REDUCTION OF METALS
BY
_SULPHURATED HYDROGEN GAS_.
Having treated of the effects of hydrogen gas, and sulphur, separately,
I shall next treat of their effects, when combined in the form of
sulphurated hydrogen gas.
I obtained this elastic fluid by pouring water, acidulated with
sulphuric acid, on alkaline sulphure, prepared in the manner mentioned
in the preceding chapter; but had it been prepared with a caustic
alkali, it is probable, the gas would be more powerful. The sulphure of
the shops is generally unfit for these experiments.
It is necessary in experiments with sulphurated hydrogen gas, hydrogen
gas, &c. to have some mode of conveying them into a chimney; as they
are offensive, and unwholesome.
EXP. 1. GOLD
A piece of silk, which was immersed in a solution of nitro-muriate
of gold in water, and dried in the air, was exposed to sulphurated
hydrogen gas: the silk got a slight tinge of brown; but no other
change occurred: it was then wetted with alcohol, and after some time,
the brown became more intense; and white films appeared on some parts
of the silk, owing to moisture collected from the gas, which is always
replete with water.
Another bit of silk was dipped in the same solution of gold, and
exposed, while wet, to the gas: the yellow colour, which the solution
of gold gave the silk, was immediately changed to a brown; and a white
metallic pellicle appeared, and covered the whole surface of the silk
opposed to the gas; but no purple, or other colours, that usually
attend the reduction of this metal by other agents, could be perceived.
This pellicle did not entirely disappear, as the silk became dry;
though it lost most of its lustre, and looked more like silver, or some
other white metal imperfectly reduced, than gold.
This elastic fluid, and alkaline sulphure, have an effect on the
reduction of gold, very different from that of the vapour of sulphur,
obtained from a burning match, which further appears from the following
experiment, made with sulphurated hydrogen gas, that happened to be
mixed with some sulphurous acid gas.
EXP. 2. GOLD.
A piece of silk, which was dipped in a solution of gold in ether, and
dried in the air, was exposed to a current of sulphurated hydrogen
gas, containing some sulphurous acid gas; but no signs of reduction
could be perceived: the silk was then wetted with alcohol; still no
visible change was produced, except a brown stain, where the alcohol
most abounded: this stain, after some minutes exposure to atmospheric
air, was covered with a white metallic film, which soon disappeared:
after some minutes, the silk was wetted with water, and exposed again
to the gas; a white metallic film immediately appeared, and the brown
stain began to change slowly to a purple; the silk was soon covered
with various colours, as blue, red, and orange; and some spots of
reduced gold of its own proper colour appeared: after some time the
white film, and all the colours, except the purple, disappeared: some
specks of the perfectly reduced gold were permanent.
Another bit of silk, which was immersed in an aqueous solution of
gold, and exposed to the same mixed gas, exhibited nearly the same
appearances.
EXP. 3. GOLD.
Having found that sulphurated hydrogen gas had a powerful effect in
reducing other metals, I was desirous of trying its effects on a larger
scale, thinking it might be attended with more success: I therefore
immersed a quarter of a yard of silk in an aqueous solution of gold,
and exposed it, while wet, to this gas in close vessels: the silk was
soon covered with a white metallic pellicle, which disappeared in a
short time: the experiment was continued about twelve hours, and, on
withdrawing the silk from the gas, I could not perceive the smallest
vestige of reduced metal, or purple, or other colours, that constantly
attend the reduction of gold.
The silk had a dull brown hue verging on a slate colour.
EXP. 4. SILVER.
A piece of silk, which was immersed in a solution of nitrate of silver
in water, and dried in the air, was exposed to a current of sulphurated
hydrogen gas; but no change could be perceived for some time; at
length, when the silk collected moisture from the gas, some particles
of reduced silver were visible, accompanied by a brown stain.
Another piece of silk was dipped in the same solution of nitrate of
silver, and exposed, while wet, to the gas: the silver was instantly
reduced all over the side of the silk opposed to the current: a great
variety of beautiful colours, as red, orange, yellow, green, and blue,
attended the reduction: most of these disappeared; but part of the
silver remained permanently reduced.
I found, that, if the silk be immersed in a solution of silver in
alcohol, and dried, and then moistened with water, and exposed, while
wet, to the gas, none of these colours appear.
This experiment was repeated on a quarter of a yard of silk in close
vessels, and immediately both sides of the silk were covered with
reduced silver, attended with the same beautiful colours: the silk was
left exposed to the gas seven or eight hours; and being then examined,
no reduced silver could be seen: nothing remained, but a strange medley
of colours, chiefly blue, green, purple, and orange, unequally mixed,
some prevailing more than others in certain parts of the silk: however
the die is by no means disagreeable.
EXP. 5. SILVER.
A piece of silk, which was immersed in a solution of fused nitrate of
silver in alcohol, and dried in the air, was exposed to a current of
the gas; but no change could be perceived for some time: it was then
wetted with alcohol, still no alteration could be observed, till the
silk attracted moisture from the gas, when small particles of reduced
silver began to appear, attended with a brown stain.
If the silk immersed in the solution of silver in alcohol, and exposed
while wet to the gas, be kept constantly wet with alcohol, these
appearances take place sooner.
A piece of silk, which was dipped in a solution of muriate of silver in
ammonia, and exposed, while wet, to the gas, had in a short time its
surface covered with reduced silver: but another piece of silk dipped
in the same solution of silver, and dried, underwent no such change.
EXP. 6. PLATINA.
A bit of silk, which was dipped in a solution of nitro-muriate of the
ore of platina in distilled water, and dried in the air, was exposed
to a current of sulphurated hydrogen gas; but underwent no perceptible
change.
Another bit of silk was immersed in the same solution of platina, and
exposed, while wet, to the gas; the whole surface of the silk, opposed
to the current, was instantly covered with reduced platina, which soon
lost its lustre; nothing remaining but a brown colour, with some faint
vestiges of the reduced metal.
This experiment was repeated on a bit of linen with the same result.
EXP. 7. MERCURY.
A bit of silk, which was dipped in a solution of oxygenated muriate
of mercury in distilled water, and dried in the air, was exposed to a
current of the gas; the only visible change produced was a slight shade
of yellow on part of the silk.
Another bit of silk was immersed in the same solution of mercury, and
exposed, while wet, to the gas: the mercury was immediately reduced;
but soon lost its lustre: the gas reacting changed most of it to a
substance of a dull white colour.
The same experiments were made with nitrate of mercury, with the same
result.
EXP. 8. COPPER.
A bit of silk was immersed in a solution of sulphate of copper in
distilled water, and suspended in the air to dry; it was then exposed
to a current of the gas: the silk assumed a brown colour; but no other
change appeared.
Another bit of silk was dipped in the same solution of copper, and
exposed, while wet, to the gas: a white metallic film, attended with
a brown stain, immediately appeared: some parts had a yellow film
approaching to the colour of copper: these films all disappeared,
leaving behind a brown stain: the parts, which were covered with the
yellow film, resembling copper, exhibited, after some time, a dull
bluish disagreeable matter.
EXP. 9. LEAD.
Having dipped a piece of silk in a solution of acetite of lead in
distilled water, and dried it in the air, I exposed it to a stream of
the gas: the silk became a little brown; but no other change could be
observed.
Another bit of silk was immersed in the same solution of lead, and
exposed while wet to the gas; no sooner did it touch the silk, than
a brown tinge diffused itself, like a passing shadow, over the whole
surface of the silk, accompanied with a bright coat of reduced lead,
which resembled silver.
The brown tinge, which the silk acquires, resembles what happens when
white paper is slightly singed by holding it near a burning body.
This experiment was repeated on a quarter of a yard of silk in close
vessels; and the silk was immediately covered with a coat of reduced
lead, which had the brilliance of silver, and was attended with a brown
tinge: the experiment was continued several hours, after which the
silk was examined, and found to have lost all the argentine lustre,
which was succeeded by a sparkling gray die: a sulphure of lead, or
galena, being formed in the fibres of the silk. The same experiment was
repeated on two other pieces of silk with the same result.
This elastic fluid is so powerful a reducer of the metals, that if
chambers, painted with white lead, have any considerable degree of
moisture, and are exposed to its influence, the lead on the surface
of the walls is reduced, and that chiefly where the humidity is most
abundant: this effect, I observed, was produced, even at the distance
of three chambers from the place, where the gas was formed: and the
lead was reduced most evidently behind the window-shutters where the
moisture of the wall was most considerable.
It is a common experiment with chymists, in order to show the effects
of this gas, to draw characters with a solution of sugar of lead on
paper, which they place over a glass of water, containing some alkaline
sulphure: in some time the characters drawn on the paper acquire a
brown, or black colour, and become visible; which amuses the spectators.
But it is surprising, that the metal has never been reduced in this
experiment even by chance; and not less so, why _lead_ should be
singled out for this trial of skill, in preference to other metals,
which are equally affected by this gas.
EXP. 10. TIN.
A piece of silk was dipped in a solution of muriate of tin in distilled
water, and dried in the air; the silk, on exposure to a current of the
gas, scarce underwent any change, except a few brown specks dispersed
over its surface.
Another bit of silk was immersed in the same solution of tin, and
exposed, while wet, to the gas, which no sooner touched the silk, than
the surface opposed to the current was covered with reduced tin of
great brightness: in some time various colours, as blue, orange, and
a faint purple, appeared, and became more evident, as the reduction
proceeded.
The gas does not react on this metal so much as on mercury, and some
other metals: part of the reduced tin remained permanent; but most of
it disappeared, and left behind a disagreeable brown stain.
EXP. 11. ARSENIC.
A bit of silk was immersed in a solution of muriate of arsenic in
distilled water, and dried in the air; it was then exposed to a stream
of sulphurated hydrogen gas; but no change could be observed, except a
faint tinge of citron yellow, which appeared on some parts of the silk.
Another bit of silk was dipped in the same solution of arsenic, and
exposed, while wet, to the gas: the arsenic was instantly reduced all
over the surface, opposed to the gas, attended with a citron yellow
colour.
The gas soon reacted on the reduced arsenic; and nothing remained but
the yellow stain.
EXP. 12. BISMUTH.
I dissolved some bismuth in nitric acid, and evaporated the solution
to dryness; the salt was then dissolved in distilled water, and the
solution deposited a copious white precipitate, but retained a quantity
of the metal sufficient for these experiments.
In this solution of bismuth a piece of silk was immersed, and dried
in the air; it was then exposed to a stream of the gas: and the only
visible effect produced was a very faint tinge of brown.
Another bit of silk was dipped in the same solution of bismuth, and
exposed wet to the gas: the silk was instantly covered with a bright
pellicle of reduced bismuth, which soon lost most of its metallic
lustre, being changed by the reaction of the gas to a sulphure.
Mr. Kirwan in his experiments on hepatic air, found, that nitrous
solution of bismuth, by mixture with a solution of hepatic air in
water, changes to a reddish brown, and even assumes a metallic
appearance.
He also found that nitrates of silver, lead, and acetite of lead, were
precipitated black.
EXP. 13. ANTIMONY.
A piece of silk, which was immersed in a solution of tartarite of
antimony in distilled water, and dried in the air, was exposed to a
stream of the gas; but suffered no visible change, except that a few
specks of yellow, and orange appeared.
A bit of silk was dipped in the same solution of antimony, and exposed,
while wet, to the gas; the antimony was immediately reduced to its
metallic form, attended with a deep orange colour: in some time, most
of the metallic lustre disappeared, a few specks only remaining
permanent.
EXP. 14. IRON.
A piece of silk was immersed in a diluted solution of sulphate of iron
in distilled water, and dried; it was then exposed to a current of the
gas; but underwent no visible change.
Another bit of silk was dipped in the same solution of iron, and
exposed, while wet, to the gas: the silk soon began to acquire a dark
colour, which gradually increased, and, at length, became black,
attended with an obscure metallic film. The silk was immersed in water,
and deposited a bright metallic film on the surface of that fluid.
EXP. 15. ZINC.
A bit of silk, which was immersed in a diluted solution of sulphate of
zinc in distilled water, and dried in the air, was exposed to a current
of the gas; but underwent no visible change.
Another bit of silk was dipped in the same solution of zinc, and
exposed, while wet, to the gas; but no change in the colour of the
silk, or sign of reduction could be perceived: the silk was then
immerged in a glass of clear water, and transferred to its surface a
bright metallic film of the colour of zinc.
When a languid stream of gas was employed in these experiments, the
films transferred to water were scarce visible.
EXP. 16. ZINC.
A piece of silk, which was immersed in a largely diluted solution of
muriate of zinc, and dried in the air, was exposed to a brisk current
of sulphurated hydrogen gas; but underwent no visible alteration.
Another bit of silk was dipped in the same solution of zinc, and
exposed, while wet, to the same brisk current of gas; instantly the
surface of the silk, opposed to the gas, was covered with a bright coat
of reduced zinc, attended with faint orange, and purple colours.
Though the reduced zinc was quite evident; yet I had the curiosity to
examine its appearance on the surface of water; I therefore immersed
the silk in a glass of that fluid, and a bright metallic pellicle,
which retained the texture of the silk, as wax retains the impression
of a seal, was transferred to its surface.
Another bit of silk, dipped in the same solution of zinc, and exposed
for a longer time to the gas, lost its metallic lustre, the zinc being
changed to a sulphure by the reaction of the gas.
These experiments on sulphate, and muriate of zinc, show, that the
acid, in which the metal is dissolved, influences the reduction in a
remarkable manner.
It is evident from the experiments related in this chapter,
1. That water is essential to the reduction of metals by sulphurated
hydrogen gas.
2. That alcohol does not promote these reductions, without the aid of
water.
3. That sulphurated hydrogen gas does not reduce the metals by giving
them phlogiston; nor by uniting with, and separating, their oxygen:
for were either of these opinions well founded; metallic solutions
in alcohol should be as effectually reduced by this gas, as metallic
solutions in water are.
When we consider, that alkaline sulphure reduces the metals by
decomposing water; we have every reason to conclude, that sulphurated
hydrogen gas reduces them in the same manner; since the same
circumstances are necessary to both: the hydrure of sulphur, which
constitutes the base of this gas, attracts the oxygen of the water,
while the hydrogen of the latter unites, in its nascent state, with the
oxygen of the metal, and reduces it.
Hence it follows, that the hydrure of sulphur is oxygenated by the
oxygen of the water, while the metal is restored to its combustible
state.