REDUCTION OF METALS
BY
_ACIDS_.
It is well known that solutions of gold throw up films of the reduced
metal to the surface of the fluid, and sides of the phials that contain
them.
It has been shown, chap. 7th, that this effect depends on the presence
of water, which is further illustrated by the following experiments.
EXP. 1. SILVER.
A dram of saturated solution of nitrate of silver in distilled
water was mixed with half an ounce of vinegar, in a few minutes, a
precipitate was formed: the whole was then poured on a filter, and
washed with distilled water: the precipitate acquired a bluish gray
metallic appearance: the liquor, which passed the filter, got a brown
colour, and became blackish: next day bright films of reduced silver
floated on the liquor; and in some months after, a considerable part
of the inner surface of the phial was coated with reduced silver.
Distilled vinegar did not produce this effect.
EXP. 2. PLATINA.
If a solution of the ore of platina in nitro-muriatic acid be
evaporated to dryness; and the salt dissolved in water, films of a
livid white metallic appearance are thrown up to the surface.
EXP. 3. TIN.
Some tin, which was dissolved in a china cup in muriatic acid, and
evaporated to dryness, was dissolved in distilled water; immediately
white metallic films appeared on the surface of the liquor; and after
some hours, the whole surface of the solution was covered with a
continuous metallic pellicle, which reflected all the colours of the
rainbow, in a beautiful manner.
EXP. 4. MERCURY.
Mr. Bergman says, “calcined mercury is reduced by digestion in acid of
salt; but the cause has not yet been sufficiently explored[25].”
As this reduction of mercury by muriatic acid is doubted of by some; I
made the following experiment.
A quantity of precipitate _per se_ was put into a china cup, placed
on hot sand, and some muriatic acid was poured on it: the oxid was
gradually dissolved, and the digestion continued, till the acid was
nearly evaporated: some distilled water was then poured on the salt,
and minute films of a bright metallic appearance floated on the surface.
A nitrate of mercury was evaporated to dryness; the salt acquired a
slight tinge of yellow; the cup was removed from the hot sand, and
some water poured on the salt: immediately bright films of reduced
mercury appeared; and in about three or four hours the whole surface
of the solution was covered with a metallic pellicle, parts of which
had the colour and splendour of the metal; and other parts reflected a
wonderful variety of the most beautiful colours.
EXP. 5. ARSENIC.
A quantity of white oxid of arsenic and muriatic acid was digested in
the same manner in a china cup; a smart heat was continued, till most
of the acid evaporated, and left behind a mass of the consistence of
tar: the surface of which had a gray appearance; after it cooled, some
distilled water was poured on, it diffused a disagreeable smell, and
films of reduced arsenic were thrown up to the surface; these were for
some time remarkably bright, but soon became of a dull gray colour,
owing to the rapid tendency of this metal to spontaneous calcination.
EXP. 6. MANGANESE.
Some concentrated sulphuric acid was poured on a quantity of the black
oxid of manganese in a china cup, placed on hot sand: an effervescence
was visible, but no sulphurous acid gas was exhaled: the cup was
removed from the sand, before any perceptible quantity of the acid
evaporated: then some water was poured on the mixture: and instantly
the surface of the fluid was covered with very brilliant metallic
films: and, at the same time, the solution acquired a rose colour.
Some of the same oxid of manganese was dissolved in nitric acid, in
which a bit of sugar was put: the solution was poured into a china cup,
placed on hot sand, and, as the evaporation proceeded, bright metallic
films appeared on the surface.
Mr. Bindheim of Moscow relates, that he reduced the aërated calx of
manganese dissolved in nitrous acid: “the solution was placed upon an
open fire in a glass retort[26].”
Happening to pour out a very largely diluted solution of sulphate of
iron, I was surprised to find the flags of the court in an hour or two
after, covered with large films of a livid white metallic aspect:
these films, after the water evaporated, remained on the flags several
days.
In consequence of this I poured a largely diluted solution of sulphate
of iron into several shallow vessels, and always found the surface of
the solution covered with abundance of bright metallic films.
I also found that a largely diluted solution of muriate of zinc throws
up bright metallic films to the surface.
The reduction of metals by acids is a very convincing proof of the
decomposition of water, and seems inexplicable on any other principle.
The reduction and calcination of metals by heat and electricity are
naturally explained on the same principle: Doctor Priestley informs us
in his History of Electricity, Vol. I. p. 344, that S. Beccaria revived
several of the metals by the electric shock. It is obvious that the
electric fluid produces this effect in the same manner, that other
combustible bodies do, viz. by decomposing water. When the red oxid
of mercury is reduced in close vessels by heat; the latter attracts
the oxygen of the water, contained both in the oxid, and in the air of
the vessels, while the hydrogen of the water unites to the oxygen of
the mercury, reduces it, and forms a quantity of water equal to that
decomposed.
It is found, that if light and caloric act both at the same time,
the reduction is effected with more facility, than if either acted
separately: in this case, the superior efficacy of the combined action
of light, and caloric resembles that of other compound reducers of the
metals; as that of sulphurated hydrogen gas, phosphorated hydrogen gas,
alkaline sulphure, &c. which are more powerful than simple hydrogen
gas, sulphur, or phosphorus.
This superior efficacy of compound reducers, is similar to what happens
with some other combinations of combustible bodies, as pyrites,
pyrophori, a mixture of lead and tin, &c. which have a more powerful
attraction for oxygen, than any of their ingredients, in a separate
state.
Other acids, as the gallic, the tartarous, the formic, &c. and also
alkalis, as ammonia, and soda, have the power of reducing some of the
metals, facts well known to chymists.