Gold is precipitated totally by fixed alkaline salts, but platina only in part. When solutions of the two metals are mixed together, so much of the platina remains suspended, after saturation with the alkali, as to be readily distinguishable by the yellow colour, which it communicates to the liquor. It has been objected, that tho' the platina was discoverable, when thus mingled superficially with the gold, it may nevertheless, when combined more intimately by fusion, elude this method of trial.
1. Mixtures of gold with small proportions of platina were therefore kept in fusion, by a very strong fire, for several hours, and afterwards dissolved in aqua-regis. The solutions being diluted with water, and a pure fixed alkaline salt gradually added, so long as any effervescence of precipitation ensued, the liquors remained manifestly coloured, tho' apparently paler than when the two metals had been dissolved by themselves.
2. A more convincing proof, that part of the platina remains suspended, after the precipitation of the gold, was obtained, by putting into the filtered liquors some plates of pure tin, which presently contracted an olive hue, and threw down a large quantity of a brownish precipitate, as from the common solutions of the crude mineral. It was observable, that the tin plates were often sensibly acted on, even whilst the liquor was overcharged with alkali.
3. It has been further suggested, and with great appearance of probability, that as a part of platina is precipitated as well as gold by alkaline salts, if only this part be mixed with gold, it will be thrown down by them again upon dissolving the compound. To determine this point, a precipitate of platina made by fixed alkali was melted with thrice its weight of fine gold, and kept in strong fusion for above an hour: they united more easily than gold does with so large a proportion of the crude mineral, and formed a smooth neat bead, which hammered well into a pretty thin plate before it cracked, and appeared internally uniform and equal. This compound being dissolved in aqua-regia, and a fixed alkaline salt added by degrees till the acid was more than saturated, the liquor became indeed pale; but tin plates put into it quickly discovered, that it held a very considerable quantity of platina. It appears therefore a constant property of this mineral to remain partially dissolved in the neutralised liquor; and that minute proportions of it, mixed with gold, are by this means distinguishable.
4. Many other experiments were made of the precipitations and precipitates of gold and platina, by alkalies both of the fixed and volatile kind. The most remarkable effects were, that volatile alkalies, added to both solutions in quantity just sufficient to saturate the acid, precipitated gold intirely, but platina only in part, so much of it remaining suspended as to give the same colour to the liquor as when fixed alkalies were made use of: that, on adding a larger quantity of the spirit after the precipitation of the gold, the liquor became yellow, a part of the metal being taken up again; and that the platina was more copiously redissolved, the liquor becoming of a deep brownish red: that the washed precipitates of both metals, whether made by volatile or fixed alkalies, proved soluble, by moderate digestion, in spirit of salt; those of platina much more difficultly and sparingly than those of gold.
3. By inflammable Liquors.
1. Inflammable spirits, which revive gold from its solutions in form of yellow films, have no such effect on solutions of platina. This experiment affords not only a criterion for distinguishing with certainty whether gold has been debased by platina, but likewise an infallible means of recovering it perfectly pure from any admixture of that mineral. If the compound be dissolved in aqua-regis, the solution mingled with twice its quantity or more of the spirit, and the mixture suffered to stand for some days in a glass slightly covered; the whole of the gold arises to the surface, leaving the whole of the platina dissolved. The golden pellicles may be collected, by pouring the matter into a filter just large enough to contain it. The dissolved platina passes thro', leaving the gold upon the paper, which is to be washed with fresh parcels of water till the liquor runs colourless. The paper is then to be carefully squeezed together, and burnt in a crucible previously lined with vitrefied borax: when fully sunk down, a little fresh borax is to be injected, and the fire raised to melt the gold. The use of lining the crucible with borax is to prevent any moleculæ of the gold from lodging in its cavities.—This process is attended with one inconvenience, the slowness of the separation of the gold from the solution: this may be in some measure expedited by employing a spirit, which has been distilled from vegetables, that give over an essential oil.
2. As essential oils take up gold from aqua-regis, and keep it dissolved for a time upon the surface of the acid; a pure colourless oil, that of rosemary, was poured into about half its quantity of a solution of platina, the mixture well shaken, and suffered to rest: the oil quickly arose, without taking up any thing from the platina, or receiving any colour: the acid liquor underneath remained coloured as at first. Compositions of platina and gold being dissolved in aqua-regis, and treated in the same manner, the whole of the gold was imbibed by the oil, and the whole of the platina remained dissolved in the acid. The oil, loaded with the gold, appeared of a fine yellow colour, and, on standing for a few hours, threw off great part of its contents, in bright yellow films, to the sides of the glass. Sundry other distilled oils were made trial of, with the same event. The gold is easily recovered, by setting the oil on fire; and, when thoroughly burnt out, melting the residuum with borax, as in the preceding experiment. After the separation of the oil employed at first, it may be proper, for the greater security, to add a little more; which, if any part of the gold should happen to have been left in the liquor, will effectually take it up.
3. The experiment was repeated likewise with the subtile fluid, prepared from vinous spirits with the vitriolic acid, called by the chemists æther. The separation succeeded in the same manner as before; the æther receiving nothing from pure platina, but instantly taking up the gold from a mixture of the two. It is observable, that the gold imbibed by this fluid is kept permanently dissolved by it; without separating or reviving, as it does from the common essential oils and vinous spirits.
4. The liquors remaining in these experiments, after the extraction of the gold, appear on all trials the same with the common solutions of platina; and readily betray their being impregnated with that mineral by their colour, by the precipitation with tin, by their yielding a sparkling red precipitate with volatile spirits, &c. A far more minute proportion of platina, mixed with gold, is more distinguishable by these processes, than by those with alkaline salts above-mentioned; these exhibiting the whole of the platina dissolved by itself, those only a part of it.