Carey Lea discovered the
preparation of so-called "allotropic" and "intermediate" silver in
1889 while he was studying reductions of silver nitrate.
"Allotropic" is however a misnomer, . In 1925, Dr. Richard
Zsigmondy, Professor of Chemistry at the University of Göttingen,
received the Nobel Prize in Chemistry for his study of Lea's
"allotropic" silver under the ultramicrosope. Dr. Zsigmondy found
that such silver actually was a monoatomic colloid of ordinary
silver, not another isotope.
Lea determined that silver occurs in "allotropic", "intermediate",
and ordinary forms. Ordinary silver is protean in nature. The
aqueous solutions are colloidal monoatoms, and give perfectly
clear solutions. The several forms of "allotropic" silver (a-Ag)
dry with their particles in optical contact with each other, thus
forming continuous films that are beautifully colored, perfect
mirrors. Strong acids and pressure will convert a-Ag to the normal
form. There are three forms of a-Ag, and all are unstable.
There is also a very stable "intermediate form" of silver (i-Ag)
which is easy to prepare. It occurs as bright gold-yellow or green
crystals with a metallic luster. Treatment with a very dilute
solution of ferric chloride will enhance the appearance of its
foliar structure, interpenetrating with plant-like ramifications,
or fine acicular crystals up to 1 inch long.
Intermediate silver is hard, tough, and unaffected by pressure. It
is nearly as indifferent to oxidizing and chlorizing agents as is
normal silver. Intermediate silver can be formed from the
allotropic varieties by light, heat, or chemical action.
Excerpts from Journals --
Amer. J. Science ( 3 rd Series )
37 ( 222 ) 476 - 491 ( June 1889 )
"On Allotropic Forms of Silver"
"... The form of allotropic silver which I have obtained may be
classified as follows : --
A. Soluble, deep red in solution, mat lilac, blue, or green whilst
moist, brilliant bluish metallic when dry.
B. Insoluble, derived from A, dark reddish brown whilst moist,
when dry somewhat resembling A.
C. Gold, silver, dark broze whilst wet, when dry exactly
resembling gold in burnished lumps. Of this form there is a
variety which is copper-colord. Insoluble in water, appears to
have no corresponding soluble form.
All these forms have several remarkable properties in common :
I. That of drying with their particles in optical contact and
consequently, forming a continuous film...
II. The halogen reaction ... very beautiful color reactions are
III. The action of acids ... instantly convert the allotropic
forms of silver into normal gray silver... absolutely without the
separation of gas...
IV. Physical Condition ... All these allotropic forms of silver
are easily reduced to an impalpable powder...
A. Soluble Allotropic Silver
A solution of ferrous citrate added to one of a silver salt
produces instantly a red liquid (Ferrous sulfate gives the same
reaction but is less advantageous). These red solutions may either
exhibit tolerable permanency or may decolorize, letting fall a
black precipitate. It is not necessary to prepare the ferrous salt
in an isolated form, a mixture of ferrous sulfate and sodium
citrate answers perfectly.
When however concentrated solutions are used with a large excess
of ferrous sulfate and a still larger one of alkaline citrate, the
liquid turns almost completely black. It should be stirred very
thoroughly for several minutes to makes sure that the whole of the
precipitated silver citrate is acted upon by the iron. After
standing for 10 or 15 minutes, the liquid may be decanted and will
leave a large quantity of a heavy precipitate of fine lilac-blue
color. It is best to adhere closely to certain proportions:
Of a 10% solution of silver, 200 cc may be placed in a
precipitating jar. In another vessel are mixed 200 of a 30%
solution of ferrous sulfate and 280 cc of a 40% solution of sodic
citrate.( The same quantity of ferrous sulfate or of sodic citrate
in a larger quantity of water will occasion much loss of the
silver product). I think some advantage is gained by neutralizing
the ferrous solution, which has a strong acid reaction, with the
solution of sodium hydroxide: as much may be added as will not
cause a permanent precipitate. To the quantities already given,
about 50 cc of 10% soda solution. The reaction takes place equally
well without the soda, but I think the product is a little more
stable with it. ) The mixed solution is to be added at once to the
The beautiful lilac shade of the precipitate is rather ephemeral.
It remains for some time if the precipitate is left under the
mother water, but when thrown upon a filter, it is scarcely
uncovered before the lilac shade disappears and the precipitate
takesa deep blue color, without losing its solubility. It may be
washed either on a filter or by decantation, with any saline
solution in which it is insoluble and which does not affect it too
much. On the whole, ammonium nitrate does best, but sodic nitrate,
citrate, or sulfate may be used, or the corresponding ammonia
salts. Although in pure water the precipitate instantly dissolves
with an intense blood red color, the presence of 5 or 10% of any
of these salts renders it perfectly insoluble. I have usually
proceeded by adding to the precipitate ( after decanting the
mother water as completely as possible...), a moderateamount of
water; for the above quantities about 150 cc. Much less would
dissolve the precipitate but for the salts present: this much will
dissolve the greater part but not the while, which is not
necessary. A little of a saturated solution of ammonium nitrate is
to be added, just enough to effect complete precipitation.
As the material appears continually to change, the amount of
washing needed must depend upon the object in view. If wanted for
analysis, the washing must be repeated many times until a ferric
salt ceases to come away, but no amount of washing will eliminate
it entirely. After 7 or 8 solutions in pure water and as many
precipitations, the material is to be filtered and then the
ammonium nitrate washed out with 95% alcohol until the filtrate
leaves nothing on evaporation. The substance at this point is
still soluble in water, though much less so than at first. During
the washing the solubility slowly but steadily diminishes, a fact
rendered noticeable by less and less ammonium nitrate being needed
to precipitate it from its solution...
... The freshly precipitated material dissolves to a blood red
liquid, by great dilution yellowish red. The purified substance
gives a darker red solution, which with dilution remains still red
B. Insoluble Form of the
The solution of the blue product just described is influenced in a
remarkable way by the addition of almost any neutral substance. So
far I hve not found any that does not precipitate it. Not only
saline solutions do this, but even a solution of gum arabic.
Neutral salts may precipitate the silver in either a soluble or an
insoluuble form. Alkaline sulfates, nitrates and citrates throw
down the soluble form, magnesium sulfate, cupric sulfate, ferrous
sulfate, nickel sulfate, potassium bichromate and ferrocyanide,
barium nitrate, even silver nitrate and other salts throw down a
perfectly insoluble form. The soluble form cosntitutes a bluish or
bluish black precipitate; the insoluble, a purple brown, which by
repeated washings continually darkens.
What is very curiosu is that the insoluble form may be made to
return to the soluble condition. many substances are capable of
effecting this change. Sodium borate does so, produing a brown
solution, potassium and sodium sulfate produce a yellowish red
solution and ammonium sulfate a red one. None of these solutions
has the same blood-red color as the original solution; the form of
silver seems to change with the slightest change of ondition.
The solutions used must be extremely dilute, otherwise the silver,
though rendered soluble in pure water by them, will not dissolve
in the solution itself, a singular complication of effects... The
insoluble substance also is readily soluble in ammonia. The
solution has a fine red color, and not the yellowish red of the
sodium sulfate solution.
Most neutral salts act in one or another ways just described,
precipitating the solution of the blue substance A in either the
soluble or the insoluble form, the latter soluble in ammonia, but
sodium nitrite is an exception; its solution effects an entire
change and renders the substance wholly insoluble, probably
reconverting it to normal silver...
Amer. J. Sci. 138 ( 223 ) p. 47 (
... The two insoluble forms of allotropic silver which I have
described as B and C; B, bluish green, C rich golden color, show
the following curious reaction. A film of B, spread on glass and
heated in a water stove to 100 C for a few minutes becomes
superficially bright yellow. A similar film of the gold-colored
substance C treated in the same way, acquires a blue bloom. In
both case it is the surface only that changes.
Sensitiveness to Light --
All these forms of silver are acted upon by light. A and B acquire
a brownish tinge after some hours' exposure to sunlight. With C
the case is quite different, the color changes from that of red
gold to that of pure yellow gold... Complete exhaustion of air and
light is certainly favorable to permanence...
Specific gravity -- The
allotropic forms of silver show a lower specific gravity that that
of normal silver.
In determining the s.g. it was found essential to keep the s.g.
bottle after placing the material in it for some hours under
vauum. Films of air attach themselves obstiantely to the surfaces
and esape but slowly even in vacuo.
... Blue substance B gave sp.gr. 9.58 and the yellow substance C,
sp.gr. 8.51. The sp.gr. of normal silver after melting is 10.5.
That of finely divided silver is 10.62 ...
Amer. J. Sci. 42 ( 250 ) Oct.,
Art. XXX. --- Notes on Allotropic Silver
Relations of the Yellow to the
Blue Forms -- ... In previous papers there has been
described a crystalline state intermediate between these active
forms and ordinary silver, which intermediate condition, while
retaining the bright yellow color of the active form is nearly as
indiferrent to reagents as ordianry silver. Into this intermediate
state both the yellow and blue forms are capable of passing, and
apparently the intermediate states of both kinds of allotropic
silver are identical: the intermediate form of blue silver is
yellow. Thus when lumps of blue silver are heated in a test tube
to about 180 C they assume a gold color and luster. The same
changes take place at the same temperature when films of blue
silver are placed in a hot air bath...
Blue silver can be converted into yellow at ordinary temperatures
and consequently with retention of its active properties. This is
accomplished through the delivery of sulfuric acid. When a
solution of silver is obtained by the action of sodium hydroxide
and dextrine on silver nitrate (*) it appears to contain the blue
variety, for if allowed to precipitate spontaneously by long
standing, or if precipitated by acetic acid, dilute nitric acid,
or by many neutral substances, it gives a form of silver which is
dark red while moist and dries with a blue surface color. (It is
always a little difficult to characterize these substances by
their colors since the surface color which they show when dry is
mostly complementary to their color when wet. The surface color is
much the more characteristic, I have adopted the course of naming
them by that.
(*) 40 gr each of naOH and of yellow or brown dextrine (not white)
are dissolved in 2 liters water and 28 gr Ag-Nitrate in solution
are added in small quantities in turn, with frequent stirring, so
that several hours shall lapse before the last portion is added.
The solution is always slightly turbid when viewed by reflected
light, by which it shows a beautiful deep green color. By
transmitted light it is deep red, and when diluted, absolutely
transparent. By diminishing the proportion of silver nitrate to
one-half, a solution nearly or quite clear by reflected as well as
by transmitted light is obtained.
The behavior of the red solution obtained by soda and dextrine
with dilute sulfuric is very interesting and instructive. When 100
cc of solution are poured into 100 cc of water to which 3 cc
sulfuric acid has been added, a dark red precipitate falls which,
when dry, especially in films, is blue. The mixed liquid from
which the precipitate is formed is acid. Increasing the proportion
of acid to 4, 5 and 6 cc successivley, the substance obtained has
a green surface color becoming more yellowish green in proportion
as the acid is increased in quantity. With 7-1/2 the
substance no longer dries green but yellow. Increased proportions
of acid produces substances drying with a coppery shade.
It will be seen that from a single solution, and using one
substnace only as a precipitant, we can obtain the whole range of
different forms of allotropic silver, simply by varying the
proportions of the precipitant.
That these forms of silver should subsist in the presence of
sulfuric acid in excess is remarkable. For the most part the
presence of this acid tends to quickly convert allotropic to
ordinary silver. For example, bright yellow allotropic silver
obtained with ferrous tartrate was washed on a filter with water
containing 1/500 its volume of sulfuric acid: in two hours the
entire mass was converted into gray ordinary silver.
...The substances precipitated with the least acid, have a very
splendid luster, and this luster diminshes steadily as the
proportion of acid is increased...
But we can also obtain the converse of this reaction. Just as the
solution which naturally would yield the blue product, can be made
to yield the yellow by the presence of excess of strong acid, so
the solution which normally yields the yellwo substance, may be
made to produce blue ( or rather green ) silver by adding alkali.
Thus a mixture of dilute solutions of ferrous sulfate and of
Rochelle salt added to mixed solutions of silver nitrate and of
Rochelle salt rsults in the formation of fold-colored silver. But
if we add a little sodium hydroxide to the iron solution or the
silver mixture, we shall get a bluish green product, whose
properties show that it belongs to the blue class and not to the
There is a well marked tendency of acids to give rise to the
formation of the yellow product and of alkalies to the blue. But
this is a tendency only. Both substances can be produced from
neutral solutions, and slight changes are sufficient to alter the
product formed. Thus, ferrous tartrate, in dilute solution acting
on silver tartrate gives rise to the formation of the gold-colored
substance, but when citrates are substituted, the blue substance
Action of Light on Blue Silver
-- This action differs with different varieties; it was more
especially exmained with the form that is obtained from the
soda dextrine silver solution already described by pouring the
solution into an equal bulk of water to which sulfuric acid had
been added in the proportion of 4 cc per 100 cc water. This form
was selected because it is easy to obtain with great constancy of
result, and because it is one of the forms of blue silver most
sensitive to light.
Exposed to light, this substance first becomes more distinctly
blue, losing a slight greensih shade. With continued exposure it
passes to a yellow-brown shade. and finally to a perfectly pure
golden-yellow of great brilliancy and luster. The last is the
intermediate or crystalline form.
The action of light on this form of silver is remarkable in this
respect, that its first effect is to increase the sensitiveness to
This result was so unexpected and a priori so improbable, that it
was subjected to the most careful verification before being
accepted... Upon this form of silver light has a reversing action,
first exalting its sensitiveness, then completely destroying it...
[ like silver bromide ]
Three of the principal modes of formation of allotropic silver
(1) reduction of silver citrate or tartrate by ferrous nitrate or
(2) acting on silver nitrae or oxide by dextrine and fixed
(3) acting on silver nitrate or carbonate by tannin and fixed
Now if in either of these cases we interrupt the action before it
is complete by adding an excess of dilute hydrochloric acid we
shall obtain a dark chestnut-brown or sometimes purple-brown
substance which on examination proves to be a mixture of silver
subchloride and protochloride. When, after complete removal of the
excess hydrochloric by boiling with distilled water, the substance
is treated with cold dilute nitric acid, that portion of the
subchloride which is not combined with the normal chloride is
broken up and there remains protochloride of a very rich and
intense rose-color. It is perhaps the best means for obtaining
silver protochloride... Hydrochloric acid, though without action
on ordinary silver, is capable of forming a variable quantity of
protochloride when placed in contact with allotropic silver.
I have not met with any exception to this general principle that
when a reaction leading to the formation of allotropic silver is
interrupted by the addition of hydrochloric acid, subchloride is
abundantly formed as one of the products.
In all such cases the reduction is evidently indirect. The silver
does not lose at once the whole of its oxygen, but appearently
passes through an intermediate form, prodbably Ag4O, the reduction
of which tends to the formation of allotropic silver.
Amer.J. Science ( July 1889 )
Art. XXXIV -- The Properties of Allotropic Silver
The three forms of allotropic silver which were described in the
June number of this Journal -- the blue soluble and the blue and
the yellow insoluble -- are not to be understood as the only forms
which exist, but as the best marked only. The substance is protean
and exhibits other modification not yet studied. No other metal
than silver appears to be capable of assuming such a remarkable
variety of appearances. Every color is represented. I have
obtained metallic silver blue, green ( many shades of both ), red,
yellow and purple, In enumerating these colors I do not refer to
interference colors produced superficially by reagents, also
wonderfully brilliant, but to body colors....
Two of the insoluble forms of allotropic silver, the gold-colored
and the blue, show in many respects a close relationship and
almost identical reactions... Blue allotropic silver (dark red
when moist, becoming blue in drying ) is very stable. It may be
exposed for weeks in a moist state on a filter, or be placed in a
pasty condition in a corked vial and so kept moist for months,
The gold-colored form on the contrary tends constantly to revert
to ordinary silver. This is especially the case when it is moist,
so that from the time of its formation, it must be separated from
its mother water and washed as rapidly as possible, otherwise it
loses its brilliancy and purity of color and changes to a dark
dull gray form of normal silver. On the filter, its proper color
is black with a sort of yellow shimmer ( the gold color appearing
as it dries ) often, especially if allowed to become uncovered by
the water during washing, it will change superficially to gray
(When well washed this form can be preserved for a time in the
moist condition in a corked vial). But if the washing is done
rapidly with the aid of a vacuum filter, the allotropic silver
obtained, when allowed to dry in lumps, or brushed on paper or
glass, is at least equal to pure gold in color and brilliancy...
When gold-colored allotropic silver is gently heated in a test
tube it undergoes a remarkable change in cohesion. Before heating
it is brittle and easily reduced to fine powder. After heating it
has greatly increased in toughness and cannot be pulverized at
Both the gold-yellow and the blue forms resemble normal silver in
disengaging oxygen from hydrogen peroxide.
Many substances which react little if at all with ordinary silver,
attack the gold-colored and the blue allotropic silver with
production of very beautiful colors due to the formation of thin
films and resulting interference of two reflected rays. In my
previous papers I called this the "halogen reaction" because first
obtained by the action of halogens... But many other reagents will
produce the same or similar effects. These are:
Sulfides -- Paper brushed over with either the gold, the
copper-colored, or the bluish green substance exposed to the vapor
of ammonium sulfide, or immersed in a dilute solution of it,
assume beautiful hues, though less brilliant than those obtained
in other ways.
Potassium permanganate -- in dilute solution produces blue, red
and green colors.
Potassium ferricyanide -- in moderately strong solution gradually
attacks allotropic silver with production of splendid blue, purple
and green coloration.
Phosphorus acid -- produces gradually a rather dull coloration.
The color reaction is produced finely by substances which readily
part with a halogen such as ferric and cupric chlorides, sodium
hypochlorite, hydrochloric acid to which potassium bichromate has
been added, and corresponding bromine and iodine compounds... I
obtained effects of the same sort but in much weaker degree with
alkaline haloids. But with purer products, the results have been
different. There is at first some darkening, but no true color
reaction and the allotropic silver appears to be gradually
converted into normal, so that it is no longer capable of giving
the brilliant color reaction with potassium ferricyanide, but,
like normal silver, takes a pale and faint coloration only.
The perchlorides of platinum, gold, and tin do not give the color
reaction, though by analogy one would expect that they should,
since they can lose chlorine with formation of a lower chloride.
Action of Light -- in a
previous paper was mentioned the remarkable fact that the gold-
and copper-colored forms of allotropic silver can be converted
first into yellow and finally into white normal silver by the
continued action of light. Earlier specimens of the blue form
became brown by exposure, but purer one since obtained are
likewise converted into yellow by exposure, becoming continually
lighter as the action is continued. The conversion from the darker
shades to a bright yellow with full metallic luster is very easy,
but [ unstable ]. Since then I have obtained the gold-colored
silver in a more sensitive form, giving a perfectly white product
by exposure for half that time.
The white silver thus obtained has all the character of ordinary
silver and does not show the color reaction with ferric and cupric
chloride, potassium ferrocyanide, etc. Just in proportion to the
exposure to light, the ability to give this color reaction
diminishes, so that after a day's exposure, when the exposed part
has become bright yellow, the color reagents scarcely affect this
yellow, whilst the protected part becomes intense blue, purple, or
green. In this way it is easy to observe the gradual effect of
light as it changes the allotropic silver into ordinary silver.
Art. XXXV. -- on Ring Systems and other Curve Systems
Produced on Allotropic Silver by iodine.
Allotropic silver, in its moist and plastic state, may be bruished
over paper and gives on drying a continuous and brilliant coating
resembling metalic leaf. When a small crystal of iodine is placed
on paper that has been this coated, rings of remarkable beauty are
obtained. A funnel or beaker should be inverted over the paper to
prevent distortion by currents of air [ unless desired --
controlled air flow produces beautiful patterns ]
That iodine is capable of producing interference rings ( Nobili's
rings ) on metalic surfaces has long been known, and Robert Hunt
has described their formation on surfaces of normal silver... The
contrast between the pale and faded-looking products produced on
normal silver, and the lustrous and glowing hues given by the
allotropic, is very striking. One cannot help wishing that this
splendid coloration could be made to do service for obtaining
natural colors by photographic processes.
As to the durability of these products... protected from light and
air they endure for several months at least. Both the bluish green
insoluble silver B, and the gold-colored C produce these effects;
the gold-colored is the better suited of the two...
The general properties of this substance can be much better
observed in the thin films obtained by brushing the moist
substance over paper than in lumps. The films thus obtained are
bright metallic green, and this green evidently results from a
mixture of blue and yellow... When the films are examined by
normal light reflected from them at a large incidence with the
normal and a Nicol's prism or an achromatized prism of calc-spar
is interposed between the film and the eye, it becomes at once
apparent that the blue and yellow light are oppositely polarized.
The yellow light is polarized in the plane of incidence, and the
blue light perpendicularly to that plane. All specimens show the
yellow light, but the quantity of blue light is very variable and
is directly connected with the amount of washing applied to the
precipitate. The more it is washed the more yellow predominates.
To see the blue form in its full beauty, a little of the red
solution may be precipitated with a very little magnesium or
aluminum sulfate and filtered. As soon as the liquid has drained
off and without any washing, the deep bronze-colored substance is
to be brushed on paper. On drying it has all the appearance of a
bright blue metal with a remarkable luster. The mirrors obtained
on glass are so beautiful and so perfect that it seems as if this
property might have useful applications...
Crystalization -- On one occasion this substance was obtained in a
crystaline form. Some crude red solution had been set aside in a
corked vial. Some weeks after, it was noticed that the solution
had become decolorized, with a crystaline deposit... [ consisting
] of short black needles and thin prisms. Evidently the saline
matters present had balanced the silver so nearly as not to cause
an immediate precipitation, but a very gradual one only. The
mother water was decanted, and a few drops of pure water added. No
sultion took pace: the crystals were therefore of material B, the
insoluble form. The contact of pure water instantly destroyed the
crystalization and the substance dried with a bright green metalic
luster. Contact with pure water evidently tends always to bring
this form of silver into the colloidal state, sometimes soluble
and sometimes not; whilst the contact with certain neutral salts
renders it crystaline...
To obtain the substance in a pure condition suitable for analysis,
it is necessary to choose a precipitant not giving an insoluble
product with either citric or sulfuric acid. Magnesium sulfate or
nickel sulfate answers well. A very dilute solution is made of it
and the red solution of A is to be filtered into it. The
preipitate soon settles. A large quantity of water is to be poured
on, and then washing by decantation can be continued to three
decantations, after which the substance remains suspended. It can
be made to subside by adding a very small quantity of magnesium
sulfate ( 0.25 gr/liter is sufficient). The substance may then be
filtered and washed....
C. Gold Yellow and Copper-Colored
It has been long known that golden-yellow specks would
occasionally show themselves in silver solutions, but could not be
obtained at will and the quantity was infinitesimal. Probably this
phenomenon has often led to a supposition that silver might be
transmuted into gold. This yellow product, however, is only an
allotropic form of silver, but it has all the color and brilliance
Amer. J. Science [ 3 ] 51 ( 244 )
p 259-267 ( April 1891 )
Art. XXVIII --- On Allotropic Silver
Part II -- Relations on
Allotropic Silver with Silver as it exists in Silver
... In the present case we have to consider three distinct forms
(1) allotropic, (2) intermediate (3) ordinary silver. We notice
that (1) can with the utmost facility and in several ways be
converted into (2) and (3), and that (2) can always be converted
into (3), but that these transformations can by no possibility be
reversed. To convert ordinary silver into allotropic we must as a
first step dissolve it in an acid: that is, convert it from a
polymerized to an atomic form, and only from this atomic form can
allotropic be obtained...
[ There may exist ] three possible moleular forms of silver, viz.:
atomic, molecular and polymerized.,,
Silver may exist in three forms: 1st. Allotropic silver which is
protean in its nature; may be soluble or insoluble in water, may
be yellow, red, blue or green, or may have almost any color, but
in all its insoluble varieties always exhibits plasticity, than
is, if brushed in a pasty state upon a smooth surface its
particles dry in contact, and with brilliant metallic luster. It
is chemically active. 2nd. The intermediate form, which may be
yellow, or green, always shows metallic luster, but is never
plastic and is almost as indifferent chemically as white silver.
3d. Ordinary silver... Further, that alotropic silver can always
be converted, either into the intermediate form, or directly into
ordinary silver; that the intermediate form, or directly into
ordinary silver; that the intermediate form, or directly into
ordinary silver, that the intermediate form can always be
converted into ordinary silver, but that these processes can never
be reversed, so that to pass from ordinary silver to allotropic it
must first be rendered atomi by combination, and then be brought
back to the metallic form under condition which chek the atoms in
uniting. Allotropic silver is affected by all forms of energy, and
this effect is always in one direction, namely, towards
Art. LVIII -- Allotropic Silver ( Part III ): Blue
Silver, soluble & insoluble forms
Allotropic Silver obtained
with Dextrine and Alkaline Hydroxide
When dextrine is dissolved in a solution of potassium or sodium
hydroxide and silver nitrate is added, keeping the hydroxide in
moderate excess, the silver is at first thrown down in the form of
the well known brown oxide. This brown color presently changes to
a reddish chocolate shade and at the same time the silver begins
to dissolve. In a few minutes the whole has dissolved to a deep
red color, so intense as to be almost black. A few drops poured
into water give it a splendid red color of perfect transparency.
Examination with the spectroscope leaves no doubt that we have to
do with a true solution.
It is interesting to obseve that silver can be held in solution in
neutral, acid and alkaline liquids. In the first process which I
which I published, in which silver citrate is reduced by a mixture
of sodic citrate and ferrous sulfate, the latter may be used
either in acid solution or it may be first neutralized with
alkaline hydroxide, so that that form of silver is held in
solution in either a neutral or an acid liquid. The form that is
obtained with the aid of dextrine dissolves most freely in the
strongly alkaline liquid in which it is produced, and when dilute
nitric or sulfuric acid is added the silver is precipitated. But
with acetic the precipitation is very incomplete: the solution
retains a brown color and contains silver. Even the addition of a
large excess of strong acetic acid fails to throw down any more
silver. it follows therefore that while this form of silver is
most freely soluble in a strongly alkaline it is also soluble to
some extent in one that is either neutral or acid.
The precipitate when once formed appears to be almost insoluble. A
small portion of it stirred up with distilled water gives no
indication of solution. But if a quantity is thrown is washed on a
filter, as soon as the mother water is washed out the liquid runs
through of a muddy red, and if this filtrate be allowed to stand
it deposits an insoluble portion and then has a fine rose-red
color and perfect transparency. Notwithstanding the beautiful
color it contains a trace of silver only, so great is the coloring
power of the metal. Sometimes if the alkaline stnads for a month
or two the silver becomes spontaneously insoluble; most of it
falls to the bottom as a deep red substance, but part remains in
suspension with a bright brick red color. The difference between
this and the true solution as originally formed is extremely well
Dextrine is a very variable substance and different specimens act
very differently. Common brown dextrine seems to do better than
the purified forms.
Convenient proportions are as follows: in two liters of water 40
grams of sodium hydroxide may be dissolved and an equal quantity
of dextrine, filtering if necessary. 28 grams of silver nitrate
are to be dissolved in a small quantity of water and added by
degrees at intervals. Complete solution readily takes place.
Although the liquid contains less than 1% of metallic silver it
appears absolutely black, when diluted, red, by great dilution
yellowish. With some specimens of dextrine the solution remains
clear, with others it soon becomes a little turbid.
Perhaps the most interesting reaction which this solution shows,
is that with disodic phosphate. A little phosphate is sufficient
to throw down the whole of the silver although both solutions are
alkaline. When a gram of phosphate in solution is added to 100 cc
of silver solution the color becomes bright red, sometimes
scarlet, and the whole of the silver is presently on the filter
has precipitated. This precipitate on the filter has a color like
that of ruby copper, which color it retains during the first
washing, but after a few hours washing with distilled water the
color changes to a deep Nile green and at the same time it becomes
slightly soluble, giving a port wine colored solution. With more
washing this solubility may disappear.
It is a general fact that all these forms of silver, however
various their color, have both a body and a surface color and
these two colors tend always to be complementary. The body color
is that shown by the precipitate while still moist; it is also
visible when a thin coat is brushed over paper, a coat so thin
that light passes through it, is reflected by the paper and
returned again through the film. But when a thick and opaque film
is applied, the body color disappears and only the complementary
surface is visible...
Allotropic Silver obtained with
Tannin and alkaline Carbonates
Tannin (gallotannic acid) in alkaline solution reduces silver
nitrate to metallic silver in the allotropic form. Tannin acts
more strongly than dextrine and therefore does best with
carbonated alkali, dextrine best with alkaline hydroxide, although
either substance will produce the reation with either form of
alkali and, though less advantageously, with ammonia. Tannin with
sodium carbonate gives a very perfect solution of silver, quite
free from the turbidity that is apt to characterize the dextrine
solution. The color of this solution is likewise very intense: one
containing 1% of silver is quite black, by dilution deep yellowish
red. It has very much the same characteristics as the preceding,
bt is rather more stable. To obtain it, 24 grams of dry sodium
carbonate may be dissolved in 1200 cc of water. A 4% solution of
tannin is to be made and filtered, of this 72 cc are to be added
to the solution just named; of silver nitrate, 24 grams dissolved
in a little water are to be added by degrees. Solution takes place
almost instantly as each successive portion is added. The solution
after standing a day or two may be decanted or filtered from a
small quantity of black precipitate.
When the solution is treated with a very dilute acid, as for
example, nitric acid diluted with 20 volumes of water, allotropic
silver is precipitated in the solid form. It dries with a
brilliant metallic surface color of a shade different from the
foregoing and somewhat dificult to exactly characterize, a sort of
I do not find that blue allotropic ( in which is included the
green and steel-gray varieties) can be reduced to any one definite
type. On the contrary, its variations are endless. Slight
differences in the conditions under which the solutions are formed
or in the mode of precipitation give quite different products. For
example, of ten products obtained give quite different products.
For example, of ten products obtained with tannin and sodium
carbonate in different proportions, several were easily and
completely soluble in ammonia, some were slightly soluble and some
not at all. Some specimens not at all soluble in water become so
by moistening with dilute phosphoric acid: they did not dissolve
in the acid but when it was removed they had beceome soluble in
water. On other specimens phosphoric acid had no such effect. Some
solutions are scarcely affected by acetic, others are partly
precipitated, others almost but not quite wholly. The films spread
on paper vary very much in their relations to light; some are
readily converted into the yellow intermediate form, whilst others
are very insensitive. The least sensitive specimens seemed to be
those for which dilute nitric acid had been used as a precipitant.
They had a steel-gray color. Precipitation by acetic acid seems to
tend to a greenish metallic surface color and greater
sensitiveness. Different specimens also vary very much as to
permanency; this character is also affected by the amount of
washing received: thorough washing tends to permanency.
In some ways the blue, gray and green forms seem more closely
related to the black or dark gray forms of normal silver, for they
tend in time to pass into them, while on the contrary,
gold-colored silver, if pure, tends with time to change to bright
white normal silver on the surface, with dark or even black silver
Action of other Carbonates
Tannin is capable of producing allotropic silver, not only in the
presence of the arbonates of potassium and sodium, but also with
those of lithium and ammonia and also with the carbonates of
calcium, magnesium, barium and strontium. The action of the last
named carbonate has been more particularly examined. it yields
allotropic silver of a dark red color while moist, drying with a
rich bluish green metallic surface color in thick films, in very
thin films transparent red. It is probable that the substances
with which tannin produces these reactions would be further
reduced by investigation...
Nature of the " intermediate
It has been mentioned in previous papers that when allotropic
silver is converted into normal silver by the action of heat it
passes through a perfectly well marked intermediate state. In this
state it retains the gold-yellow color and high luster but none of
the other properties of the original form. Oxidizing and
chlorizing agents show nearly the same indifference as with
ordinary silver. While other allotropic silver is soft and easily
reduced to powder the intermediate substnace is hard and tough.
When a glass rod is drawn over a film of allotropic silver it
leaves behind it a white trace of ordinary silver. The
intermediate substance shows no such reaction: the trace of a
glass rod does not differ from the rest of the film and even hard
burnishing produces no change in the color. Continued exposure to
sublight brings about the same alteration to the intermediate form
and it takes place spontaneously with time.
At that time no explanation could be found as to the nature of the
change. It proves however to be a passage into a crystalline form.
Some films spread on paper were exposed to the action of a very
dilute solution of ferric chloride. It chanced that one of these
films had undergone a partial change into the intermediate; the
unchanged portion was darkened by the ferric solution, while the
portion that had passed into the intermediate form retained its
bright gold-yellow color and luster rendering it thus
distinguishable. The figures which it exhibited were strikingly
crystaline. One portion showed a foliated struture such as if
formed by interpenetrating crystals, other parts showed
ramifications, with something of a plant-like form. Another part
exhibited a sheaf of acicular cystals nearly parallel in
direction, half an inch to an inch long and fine as hairs...
This change to the crystaline condition does not seem to be
peculiar to gold-colored silver...
The stable "intermediate form" of silver (i-Ag) is easy to
prepare. It occurs as bright gold-yellow or green crystals with a
metallic luster. Treatment with a very dilute solution of ferric
chloride will enhance the appearance of its foliar structure,
interpenetrating with plant-like ramifications, or fine acicular
crystals up to 1 inch long.
Intermediate silver is hard, tough, and unaffected by pressure. It
is nearly as indifferent to oxidizing and chlorizing agents as is
normal silver. Intermediate silver can be formed from the
allotropic varieties by light, heat, or chemical action. The
simplest preparation is as follows:
"...It is a little curious that its permanency seems to depend
entirely on details in the mode of preparation. I have found many
ways of obtaining it, but in a few months the specimens preserved
changed spontaneously, to normal silver. This happened even in
well closed tubes. The normal silver produced in this way is
exquisitely beautiful. It has a pure and perfect white color like
the finest frosted jewelers' silver, almost in fact exceeding the
jeweler's best products. I found, however, one process by which a
quite permanent result could be obtained...
In forming the blue product which I have called A, very
concentrated solutions were necessary. C on the contrary is best
obtained from very dilute ones. The following proportions give
Two mixtures are required: No. 1 containing 200 cc of a 10%
solution of silver nitrate, 200 cc of 20% solution of Rochelle
Salt [Sodium potassium tartrate] and 800 cc of distilled water.
No. 2, containing 107 cc of a 30% solution of ferrous sulfate, 200
cc of a 20% solution of Rochelle salt and 800 cc of distilled
water. The second solution (which must be mixed immediately before
using only) is poured into the first with constant stirring. A
powder, at first glittering red, then changing back to black,
falls, which on the filter has a beautiful bronze appearance.
After washing it should be removed whilst in a pasty condition and
spread over watch glasses or flat basins and allowed to dry
spontaneously. It will be seen that this is a reduction of silver
nitrate by ferrous sulfate. The metallic silver formed by
reduction with ferrous citrate and ferrous tartrate is in an
allotropic condition; with ferrous oxalate this result does not
seem to be produced.
Although the gold-colored silver (into which the nitrate used is
wholly converted) is very permanent when dry, it is less so when
wet. In washing, the filter must be kept always full of water;
this is essential. It dries into lumps exactly resembling highly
polished gold... By brushing a thick paste of this substance
evenly over clean glass, beautiful cold-colored mirrors are
obtained; the film seems to be entirely continuous and the mirror
is very perfect.
By continuous washing the precipitate changes somewhat, so that in
drying it takes on a coppery rather than a golden color, and is
rather less lustrous, though still bright and brilliant...
Art. XX -- On Gold-Colored Allotropic Silver ( Part I
The most characteristic reactions of gold-colored allotropic
silver are those with the strong acids. When normal silver reduced
with milk sugar and alkaline hydroxide is left in contact with
strong hydrochloric acid even for several hours there is no
action, and the silver after thorough washing dissolves in warm
dilute nitric acid without residue. With allotropic silver
similarly treated chloride is always formed. But strong
hydrochloric acid instantly converts allotropic to ordinary silver
and consequently only atrace of chloride is produced. By largely
diluting the acid the conversion is retraded and the proportion of
chloride is greatly increased. Thus for example when ordinary
hydrochloric acid is diluted with 50 times its volume of water and
is made to act on allotropic silver, about one-third is converted
to chloride. Probably the whole would be but for the simultaneous
conversion to normal silver. This double action is curious and
strongly differentiates allotropic from ordinary silver. Even with
the same acid diluted with 100 volumes of water, there is a
gradual but complete conversion to white silver accompanied by the
production of a not inconsiderable quantity of silver chloride.
Neutral chlorides also act strongly upon allotropic silver even
when much diluted...
Sulfuric acid diluted with 50 volumes of water has no action upon
n ormal silver. It quickly converts allotropic silver to normal
but at the same time dissolves a little of it...
Ammonia seems to be without a converting action but dissolves a
trace. It will be shown in a future paper that there exists a form
of allotropic silver abundantly soluble in ammonia...
Allotropic silver presents itself in an almost endless variety of
forms and colors... Most of these varieties seem to be capable of
existing in two conditions, of which one is more active than the
If we coat a chemically clean glass plate with a film of
gold-colored allotropic silver, let it dry, first in the air, then
for an hour or two in a stove at 100 C, and then heat the middle
of the plate carefully over a spirit lamp, we shall obtain with
sufficient heat a circle of whitish gray with a bright, lustrous
golden ring round it, somewhat lighter and brighter than the
portion of the plate that has not been changed by heat. This ring
consists of what I propose to call the "intermediate form".
With sulfuric acid diluted with four times its bulk of water and
allowed to cool, an immersion of one or two seconds converts a
film on glass or on pure paper wholly to the intermediate form...
Its properties are better seen by using a film formed on pure
paper, one end of which is heated over a spirit lamp to a
temperature just below that at which paper scorches. The change is
sudden and passes over the heated portion like a flash. Examining
the changed part, we find :
1st. That it has changed from a deep gold to a bright yellow gold
2nd. When subjected to a shearing stress it does not whiten or
change color in the slightest degree.
3rd. It is much harder, as is readily perceived in burnishing it.
4th. It no longer shows the color reaction with potassium
ferricyanide and ferric chloride, changing only by a slight
deepening of color.
Of these characteristic changes the second is the most remarkable.
The gold-colored silver in its original condition changes with
singular facility to white silver; almost any touch, any friction
or pressure effects the conversion... Heat effects the same change
but with an intermediate stage at which pressure no longer
produces any action.
The intermediate form is distinguished from normal silver almost
solely by its bright yellow color and its higher luster. This last
difference is very striking when a film on glass is heated in the
same manner as above. The central parts in changing to white
silver become wholly lusterless, while the circle of intermediate
retains all its original luster. Its continuity is so complete,
that if viewed through glass, it still acts as a mirror.
This change may be either molecular or depend on dehydration.
The latter seems doubtful for the change cannot be brought about
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