_a._ _From hydrated ferrocyanide of potassium._--By heating it in a glass
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 potassium yield 81 parts of hydrocyanic
acid. The greater part of the hydrocyanic acid passes 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 necessary to employ a good condensing apparatus, or the
hydrocyanic acid which passes over at first will for the most part be
dissipated in vapour mixed with the air of the apparatus. This loss may
also be obviated by placing water in the receiver. The residue need not be
boiled down to dryness; 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 necessary to dissolve the ferrocyanide in water previous to
adding the sulphuric acid, as it readily dissolves in the water during the
process 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 possible amount of surface to be cooled.
If sulphate of potassium and prussian 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 potassium_ (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
potassium are added; the vessel 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
potassium, is poured off from the crystallised tartrate.[351] This acid
contains 3·6 per cent. of anhydrous hydrocyanic acid.
[Footnote 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. Excess 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
possesses 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 assists 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 excess 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
excess of hydrochloric acid the liquid turns to a blue colour (more or
less intense according to the quantity of acid present), owing to the
formation of Prussian 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 redness, 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-glass; then invert a second
watch-glass, moistened with a drop of solution of ammonic sulphide over
this. After a few minutes remove the upper watch-glass, and evaporate the
liquid to dryness 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 hydrocyanide, 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 dissolved in about an
ounce of distilled water, and directly afterwards, ten grains of sulphate
of iron, also dissolved 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.
=Hydrocyanic Acid, Diluted.= _Syn._ ACIDUM HYDROCYANICUM DILUTUM. (B. P.)
The 'British Pharmacop[oe]ia' gives the following simple directions for
the preparation of this acid:--
Dissolve two and a quarter ounces (avoir.) of ferro-cyanide of potassium
in ten fluid ounces of distilled water, then add one fluid ounce of
sulphuric acid, previously diluted with four fluid ounces of distilled
water, and cooled. Put them into a retort and adapt this to a receiver,
containing eight fluid ounces of water, which must be kept carefully cold.
Distil with a gentle heat until the fluid in the receiver measures
seventeen fluid ounces. Add to this three fluid ounces of the water, or as
much as may be sufficient to bring the acid to the required strength of
two per cent. by weight.
_Prop._ Specific gravity, ·997. 100 grains, or 110 minims, precipitated
with a solution of nitrate of silver, give a precipitate of cyanide of
silver, which when dried, weighs 10 grains. 270 grains rendered alkaline
by liquor sodæ, require a 1000 grain measures of volumetric solution of
nitrate of silver, before a permanent precipitate begins to form.
_Antidotes._ See HYDROCYANIC ACID.
=Hydrocyanic Acid, Scheele's.= _Syn._ ACIDUM HYDROCYANICUM SCHEELII. The
original process of Scheele does not yield an acid of uniform strength,
and is probably never followed. It is therefore impossible to state
precisely what is intended when Scheele's acid is prescribed, or to
understand why it should be preferred by certain physicians to the British
Pharmacop[oe]ia preparation, which is of a known and definite strength. As
prepared by different makers it has been found to contain from three to
five per cent. of anhydrous acid. The following is Scheele's process:--
Mix two ounces of Prussian blue, with six ounces of red precipitate of
mercury, and add six ounces of water. Boil for some minutes, constantly
agitating; pour the whole on a filter and wash the residuum on the filter
with two ounces of hot water, which is to be added to the filtered liquor.
Add to this an ounce and a half of clean iron filings, and three drachms
of sulphuric acid; shake well and let it settle; then pour the clear
liquor into a retort, and distil a fourth part into a receiver well luted
and kept cold.
=HYDROFLUORIC ACID. (H. F.)= _Syn._ HYDRIC FLUORIDE, HYDROGEN FLUORIDE.
_Prep._ 1. From _fluor-spar_ (free from silica and metallic sulphides) and
oil of vitriol. The fluor-spar being reduced to fine powder and placed in
a leaden retort, is mixed with twice its weight of concentrated oil of
vitriol, and on applying heat, an acid and highly acid vapour distils
over, which condenses to a liquid if passed into a receiver of the same
metal, standing in a freezing mixture at a temperature of 4° Fahr. Louyet
has shown that the liquid acid, obtained as above, is not (as once
believed) anhydrous.
2. From the double fluoride of potassium and hydrogen. Fremy first renders
the salt anhydrous by careful drying; and by the subsequent application of
a strong heat, expels the equivalent of hydrofluoric acid contained in it;
condensing it into a colourless, mobile, very volatile liquid by the
application of a freezing mixture of ice and salt.