prevent the receiver coming into actual contact with the hot neck of the retort, some asbestos in large fibres is placed between them. When about 10 or 15 grm. has been drawn over, change the receiver and distil off three-fourths of the contents of the retort. This method depends on the fact discovered by Bussy and BUIGNET, that on distilling sulphuric acid which contains arsenic in the form of arsenic acid, an arsenic-free distillate is obtained.

B. Pour into 4 parts of water 1 part of concentrated sulphuric acid, and conduct into the mixture for some time a slow stream of hydrosulphuric acid, keeping the fluid heated to 70°. Let the mixture stand at rest for several days, then decant the clear supernatant fluid from the precipitate, which consists of sulphur, sulphide of lead, perhaps also sulphide of arsenic, and heat the decanted fluid in a tubulated retort with obliquely upturned neck and open tubulature until sulphuric acid fumes escape with the aqueous vapor. The acid so purified is fit for many purposes of chemical analysis; if it is wished, however, to free it also from non-volatile substances, it may be distilled from a coated retort as As soon as the drops in the neck of the retort become oily, the receiver is changed, and the concentrated acid which now passes over is kept in a separate vessel.

in a.

c. Common dilute sulphuric acid. This is prepared by adding to 5 parts of water in a leaden or porcelain dish gradually, and whilst stirring, 1 part of concentrated sulphuric acid. The sulphate of lead which separates is allowed to subside, and the clear fluid finally decanted.

Tests. Pure sulphuric acid must be colorless; when colorless solution of sulphate of protoxide of iron is poured upon it in a test tube, no brown tint must mark the plane of contact of the two fluids (nitric acid, hyponitric acid); when diluted with 20 parts of water, it must not impart a blue tint to a solution of iodide of potassium mixed with starch (hyponitric acid). Mixed with pure zinc and water, it must yield hydrogen gas, which, on being passed through a red-hot tube, must not on a deposit the slightest trace of arsenic. It must leave no residue upon evaporation on platinum, and must remain perfectly clear upon dilution with four or five parts of spirit of wine (oxide of lead, sesquioxide of iron, lime). The presence of small quantities of lead is detected most easily by adding some hydrochloric acid to the sulphuric acid in a test tube. If the plane of contact is marked by turbidity (chloride of lead), lead is present. Sulphurous acid is discovered by the odor after shaking the acid in a half-filled bottle.

Uses.-Sulphuric acid has for most bases a greater affinity than almost any other acid; it is therefore used principally for the liberation and expulsion of other acids, especially phosphoric, boracic, hydrochloric, nitric, and acetic acids. Several substances which cannot exist in an anhydrous state (e.g. oxalic acid), are decomposed when brought into contact with concentrated sulphuric acid; this decomposition is owing to the great affinity which sulphuric acid possesses for water. The nature of the decomposed body may in such cases be inferred from the liberated products of decomposition. Sulphuric acid is also frequently used for the evolution of certain gases, more particularly of hydrogen and hydrosulphuric acid. It serves also as a special reagent for the detection and precipitation of baryta, strontia, and lead. What kind of sulphuric acid is to be used, whether the pure acid or the ordinary acid of commerce, whether concentrated or dilute, depends upon what the circumstances

in each case may require. It will, however, be found that the necessary directions on this point are generally given in the present work.

§ 25.

2. NITRIC ACID (HO, NO,).

Preparation.-a. Heat crude nitric acid of commerce, as free as possible from chlorine, and of a specific gravity of at least 131,* in a glass retort to boiling, with addition of some nitrate of potassa; let the distillate run into a receiver kept cool, and try from time to time whether it still continues to precipitate or cloud solution of nitrate of silver. As soon as this ceases to be the case, change the receiver, and distil until a trifling quantity only remains in the retort. Dilute the distillate with water until the specific gravity is 1.2.

b. Dilute crude nitric acid of commerce of about 1:38 specific gravity with two-fifths of its weight of water, and add solution of nitrate of silver as long as a precipitate of chloride of silver continues to form; then add a further slight excess of solution of nitrate of silver, let the precipitate subside, decant the perfectly clear supernatant acid into a retort or an alembic with ground head; add some nitrate of potassa free from chlorine, and distil until only a small quantity remains, taking care to attend to the proper cooling of the fumes distilling over. Dilute the distillate, if necessary, with water, until it has a specific gravity of 1.2.

Tests.-Pure nitric acid must be colorless and leave no residue upon evaporation on platinum foil. Addition of solution of nitrate of silver or of nitrate of baryta must not cause the slightest turbidity in it. It is advisable to dilute the acid with water before adding these reagents, as otherwise nitrates will precipitate. Silver should be tested for by hydrochloric acid.

Uses.-Nitric acid serves as a chemical solvent for metals, oxides, sulphides, oxygen salts, &c. With metals and sulphides of metals the acid first oxidizes the metal present, at the expense of part of its own oxygen, and then dissolves the oxide to a nitrate. Most oxides are dissolved by nitric acid at once as nitrates; and so are also most of the insoluble salts with weaker acids, the latter being expelled in the process by the nitric acid. Nitric acid dissolves also salts with soluble nonvolatile acids, as e.g. phosphate of lime, with which it forms nitrate of lime and acid phosphate of lime. Nitric acid is used also as an oxidizing agent: for instance, to convert protoxide of iron into sesquioxide, protoxide of tin into binoxide, &c.

A highly concentrated acetic acid is not required in qualitative analytical processes; the acidum aceticum of the British Pharmacopoeia, which contains 33 per cent. of H O, C ̧H ̧O,, and has a specific gravity of 1.044, fully answers the purpose.

Tests.-Pure acetic acid must leave no residue upon evaporation, and -after saturation with carbonate of soda-emit no empyreumatic odor. Hydrosulphuric acid, solution of nitrate of silver, and solution of nitrate

A weaker acid will not answer the purpose.

of baryta must not color or cloud the dilute acid, nor must sulphide of ammonium after neutralization of the acid by ammonia. Solution of indigo must not lose its color when heated with the acid. Empyreumatic matter is best detected by neutralizing the acid with carbonate of soda, and adding permanganate of potash solution. If the solution loses its color and afterwards deposits a brown precipitate, empyreumatic matter is present.

If the acid is not pure, add some acetate of soda and redistil from a glass retort not quite to dryness; if it contains sulphurous acid (in which case hydrosulphuric acid will produce a white turbidity in it), digest it first with some binoxide of lead or finely-pulverized binoxide of manganese, and then distil with acetate of soda.

Uses. Acetic acid possesses a greater solvent power for some substances than for others; it is used therefore to distinguish the former from the latter; thus it serves, for instance, to distinguish oxalate of lime from phosphate of lime. Acetic acid is occasionally used also to acidulate fluids where it is wished to avoid the employment of mineral acids.

§ 27.

4. TARTARIC ACID (2 HO, C,H,O,, = 2 HO, T).

4 10

The tartaric acid of commerce is sufficiently pure for the purposes of chemical analysis. It is kept best in powder, as its solution suffers decomposition after a time. For use it is dissolved in a little water with the aid of heat.

Uses. The addition of tartaric acid to solutions of sesquioxide of iron, alumina, and various other oxides of metals, prevents the usual precipitation of these metals by an alkali; this non-precipitation is owing to the formation of double tartrates, which are not decomposed by alkalies.

Tartaric acid may therefore be employed to effect the separation of these metals from others the precipitation of which it does not prevent. Tartaric acid forms a difficultly soluble salt with potassa, but not so with soda; it is therefore one of our best reagents to distinguish between the two alkalies. Bitartrate of soda answers this latter purpose still better than the free acid. This reagent is prepared by dissolving one of two equal portions of tartaric ac in water, neutralizing the solution with carbonate of soda, then adding the other portion of the acid, and evaporating the solution to the crystallization point. For use, 1 part of the salt is dissolved in 10 parts of water.

b. HYDROGEN ACIDS AND HALOGENS.

§ 28.

1. HYDROCHLORIC ACID (HCI).

Preparation. Pour a cooled mixture of seven parts of concentrated sulphuric acid and two parts of water over four parts of chloride of sodium in a retort; expose the retort, with slightly raised neck, to the heat of a sand-bath until the evolution of gas ceases; conduct the evolved gas, by means of a bent tube, into a flask containing six parts of water, and take care to keep this vessel constantly cool. To prevent the gas from

receding the tube ought only to dip about one line into the water of the flask. When the operation is terminated, try the specific gravity of the acid produced, and dilute with water until it marks from 1.11 to 1.12. If you wish to ensure the absolute purity of the acid, and its perfect freedom from every trace of arsenic and chlorine, you must take care to free the sulphuric acid intended to be used in the process from arsenic and the oxygen compounds of nitrogen, according to the directions of § 24. A pure acid may also be prepared cheaply from the crude hydrochloric acid of commerce by diluting the latter to a specific gravity of 1·12, and distilling the fluid, with addition of some chloride of sodium. Or you may put the acid into the retort in the concentrated form, placing 60 parts of water into the receiver for every 100 parts of concentrated acid, and not luting the receiver to the retort. If the crude acid contains chlorine this should be removed first by cautious addition of solution of sulphurous acid, before proceeding to the distillation; if, on the other hand, it contains sulphurous acid, this is removed in the same way by cautious addition of some chlorine water. Hydrochloric acid not unfrequently contains chloride of arsenic, owing to the presence of arsenic in the sulphuric acid employed. To free it from this impurity, the acid is mixed with twice its volume of water, hydrosulphuric acid is conducted into it, the mixture allowed to stand at rest for some time, the clear fluid then decanted from the sulphur and sulphide of arsenic, and heated, to expel the sulphuretted hydrogen.

Tests.-Hydrochloric acid must be perfectly colorless and leave no residue upon evaporation. If it turns yellow on evaporation, sesquichloride of iron is present. It must not impart a blue tint to a solution of iodide of potassium mixed with starch paste (chlorine or sesquichloride of iron), nor discolor a fluid made faintly blue with iodide of starch (sulphurous acid). Chloride of barium ought not to produce a precipitate in the highly diluted acid (sulphuric acid). Hydrosulphuric acid must leave the diluted acid unaltered (arsenic). After neutralization with ammonia, sulphide of ammonium must produce no change in it (iron, thallium).

Uses.-Hydrochloric acid serves as a solvent for a great many subIt dissolves many metals and sulphides of metals as chlorides, with evolution of hydrogen or of hydrosulphuric acid. It dissolves oxides and superoxides in the form of chlorides, in the latter case mostly 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, and likewise for the detection of free ammonia, with which it produces in the air dense white fumes of chloride of ammonium.

§ 29.

2. CHLORINE (Cl) AND CHLORINE WATER.

Preparation. Mix 18 parts of common salt in lumps with 15 parts of finely pulverized good binoxide of manganese, free from carbonate of lime; 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. Where it is desired to obtain chlorine water quite free from bromine, the washing flask is changed after about one-half of the chlorine has been expelled, and the gas which now passes over is conducted into a fresh bottle filled with water. If the chlorine water is to be quite free from hydrochloric acid, the gas must be passed through a U tube containing binoxide of manganese. 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 effect the solution of certain metals (gold, platinum), to decompose metallic sulphides, to convert sulphurous acid into sulphuric acid, protoxide of iron into sesquioxide, &c.; and also to effect the destruction of organic substances, as in presence of these it withdraws hydrogen from the water, 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, and then adding chlorate of potassa. This gives rise to the formation of chloride of potassium, water, free chlorine, and bichlorate of chlorous acid, which acts in a similar manner to chlorine.

§ 30.

3. NITRO-HYDROCHLORIC ACID. Aqua regia.

Preparation. Mix 1 part of pure nitric acid with from 3 to 4 parts of pure hydrochloric acid.

Uses. Nitric acid and hydrochloric acid decompose each other, the decomposition mostly resulting, as GAY-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. Thus, H O, NO,