$128.

SIXTH GROUP.

Of common occurrence: TEROXIDE OF GOLD, BINOXIDE OF PLATI. NUM, PROTOXIDE OF TIN, BINOXIDE OF TIN, TEROXIDE OF ANTIMONY, ARSENIOUS ACID AND ARSENIC ACID.

Of rare occurrence: OXIDES OF IRIDIUM, MOLYBDENUM, TUNG STEN, TELLURIUM AND SELENIUM.

The higher oxides of the elements belonging to the sixth group all possess more or less marked acid characters; they are however noticed here because they cannot be conveniently considered apart from the lower oxides of the same elements, especially as they deport themselves similarly towards hydrosulphuric acid.

Properties of the group.-The sulphides corresponding to the oxides of the sixth group are insoluble in dilute acids. These combine with alkaline sulphides either directly, or by passing into a higher degree of sulphuration, forming soluble sulphur salts, in which they perform the part of the acid. Hydrosulphuric acid precipitates these oxides therefore, like those of the fifth group, completely from acidified solutions. The precipitated sulphides differ, however, from those of the fifth group in this, that they dissolve in sulphide of ammonium, sulphide of potassium, &c., and are reprecipitated from these solutions on the addition of acids. We divide the oxides of this group into two classes, and distinguish,

1. OXIDES WHOSE CORRESPONDING SULPHIDES ARE INSOLUBLE IN HYDROCHLORIC ACID AND IN NITRIC ACID, and are reduced to the metallic state upon fusion in conjunction with nitrate and carbonate of soda: viz., teroxide of gold and binoxide of platinum.

2. OXIDES WHOSE CORRESPONDING SULPHIIDES ARE SOLUBLE IN BOILING HYDROCHLORIC ACID OR NITRIC ACID, and are upon fusion with nitrate and carbonate of soda converted into oxides or acids, which combine with the soda: viz., Teroxide of antimony, protoxide and binoxide of tin, arsenious and arsenic acids.

FIRST DIVISION.

Special Reactions.
§ 129.

a. TEROXIDE OF GOLD (Au O3).

1. METALLIC GOLD has a reddish yellow color, and a high metallic lustre it is rather soft, exceedingly malleable, ductile, difficultly fusible: it does not oxidize upon ignition in the air, and is insoluble in hydrochloric, nitric, and sulphuric acids; but it dissolves in fluids containing or evolving chlorine, e. g. in nitro-hydrochloric acid. The solution contains terchloride of gold.

2. TEROXIDE OF GOLD is a blackish-brown, its hydrate a chesnutbrown powder. Both are reduced by light and heat, and dissolve readily in hydrochloric acid, but not in dilute oxygen acids. Concentrated nitric and sulphuric acids dissolve a little teroxide of gold; water reprecipitates it from these solutions. The protoxide of gold (Au O) in violet-black, on heating it is resolved into gold and oxygen.

3. SALTS OF GOLD with oxygen acids are nearly unknown. The haloid salts of gold are yellow, and their solutions continue to exhibit this color up to a high degree of dilution. The whole of them are readily decomposed upon ignition. Neutral solution of terchloride of gold reddens litmus-paper.

4. Hydrosulphuric acid precipitates from neutral and acid solutions of gold the whole of the metal in the cold, as brown black TERSULPHIDE OF GOLD (Au S), when hot, as PROTOSULPHIDE OF gold (Au S). These precipitates are insoluble in nitric or hydrochloric acid; but dissolve in aqua regia. They are insoluble in colorless sulphide of ammonium, but dissolve in the yellow alkaline sulphides, especially in yellow sulphides of potassium and sodium.

5. Sulphide of ammonium throws down TERSULphide of gold. This redissolves in an excess of the precipitant only if the latter contains an excess of sulphur.

6. Ammonia produces, but only in tolerably concentrated solutions of gold, reddish-yellow precipitates of AURATE OF AMMONIA (fulminating gold). The more acid the solution and the greater the excess of ammonia added, the more gold remains in solution.

7. Protochloride of tin containing an admixture of bichloride (procured by adding a little chlorine water to protochloride), produces even in extremely dilute solutions of gold, a purple-red precipitate (or coloration at least), which sometimes inclines rather to violet or to brownish-red. This precipitate, which has received the name of PURPLE OF CASSIUS, is insoluble in hydrochloric acid. It is assumed to be a hydrated compound of binoxide of tin and protoxide of gold with protoxide and binoxide of tin (Au O, Sn O2+ Sn O, Sn O + 4 H O).

8. Salts of protoxide of iron reduce the teroxide of gold, when added to its solutions, and precipitate metallic gold in form of a most minutely divided brown powder, which shows metallic lustre when pressed with the blade of a knife, or when rubbed. The fluid in which the precipitate is suspended appears of a blackishblue color by transmitted light.

9. If to a solution of gold, potassa or soda be added in excess, it remains clear; on addition of tannic acid to the alkaline liquid a deep black precipitate of protoxide of gold (Au O) is formed, which separates perfectly after a little time.

$130.

b. BINOXIDE OF PLATINUM (Pt O.).

1. METALLIC PLATINUM has a light steel-gray color; it is very lustrous, moderately hard, very malleable and ductile, difficultly fusible; it does not oxidize upon ignition in the air, and is insoluble in hydrochloric, nitric, and sulphuric acids. It dissolves in nitrohydrochloric acid, especially upon heating. The solution contains bichloride of platinum.

2. BINOXIDE OF PLATINUM is a blackish-brown, its hydrate a reddish-brown powder. Both are reduced by heat; they are both readily soluble in hydrochloric acid, and difficultly soluble in oxygen acids. HYDRATED PROGTOXIDE OF PLATINUM is black, and when heated, yields metallic platinum.

3. The SALTS OF BINOXIDE OF PLATINUM are decomposed at a red heat. They are yellow; bichloride of platinum is reddishbrown, its solution reddish-yellow, which tint it retains up to a high degree of dilution. The solution reddens litmus-paper. Exposure to a very low red heat converts bichloride of platinum to protochloride; application of a stronger red heat reduces it to the metallic state. Solution of bichloride of platinum, containing protochloride, has a dark-brown color.

4. Hydrosulphuric acid throws down from acid and neutral solutions (but not from alkaline solutions, or, at all events, not completely)-after the lapse of some time a blackish-brown precipitate of BISULPHIDE OF PLATINUM (Pt S2). If the solution is heated after the addition of the hydrosulphuric acid the precipitate forms immediately. It dissolves in a great excess of alkaline sulphides, more particularly of the higher degrees of sulphuration. Bisulphide of platinum is insoluble in hydrochloric acid and in nitric acid; but it dissolves in nitrohydrochloric acid.

5. Sulphide of ammonium produces the same precipitate; this redissolves slowly and with difficulty, but at last completely in a large excess of the precipitant, if the latter contains an excess of sulphur. Acids reprecipitate the bisulphide of platinum unaltered from the red-brown solution.

6. Chloride of potassium and chloride of ammonium (and also potassa and ammonia in presence of hydrochloric acid), produce in not too highly dilute solutions of salts of platinum, yellow crystalline precipitates of PLATINCHLORIDES OF POTASSIUM and AMMONIUM. From dilute solutions these precipitates are obtained by evaporating with chloride of potassium or chloride of ammonium on the water-bath to dryness, and treating the residue with dilute spirit of wine until the alkaline chlorides are dissolved. The precipitates are no more soluble in acids than in water, they dissolve, however, by heating with soda or potassa-lye. Upon ignition platin chloride

of ammonium leaves spongy platinum behind. Platinchloride of potassium leaves platinum and chloride of potassium, but its decomposition is only complete when it is gently heated in a stream of hydrogen gas, or ignited with oxalic acid.

7. Protochloride of tin imparts to solutions of salts of binoxide of platinum which contain much free hydrochloric acid an INTENSELY DARK BROWNISH-RED COLOR, Owing to a reduction of the binovide or bichloride of platinum to protoxide or simple chloride. But the reagent produces no precipitate in such solutions.

8. Sulphate of protoxide of iron does not precipitate solution of bichloride of platinum, unless the two are boiled together for a long time, when metallic platinum separates.

§ 131.

Recapitulation and remarks.-The reactions of gold and platinum enable us, at least partially, to detect these two metals in the presence of many other oxides, and also when platinum and gold are present in the same solution. In the latter case the solution is evaporated to dryness with chloride of ammonium, and the residue treated with spirit of wine, in order to obtain the gold in solution, and the platinum in the residue. The latter yields metallic platinum on ignition, while gold may be thrown down from the solution after evaporating off the alcohol, by means of protosulphate of iron.

SECOND DIVISION OF THE SIXTH GROUP.

Special Reactions.

§ 132.

a. PROTOXIDE OF TIN (Sn O).

1. TIN has a light gray-white color and a high metallic lustre ; it is soft and malleable; when bent, it produces a crackling sound. When heated in the air, it absorbs oxygen, and is converted into grayish-white binoxide; heated on charcoal before the blowpipe, it forms a white coating on the support. Concentrated hydrochloric acid dissolves tin to protochloride, with evolution of hydrogen gas; nitrohydrochloric acid dissolves it, according to circumstances, to bichloride or to a mixture of proto- and bichloride. Tin dissolves with difficulty in dilute sulphuric acid; concentrated sulphuric acid converts it, with the aid of heat, into sulphate of binoxide; moderately concentrated nitric acid oxidizes it readily, particularly with the aid of heat; the white binoxide (hydrated metastannic acid Sn O2, 2 H O) formed, does not redissolve in an excess of the acid.

2. PROTOXIDE OF TIN is a black or grayish-black powder; its hydrate is white.

Protoxide of tin is reduced by fusion with

cyanide of potassium. It is readily soluble in hydrochloric acid Nitric acid converts it into hydrated metastannic acid, which is insoluble in an excess of the acid.

3. The SALTS OF PROTOXIDE OF TIN are colorless, and are decomposed by heat. The soluble salts, in the neutral state, redden litmus-paper. The salts of protoxide of tin rapidly absorb oxygen from the air, and are partially or entirely converted into salts of binoxide; hence a solution of protochloride of tin becomes speedily turbid (from ensuing separation of oxychloride of tin), if the glass is often opened and there is only little free acid present; hence it is only quite recently prepared protochloride of tin which will completely dissolve in water free from air, and the crystallized protochloride of tin dissolves to a clear liquid only in water acidified with hydrochloric acid.

4. Hydrosulphuric acid throws down from neutral and acid solutions of salts of protoxide of tin a dark brown precipitate of hydrated PROTOSULPHIDE OF TIN (Sn S), which is insoluble, or nearly so, in protosulphide of ammonium, but dissolves readily in the higher yellow sulphide. Acids precipitate from this solution yellow bisulphide of tin, mixed with sulphur. Protosulphide of tin dissolves also in solution of soda or potassi. Acids precipitate from these solutions brown protosulphide. Boiling hydrochloric acid dissolves it, with evolution of hydrosulphuric acid gas; boiling nitric acid converts it into insoluble hydrated metastannic acid. Alkaline solutions of protosalts of tin are not, or at least only imperfectly, precipitated by hydrosulphuric acid.

5. Sulphide of ammonium produces the same precipitate of hydrated PROTOSULPHIDE OF TIN.

6. Soda, potassa, ammonia, carbonate of potassa, and carbonate of ammonia produce in solutions of salts of protoxide of tin a white, bulky precipitate of HYDRATE OF PROTOXIDE OF TIN (Sn O, H O), which redissolves readily in an excess of soda or potassa, but is insoluble in an excess of the other precipitants. If the solution of hydrate of protoxide of tin in potassa is briskly evaporated, a compound of binoxide of tin and potassa is formed, which remains in solution, whilst metallic tin precipitates; but upon evaporating slowly, crystalline anhydrous protoxide of tin separates.

7. Terchloride of gold produces in solutions of protochloride or protoxide of tin, upon addition of some nitric acid (without appli cation of heat), a precipitate or coloration of PURPLE OF CASSIUS. (Compare § 129. 7.)

8. Solution of chloride of mercury, added in excess, produces in solutions of protochloride of tin or of protoxide of tin in hydrochloric acid, a white precipitate of SUBCHLORIDE OF MERCURY, owing to the protosalt of tin withdrawing from the chloride of mercury half of its chlorine.