11. The compounds of chromic acid show the same reactions with phosphate of soda and ammonia and with borax in the blowpipe flame, as the compounds of sesquioxide of chromium. 12. Very minute quantities of chromic acid may be detected by one of the following methods: a. mix with the fluid, slightly acidified with sulphuric acid, a little tincture of guaiacum (1 part of the resin to 100 parts of alcohol of 60 per cent.) when an intense blue coloration of the fluid will at once make its appearance, speedily vanishing again, however, where mere traces of chromic acid are present (H. SCHIFF); b. mix the solution of the alkaline chromate, which must be as neutral as possible with some dilute decoction of logwood, when a very intense black coloration will be produced; in the presence of exceedingly small quantities of chromic acid the color is violet-red (R. WILDENSTEIN). Chromic acid being reduced by hydrosulphuric acid to sesquioxide. of chromium, this acid is in the course of analysis always found in the examination for bases. The intense color of the solutions containing chromic acid, the excellent reaction with peroxide of hydrogen, and the characteristic precipitates produced by solutions of salts of lead and salts of silver, afford moreover ready means for its detection. the discovery of traces of chromium present in many minerals, for instance in serpentine, the reactions in 12 may be used after the mineral has been fused with alkaline carbonate and nitrate. Rarer Acids of the First Division. a. SULPHUROUS ACID (SO). For SULPHUROUS ACID 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 are colorless. Of the neutral sulphites, those with alkaline base only are readily soluble in water; many of the sulphites insoluble or sparingly soluble in water dissolve in an aqueous solution of the sulphurous acid, but fall down again on boiling. All the sulphites evolve sulphurous acid when treated with sulphuric 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 the free sulphurous acid, water and pentathionic acid being formed and free sulphur eliminated. If to a solution of sulphurous acid mixed with an equal volume of hydrochloric acid, a piece of clean copper wire is added, and the mixture is boiled, the copper appears black, as if covered with soot, if much sulphurous acid is present; but only dull if a little is present (H. REINSCH). If a trace of sulphurous acid or of a sulphite is introduced into a flask in which hydrogen is being evolved from zinc or aluminium 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 Sulsufficient quantity of soda to redissolve the precipitate which forms at first. phurous 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 yellow precipitate of BISULPHIDE OF TIN is formed after some time. If an aqueous solution of an alkaline sulphite is mixed with acetic acid just to give it an incipient acid reaction, and is then added to a relatively large amount of solution of sulphate of zinc, mixed with a very small quantity of nitroprusside of sodium, the fluid acquires a red color if the quantity of the sulphite present is not too inconsiderable, but when the quantity of the sulphite is very minute the coloration makes its appearance only after addition of some solution of ferrocyanide of potassium. If the quantities are not altogether too minute, a purple-red precipitate will form upon the addition of the ferrocyanide of potassium (BÖDEKER). Hyposulphites of the alkalies do not show this reaction. b. HYPOSULPHUROUS ACID (S2O2). This acid does not exist in the free state. Most of its salts are 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) this 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 after some time, and immediately upon boiling, 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. Ferric chloride colors the solutions of alkaline hyposulphites reddish-violet (here they differ from alkaline sulphites); on standing the liquid loses its color, especially when heated, protochloride of iron being formed. Acidified solution of chromic acid is immediately reduced by hyposulphites, iodide of starch is at once decolorized. With zinc or aluminium and hydrochloric acid the hyposulphites behave like the sulphites. Where it is required to find sulphites and hyposulphites of the alkalies in presence of alkaline sulphides, as is often the case, solution of sulphate of zinc is first added to the fluid until the sulphide is decomposed; the sulphide of zinc is then filtered off, and one part of the filtrate is tested for hyposulphurous acid by addition of acid, another portion for sulphurous acid with nitroprusside of sodium, &c. c. IODIO ACID (I 0 ̧). 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 IODINE, which then dissolves in hydriodic acid; the precipitation is attended with separation of sulphur. If an excess of hydrosulphuric acid is added, the fluid loses its color, and a further separation of sulphur takes place, the iodine being converted into hydriodic acid. Iodic acid combined with bases is also decomposed by hydrosulphuric acid. Sulphurous acid throws down JODINE, which upon addition of an excess of the acid is converted into hydriodic acid. Second Division of the First Group of the Inorganic Acids. SULPHURIC ACID (S O1). § 140. 1. ANHYDROUS SULPHURIC ACID is a white feathery-crystalline mass which emits strong fumes upon exposure to the air; HYDRATED SULPHURIC ACID (which contains rather more water than corresponds to the formula HO,SO,) 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. The neutral SULPHATES are readily soluble in water with the exception of the sulphates of baryta, strontia, lime and lead. The basic sulphates of the oxides of the heavy metals which are insoluble in water dissolve in hydrochloric acid or in nitric acid. Most of the sulphates are colorless or white. The sulphates of the alkalies are not decomposed by ignition. The other sulphates are acted upon in different ways by a red heat, some of them being readily decomposed, others with difficulty, and some resisting decomposition altogether. 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, SO1), insoluble in dilute hydrochloric and nitric acids. From very dilute solutions the precipitate separates only after standing for some time. Concentrated acids and concentrated solutions of many salts impair the delicacy of the reaction. 4. Acetate of lead produces a heavy white precipitate of SULPHATE OF LEAD (PbO, 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 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 by digestion or boiling with concentrated solutions of carbonates of the alkalies (comp. §§ 95, 96, 97). 6. Upon fusing sulphates with carbonate of soda on charcoal in the inner flame of the blowpipe, or heating them in the stick of charcoal (p. 24) in the lower reducing flame, 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. (Compounds of tellurium and selenium give the same reaction.) 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. It is a rule that should never be departed from, in testing for sulphuric acid with chloride of barium, to dilute the fluid largely; a little hydrochloric acid should also be added, which counteracts the adverse influence of many salts, as, for instance, citrates of the alkalies. Where very minute quantities of sulphuric acid are to be detected the fluid should be allowed to stand several hours at a gentle heat; the trace of sulphate of baryta formed will in that case be found deposited at the bottom of the vessel. When the least uncertainty exists about the nature of the precipitate produced by chloride of barium in presence of hydrochloric acid, the reaction in 6 will at once set all doubt at rest. In looking for very small quantities of sulphuric acid in the presence of much hydrochloric or nitric acid, the greater part of the latter should first be evaporated off or neutralized. To detect free sulphuric acid in presence of a sulphate the fluid is mixed with a very little cane-sugar, and evaporated to dryness in a porcelain dish at 100°. 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. § 141. HYDROFLUOSILICIO ACID (H F, Si F2). Hydrofluosilicic acid is a very acid fluid; upon evaporation on platinum it volatilizes completely as fluoride of silicon and hydrofluoric acid. When evaporated in 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 (§ 95, 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. metallic silico-fluorides with concentrated sulphuric acid dense fumes are emitted in By heating the air, arising from the evolution of hydrofluoric and silicofluoric gas. ment is conducted in a platinum vessel covered with glass the fumes ETCH the glass If the experi(§ 146, 5); the residue contains the sulphates formed. Third Division of the First Group of the Inorganic Acids. § 142. a. PHOSPHORIC ACID (PO). 1. PHOSPHORUS is a colorless, transparent, solid body, of 1-84 specific gravity; it has a waxy appearance. Taken internally it acts as a virulent poison. It fuses at 44·3°, and boils at 290°. 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 highly characteristic and most disagreeable odor, copious fumes being evolved which are luminous in the dark. These fumes are formed by oxidation of the vapor of phosphorus, and consist of phosphoric and phosphorous acids. When the air is moist, ozone, peroxide of hydrogen and nitrite of ammonia are produced at the same time. Phosphorus very readily takes fire, burning with a luminous flame to phosphoric acid, which appears in the form of white fumes. By the protracted influence of light, or by heating to 250°, phosphorus is converted into a red variety, the so-called amorphous phosphorus. In this state it does not alter in the air, it is not luminous, its inflammability is much decreased, and it has a specific gravity of 21. Nitric acid and nitrohydrochloric acid dissolve phosphorus pretty readily upon heating. The solutions contain at first, besides phosphoric acid, also phosphorous acid. Hydro chloric 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 substance containing unoxidized phosphorus is placed at the bottom of a flask, and a slip of paper moistened with solution of nitrate of silver is by means of a cork loosely inserted into the mouth suspended inside the flask, and a gentle heat applied (from 30° to 40°), the paper slip will turn black in consequence of the reducing action of the phosphorous fumes, even though only a most minute quantity of phosphorus should be present. If after the termination of the reaction the blackened part of the paper is boiled with water, the undecomposed portion of the silver salt precipitated with hydrochloric acid, the fluid filtered, and the filtrate evaporated as far as practicable on the water-bath, the presence of phosphoric acid in the residue may be shown by means of the reactions described below. (J. SCHERER.) It must be borne in mind that the silver salt is blackened also by hydrosulphuric acid, formic acid, volatile products of putrefaction, &c.; and also that the detection of phosphoric acid in the slip of paper can be of value only where the latter and the filtering paper were perfectly free from phosphorus. As regards the deportment of phosphorus upon boiling with dilute sulphuric acid, and in a hydrogen evolution apparatus supplied with zinc and dilute sulphuric acid, see § 227. 2. Anhydrous PHOSPHORIC ACID is a white, snowlike mass, which rapidly deliquesces in the air. When treated with water it hisses like a red-hot iron, and is at first only partially dissolved, in time, however, the solution is complete. It forms with water and bases three series of compounds, viz., with three equivalents of water or base hydrate of tribasic phosphoric acid or common phosphates; with two equivalents of water or base hydrate of pyrophosphoric acid or pyrophosphates; with one equivalent of water or base hydrate of metaphosphoric acid or metaphosphates. As the meta- and pyrophosphoric acids are of comparatively rare occurrence they will be treated less fully in a supplemental paragraph. 3. The HYDRATE of TRIBASIC PHOSPHORIC ACID (3 HO, PO1) forms colorless and pellucid crystals, which deliquesce rapidly in the air to a syrupy non-caustic liquid. The action of heat changes it into hydrated meta- or pyrophosphoric acid, according as either one or two equivalents of water are expelled. Heated in an open platinum dish the hydrate of common phosphoric acid, if pure, volatilizes completely, though with difficulty, in white fumes. 4. The action of heat fails to decompose the TRIBASIC PHOSPHATES with fixed bases, but converts them into pyrophosphates if they contain one equivalent of basic water or ammonia, and into metaphosphates if they contain two equivalents. Of the tribasic phosphates those with alkaline base alone are soluble in water, in the neutral state. The solutions manifest alkaline reaction. If pyro- or metaphosphates are fused with carbonate of soda the fused mass contains the phosphoric acid invariably in the tribasic state. 5. Chloride of barium produces in aqueous solutions of the neutral or basic phosphates of the alkalies, but not in solutions of the hydrate, a white precipitate of PHOSPHATE OF BARYTA (2 Ba O, HO PO; or |