On taking up with water the protochloride is decomposed into gold and trichloride, but the hydrochloric acid can only be eliminated by shaking with ether, which withdraws the tri¬chloride from its solution in water. A brownish-red mass is thus formed, consisting of AuCl3 mixed with more or less of the protochloride and of hydrochloric acid. The usual method adopted for the preparation of auric chloride is to dissolve gold in aqua regia, and then to evaporate the liquid to dryness, keeping the temperature as low as possible. However this may be, it is certain that when gold is heated in chlorine at atmospheric pressure, trichloride of gold is formed and volatilised at all temperatures above 180°, up to, and probably far beyond, 1,100°. The second change is perhaps caused by the change of sign of the heat of formation of the trichloride AuCl3 when this becomes negative, the pressure of dissociation of the compound would decrease, in accordance with the law of van’t Hoff and Le Chatelier. The first named change in the direction of the curve probably occurs at the melting point of the chloride-namely, 288°. 5) showing the variation of volatilisation with temperature is irregular, passing through a maximum near 300°, and a minimum at a point somewhere below the melting point of gold. The rise of vapour pressure tends to raise, and that of the pressure of dissociation to reduce, the amount of gold volatilised as chloride. The vapour pressure increases more rapidly than the pressure of dissociation at temperatures below 300°, and also above 900°, but less rapidly at intermediate temperatures. the pressure of dissociation of the trichloride, which also rises continuously with the temperature, but not at the same rate as the vapour pressure.The vapour pressure of gold trichloride, AuCl3, which of course increases continuously as the temperature rises and.The amounts volatilised vary according to two different factors: The rate of volatilisation at various temperatures is as follows:įor purposes of comparison, it may be added that when gold is heated in air or coal gas, no gold is volatilised below 1,050°, and only about 0.02 per cent, in 30 minutes at 1,100°, or about one-hundredth part of that volatilised in chlorine at the same temperature. The fact that gold is attacked by chlorine, and that the chloride is subsequently volatilised at all temperatures between 180° and 1,100°, was proved by means of Deville’s hot and cold tubes, which enable part of the sublimed chloride to be collected. The absorption of chlorine by gold with the formation of chlorides at first increases in rapidity as the temperature rises, and reaches its maximum at about 225°. The matter has, therefore, been investigated by the author with the following results:-Gold unites with chlorine if placed in the gas at atmospheric pressure at all temperatures up to a white heat, but the action is slight at a red heat. The exact point at which gold ceases to be attacked by chlorine and the rate of volatilisation of the chloride are of great importance in connection with the loss of gold on roasting auriferous materials with salt. At 220° the AuCl was completely decomposed, leaving metallic gold, while up to this temperature a little AuCl3 continued to be sublimed, but towards 300° all action ceased, and the gold remained unattacked by the chlorine at all higher temperatures.

He found that AuCl3 was formed at 140° to 150°, and was converted to AuCl at about 180° in spite of the presence of an excess of chlorine. Kruss tried to repeat Debray’s experiments without confirming his results. The chloride formed sublimes at this temperature and is deposited in the cooler part of the tube in fine red prisms and needles crystallising in the triclinic system. How Gold Trichloride AuCl3 is prepared: Trichloride of gold can be prepared, according to Debray, by heating finely-divided gold in a current of pure dry chlorine in a glass tube at a temperature of 300°. It yields gold and AuCl3 if brought in contact with water, ether, or alcohol. According to Kruss and Lindet it is merely a mixture of AuCl and AuCl3. Auro AurichlorideĪu2Cl4 is a dark red compound having this composition is said, by Thomsen, to be obtained by heating finely-divided gold in a current of chlorine to 140° to 170°. Water converts aurous chloride into a mixture of gold and gold trichloride. It is non-volatile and unaltered at ordinary temperatures and pressure by dry air, even when exposed to light, but begins to decompose at temperatures above 160°, and the decomposition is complete if it is heated at 175° to 180° for six days, or at 250° for one hour.

Gold Monochloride or Aurous Gold Chloride “ AuCl” is a salt is prepared by heating the trichloride to 185° in air for twelve hours.

Precipitate gold from chloroauric acid