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.