CHLORIDE OF GOLD. There are two chlorides of gold, viz., Au. Cl. and Au. C1.2: the former is prepared by heating the latter to the temperature of melting tin, till no more chlorine is evolved, and is decomposed by water into the latter and metallic gold. The second of these two only is soluble, and is the salt generally called the chloride of gold. Gold unites directly with chlorine, and it also dissolves in an aqueous solution of chlorine. Gold cannot deeompose hydrochloric acid, either dry or in solution, because the attraction of chlorine for hydrogen is greater than for gold. It can liberate the hydrogen in neither case : but if nitric acid be also present in the solution, then the hydrogen, instead of escaping, forms water with the oxygen of the nitric acid and disengages N 02, which is held by a much more feeble force than the hydrogen of either hydro chloric acid or water. The attraction of chlorine for gold is stronger than that of N 02 for 03. But the affinity of chlorine for gold is not great, and hence it is easily driven off by heat. It is also reduced to the metallic state by protosulphate of iron, and by gallic, acetic, citric, oxalic, and other acids which take oxygen, and then forms hydrochloric acid from the elements of water. When neutral, the compound and its solution are red; but if hydrochloric acid be present, yellow. It is of important use in photography : in the art it is constantly made use of for toning the red newly-fixed images, for making the sel d'or, or double hyposulphite of gold and soda, for gilding daguerreotypes, and even in chloridizing collodion, and, mixed with other chlorides, in preparing papers for the printing frame. It has been fm-ther applied to the reduction in strength of over-printed positives, and to the restoration of those which have faded. Solution of chlorine, or any easily decomposed chloride, as the perchloride of copper, or bichloride of mercury, will reduce a print, when quite bronzed, to a reasonable condition as well as the chloride of gold. To revive a print, immerse it in a bath composed of 40 grains of the chloride to a pint of water rendered slightly acid with hydrochloric acid : in the light of the sun a few minutes are sufficient, but in the dark three or four hours are necessary to com plete the restoration, when the print must be fixed again with fresh hyposulphite. The other uses are explained elsewhere. It is best kept in solution. To make it, place a half-sovereign in a vessel containing nitro-hydrochloric acid and digest with a gentle heat : add fresh aqua regia when action has ceased if it be not entirely dis solved. The solution will contain chloride of gold and chloride of copper, and free acid : add, therefore, carbonate of soda enough to neutralize the acid and precipitate a green salt of copper, which may be left till the next day to settle. The solution will contain about 85 grains of chloride of gold, and if rendered slightly acid, and made up to 85 fl. drachms, will be stable, and e,ach drachm vvill represent a grain of the chloride.
Chloride of gold is a photo-sensitive salt : a piece of paper moistened with it and exposed to light becomes purple in conse quence of its decomposition. The oxides of gold are more or less purple, and the finely divided metallic gold reddish brown : the chloride of gold would, therefore, appear to be reduced to an oxide, Au. C1.3, + 3 H 0 = Au. 0 + 3 H Cl. This oxide has not the
instability of the teroxide in the light. Pictures toned with gold.
in the form of teroxide have been known to become redder in time owing to the reduction of the gold to the state of pure metal. The terchloride of gold is the basis of the chrysotype process, but it is merely a developer in this case. A persalt of iron in the pores of the paper is reduced, by light, to the state of protosalt in the sunned portions, and upon this, by the reducing power of the iron itself, gold is precipitated in the metallic state, or silver if nitrate of silver be brushed over the paper instead of terchloride. If chlo ride of gold and chloride of barium are both in the paper when the exposure takes place, then instead of the gold appearing in the purple colour of the oxide, the first effect is to remove the slight pinkey or yellowish colour of the gold salt entirely and then slowly and feebly darken it. But before any decidedly visible image is formed if the paper be immersed in water, the picture is rapidly developed to a full purple brown. Hot steam has the same effect as water and is more energetic.. The following is probably the rationale of this interesting change which has not hitherto been explained. Chlo ride of gold (Au. forms with chloride of barium, ammonium, &c., double salts containing an atom of each, and which crystallize in a regular manner with four or five atoms of water. The gold cannot be precipitated by alkalis from the solutions of these salts : and in this and their constitution they resemble the double salts of citrate of iron and citrate of ammonia and the like, such as the potassio tartrate of iron, the ammonio-oxalate of iron and many others. When these double salts of iron are exposed to light the oxygen and carbon of the organic acid are so excited—See " Chemistry of Photography,"—that carbonic.acid is formed and escapes, and as a consequence the salt is reduced to the state of protoxide. Similarly when the double chloride of gold and barium is solarized, hydro chloric acid escapes and the gold is reduced to the state of proto chloride. But protochloride of gold, though permanent when dry, is quickly resolved by water, and especially by hot water into metallic gold and perchloride. The first whitening effect observed is the formation of the protocloride and the subsequent darkening is the commencement of the resolution of the double compound by the action of the atmosphere. Similar effects occur with oxalate of ammonia, acetate of lead, bichromate of potash and iodide of of potassium with this chloride.
Chloride of gold can be entirely separated in the neutral state from the acid solution by means of ether, which dissolves it more readily than water. The hydrochloric acid remains with the water. This is a neat way of neutralizing it for the preparation of toning baths. The salt is extremely deliquescent, and, therefore, cannot be conveniently kept dry : its solution in pure water, slightly acid, will not decompose, hut an alcoholic or ethereal solution might decom pose in the light, if kept long.