Cent. Eur. J. Chem. • 8(2) • 2010 • 448–452
DOI: 10.2478/s11532-009-0148-2
Central European Journal of Chemistry
Novel modification of elemental silver
formed in Ag4[Fe(CN)6]-gelatin-immobilized
matrix Implants
Research Article
Oleg V. Mikhailov1*, Natal’ya I. Naumkina2
1 Kazan State Technological University,
420015 Kazan, Russia
2
Central Scientific-Research Institute of Geology Non-Ore Minerals,
420097 Kazan, Russia
Received 30 September 2009; Accepted 10 December 2009
Abstract: A
g4[Fe(CN)6]→Ag reduction process occurring in Ag4[Fe(CN)6]- gelatin-immobilized matrices, has been studied. In the course of
reduction, these matrices are in a contact with water-alkaline solu-tion containing tin(II) dichloride along with an organic or inorganic
compound capable of forming a stable soluble complex with Ag(I). It has been found that the substance formed in the gelatin matrix,
is almost pure elemental silver, which consists two various phases which are distinguished by their optical and X-ray diffraction
parameters.
Keywords: Elemental silver • Silver(I)hexacyanoferrate(II) gelatin-immobilized matrix
© Versita Sp. z o.o.
1. Introduction
Over 40 years ago in [1] it was mentioned that small
particles of colloidal elemental silver formed in gelatin
layers during development of silver halide photographic
emulsions. A number of works, in which there are
indications of the existence of a separate phase of
elemental silver consisting of nano-particles being
generated as a result of photochemical reduction of
Ag (I) salts, have appeared lately in the literature, in
particular [2-13]. Before in [12,13] it had been noted
that during development of gelatin layers of silver halide
photographic emulsions by water-alkaline solutions
containing tin(II) dichloride and some inorganic or
organic substance forming rather stable coordination
compounds with Ag(I), the gelatin layer is tinged brown
or red. However, formation only of element silver occurs.
Therefore, it should be noted that under standard
development of using hydroquinone developers, gelatin
layer is tinged black or grey.
It is significant that with an increase of optical density
of the gelatin layer containing such an elemental silver,
a red tone in the coloring of the gelatin layer becomes
more and more clearly expressed. A similar phenomenon
takes place, too, when instead of silver halide (AgHal) in
a gelatin matrix there are silver(I) compounds such as
silver(I) hexacyanoferrate(II) Ag4[Fe(CN)6]. Whether this
totality of particles constitutes a novel phase of element
silver or is only a variety of previously known phases of
the given simple substance has remained open till now.
In the present work we have tried to decide it.
2. Experimental Procedure
As the initial material to obtain silver-containing gelatinimmobilized matrix implants (hereafter GIM), X-ray
film Structurix D-10 (Agfa-Gevaert, Belges) was used.
Samples of the given film (which actually is nothing but
AgHal-GIM) having format 20x30 cm2 were exposed
to X-ray radiation with an irradiation dose the range
0.05-0.50 Röntgen. These exposed films were further
* E-mail: [email protected]
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O. V. Mikhailov, N. I. Naumkina
subjected to processing according to the following
technology [12,13]:
• Development in D-19 standard developer as
indicated in [12,13], for 6 min at 20-25оC;
• Washing with running water for 2 min at 20-25оC;
• Fixing in 25% water solution of sodium
trioxosulphidosulphate(VI) (Na2S2O3) for 10 min at
20-25°C;
• Washing with running water for 15 min at 18-25оC.
The first three stages of standard processing indicated
(development, washing and fixing) were carried out in
non-actinic green-yellow light, final washing – at natural
light. The samples of GIM obtained which contained
elemental silver (Ag-GIM), then were processed
according to the following protocol:
1) Oxidation in water solution containing (g L–1)
Potassium hexacyanoferrate(III)
Potassium hexacyanoferrate(II)
Potassium hydroxide
Sodium trioxocarbonate(IV) (Na2CO3)
Water
50.0
20.0
10.0
5.0
up to 1000 mL
for 6 min at 20-25оC;
2) Washing with running water for 2 min at 20-25оC;
3) Reduction in water solution containing (g L–1)
Tin(II) chloride
Sodium N,N′-ethylenediaminetetraacetate
Potassium hydroxide
Reagent formed water-soluble complex with Ag(I)
Water
50.0
35.0
50.0
1.0-100.0
up to 1000 mL
for 1 min at 20-25оC;
4) Washing with running water for 15 min at 18-25оC;
5) Drying for 2-3 hours at 20-25оC.
As reagents for forming water-soluble complexes
with Ag(I) (hereafter CR from Complexation Reagent),
ammonia NH3, potassium thiocyanate KSCN, sodium
trioxosulphidosulphate(VI) Na2S2O3, ethanediamine-1,2
H2N–CH2–CH2–NH2, 2-aminoethanol H2N–CH2–CH2–
OH and 3-(2-hydroxyethyl)-3-azapentanediol-1,5 N(CH2–
CH2–OH)3, were used. During the first stage of processing
of Ag-GIM, conversion of Ag-GIM into Ag4[Fe(CN)6]-GIM
occurred, while during the second stage, reduction of
Ag4[Fe(CN)6]-GIM with Sn(II) to elemental silver took
place. And so, peculiar “re-precipitation” of elemental
silver into gelatin matrix occurred incidentally.
Isolation of particulate substances from Ag-GIM
was carried out by hydrolyzing the protein matrix with
proteolytic enzymes (for example, trypsin) destroying
the polymeric gelatin carrier of a GIM. Subsequently
the solid phase was separated from the mother
solution according to a technique described in [14]. The
substances isolated thus from GIM were further analyzed
by X-ray diffraction using a D8 Advance spectrometer
(Bruker, Germany) scanning from 3 up to 65o 2q, with a
step size of 0,05 2q.
The calculation of intensities of reflexes (I) and interplane distances (d) was carried out using the standard
software package EVA. Theoretical XRD spectra (X-ray
patterns) were calculated with the PowderCell program
described in [15]. Optical densities of the Ag-GIMs were
measured by means of a Macbeth TD504 photometer
(Kodak, USA) in a range 0.1-5.0 units with an accuracy
of +2 % (rel.). The optical densities before and after
exposure, DAg and DAg , respectively, were measured
with a blue light-filter with a transmission maximum at
450 nm.
3. Results and Discussion
Striking visual changes were observed over a course
of processing during which Ag4[Fe(CN)6]-GIM is
transformed into Ag-GIM. Ag-GIMs obtained as a result
of standard processing of exposed AgHal-GIM, at rather
small optical density (DAg) have grey colour, while at
large DAg, black colour. The colouring of the Ag-GIM
containing the “re-precipitated” element silver, varied
from black-brown to brick-red depending on the nature
and concentration of the CR.
It is significant, however, that absorption spectra of
both initial and the “re-precipitated” element silver in the
visible area do not contain any accurately expressed
maxima. The qualitative difference observed is that the
optical density of the Ag-GIM with the “re-precipitated”
silver (DAg), at the same volume concentration of
element silver (CAgV) in GIM, is greater than the DAg
values, although DAg depends on both the nature and
quantity of the CR in solution contacting the GIM.
Examples of DAg = f(CAgV) and DAg = f(CAgV) dependences
for inorganic and organic CR are presented in Fig. 1 and
Fig. 2, respectively.
At the first stage of the given process, the reaction
may be described by general Eq. 1 (the braces {….}
indicate formulas of gelatin-immobilized chemical
compounds):
4{Ag} + 4[Fe(CN)6]3– → {Ag4[Fe(CN)6]} + 3[Fe(CN)6]4– (1)
Each of the CR under examination forms with Ag(I) a
soluble complex having a metal ion: ligand ratio of
1:2. That is why, formation of silver(I) complexes with
the corresponding CR will occur to some extent when
Ag4[Fe(CN)6]-GIM is in contact with a solution containing
any of the CR indicated. Gelatin-immobilized silver(I)
hexacyanoferrate(II) as well as any of these soluble
complexes, can be reduced with Sn(II). Consequently,
two parallel processes Ag(I)→Ag(0) will take place
during the contact of Ag4[Fe(CN)6]-GIM with solutions
indicated above, containing Sn(II) and CR:
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Novel modification of elemental silver
formed in Ag4[Fe(CN)6]-gelatin-immobilized
matrix implants
gelatin-immobilized silver(I) hexacyanoferrate(II)
reduction proceeding in a polymer layer,
• Ag(I) complex with CR reduction proceeding at the
interface of phases a GIM/solution.
In water solutions at pH=12-13, Sn(II) is mainly present
as a hydroxo-complex [Sn(OH)3]–. General Eq. 2
represents the first of these processes:
{Ag4[Fe(CN)6]}+2[Sn(OH)3]– + 6OH– →
→ 4{Ag} + 2[Sn(OH)6]2– + [Fe(CN)6]4–
(2)
while the second process is represented by general
Eq. 3 with uncharged L:
2[AgL2]+ + [Sn(OH)3]– + 3OH– → 2Ag + 4L + [Sn(OH)6]2– (3)
and more generally by Eq. 4:
2[AgL2](2z+1) + [Sn(OH)3]– + 3OH– → 2Ag + 4Lz + [Sn(OH)6]2– (4)
where L represents a monodentate ligand with charge z).
The particles of elemental silver formed as a result of
Reactions 3 and 4, theoretically should have smaller
sizes than the particles of elemental silver arising in
the polymer layer of GIM. To be a part of a substance
immobilized in GIM, these particles should place freely
in intermolecular cavities of a gelatin layer. Only in this
case, they may diffuse in GIM and may be immobilized
in gelatin mass. According to data in [16], these cavities
are nano-sized. Therefore, only nano-particles can gain
entry into these cavities. By entering into such cavities,
nano-particles of elemental silver are isolated from each
other minimizing their aggregation with each other.
Increasing the concentration of any CR, will increase
the concentration of coordination compounds formed
by the CR with Ag(I). Correspondingly, the quantity
of nano-particles of the elemental silver formed as a
result of reduction of these coordination compounds
by [Sn(OH)3]– complex, should increase, as well. We
anticipate that when the concentration of these CR
increases, the share of nano-particles contained in
the “re-precipitated” elemental silver should increase
correspondingly. Thus, at the same Ag concentration,
nano-particles of elemental silver owing to their higher
degree of dispersion should result in a higher degree of
absorption of visible light (and, accordingly, higher optical
density) relative to micro-particles in the polymeric layer
GIM. The experimental data presented in Figs. 1 and 2,
are in full conformity with the given prediction.
The particles of elemental silver formed as a result
of Reactions 3 and 4, are one- or two-nuclear. When
their concentration is insufficient to aggregate at the
interface [which occurs when the concentration of Ag(I)
complexes at the interface of phases a GIM/solution is
low enough], these particles, owing to their remoteness
from each other, diffuse into the polymeric layer of GIM,
where they become immobilized without a change of
their size. With an increase in the concentration of CR
[and, accordingly, of concentration of Ag(I) complex with
•
D
CAgV, mole dm–3
Figure 1.
Optical Density vs. [Ag]. The dependenc of DAg = f(CAgV)
(-------) and DAg = f(CAgV) following reprecipitation
by thereduction process, Ag4[Fe(CN)6]→Ag using
sodium trioxosulphidosulphate(VI) as CR at varying
concentrations: 2.0 g L–1 (curve 1), 4,0 g L–1 (2), 8,0 g L–1
(3), 24.0 g L–1 (4) and 40.0 g L–1 (5). The optical densities
were measured with a blue light-filter with a transmission
maximum at 450 nm.
D
CAgV, mole dm–3
Figure 2.
Optical Density vs. [Ag]. The dependenc of DAg = f(CAgV)
(-------) and DAg = f(CAgV) following reprecipitation by the
reduction process Ag4[Fe(CN)6]→Ag using 1,2-diaminoethane as CR -at varying concentrations: 5.0 g L–1 (curve
1), 10,0 g L–1 (2), 20,0 g L–1 (3), 40.0 g L–1 (4) and 80.0 g L–1
(5). The optical densities were measured with blue lightfilter with a transmission maximum at 450 nm.
given CR], the quantity of nano-particles indicated at the
interface of phases the GIM/solution accrues. This leads
to an increase in the number of such particles diffused
into GIM. However, at some rather high concentration
CR in a solution, the effect of aggregation of nanoparticles of elemental silver starts to affect. One- and twonuclear particles of element silver formed at reduction
of corresponding Ag(I) complex, begin to unite to some
extent with each other in larger particles. Polynuclear
particles of elemental silver resulting from such an
association, are not so mobile and, consequently, will
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O. V. Mikhailov, N. I. Naumkina
Figure 3.
XRD-pattern of elemental silver isolated from initial
Ag-GIM.
Figure 5. XRD-pattern of substance isolated from Ag-GIM containing
«re-precipitated» elemental silver and obtained with
using of solution containing ethanediamine-1,2 in
concentration 20.0 g L–1.
not diffuse into the polymeric layer of GIM. They will be
precipitated near the interface GIM/solution (or even to
form as a solid phase on the surface of the GIM). As a
result, rates of an increment of number of one-and twonuclear particles of elemental silver with further growth
of concentration CR begins to be slowed down. Thus,
inevitably there should come the moment when the
number of similar particles will reach some limiting value.
That is why at certain “threshold” concentrations of CR
in a solution, growth of DAg values must stop. Moreover,
when present in excess of this “threshold” concentration,
a decrease DAg should begin. The point is that the
correspondence between the number of aggregated
particles and number of one- and two-nuclear ones with
growth of concentration of Ag(I) complex continuously
grows and has no restrictions. These polynuclear particles
are precipitated in the frontier zone GIM at minimal depth
forming, the same kind of elemental silver as the microparticles of elemental silver formed as a result of reduction
of gelatin-immobilized Ag4[Fe(CN)6] according to Reaction
2. That is why, DAg values with increasing CR concentration
at first increase, reach a maximum and then decrease.
Taking into consideration the foregoing, it may be
assumed that “re-precipitated” gelatin-immobilized
Figure 4.
XRD-pattern of substance isolated from Ag-GIM containing «re-precipitated» elemental silver and obtained with
using of solution containing Na2S2O3 in concentra-tion
20.0 g L–1.
silver should contain, as a minimum, two phases
of silver particles, one of which is formed by nanoparticles, and another, by micro-particles. In order to
corroborate this conclusion, we analyzed the elemental
silver isolated from initial Ag-GIM and elemental silver
isolated from Ag-GIM following the “re-precipitation”
process by X-ray powder diffraction. The X-ray powder
diffraction patterns (XRD-patterns) of samples obtained
are presented in Figs. 3-5. As may be seen, the XRDpattern of initial elemental silver with grey-black color
of the gelatin layer (Fig. 3) and XRD-patterns of “reprecipitated” elemental silver (Figs. 4-5) essentially differ
from each other.In XRD-patterns of “re-precipitated”
elemental silver obtained from all of the studied CR in a
solution in contact with GIM, there are accurate reflexes
having d = 333.6, 288.5, 166.7 and 129.1 pm that are
absent in the XRD-pattern of initial elemental silver. At
the same time, reflexes with d = 235.7, 204.1, 144.4,
123.1 and 117.9 pm are observed. These reflexes are
characteristic for the known phase of elemental silver
isolated from initial Ag-GIM. These XRD data are
consistent with the “re-precipitated” elemental silver
obtained with any of the CR indicated above, containing
at least two structural modifications of elemental silver.
Curiously, the reflexes with d = 333.6, 288.5,
204.2, 166.7 and 129.1 pm approximate the d values
of reflexes of silver(I) bromide AgBr (number of
card PDF 06-0438, parameter of an elementary cell
a0 = 577.45 pm, face-centered lattice, cubic syngonia,
Fm3m group of symmetry according to the international
classification [17]). In this connection, it may be
assumed that the structure of the novel phase contained
in “re-precipitated” elemental silver, at least in outline,
resembles the structure of AgBr and its crystal lattice is
similar to a lattice of silver(I) bromide where the Br atom
positions are occupied by atoms of silver.
To determine whether the reflexes indicated can
belong to elemental silver with such a space structure in
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Novel modification of elemental silver
formed in Ag4[Fe(CN)6]-gelatin-immobilized
matrix implants
inter-plane distances in which are 235.7, 204.1, 144.4,
123.1 and 117.9 pm.
Thus, formation of a novel phase of elemental silver
which, probably, has not been described in the literature
up to now, takes place here indeed.
4. Conclusion
Figure 6.
Theoretical XRD-patterns of elemental silver contained in
initial Ag-GIM (a) and of elemental silver formed in GIM
as a result of “re-precipitation” process (b).
principle, theoretical XRD-patterns of assumed structure
of element silver were generated using PowderCell
as described in work [15]. These XRD-patterns are
presented in Fig. 6. As may be seen, the theoretical d
values calculated (333.6, 288.7, 204.2, 174.1, 166.7,
144.4, 132.5, 129.1, 117.9 pm) for the structure described
above with an elementary cell parameter a = 288.72 pm
and Pm3m symmetry group, practically coincide with d
values experimentally observed in the XRD-pattern of
the “re-precipitated” elemental silver (d = 333.6, 288.5,
166.7 and 129.1 pm). It should be noted in this connection
that d values calculated theoretically for the elemental
silver isolated from initial Ag-GIM (235.4, 204.3, 144.5,
123.2 and 118.0 pm), correspond to compact-packed
crystal structure having an elementary cell parameter
a = 408.62 pm belonging to the Fm3m symmetry group,
Formation of novel phase of elemental silver occurs
as a result of redox-processes proceeding into AgHalgelatin-immobilized matrices during their contact with
water-alkaline solutions containing Sn(II) and NH3,
SCN–, S2O32–, ethanediamine-1,2, 2-aminoethanol or
3-(2-hydroxyethyl)-3-azapenthanediol-1,5. XRD data
show that this novel phase consists of nano-particles
of elemental silver. The symmetry group of this phase
is Pm3m with elementary cell parameter a = 288.72 pm
whereas symmetry group of elemental silver is Fm3m
with elementary cell parameter a = 408.62 pm.Besides,
nano-particles of the new phase are formed in
intermolecular cavities of gelatin mass and give red
color to gelatin layer.
Acknowledgements
The Russian Foundation of Basic Researches (RFBR)
is acknowledged for the financial support of given work
(grant No. 09-03-97001).
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