Special Reactions.

$100.

a. ALUMINA (AI, 0.).

1. Alumina is non-volatile and colorless; the hydrate is also colorless. Alumina dissolves in acids (particularly when dilute) slowly and with very great difficulty; in fusing bisulphate of potassa it dissolves readily to a mass soluble in water. The hydrate in an amorphous state, and recently precipitated, is readily soluble in acids; but after being left some time in the fluid from which it was precipitated, its solubility decreases, and in the crystallized state it dissolves with very great difficulty in acids. After previous ignition with alkalies, which leads to the formation of aluminates of the alkalies, alumina is readily dissolved by acids.

2. The salts of alumina are colorless, and most of them are nonvolatile some of them are soluble, others insoluble. The soluble salts have a sweetish, astringent taste, redden litmus-paper, and lose their acids upon ignition. The insoluble salts are dissolved by hydrochloric acid, with the exception of certain native compounds of alumina ; the compounds of alumina which are insoluble in hydrochloric acid are decomposed and made soluble by ignition with carbonate of soda and potassa, or bisulphate of potassa.

3. Potassa and soda throw down from solutions of alumina a bulky precipitate of HYDRATE OF ALUMINA (Al, O,, 3 H O), which contains alkali, and generally also an admixture of basic salt; this precipitate redissolves readily and completely in an excess of the precipitant, but from this solution it is reprecipitated by addition of chloride of ammonium, even in the cold, but more completely upon application of heat (compare § 51). The presence of salts of ammonia does not prevent the precipitation by potassa or soda.

4. Ammonia also produces in solutions of alumina a precipitate of HYDRATE OF ALUMINA, containing ammonia and an admixture of basic salt; this precipitate also redissolves in a very considerable excess of the precipitant, but with difficulty only, which is the greater the larger the quantity of salts of ammonia contained in the solution. It is this deportment which accounts for the complete precipitation of hydrate of alumina from solution in potassa, by an excess of chloride of ammonium.

5. If the solution of a salt of alumina is digested with finely pulverized carbonate of baryta, the greater part of the acid of the alumina salt combines with the baryta, the liberated carbonic acid escapes, and the alumina precipitates completely as HYDRATE mixed with BASIC SALT OF ALUMINA; even digestion in the cold suffices to produce this reaction.

6. If alumina or one of its compounds is ignited upon charcoal before the blowpipe, and afterwards moistened with a solution of nitrate of protoxide of cobalt, and then again strongly ignited, an unfused mass of a deep SKY-BLUE color is produced, which consists of a compound of the two oxides. The blue color becomes distinct only upon cooling. By candlelight it appears violet. This reaction is decisive only in the case of infusible or difficultly fusible compounds of alumina pretty free from other oxides, as solution of cobalt imparts a blue tint to readily fusible salts, even though no alumina be present.

§ 101.

b. SESQUIOXIDE OF CHROMIUM (Cr, O2).

1. Sesquioxide of chromium is a green, its hydrate a bluish gray-green powder. The hydrate dissolves readily in acids; the non-ignited sesquioxide dissolves more difficultly, and the ignited sesquioxide is almost altogether insoluble.

2. The salts of sesquioxide of chromium have a green or violet color. Many of them are soluble in water. Most of them dissolve in hydrochloric acid. The solutions usually exhibit a fine green color. Many double salts of sesquioxide of chromium, e.g., sulphate of sesquioxide of chromium and potassa (chrome alum), have a deep violet color, and give with cold water bluish-violet solutions which, however, acquire a green tint when even moderately heated. The salts of sesquioxide of chromium with volatile acids are decomposed upon ignition, the acids being expelled. The salts of sesquioxide of chromium which are soluble in water redden litmus.

3. Potassa and soda produce in solutions of salts of sesquioxide of chromium a bluish-green precipitate of HYDRATE OF SESQUIOXIDE OF CHROMIUM, which dissolves readily and completely in an excess of the precipitant, imparting to the fluid generally an emerald-green tint, but in cases where the solution originally precipitated was bluish-violet, a violet color. Upon long-continued ebullition of this solution, the whole of the hydrated sesquioxide separates again, and the supernatant fluid appears perfectly colorless. The same reprecipitation takes place if chloride of ammonium is added to the alkaline solution, and the mixture heated. If the alkaline solution of sesquioxide of chromium is gently heated with binoxide of lead, the sesquioxide of chromium is converted into chromic acid, and the alkaline fluid now holds chromate of lead in solution; upon supersaturation with acetic acid, chromate of lead separates in form of a yellow precipitate.

4. Ammonia also produces in solutions of salts of sesquioxide of chromium a precipitate of HYDRATE OF SESQUIOXIDE OF CHROMIUM: this precipitate contains more or less water, according as the ammonia has been added gradually or suddenly, and appears, therefore, sometimes grayish-green, sometimes grayish-blue. A small portion of it redissolves in an excess of the precipitant in the cold, imparting to the fluid a peach-blossom red tint; but if, after the addition of ammonia in excess, heat is applied to the mixture, the precipitation is complete.

5. Carbonate of baryta precipitates from solutions of sesquioxide of chromium the whole of the sesquioxide as a GREENISH HYDRATE mixed with BASIC SALT. The precipitation takes place in the cold, but is complete only after long-continued digestion.

6. The fusion of sesquioxide of chromium or of any of its compounds with nitrate of soda and some carbonate of soda gives rise to the formation of yellow CHROMATE OF SODA, part of the oxygen of the nitric acid separating from the nitrate of soda, and converting the sesquioxide of chromium into chromic acid, which then combines with the soda. For the reactions of chromic acid, see § 138.

7. Phosphate of soda and ammonia dissolves sesquioxide of chromium and its salts, both in the oxidizing and reducing flame of the blowpipe, to clear beads of a faint YELLOWISH-GREEN tint, which upon cooling

changes to EMERALD-GREEN. The sesquioxide of chromium and its salts show a similar deportment with biborate of soda.

§ 102.

Recapitulation and remarks.-The solubility of hydrate of alumina in solutions of potassa and soda, and its reprecipitation from the alkaline solutions by chloride of ammonium, afford a safe means of detecting alumina only in the absence of sesquioxide of chromium. If the latter is present therefore, which is seen either from the color of the solution, or by the reaction with phosphate of soda and ammonia, it must be removed before we can proceed to test for alumina. The separation of sesquioxide of chromium from alumina is effected the most completely by fusing 1 part of the mixed oxides together with 2 parts of carbonate and 2 parts of nitrate of soda, which may be done in a platinum crucible. The yellow mass obtained is boiled with water; by this process the whole of the chromium is dissolved as chromate of soda, and part of the alumina as aluminate of soda, the rest of the alumina remaining undissolved. If the solution is acidified with nitric acid, it acquires a reddish tint; if ammonia is then added to feebly alkaline reaction, the dissolved portion of the alumina separates.

The precipitation of sesquioxide of chromium, effected by boiling its solution in solution of potassa or soda, is also sufficiently reliable if the ebullition is continued a sufficient length of time; still it is often liable to mislead in cases where only little sesquioxide of chromium is present, or where the solution contains organic matter, even though in small proportion only. The deportment of a solution of sesquioxide of chromium with solution of potassa or soda is completely changed by the presence of certain other metallic oxides, especially oxide of zinc, oxide of lead, sesquioxide of iron; in presence of these oxides, and according to the greater or less relative proportion in which they happen to be present, sesquioxide of chromium does not dissolve, or dissolves only incompletely in an excess of solution of potassa. This circumstance should never be lost sight of in the analysis of compounds containing sesquioxide of chromium. Lastly, it must be borne in mind, also, that alkalies produce no precipitates in the solutions of alumina if non-volatile organic substances are present, such as sugar, tartaric acid, &c. ; whilst the precipitation of sesquioxide of chromium is less seriously interfered with by the presence of such bodies.

§ 103.

SUPPLEMENT TO THE THIRD GROUP.

TITANIC ACID (Ti O2).

From among the other oxides enumerated in § 86 as belonging to the third group, we have selected titanic acid as deserving of special notice, since this substance is met with more frequently than the others in analytical investigations, more especially also in the analysis of the slag adhering to the bottom of blast furnaces in which iron ore is reduced, which slag is often found to contain small copper-coloured cubes, consisting of a combination of cyanide of titanium with nitride of titanium (Wöhler).

Titanium burns in the air with considerable brilliancy to titanic acid

(Ti 02). Besides the acid, another oxide of titanium is known, the sesquioxide (Ti, O). According to the different mode of its preparation, titanic acid appears sometimes as a white powder, which, when heated, transiently acquires a yellow tint, sometimes in the form of small lumps of a reddish-brown color. It is infusible, and insoluble in water and acid, with the exception of concentrated sulphuric acid. With bisulphate of potassa it fuses into a mass, which dissolves in a large proportion of cold water to a clear fluid. Fusion of titanic acid with carbonate of soda gives rise to the formation of titanate of soda, which is by addition of water converted into soda and bititanate of soda; the latter salt is insoluble in water, but soluble in hydrochloric acid. The hydrate of titanic acid is white; it dissolves, both in the moist state and when dried without the aid of heat, in dilute acids, especially hydrochloric and sulphuric acids. Solutions of titanic acid in hydrochloric or sulphuric acid, but more especially the latter, when subjected in a highly dilute state to long-continued ebullition, deposit titanic acid as a white powder, insoluble in dilute acids. The precipitate thus formed in hydrochloric acid cannot be separated by filtration, unless an acid or chloride of ammonium is added, as it will pass through the filter with the washing water. Solution of potassa throws down from solutions of titanic acid in hydrochloric or sulphuric acid, hydrate of titanic acid in form of a bulky, white precipitate, which is insoluble in an excess of the precipitant; ammonia, sulphide of ammonium, carbonates of the alkalies, and carbonate of baryta act in the same way. The precipitate, if thrown down without application of heat, and washed with cold water, is soluble in hydrochloric acid and dilute sulphuric acid; presence of tartaric acid prevents its formation. Ferrocyanide of potassium produces in acid solutions of titanic acid a dark brown precipitate; infusion of galls a brownish precipitate, which speedily turns orangered. Metallic zinc produces, in consequence of the ensuing reduction of titanic acid to sesquioxide of titanium, at first a blue coloration of the solution, afterwards a blue precipitate of hydrate of sesquioxide of titanium. Phosphate of soda and ammonia readily dissolves titanic acid in the outer flame of the blowpipe to a clear bead of a yellowish color whilst hot, but colorless when cold. If this bead is exposed to the reducing flame, it acquires a yellow tint, which turns to red when the bead is half cold, and to violet when quite cold.

§ 104.

FOURTH GROUP.

OXIDE OF ZINC, PROTOXIDE OF MANGANESE, PROTOXIDE OF NICKEL, PROTOXIDE OF COBALT, PROTOXIDE OF IRON, SESQUIOXIDE OF IRON.

Properties of the group.-The solutions of the oxides of the fourth group, when containing a stronger free acid, are not precipitated by hydrosulphuric acid; neutral solutions also are not, or only very incompletely, precipitated by that reagent; but alkaline solutions are completely precipitated by hydrosulphuric acid; and other solutions also if a sulphide of an alkali metal is used as the precipitant, instead of hydrosulphuric acid. The precipitated metallic sulphides corresponding

to the respective oxides are insoluble in water; some of them are readily soluble in dilute acids; others (sulphide of nickel and sulphide of cobalt) dissolve only with very great difficulty in these menstrua. Some of them are insoluble in sulphides of the alkali metals; others are sparingly soluble in them, under certain circumstances.

Special Reactions.

§ 105.

a. OXIDE OF ZINC (Zn O).

1. Metallic zinc is bluish-white and very bright; when exposed to the air, a thin coating of basic carbonate of zinc forms on its surface. It is of medium hardness, ductile at a temperature of between 212° and 302° Fah., and under ordinary circumstances more or less brittle; it fuses readily on charcoal before the blowpipe, boils afterwards, and burns with a bluish-green flame, giving off white fumes, and coating the charcoal support with oxide. Zinc dissolves in hydrochloric and sulphuric acids, with evolution of hydrogen gas; in dilute nitric acid, with evolution of nitrous oxide; in more concentrated nitric acid, with evolution of nitric oxide.

2. The oxide of zinc and its hydrate are white powders, which dissolve readily in hydrochloric, nitric, and sulphuric acids. The oxide of zinc acquires a lemon-yellow tint when heated, but it reassumes its original white color upon cooling. When ignited before the blowpipe, it shines with considerable brilliancy.

3. The compounds of oxide of zinc are colorless; part of them are soluble in water, the rest in acids. The neutral salts of zinc which are soluble in water redden litmus-paper, and are readily decomposed by heat, with the exception of sulphate of zinc, which can bear a dull red heat, without undergoing decomposition. Chloride of zinc is volatile at a red heat.

4. Hydrosulphuric acid precipitates from neutral solutions of salts of zinc a portion of the metal as white hydrated SULPHIDE of ZINC (Zn S). In acid solutions this reagent fails altogether to produce a precipitate if the free acid present is one of the stronger acids; but from a solution of oxide of zinc in acetic acid, it throws down the whole of the zinc, even if the acid is present in excess.

5. Sulphide of ammonium throws down from neutral, and hydrosulphuric acid from alkaline solutions of salts of zinc, the whole of the metal as hydrated SULPHIDE OF ZINC, in form of a white precipitate. This precipitate is not redissolved by an excess of sulphide of ammonium, nor by potassa or ammonia; but it dissolves readily in hydrochloric acid, nitric acid, and dilute sulphuric acid.

6. Potassa and soda throw down from solutions of salts of zinc HYDRATED OXIDE OF ZINC (Zn O, H O), in form of a white, gelatinous precipitate, which is readily and completely redissolved by an excess of the precipitant. Upon boiling these alkaline solutions they remain, if concentrated, unaltered; but from dilute solutions nearly the whole of the oxide of zinc separates as a white precipitate. Chloride of ammonium does not precipitate alkaline solutions of oxide of zinc. If a solution of oxide of zinc in solution of potassa or soda is mixed with a solution of sesquioxide of chromium also in solution of potassa or soda,