionary.’ ⁂ The observance of the greatest care and caution are nece ary in the preparation of this most potent poison. The operation is most safely performed in winter. The apparatus should be so arranged as to allow of any vapours given off being carried from the operator by a brisk current of air. Prop. At ordinary temperatures anhydrous hydrocyanic acid is a colourle liquid, having a specific gravity of 0·7058 at 44·6° Fahr. It is very inflammable, burning with a violet flame resembling that of cyanogen, but somewhat whiter in colour. It is soluble in all proportions in water, the resulting mixture being lighter than that fluid, and miscible with alcohol. It is very feebly acid; pota ic cyanide always having an alkaline reaction. Red oxide of mercury is readily di olved by it, and when added to a solution of argentic nitrate it precipitates white flocculi of cyanide of silver. Anhydrous hydrocyanic acid is an extremely volatile liquid; if a drop be let fall on a gla plate, part of it 2. Preparation of aqueous hydrocyanic acid. a. From hydrated ferrocyanide of pota ium. —By heating it in a gla retort with oil of vitriol and water, Everitt states that the best proportions are nearly ten parts of the salt to seven of oil of vitriol (diluted with any convenient amount of water). Adopting these proportions, 422·4 parts of ferrocyanide of pota ium yield 81 parts of hydrocyanic acid. The greater part of the hydrocyanic acid pa es over at the beginning of distillation, at a temperature a little above 212° Fahr.; and when the residual liquid reaches a higher temperature, the water (which then contains but little hydrocyanic acid) is then carried over. It is therefore nece ary to employ a good condensing apparatus, or the hydrocyanic acid which pa es over at first will for the most part be di ipated in vapour mixed with the air of the apparatus. This lo may also be obviated by placing water in the receiver. The residue need not be boiled down to dryne ; it will be found best to distil off from two thirds to three fourths of the liquid, according to the amount of water present. It is not nece ary to di olve the ferrocyanide in water previous to adding the sulphuric acid, as it readily di olves in the water during the proce of distillation. Three conditions are important to be observed in the arrangements of the apparatus:—1. The mixture in the retort should not be allowed to spirt over. 2. It should contain but little air. 3. It should present the greatest po ible amount of surface to be cooled. If sulphate of pota ium and pru ian blue are spirted over into the distillate, this must be carefully rectified over a small quantity of magnesia, chalk, or carbonate of barium. b. From cyanide of pota ium (without distillation).—To a solution of nine parts of tartaric acid in sixty parts of water, contained in a well-stoppered bottle nearly filled with it, four parts of pure cyanide of pota ium are added; the ve el is shaken, frequently dipped into cold water, and then left in the cold for twelve hours; and the aqueous hydrocyanic acid, which contains but a very small quantity of tartrate of pota ium, is poured off from the crystallised tartrate. [351] This acid contains 3·6 per cent. of anhydrous hydrocyanic acid. [351] ‘London Med. Surg. Journ.,’ vi, 524. c. From cyanide of mercury. —126 (accurately weighed) parts of cyanide of mercury are agitated with at least 28 parts of iron filings, in a well-stoppered bottle, containing 49, or rather more, parts of oil of vitriol, diluted with a considerable quantity of water. The agitation is continued until a portion of the liquid taken out is not blackened by sulphuretted hydrogen. The solution, decanted from the iron and mercury, is then placed in a retort and distilled; the acid coming over at a gentle heat. Exce of iron accelerates the decomposition. d. From cyanide of silver. —Everitt recommends 200 parts of pure cyanide of silver to be shaken up with 240 parts of hydrochloric acid of specific gravity of 1·129, and when the decomposition is complete, the hydrocyanic acid to be separated from the chloride of silver by decomposition. This hydrocyanic acid may contain a small quantity of hydrochloric acid, a not very objectionable admixture, since it retards decomposition. It po e es the advantage of definite strength. Prop. The aqueous is very similar in properties to the anhydrous acid, differing in taste, odour, poisonous and combustible properties, according to its degree of concentration. Like the anhydrous, the aqueous acid decomposes, but not so readily; when perfectly pure, becoming brown, and at last black. As before stated, a little free mineral acid a ists to preserve it. It should be always kept in a dark place. Detection and estimation of hydrocyanic acid and soluble cyanides. —The presence of hydrocyanic acid, indicated by the characteristic smell, which is given off by the contents of the stomach, or of any fluid containing it (provided this is not disguised by any substance of stronger odour) may be confirmed by the following tests: 1. To the filtered suspected fluid add a slight exce of caustic potash, and then a solution containing ferrous and ferric sulphates. If hydrocyanic acid, or a soluble cyanide be present, upon the addition of an exce of hydrochloric acid the liquid turns to a blue colour (more or le intense according to the quantity of acid present), owing to the formation of Pru ian blue. 2. Add the suspected fluid solution of nitrate of silver; if hydrocyanic acid be present, a white cyanide of silver is formed, which is nearly insoluble in cold nitric acid, but is soluble in ammonia and cyanide of potash, and which, when heated to redne , gives off the inflammable violet-flamed cyanogen. 3. Acidulate a small quantity of the suspected liquid with a few drops of hydrochloric acid, and place it in a watch-gla ; then invert a second watch-gla , moistened with a drop of solution of ammonic sulphide over this. After a few minutes remove the upper watch-gla , and evaporate the liquid to dryne over a water bath; let the dry residue be treated with a drop of a weak solution of ferric chloride. If hydrocyanic acid be present, a blood-red colour is produced, owing to the formation of red ferric hydro cyanide, which may be discharged by chloride of mercury; a reaction which distinguishes it from a similar colour given by meconic acid. Where large quantities of material have to be examined, it is desirable that the acid should be distilled off by the heat of a water-bath, acidulating the liquid with tartaric acid if it be alkaline. The distillate is then to be tested by any of the above methods. Antidotes. —Give a scruple of carbonate of potash di olved in about an ounce of distilled water, and directly afterwards, ten grains of sulphate of iron, also di olved in the same quantity of distilled water, to which should be added one drachm of tincture of perchloride of iron. Whilst this is being prepared, and subsequently, apply cold affusion to the head and neck, artificial respiration, and, if practicable, give strong coffee and brandy. A more ready remedy is ammonia, given both internally and applied to the nostrils.