Gold is readily soluble in aqua regia, or in any other mixture producing nascent chlorine, among such mixtures being solutions of:

1. nitrates, chlorides, and sulphates — e.g., bisulphate of soda, nitrate of soda, and common salt;
2. chlorides and some sulphates—e.g., ferric sulphate;
3. hydrochloric acid and potassium chlorate;
4. bleaching powder and acids, or salts such as bicarbonate of soda.

The action is much more rapid if heat is applied or if the gold is alloyed with one of the base metals than if it is pure. The presence of silver in the gold retards the process, a scale of insoluble chloride of silver being formed over the metal, and the action may eventually be completely stopped if the percentage of silver present is large. Gold is also dissolved by liquids containing chlorine and bromine, but the action is much slower than that of aqua regia; and subject to the same difficulties if silver is present; heat assists the dissolution. Iodine only dissolves gold if it is nascent, or if heated with gold and water in a sealed tube to 50°. Metallic gold dissolves in hot strong sulphuric acid, especially if a little nitric acid is added (the precipitated metal dissolving most readily), forming a yellow liquid, which, when diluted with water, deposits the metal as a violet or brown powder. The solution also becomes covered with a shining film of reduced metal on exposure to moist air. On addition of hydrochloric acid or a metallic chloride, auric chloride is formed, no longer precipitable by water. Gold is also attacked when used as the positive pole of the battery in the electrolysis of strong sulphuric acid, but is immediately reduced again by the evolved hydrogen.

The easily decomposable metallic perchlorides, perbromides and periodides are capable of dissolving gold, lower chlorides, &c., of the base metals being formed, and gold chloride, &c., produced. The higher chlorides and bromides of manganese and cobalt (Co2Cl6, &c.) act well, and a hot strong aqueous solution of ferric bromide or of ferric chloride will also readily dissolve gold. Ferric iodide is decomposed by gold under ordinary conditions, aurous iodide being produced. Some other haloid compounds only attack gold in the presence of ether, in which case even hydriodic acid itself is decomposed and aurous iodide formed. Selenic and iodic acids have also been mentioned as solvents for gold, and the effect of a mixture of nitric and nitrous acids is described elsewhere. Alkaline sulphides attack gold slowly in the cold, and more rapidly if heated, producing sulphide of gold which is subsequently dissolved. Ditte denies this. Gold is also soluble in the hyposulphites of calcium, sodium, potassium, and magnesium.

Spring has shown that gold is soluble in hydrochloric acid if heated with it to 150° in a closed tube, and is subsequently reduced by the liberated hydrogen and deposited as microscopic crystals on the side of the tube. C. Lossen pointed out in 1895 that if a solution of potassium bromide is electrolysed, the resulting alkaline solution, containing hypobromite and bromate of potassium, is capable of dissolving gold. Gold is dissolved by aqueous solutions of simple cyanides and by certain double cyanides, such as sulphocyanides and ferrocyanides, which act very slowly except in presence of oxidising agents and with the aid of heat.

How to Prepare Pure Gold

The purest gold obtainable is required for use as standards or check pieces in the assay of gold bullion. The following method of preparing it was adopted by Roberts-Austen in the manufacture of the Trial-Plate, by which the imperial gold coinage is tested. Gold assay cornets, from the purest gold which can be obtained, are dissolved in nitrohydrochloric acid, the excess of acid expelled, and alcohol and chloride of potassium added to precipitate traces of platinum. The chloride of gold is then dissolved in distilled water in the proportion of about half an ounce of the metal to one gallon, and the solution allowed to stand for three weeks. At the end of this time the whole of the precipitated silver chloride will have subsided to the bottom, and the supernatant liquid is removed by a glass siphon. Crystals of oxalic acid are then added from time to time, and the liquid gently warmed until it becomes colourless, when precipitation is complete, a point reached in three or four days if ten-gallon vessels are used. The spongy and scaly gold so obtained is washed repeatedly with hydrochloric acid, distilled water, ammonia, and distilled water again, until no reaction for silver or chlorine can be obtained, after which it is melted in a clay crucible with bisulphate of potash and borax, and poured into a stone mould. Lack of care in any one of the operations will result in gold containing one or two parts of impurity in ten thousand.

With regard to the above method, it may be observed that carefully purified sulphurous acid gas is a more convenient precipitant than oxalic acid, and may be substituted for it without any ill effects, as any foreign metals that may be present are in such small quantities that their sulphites, even if formed, would remain dissolved. It should be added that, according to the recent researches of Kohlrausch, Rose, and Holleman, silver chloride is soluble in 600,000 to 700,000 parts of pure water at the ordinary temperature. It follows that, under the conditions given by Roberts-Austen, the solution contains at least 0.3 to 0.4 part of silver per 1,000 parts of gold, and this proportion is doubtless higher in practice, owing to the greater solubility of silver chloride in solutions of other chlorides than in pure water. There can be no doubt that part of this silver is precipitated with the gold, and that, by re-dissolving and re-precipitating, a purer product is obtained The amount of silver remaining in solution can, moreover, be reduced to about one-fifth of the amount noted above by adding a small quantity of hydrobromic acid to the solution, silver bromide being far less soluble than silver chloride. Another additional precaution is to remove the gases taken up by the gold during the process of melting by heating it to redness in vacuo.

Solubility of Gold

Gold is readily soluble in aqua regia, or in any other mixture producing nascent chlorine, among such mixtures being solutions of (1) nitrates, chlorides, and acid sulphates—e.g., bisulphate of soda, nitrate of soda, and common salt; (2) chlorides and some sulphates—e.g., ferric sulphate; (3) hydrochloric acid and nitrates, peroxides such as permanganate, or chlorates; (4) bleaching powder and acids, or salts such as bicarbonate of soda. Speaking generally, almost any chloride, bromide, or iodide will dissolve gold in presence of an oxidising agent. The action is much more rapid if heat is applied, or if the gold is alloyed with one of the base metals. The presence of silver in the gold retards the process, a scale of insoluble chloride of silver being formed over the metal, and the action may eventually be completely stopped if the percentage of silver present is large. Gold is also dissolved by liquids containing chlorine and bromine or a mixture producing bromine. The action is much slower than that of aqua regia, and subject to the same difficulties if silver is present; heat assists the dissolution. Iodine dissolves gold only if it is nascent, or if dissolved in iodides or in ether or alcohol. Metallic gold does not dissolve in strong sulphuric acid unless a little nitric acid is added when a yellow liquid is formed, which, when diluted with water, deposits the metal as a violet or brown powder. The mixture of nitric and sulphuric acids is a more rapid solvent for gold than nitric acid alone.

According to Nickles, the easily decomposable metallic perchlorides, perbromides, and periodides are capable of dissolving gold, lower chlorides, &c., of the base metals being formed, and gold chloride, &c., produced. Some of these so-called persalts are, however, often regarded merely as solutions of chlorine, bromine, or iodine in the protosalts. A boiling concentrated solution of ferric chloride readily dissolves gold. Some other haloid compounds only attack gold in the presence of ether, in which case even hydriodic acid has a slight effect. Pure hot selenic acid dissolves gold with the formation of auric selenate (Mitscherlich, also Victor Lenher). Iodic acid has also been mentioned as a solvent for gold, but its action is very slight, much less, for example, than that of concentrated hydrochloric acid under similar conditions. A mixture of iodic and sulphuric acids dissolves gold when heated to 300° (Prat, also Victor Lenher). The effect of a mixture of nitric and nitrous acids is described in Chapter xx. Alkaline sulphides attack gold slowly in the cold, and more rapidly if heated, producing sulphide of gold which is subsequently dissolved. Gold is also soluble in the thiosulphates (hyposulphites) of calcium, sodium, potassium, and magnesium, in the presence of an oxidising agent.

Spring has shown that gold is soluble in hydrochloric acid if heated with it to 150° in a closed tube, and is subsequently reduced by the liberated hydrogen and deposited as microscopic crystals on the side of the tube. The author has found that boiling concentrated hydrochloric acid dissolves gold and maintains it in solution. Victor Lenher showed that in presence of sulphuric acid many oxidising substances, such as telluric acid, manganese dioxide, lead dioxide, red lead, chromium trioxide, and nickelic oxide, cause gold to pass into solution. In some cases phosphoric acid may be substituted for sulphuric acid. When gold is used as an anode, gold is oxidised, and if the electrolyte is strong sulphuric acid, phosphoric acid or caustic soda or potash, part of the oxide is dissolved. C. Lossen pointed out in 1895 that if a solution of potassium bromide is electrolysed, the resulting alkaline solution containing hypobromite and bromate of potassium, is capable of dissolving gold. Gold is dissolved by aqueous solutions of simple cyanides in presence of an oxidising agent. Sulphocyanides, ferrocyanides, and some other double cyanides also dissolve gold, but the action is very slow, even in presence of oxidising agents, unless the solutions are heated.

The table on p. 12 gives the relative rate of dissolution of gold by a number of solvents. The gold used in each case consisted of a single “ cornet ” of “ parted ” gold, weighing about half a gramme, and consisting of gold 99.93 per cent., silver 0.07 per cent. Cornets offer a large surface to attack, as they consist of spongy gold, and those used did not differ in physical state. They were prepared by the method described under bullion assaying. The solutions were in excess, but no stirring or agitation was used.

Allotropic Forms of Gold

Little is known of Allotropic Forms of Gold. The marked influence of traces of other metals on the properties of gold has already been touched on; from this and from the variations in colour and other properties the existence of several allotropic modifications of gold might be inferred. In alloys containing appreciable quantities of other metals, evidences of allotropy are not met with so frequently. The potassium alloy, however, containing 10 per cent, of gold, on being attacked by water, leaves a black finely-divided gold powder, and there is reason to believe that this combines with water to form a hydrate.

Wilm states that if gold is dissolved in dilute sodium amalgam under water, the aqueous liquid becomes dark violet, and when this is acidulated with hydrochloric acid, a black precipitate of pure gold is obtained. The black gold differs from the ordinary modifications in its extreme lightness; moreover, it is soluble in alkaline solutions, and does not amalgamate with mercury or with sodium amalgam. When heated, it yields the ordinary modification as a violet red powder. This form of gold appears, from Wilm’s account, to resemble the black precipitate obtained on digesting certain aluminium-gold alloys with hydrochloric acid.

Gold Amalgams

Mercury rapidly dissolves gold at ordinary temperatures, forming liquid, pasty, or solid amalgams, according to the proportions of the metals present, and their purity. A piece of gold rubbed with mercury is immediately penetrated by it and becomes exceedingly brittle. The ductility is not always restored when the mercury is removed by distillation, a crystalline structure being often induced. Some forms of precipitated gold are not readily taken up by mercury, the particles tending to float on the surface of the latter. An amalgam containing 90 per cent, of mercury is liquid, that containing 87.5 per cent, is pasty, and that containing 85 per cent, crystallises in yellowish-white easily fusible prisms. On dissolving precipitated gold in mercury heated to 120°, and then cooling the mass, white crystalline plates having a composition corresponding to the formula AuHg4 separate out. Amalgams with smaller proportions of mercury can be obtained in various ways by heating gold and mercury to different temperatures up to a low red heat, and acting on the products with nitric acid. Gold amalgam dissolves readily in excess of mercury, forming a liquid mass from which it may be partially separated by straining through chamois leather, when a white pasty amalgam containing about 33 per cent, gold remains behind, while the mercury which filters through contains some gold, the amount varying with the temperature, but not with the pressure applied. Kasentseff has shown that this liquid amalgam contains 0.11 per cent, of gold if it is filtered at 0°, 0.126 per cent, at 20°, and 0.650 per cent, at 100° C.; these amalgams, therefore, behave like aqueous solutions.

When amalgams are gradually heated, the mercury is distilled off by degrees, the action soon ceasing if the temperature is allowed to become stationary, and distillation recommencing if it is again raised. At 440° (somewhat below a red heat), an amalgam containing about three parts of gold to one of mercury is obtained, and at a bright red heat almost all the mercury is expelled, and if the heating has not been pushed too rapidly the vapours contain but little gold. The gold obstinately retains about 0.1 per cent, of mercury, which is not driven off below the melting point of gold.