Uses.-Hydrochloric acid serves as a solvent for a great many substances. It dissolves many metals and sulphides of metals, forming chlorides, with evolution of hydrogen or of hydrosulphuric acid. It dissolves lower and higher oxides in the form of chlorides, the solution being, in the case of the higher oxides, mostly attended with liberation of chlorine. Salts with insoluble or volatile acids are also converted by hydrochloric acid into chlorides, with separation of the original acid; thus carbonate of lime is converted into chloride of calcium, with liberation of carbonic acid. Hydrochloric acid dissolves, salts with non-volatile and soluble acids, apparently without decomposing them (e. g., phosphate of lime); but the fact is that, in cases of this kind, a metallic chloride and a soluble acid salt of the acid of the dissolved compound are formed; thus, for instance; in the case of phosphate of lime, chloride of calcium and acid phosphate of lime are formed. With salts of acids forming no soluble acid compound with the base present, hydrochloric acid forms metallic chlorides, the liberated acids remaining free in solution (borate of lime). Hydrochloric acid is also applied as a special reagent for the detection and separation of oxide of silver, suboxide of mercury, and lead (see below); and likewise for the detection of free ammonia, with which it produces in the air dense white fumes of chloride of ammonium,

§ 25.

2. CHLORINE (Cl) AND CHLORINE WATER.

Preparation. Mix 18 parts of finely ground common salt with 15 parts of finely pulverized good binoxide of manganese; put the mixture in a flask, pour a completely cooled mixture of 45 parts of concentrated sulphuric acid and 21 parts of water upon it, and shake the flask: a uniform and continuous evolution of chlorine gas will soon begin, which, when slackening, may be easily increased again by the application of a gentle heat. This method of Wiggers is excellent, and can be highly recommended. Conduct the chlorine gas evolved, first through a flask containing a little water, then into a bottle filled with cold water, and continue the process until the fluid is saturated. The chlorine water must be kept in a cellar, and carefully protected from the action of light; since, if this precaution is neglected, it speedily suffers complete decomposition, being converted into dilute hydrochloric acid, with evolution of oxygen (resulting from the decomposition of water). Smaller quantities, intended for use in the laboratory, are best kept in a stoppered bottle, protected from the influence of light by a case of pasteboard. Chlorine water which has lost its strong peculiar odor is unfit for use.

Uses.-Chlorine has a greater affinity than iodine and bromine for metals and for hydrogen. Chlorine water is therefore an efficient agent to effect the expulsion of iodine and bromine from their compounds. Chlorine serves, moreover, to convert sulphurous acid into sulphuric acid, protoxide of iron into sesquioxide, &c.; and also to effect the destruction of organic substances, in presence of water, as it withdraws hydrogen from the latter, enabling thus the liberated oxygen to combine with the vegetable matters and to effect their decomposition. For this latter purpose it is most advisable to evolve the chlorine in the fluid which contains the organic substances; this is effected by adding hydrochloric acid to the fluid, heating the mixture to boiling, and then adding chlorate of potassa. This gives rise to the formation of chloride of potas

sium, water, free chlorine, and bichlorate of chlorous acid, which acts ina similar manner to chlorine.

$ 26.

3. NITRO-HYDROCHLORIC ACID. Aqua regia.

Preparation. Mix one part of pure nitric acid with from three to four parts of pure hydrochloric acid.

Nitric acid and hydrochloric acid decompose each other, the decomposition resulting, in most cases, as Guy-Lussac has shown, in the formation of two compounds which are gaseous at the ordinary temperature, NO, Cl, and N O, Cl, and of free chlorine and water. one equivalent of N O, is used to three equivalents of Cl H, it may be assumed that only chloro-hyponitric acid (N O, Cl,), chlorine and water, are formed (NO, + 3 H Cl NO, CI, CI+ 3 HO).

2

5

=

2

If

This decomposition occurs as soon as the fluid is saturated with the gas; but it recommences the instant this state of saturation is disturbed by the application of heat or by decomposition of the acid. The presence of the free chlorine, and also, but in a very subordinate degree, that of the acids named, makes aqua regia our most powerful solvent for metals (with the exception of those which form insoluble compounds with chlorine). Nitro-hydrochloric acid serves principally to effect the solution of gold and platinum, which metals are insoluble both in hydrochloric and in nitric acid; and also to decompose various metallic sulphides, e. g., cinnabar, pyrites, &c.

$ 27.

4. HYDROFLUOSILICIC ACID (HF, SIF).

Preparation-Take quartz sand, wash off every particle of dust, and dry thoroughly. Mix one part of the dry sand intimately with one part of perfectly dry fluor spar in powder; pour six parts of concentrated sulphuric acid over the mixture in a strong glass flask or in a mineral water bottle dry inside, and mix carefully by shaking the vessel. As the mixture swells up, when getting warm, it must at first fill the vessel only to one-third. The mouth of the flask or bottle must be closed with a perforated cork, or a cap of india-rubber, into which one end of a somewhat wide glass tube, twice bent at a right angle, is fitted air-tight; the other limb reaching to the bottom of a tall flat-bottomed glass jar with just sufficient mercury to allow the end of the tube to dip into it to the extent of several lines; the mercury in the jar is covered with four parts of water. Promote the disengagement of fluosilicic gas, which commences even in the cold, by exposing the flask or bottle to a moderate heat in the sand-bath. Towards the end of the process a pretty strong heat should be applied. Every gas bubble ascending through the mercury produces in the water a precipitate of hydrated silicic acid. The rationale of this process is that, of every three equivalents of fluoride of silicon (Si Fl), one equivalent decomposes with two equivalents of water into silicic acid (Si O2), which separates, and hydrofluoric acid, which combines with the two undecomposed equivalents of fluoride of silicon, forming hydrofluosilicic acid.

[3 Si F, + 2 HO = 2 (HF, SIF) + Si O2].

The precipitated hydrate of silicic acid renders the liquid gelatinous, and it is for this reason that the aperture of the exit tube must be placed

under mercury, since it would speedily be choked if this precaution were neglected. The same end may be attained also by attaching a funnel to the exit tube, by means of vulcanized india-rubber, and letting the funnel alone dip into the water. It sometimes happens in the course, and especially towards the end of the operation, that the gas forms complete channels of silica in the gelatinous liquid, through whieh it gains the surface without undergoing decomposition, if the liquid is not occasionally stirred. When the evolution of gas has completely ceased, throw the gelatinous paste upon a linen cloth, squeeze the fluid through, and filter it afterwards. Keep the filtrate for use.

Tests.-Hydrofluosilicic acid, mixed with two parts of water, must produce no precipitate in solutions of salts of strontia.

Uses.-Bases decompose with hydrofluosilicic acid, forming water and metallic silicofluorides. Many of these are insoluble, whilst others are soluble; the latter may therefore by means of this reagent be distinguished from the former. In the course of analysis, hydrofluosilicic acid is applied simply for the detection of baryta.

c. SULPHUR ACIDS.

$ 28.

5. HYDROSULPHURIC ACID (Sulphuretted Hydrogen) (H S). Preparation.-Hydrosulphuric acid gas is evolved best from sulphide of iron, which is broken into small lumps, and then treated with dilute sulphuric acid. Fused sulphide of iron may be procured so cheaply in commerce that it is hardly worth while to take the trouble of preparing it expressly. However, if you wish to prepare it yourself, this may be done by heating iron turnings or nails, from 1 to 1 inch long, in a Hessian crucible to a white heat, and then adding small lumps of rollsulphur until the entire contents of the crucible are in fusion. As soon as this is the case, pour the fused

mass out on sand, or into an old Hessian crucible. Or make a hole in the bottom of the crucible in which you fuse the mass, when the sulphide of iron will, as fast as it forms, run through the hole in the bottom of the crucible, and may thus be easily received in a coal-shovel placed in the ash-pit. Or, introduce an intimate mixture of thirty parts of iron filings and twenty-one parts of flowers of sulphur, gradually and in small portions at a time, into a red-hot crucible, awaiting always the incandescence of the portion last introduced, before proceeding to the addition have thus put the whole mixture into the crucible, cover the latter closely, and expose it to a more intense heat, sufficient to make the sulphide of iron fuse more or less.

Fig. 18.

of a fresh one. When you

The evolution of the gas is effected in the apparatus illustrated by Fig. 18.

Pour water over the sulphide of iron in a, add concentrated sulphuric acid, and shake the mixture; the evolved gas is washed in c. When a sufficient quantity of gas is evolved, pour the fluid from off the still undecomposed sulphide of iron, wash the bottle repeatedly with water, fill it with that fluid, and keep it for next operation. If you neglect this, the apparatus will speedily become incrusted with crystals of sulphate of protoxide of iron, which is apt to interfere injuriously with subsequent processes of evolution of gas.

For larger laboratories, or for chemists, having to operate often and largely with hydrosulphuric acid, I can recommend the lead apparatus designed by myself, which I have now for several years employed with the most satisfactory results in my own laboratory* (see Figs. 19 and 20).

abcd and e f g h (Fig. 20) are two cylindrical leaden vessels, soldered with pure lead. They are both of the same size (in my own apparatus 33

centimètres high, and 30 centimètres in diameter). i is a false bottom of lead, perforated like a sieve, placed from 4 to 5 centimètres above the actual bottom of the vessel, and resting on leaden feet, which support it on the sides as well as also more particularly in the middle. The numerous holes in the sieve-like bottom have a diameter of 1 milli

*The apparatus is made by Mr. Stumpf, of Wiesbaden, mechanist, and fully answers all reasonable demands, both as regards workmanship and price.

mètres; k shows the opening through which the sulphide of iron is introduced into the vessel.

it

In my apparatus this aperture has a diameter of 7 centimètres, and is closed by putting a greased leather ring on its broad smooth rim, and pressing down upon this by means of three winged screws, the broad rim of the smoothturned cover. 7 shows the opening through which the solution of sulphate of protoxide of iron is drawn off; it will be seen by the drawing that the bottom of the vessel (g h) slants towards the part where this opening is placed. The aperture has a diameter of 3 centimètres; it is closed by means of a smooth-turned broad and thick leaden cap, fitting on the smooth-turned broad rim, and pressed down upon with a winged screw. The semi-elliptical bar in which the female screw is set, is moveable, and hinged to the sides of in a manner to admit of its being bent out of reach of the liquid on drawing off the latter. The construction of the filling tube, m, may be learned from the drawing, and equally so that of the tube d h, which is intended to convey the acid from the upper to the lower vessel, and vice versa. It will be seen from the drawing that this tube reaches down into the slanting and deepened part of the bottom g h, without, however, actually touching the latter. The tube ce is closed at the top, and has, therefore, no communication with the upper vessel, being simply intended to let off the gas evolved in efgh; to which end it is connected laterally by a branch tube, with the tube o; this latter tube is fitted with a stop-cock (n). The tube q is closed at both ends, and serves simply as an additional support for the upper vessel. The tubes in my apparatus have an inner diameter of 16 millimètres.

Fig. 20.

The process of filling is conducted as follows: put 3.3 kilogrammes* of fused sulphide of iron, broken in lumps, through the mouth k, upon the perforated bottom i; screw the covers properly down upon k and l, shut the cock n, and pour through the funnel of m first 7 litres of water, then 1 litre of concentrated sulphuric acid, and then again 7 litres of water. The air in a b c d escapes in this operation through p, even when the latter tube is already connected with the flasks r, s, t.

If the cock n is now opened, and one of the cocks u, the acid will flow through the tube dl into e f g h; and through o air will escape at first, followed by the hydrosulphuric acid evolved in e f g h. As is seen in the figure, the tube o rises only to a certain elevation, when it makes a bend, running on thence in a horizontal direction. As many cocks, u u, are added as is thought desirable; these cocks are common brass gas stop

* The quantities here given are calculated for an apparatus of the dimensions stated.