First Division of the First Group of the Inorganic Acids.

§ 138.

a. ARSENIOUS ACID and ARSENIC ACID are, as we have seen above, decomposed by hydrosulphuric acid, and are precipitated by that reagent respectively as ter- and pentasulphide of arsenic. As this would lead to confounding them rather with the metallic oxides thau with other acids, it has been deemed more judicious to class these two acids with the oxides of the sixth group. (See § 131 and § 132.)

b. CHROMIC ACID (Cr O2).

1. Chromic acid presents the appearance of a scarlet-red crystalline mass, or of distinct acicular crystals. Upon ignition it is resolved into sesquioxide of chromium and oxygen. It deliquesces rapidly upon exposure to the air. It dissolves in water, imparting to the fluid a deep reddish-brown tint, which remains still visible in very dilute solutions.

2. The chromates are all red or yellow, and for the most part insoluble in water. Part of them are decomposed upon ignition; those with alkaline bases are fixed, and soluble in water; the solutions of the neutral alkaline chromates are yellow, those of the alkaline bichromates are red. These tints are still visible in highly dilute solutions. The yellow color of the solution of a neutral salt changes to red on the addition of a mineral acid, owing to the formation of an acid chromate.

3. Hydrosulphuric acid reduces chromic acid readily when present in the solution in the free state, more difficultly when present in form of a chromate; sesquioxide of chromium, water, and sulphuric acid are formed in this process, and sulphur precipitates. Heat promotes this decomposition. If no free acid is present, a greenish-gray precipitate is produced, consisting of a mixture of hydrated sesquioxide of chromium and sulphur. But if free acid is present, the precipitate is far less considerable, and consists of pure sulphur. In the latter case, the salt of sesquioxide of chromium formed imparts a green tint to the fluid, which may lead to the erroneous impression that the precipitate itself is green.

4. Chromic acid may also be reduced to sesquioxide of chromium by means of many other substances, and more particularly by sulphurous acid, or by heating with hydrochloric acid, especially upon the addition of alcohol (in which case chloride of ethyle and aldehyde are evolved); also by metallic zinc, or by heating with tartaric acid, oxalic acid, &c. All these reactions are clearly characterized by the change of the red or yellow color of the solution to the green tint of the salt of sesquioxide of chromium.

5. Chloride of barium produces a yellowish-white precipitate of CHROMATE OF BARYTA (Ba O, Cr O), which is soluble in dilute hydrochloric acid and nitric acid.

6. Nitrate of silver produces a dark purple-red precipitate of CHROMATE OF SILVER (Ag O, Cr O1), which is soluble in nitric acid and in ammonia; in acid solutions it produces a precipitate of BICHROMATE OF SILVER (Ag O, 2 Cr 01).

7. Acetate of lead produces a yellow precipitate of CHROMATE OF LEAD (Pb O, Cr 01), which is soluble in potassa, but only difficultly soluble in

dilute nitric acid. Upon heating with alkalies, the yellow neutral salt is converted into basic red chromate of lead (2 Pb O, Cr 0 ̧).

8. If insoluble chromates are fused together with carbonate of soda and nitrate of soda, and the fused mass is treated with water, the fluid produced appears YELLOW from the alkaline chromate which it holds in solution; upon the addition of an acid the yellow color changes to red. The oxides remain behind either in the pure state or as carbonates, if they are not soluble in the caustic soda formed from the nitrate.

§ 139.

Remarks. When testing for bases, we always find the chromic acid as sesquioxide of chromium, since hydrosulphuric acid reduces it to that state. The characteristic color of the solution frequently renders the application of any further test unnecessary. The reactions with salts of silver and salts of lead afford positive and confirmatory proof of the presence of chromic acid in aqueous solutions. Acid solutions are boiled with carbonate of soda in excess, and filtered; the filtrate is acidified with acetic acid, and acetate of lead then added to it. If there is reason to suppose that chromic acid is present in a solution, metallic oxides being also present, it is preferable to effect the reduction of the chromic acid by means of hydrochloric acid and alcohol, or by sulphurous acid, instead of reducing it by hydrosulphuric acid.

§ 140.

Supplement to the First Division of the First Group.

a. Selenious acid (Se O2). Sublimed anhydrous selenious acid appears in form of white four-sided needles, its hydrate in form of crystals resembling those of nitrate of potassa. Both the acid and its hydrate dissolve readily in water to a strongly acid fluid. Of the neutral salts only those with the alkalies are soluble in water. All selenites dissolve readily in nitric acid, with the exception of the selenites of lead and silver, which dissolve with difficulty. Hydrochloric acid, even when boiling, fails to decompose selenites; sulphuric acid decomposes them readily with the aid of heat. Hydrosulphuric acid produces in solutions of selenious acid or of selenites (in presence of free hydrochloric acid) a yellow precipitate of SULPHIDE OF SELENIUM, which, upon heating, turns reddish-yellow, and is soluble in sulphide of ammonium. Chloride of barium produces (after neutralization of the free acid, should any be present) a white precipitate of selenite of baryta, which is soluble in hydrochloric acid and in nitric acid. Protochloride of tin produces a red precipitate of SELENIUM, which turns gray at a high temperature; sulphurous acid produces the same precipitate. When exposed on a charcoal support to the reducing flame, the selenites evolve selenium, exhaling at the same time a characteristic odor of decaying horseradish.

b. Sulphurous acid (SO) is a colorless, uninflammable gas, which exhales the stifling odor of burning sulphur. It dissolves copiously in water. The solution has the odor of the gas, reddens litmus paper, and bleaches Brazil-wood paper. It absorbs oxygen from the air, and is

thereby converted into sulphuric acid. The salts of sulphurous acid are colorless. Of the neutral sulphites those with alkaline base only are readily soluble in water; many of the sulphites insoluble or difficultly soluble in water dissolve in an aqueous solution of sulphurous acid, but fall down again upon boiling. All the sulphites evolve sulphurous acid when treated with sulphuric acid or hydrochloric acid. Chlorine water dissolves most sulphites to sulphates. Chloride of barium precipitates neutral sulphites, but not free sulphurous acid. The precipitate dissolves in hydrochloric acid. Hydrosulphuric acid decomposes free sulphurous acid, water being formed and free sulphur eliminated, which latter separates from the fluid. If a trace of sulphurous acid or of a sulphite is introduced into a flask in which hydrogen is being evolved from zinc and hydrochloric acid, hydrosulphuric acid is immediately evolved along with the hydrogen, and the gas now produces a black coloration or a black precipitate in a solution of acetate of lead to which has been added a sufficient quantity of solution of soda to redissolve the precipitate which forms at first. Sulphurous acid is a powerful reducing agent; it reduces chromic acid, permanganic acid, chloride of mercury (to subchloride), decolorizes iodide of starch, produces a blue precipitate in a mixture of ferricyanide of potassium and sesquichloride of iron, &c. With a hydrochloric acid solution of protochloride of tin, a brown precipitate of PROTOSULPHIDE OF TIN is formed after some time.

c. Hyposulphurous acid (S, O2). This acid does not exist in the free state. Its salts are generally soluble in water. The solutions of most hyposulphites may be boiled without suffering decomposition; hyposulphite of lime is resolved upon boiling into sulphite of lime and sulphur. If hydrochloric acid or sulphuric acid is added to the solution of a hyposulphite, the fluid remains at first clear and inodorous, but after a short time-the shorter the more concentrated the solution-it becomes more and more turbid, owing to the separation of sulphur, and exhales the odor of sulphurous acid. Application of heat promotes this decomposition. Nitrate of silver produces a white precipitate of HYPOSULPHITE OF SILVER, which is soluble in an excess of the hyposulphite; after a little while (upon heating almost immediately) the precipitate turns black, being decomposed into sulphide of silver and sulphuric acid. Hyposulphite of soda dissolves chloride of silver; upon the addition of an acid the solution remains clear at first, but shortly, and upon boiling, immediately, sulphide of silver separates. Chloride of barium produces a white precipitate, which is soluble in much water, more especially hot water, and is decomposed by hydrochloric acid.

d. Iodic acid (IO). Iodic acid crystallizes in white, six-sided tables ; at a moderate heat it is resolved into iodine vapor and oxygen; it is readily soluble in water. The salts are decomposed upon ignition, being resolved either into oxygen and a metallic iodide, or into iodine, oxygen, and metallic oxide; the iodates with an alkaline base alone dissolve readily in water. Chloride of barium throws down from solution of iodates of the alkalies a white precipitate of IODATE OF BARYTA, which is soluble in nitric acid; nitrate of silver, a white, granular-crystalline precipitate of IODATE OF SILVER, which dissolves readily in ammonia, but only sparingly in nitric acid. Hydrosulphuric acid throws down from solutions of iodic acid and iodates IODINE, which then dissolves in hydriodic acid; if an excess of hydrosulphuric acid is added, the fluid loses its color, and sulphur separates, the iodine being converted into hydriodic acid. Sul

phurous acid throws down IODINE, which upon addition of an excess of the acid is converted into hydriodic acid.

Second Division of the First Group of the Inorganic Acids..

§ 141.

SULPHURIC ACID (S 0 ̧).

1. Anhydrous sulphuric acid is a white, feathery-crystalline mass, which emits strong fumes upon exposure to the air; hydrated sulphuric acid forms an oily liquid, colorless and transparent like water.

Both the

anhydrous and hydrated acid char organic substances, and combine with water in all proportions, the process of combination being attended with considerable elevation of temperature, and in the case of the anhydrous acid, with a hissing noise.

2. Most of the sulphates are soluble in water; the insoluble sulphates are generally white; the soluble sulphates, in the crystalline state, are for the most part colorless. The sulphates of the alkalies and alkaline earths are not decomposed upon ignition.

3. Chloride of barium produces even in exceedingly dilute solutions of sulphuric acid and of the sulphates a finely-pulverulent, heavy, white precipitate of SULPHATE OF BARYTA (Ba O, SO), which is insoluble in hydrochloric acid and in nitric acid. From very dilute solutions the precipitate separates only after standing some time. In presence of the citrate of an alkali, the precipitate forms only after addition of hydrochloric acid (Spiller).

4. Acetate of lead produces a heavy, white precipitate of SULPHATE OF LEAD (Pb O, SO,) which is sparingly soluble in dilute nitric acid, but dissolves completely in hot concentrated hydrochloric acid.

5. The salts of sulphuric acid with the alkaline earths which are insoluble in water and acids are converted into CARBONATES, by fusion with alkaline carbonates. But the sulphate of lead is reduced to the state of PURE OXIDE when treated in this manner. Both the conversion of the former into carbonates, and the reduction of the latter to the state of oxide, are attended with the formation of an alkaline sulphate. The sulphates of the alkaline earths and sulphate of lead, are also resolved into insoluble carbonates and soluble alkaline sulphate when boiled with concentrated solutions of carbonates of the alkalies.

6. Upon fusing sulphates with carbonate of soda on charcoal in the inner flame of the blowpipe, the sulphuric acid is reduced, and sulphide of sodium formed, which may be readily recognised by the odor of hydrosulphuric acid emitted upon moistening the sample and the part of the charcoal into which the fused mass has penetrated, and adding some acid. If the fused mass is transferred to a clean silver plate, or a polished silver coin, and then moistened with water and some acid, a black stain of sulphide of silver is immediately formed.

Remarks.-The characteristic and exceedingly delicate reaction of sulphuric acid with salts of baryta renders the detection of this acid an easier task than that of almost any other. It is simply necessary to take care not to confound with sulphate of baryta, precipitates of chloride of barium, and particularly of nitrate of baryta, which are formed upon

mixing aqueous solutions of these salts with fluids containing a large proportion of free hydrochloric acid or free nitric acid. It is very easy to distinguish these precipitates from sulphate of baryta, since they redissolve immediately upon diluting the acid fluid with water. To detect free sulphuric acid in presence of a sulphate, the fluid under examination is mixed with a very little cane-sugar, and the mixture evaporated to dryness in a porcelain dish at 212° Fah. If free sulphuric acid was present, a black residue remains, or, in the case of most minute quantities, a blackish-green residue. Other free acids do not decompose cane-sugar in this way (Runge).

§ 142.

Supplement to the Second Division of the First Group.

HYDROFLUOSILICIC ACID (HF, Si F).

Hydrofluosilicic acid is a very acid fluid; when evaporated on platinum, it volatilizes completely as fluoride of silicon and hydrofluoric acid. When evaporated on glass, it etches the latter. With bases it forms water and silico-fluorides of the metals, which are most of them soluble in water, redden litmus paper, and are resolved upon ignition into metallic fluorides and fluoride of silicon.

Chloride of barium forms a crystalline precipitate with hydrofluosilicic acid (§ 94, 6). Chloride of strontium and acetate of lead form no precipitates with this acid; salts of potassa precipitate transparent GELATINOUS SILICO-FLUORIDE OF POTASSIUM; ammonia in excess precipitates HYDRATED SILICIC ACID, with formation of fluoride of ammonium. When metallic silico-fluorides are heated with concentrated sulphuric acid in a platinum vessel covered with glass, the glass is ETCHED (§ 147, 5).

Third Division of the First Group of the Inorganic Acids.

§ 143.

a. PHOSPHORIC ACID (PO).

1. Phosphorus is a colorless, transparent, solid body, of 2.089 specific gravity; it has a waxy appearance. Taken internally, it acts as a virulent poison. It fuses at 113°, and boils at 554° Fah. By the influence of light, phosphorus kept under water turns first yellow, then red, and is finally covered with a white crust. If phosphorus is exposed to the air, at the common temperature, it exhales a garlic odor, shines in the dark, and emits fumes of phosphorous acid, into which substance it is gradually entirely converted. Phosphorus very readily takes fire spontaneously, and burns with a luminous flame, being converted into phosphoric acid, which is dissipated for the most part in white fumes through the surrounding air. Nitric acid and nitrohydrochloric acid dissolve phosphorus pretty readily upon heating. The solutions contain at first, besides phosphoric acid, also phosphorous acid. Hydrochloric acid does not dissolve phosphorus; if phosphorus is boiled with solution of soda or potassa, or with milk of lime, hypophosphites and phosphates are formed, whilst spontaneously inflammable phosphuretted hydrogen gas escapes. If a trace of phosphorus is put into a hydrogen gas-evolution apparatus supplied with pure zinc and pure dilute sulphuric acid, phosphide of hydrogen is evolved with the hydrogen, and the gas burns now with