Note: Descriptions are shown in the official language in which they were submitted.
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ONE-PART BLEACH-FIX LIQUID CONCENTRATES
FIELD OF THE INVENTION
This invention relates in general to photographic
processing, and in particular to compositions and methods of
making bleach-fix concentrates. More specifically, this
invention relates to one-part bleach-fix liquid concentrates that
remain free of crystallized precipitates and retain optimal
oxidizing capability during extended storage.
BACKGROUND OF THE INVENTION
Color developers produce a combined silver and dye image,
however, to obtain clear colors the silver image must be removed.
This is accomplished by oxidizing the metallic silver with a
suitable oxidizing agent, commonly referred to as a bleaching
agent. Any remaining light-sensitive silver halide is then
dissolved in a silver halide solvent, commonly referred to as a
fixing agent. These bleaching and fixing processes may be
performed sequentially in two separate solutions, or in the
alternative, the bleaching agent and fixing agents can be
combined into a single bleaching-fixing solution.
The form in which the bleaching and fixing agents are
typically employed is a dilute aqueous solution. Thus, it is not
generally feasible from an economic standpoint to package,
transport, and store solutions of working strength because this
would involve the packaging, transport, and storage of large
amounts of water. Therefore, the bleaching and fixing solutions
are prepared immediately before use by dissolving the necessary
ingredients in water.
The prevalence of small photographic service laboratories
using automatic processing systems that are supervised by
technicians who have limited technical background requires nearly
foolproof premixed solutions. However, as stated above, full
working ready-to-use solutions are expensive to ship, and
further, the bulky containers take up valuable storage space.
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To overcome these disadvantages, there has been a trend to reduce
the size of packaging, and thus, provide a savings in shipping
costs. One way to achieve this goal is to make either all-powder
or liquid bleaching, fixing, and/or bleach fixing concentrates.
These concentrates merely have to be dissolved in or diluted with
water to form a working solution.
DESCRIPTION OF THE RELATED ART
A bleach/fix solution that is prepared by mixing pre
prepared bleaching and fixing solutions is disclosed in U.S.
Patent No. 5,070,004 issued to Yoshihiro Fujita et al.
Preparation immediately before use is necessary because the
individual components of the separate solutions chemically react
and quickly decompose and/or oxidize reactive components thereby
diminishing the useful life of the working solution.
Because of the serious problem of unwanted chemical
reactions occurring between the components, the bleaching and
fixing concentrates are usually separated into two or more parts
that are subsequently combined to form a working solution. For
instance, a two-part bleach-fixing liquid concentrate is
disclosed in U.S. Patent No. 4,956,268 issued to Kiyoshi Nakazyo
et al, wherein a first concentrate comprises a silver halide
solvent and the second concentrate contains an oxidizing agent.
The components are packaged separately from each other to prevent
chemical reactions that form undesirable reaction products, such
as crystallized precipitates. For instance, when a bleaching
concentrate containing a ferric ion solution is combined with a
fixing concentrate containing a thiosulfate solution the
concentrate will have a very short shelf life because the ferric
ion can be reduced to a ferrous ion and the thiosulfate can
sulphurize to inactivate the concentrate.
While the higher degree of concentration will reduce some
of the volume of water the two separate concentrates are still
not sufficiently compact to substantially reduce the cost of
shipping and/or the necessary storage space.
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In an attempt to provide a one-part concentrate, U.S. Patent
No . 5, 310, 633, issued to Yoshida et al . , discloses a one-part
powdery bleach-fixing composition. Powdered components tend to
solve the disadvantages of shipping and storing separate liquid
concentrates, but lumping or caking of the dry components,
especially of the thiosulfate, can occur during shipping and/or
storage thereby making it difficult to provide a homogenous
solution upon dilution.
U.S. Patent No. 3,751,251 provides for a one-part liquid
bleach-fix concentrated regenerator. As described in this patent
an aqueous solution of an aminopolycarboxylic acid and a
thiosulfate ion are mixed with a sufficient amount of a sulfite
ion, in the form of gaseous sulfur dioxide, to adjust the pH of
the solution to between 5.4 and 6Ø However, this product still
has a limited shelf life due to undesirable crystal formation in
the concentrate.
Accordingly, there is a need for improved one-part bleach-
fix liquid concentrates which can be shipped economically,
require minimal storage space, eliminate the need for mixing
several different components, retain oxidizing ability, and
prevent crystals from forming in the concentrate.
SUMMARY OF INVENTION
It is, therefore, a primary object of the present invention
to provide one-part bleach-fix liquid concentrates having
stability for extended storability.
It is another object to provide one-part bleach-fix liquid
concentrates wherein crystallized precipitates are essentially
non-existent after a year or more of storage.
It is yet another object of the invention to provide one
part bleach-fix liquid concentrates wherein the polyvalent metal
complexed oxidizing agent maintains its oxidizing ability for an
extended time period.
A further object of the present invention is to provide
stable one-part bleach-fix liquid concentrates that upon dilution
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provide working strength solutions from which silver can be
recovered with reduced risk of toxic hydrogen cyanide gas
formation.
A yet still further object of the present invention is to
provide one-part liquid beach-fix concentrates that reduce
transportation and packaging costs.
These and other objects of the present invention will be
clear from the following, more detailed, description.
This inventor found that one-part bleach-fix liquid
concentrates suitable for dilution to working strength solutions
can be formulated to have an extended shelf life without forming
crystallized precipitates by combining a bleaching agent with a
thiocyanate salt fixing agent. After further investigation, it
was found that when a thiocyanate salt is used and/or combined
in a specific ratio with another fixing agent, crystal formation
is essentially eliminated thus increasing the stability of the
one-part liquid bleach-fix concentrate. Surprisingly, it was
found that when the amount of the thiocyanate salt is less than
the amount of the other fixing agent the oxidizing ability of the
diluted working solution is not decreased even after many months
of storage.
In accordance with the present invention there is provided
one-part bleach-fix liquid concentrates which comprise:
a) a bleaching agent for oxidizing metallic silver; and
b) a fixing mixture comprising at least a thiocyanate salt
in an amount to substantially inhibit crystal formation for
extending the stability and shelf-life of the liquid concentrate.
The concentrate remains essentially free of crystallized
precipitates during the extended shelf life.
More preferably, there is provided a first and second fixing
agent wherein one fixing agent is a thiocyanate salt and the
other fixing agent is selected from the group which includes
thiosulfate salts, thioethers compounds, thiourea, thioglycolic
acid, and the like.
The bleaching agent may be selected from the group
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consisting of compounds of polyvalent metals such as iron, (III),
cobalt (III), chromium (VI), and copper (II); peracids; quinones;
nitro compounds and mixtures thereof. The compounds of the
polyvalent metals may include iron (III) compounds such as ferric
chloride; ferricyanide; bichromates; and organic complexes with
aminopolycarboxylic acids, aminopolyphosphonic acids and organic
phosphonic acids.
It was found that the polyvalent metal oxidizing agent does
not precipitate out of the concentrate when a thiocyanate fixing
agent is used as the sole fixing agent, or when combined with
another fixing agent. Generally, the shelf life of the
concentrate increases as the amount of thiocyanate salt increases
in the one-part bleach-fix liquid concentrate. The thiocyanate
salt in the one-part bleach-fix liquid concentrate is generally
used in an amount greater than 0.1 mol/liter to less than 1
mole/liter of concentrate.
The present invention can include additional photographic
processing agents; such as preservatives, alkaline agents,
buffering agents, bleaching accelerators, brightening agents,
anti-staining agents, defoaming agents, surfactants, fungicides,
anticorrosion agents and organic solvents.
The present invention also provides for a method for
processing a color photographic silver halide photosensitive
material after image wise exposure using a one-part bleach-fix
liquid concentrate, comprising the steps of color developing and
processing with a bleach-fix solution which is a one-part bleach-
fix liquid concentrate diluted with a sufficient amount of water
to prepare a working strength solution. The one-part bleach-fix
liquid concentrate is an aqueous solution comprising:
a) a bleaching agent which is a polyvalent metal oxidizing
agent; and
b) a mixture of at least a first and second fixing agent,
one fixing agent being a thiocyanate salt present in an amount
sufficient to inhibit crystal formation in the one-part bleach-
fix liquid concentrate, to extend the shelf-life of the
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concentrate.
The present invention further provides a method of
inhibiting the formation of crystallized precipitates in one-part
bleach-fix liquid concentrates comprising the steps of:
a) providing a bleaching agent which is a polyvalent metal
complexed oxidizing agent, and
b) combining the bleaching agent with a mixture of at least
a first and second fixing agent. The first fixing agent is a
thiocyanate salt present in an amount sufficient to inhibit
crystal formation in the one-part bleach-fix liquid concentrate.
Still further, the present invention provides for a method
of making one-part bleach-fix liquid concentrates comprising the
step of combining a bleaching agent, which is a metal complexed
oxidizing agent and at least a first and second fixing agent, at
least one fixing agent being a thiocyanate salt present in an
amount sufficient to inhibit crystal formation in the one-part
bleach-fix liquid concentrate. The metal complexed oxidizing
agent is preferably a polyvalent metal complexed oxidizing agent,
and more preferably used, a polyvalent metal selected from the
group of iron, copper, cobalt and chromium. When using at least
two different fixing agents the thiocyanate salt is preferably
used in an amount by weight less than the amount by-weight of the
first fixing agent. More preferably, the thiocyanate salt is
used in a sufficient amount to substantially inhibit crystal
formation in the one-part bleach-fix liquid concentrate, but not
exceeding an amount that will produce toxic hydrogen cyanide gas
during silver recovery using electrolytic systems. The other
fixing agent may be a thiosulfate salt.
Yet another embodiment of the present invention provides for
a photographic bleach-fix kit comprising a one-part bleach-fix
liquid concentrate, which upon dilution with a sufficient amount
of water provides a working strength bleach-fix solution.
In one embodiment, the kit comprises a single package
containing a concentrated aqueous solution comprising a bleaching
agent, which oxidizes metallic silver, and at least a thiocyanate
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salt present in an amount sufficient to inhibit crystal formation
in the one-part bleach-fix liquid concentrate.
Another embodiment of the one-part bleach-fix liquid
concentrate kit may comprise:
a concentrated aqueous solution comprising:
i) a bleaching agent which oxidizes metallic silver; and
ii) a mixture of at least a first and second fixing agents.
One fixing agent is a thiocyanate salt present in an amount
sufficient to inhibit crystal formation in the one-part bleach
-fix liquid concentrate.
The bleach-fix components in the single packaging kit have
excellent shelf life characteristics because any adverse
interactions between the components are effectively avoided by
the optimal quantitative ratio of the fixing agents with one
fixing agent being a thiocyanate salt.
The one-part bleach-fix liquid concentrates formulated with
thiocyanate salt have pH values of from about 5.6 to about 5.9.
When the liquid concentrates are diluted with water to yield
working strength bleaching and fixing photographic solutions the
pH values are from about 5.5 to about 6Ø
To maintain the pH one of the preferred acids is phosphoric
acid. In using the phosphoric acid it was found that in addition
to maintaining an acid pH in the stable bleach-fix liquid
concentrates of this invention, phosphoric acid is particularly
useful in inhibiting the formation of crystals and to extend the
shelf-life of the concentrates disclosed herein.
Accordingly, it is an alternative embodiment of the present
invention to provide for a stable one-part bleach-fix liquid
concentrate, which comprises:
a) a photographic bleaching agent which oxidizes metallic
silver;
b) at least one fixing agent, and
c) a source of phosphate ions present in an amount
sufficient to inhibit precipitation and to form a storage-stable,
liquid concentrate having a pH value of between about 4 and about
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7. The resulting one-part bleach-fix liquid concentrate is
suitable for dilution with water to a working strength solution.
While phosphoric acid is a good source of phosphate ion,
other available sources of phosphate ion, such as a phosphate
salt, can be used as well. The silver metal oxidizing agent is
preferably a polyvalent metal, and more preferably, a polyvalent
metal selected from the group of iron, copper, cobalt and
chromium.
The present invention further provides for a method of
making a stable one-part bleach-fix liquid concentrate comprising
the steps of combining a photographic bleaching agent, which
oxidizes metallic silver, at least one fixing agent, and a source
of phosphate ions in an amount sufficient to inhibit crystal
formation. This method forms storage stable liquid concentrates
with pH values of between about 4.5 and about 6.5 suitable for
dilution with water to a working strength solution.
In addition, the present invention provides for stabilizing
one-part bleach-fix liquid concentrates by introducing a source
of phosphate ions into the concentrates in amounts sufficient to
inhibit crystal formation, and to maintain the concentrates
within a range of pH values that are between about 4 and about
7.
Whereas, the one-part bleach-fix liquid concentrates of the
present invention can be diluted into working solutions that have
excellent bleach-fixing power, they are effectively applicable
to the processing of not only low speed, but also high speed
light-sensitive silver halide color photographic materials.
Further, the concentrates can be applied in common to the
processing of all types of light-sensitive silver halide color
photographic materials that are required to be bleached and
fixed, such as photographic printing papers, negative color
films, and reversal color films.
Concentrates of the present invention can be diluted
according to the needs of the end user. The concentration of the
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working strength solutions, prepared from the stable
concentrates, will depend on a variety of factors, such as
controlling environmental regulations or the requirements of the
particular automatic processor being used. In general, the
dilution factor ranges from about one part concentrate to one
part of water (1:1) by volume to about 1:6, and more preferably
from about 1:2 to 1:4 by volume.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides compositions and methods of
making one-part bleach-fix liquid concentrates packaged in a
single unit, wherein the bleaching agent is a polyvalent metal
oxidizing agent protected from reduction for an extended period
of time by incorporating a thiocyanate fixing agent. More
preferably, a quantitative ratio of at least two fixing agents
are combined wherein one of the fixing agents is a thiocyanate
salt used in a sufficient amount to provide homogeneous one-part
liquid concentrates free, or virtually free, of crystallized
components and which retain effective oxidizing potential during
extended storage as a concentrate.
As stated above, if clear colors are to be obtained the
silver image must be removed leaving only the dye image. This
can be accomplished by oxidizing the metallic silver with a
suitable oxidizing agent, commonly referred to as a bleaching
agent. The one-part bleach-fix liquid concentrates as described
above comprise a bleaching agent. Generally any bleaching agent,
which oxidizes metallic silver, may be used in the present
invention. Preferably, the bleaching agents used herein include
compounds of polyvalent metals, such as iron (III), cobalt (III),
chromium (VI), and copper (II); peracids; quinones and nitro
compounds. Typical bleaching agents used herein are iron salts,
such as ferric chloride; ferricyanides; bichromates; and organic
complexes with aminopolycarboxylic acids, aminopolyphosphonic
acids and organic phosphonic acids for forming the organic
complex salts of iron (III) include:
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nitrilotriacetic acid,
nitrilotripropionic acid,
1,2-Diaminopropanetetraacetic acid,
cyclohexanediaminetetraacetic acid,
ethylenediaminetetraacetic acid,
propylenediaminetetraacetic acid,
diethylenetriaminepentaacetic acid,
ortho-diaminecyclohexanetetraacetic acid,
ethylene glycol bis(aminoethyl ether) tetraacetic acid,
diaminopropanol tetraacetic acid,
N-(-2-hydroxyethyl)ethylenediaminetriacetic acid,
ethyliminodipropionic acid,
iminodiacetic acid,
methyliminodiacetic acid,
ethyliminodiacetic acid,
phenylenediaminetetraacetic acid,
2-Phosphonobutane-1,2,4-triacetic acid,
1,3-Diaminopropanol-N,N,N',N'-tetramethylenephosphonic acid,
and the like.
Among the above, ferric aminopolycarboxylic acid is
preferred, and even more preferably, ferric ethylenediamine
tetraacetic acid. The ferric complex salts of the
aminopolycarboxylic acids may be used in the form of salts with
sodium, potassium, ammonium, and the like, although ammonium
salts are preferred for fastest bleaching.
Ferric complex salts of the aminopolycarboxylic acids may
be used in the form of a complex salt. Alternatively, the
complex salt of the ferric ion may be formed in a solution of an
aminopolycarboxylic acid with a ferric salt, such as ferric
sulfate, ferric chloride, ferric nitrate, ferric ammonium sulfate
or ferric phosphate. When a complex salt is used, it is possible
to use one type of complex salt, or a mixture of two or more
types of complex salt. On the other hand, when the complex salt
is formed in the solution using a ferric salt and an
aminopolycarboxylic acid, it may be formed using one or more than
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one type of ferric salt. Moreover, the ferric complex salt may
also be formed using one or more than one type of
aminopolycarboxylic acid.
Generally, the bleaching agent should be used in a
sufficient amount to effectively oxidize any metallic silver
remaining on the photographic material. More specifically, the
bleaching agent may be present in an amount from about 10 percent
to about 25 percent, and more preferably from about 15 percent
to about 20 percent based on total weight of the concentrate.
As described above, any remaining light-sensitive silver
halide must be dissolved in a silver halide solvent, commonly
referred to as a fixing agent. Because the silver halides are
almost insoluble, they cannot be removed from the emulsion by
water washing, but instead must be treated with a fixing agent,
which reacts with the silver halides to form soluble complex
salts. Thus, in addition to the bleaching agent, at least one
fixing agent must be incorporated into the one-part bleach-fix
liquid concentrates of the present invention.
In the present invention, any fixing agent that dissolves
light-sensitive silver halides without affecting the silver image
without causing excessive swelling or softening of the gelatin
layer, may be used. Thiosulfates, thiocyanates, thioether
compounds, thiourea, and thioglycolic acid are the most important
with the proviso that when thiocyanate salt is used, it is used
either alone or in combination with a different and distinct
fixing agent.
Although thiocyanate salts are used as fixing agents, they
have limited applications. Thiocyanate salts find some
application in ultra-rapid processing systems, as they are more
rapid than ammonium thiosulfate. However, because they tend to
soften gelatin, the emulsion should be well-hardened and the
fixing time very short. Thiocyanate salts also are much more
expensive than thiosuliate salts. Even more important, during
silver recovery using electrolytic methods there is a possibility
of producing toxic hydrogen cyanide gas. In the past, these
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disadvantages rendered the use of thiocyanate salts as a fixing
agent undesirable, as it was neither economical nor reliable.
In spite of the above disadvantages, it has been discovered
that using a thiocyanate salt either alone or in combination with
another fixing agent provides unexpected advantages and benefits.
Further, it has been found that mixing a quantitative amount of
a thiocyanate salt with another fixing agent, in a ratio as
disclosed herein, provides one-part bleach-fix liquid
concentrates with excellent properties. These include both a
long shelf life free from crystalline precipitates and the
retention of oxidizing capacity when the concentrates are diluted
to working strength solutions. This is surprising and
unexpected. That is, when a mixture of fixing agents contains
an amount of a thiocyanate salt, ranging from about 5 to about
20 percent, and more preferably from about 7 percent to about 15
percent by weight based on the total weight of the fixing agents,
a homogenous concentrate essentially free of crystals even after
months of storage is formed.
Typically, any thiocyanate salt may be used having a cation
selected from sodium, potassium, ammonium, lithium and the like,
and preferably ammonium thiocyanate.
Generally, the thiocyanate salt should be used in an amount
sufficient to inhibit crystallized precipitates for an extended
period of time. More specifically, the thiocyanate salt may be
present in an amount from about 1 percent to about 7 percent, and
more preferably, from about 1.5 percent to about 5 percent based
on total weight of the bleach-fix concentrate.
Although the theory of the crystallization is not perfectly
clear, it is known that the precipitate is comprised primarily
of salts of the aminopolycarboxylic acid, such as ferrous
complexes of the aminopolycarboxylic acid. It is believed that
the ferrous complex has a lower solubility than a ferric complex.
A solution containing these crystals demonstrates unsatisfactory
bleaching capability. That is, the ability to oxidize metallic
silver to a silver ion is greatly reduced. While not wishing to
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be bound by any particular theory of mechanism, it is believed
that the inclusion of thiocyanate ions keeps the ferrous ions in
solution, thus preventing their crystallization and
precipitation.
When using two different fixing agents (where one is
thiocyanate) in the concentrates of the present invention, any
fixing agent (other than a thiocyanate salt) which acts as a
silver halide solvent may be used, including thiosulfates,
thioether compounds, thiourea, thioglycolic acid and mixtures
thereof. Preferably, a thiosulfate salt is used such as sodium
thiosulfate, potassium thiosulfate, ammonium thiosulfate, lithium
thiosulfate, and guanidine thiosulfate, and more preferably,
ammonium thiosulfate. Ammonium thiosulfate is preferred because
solutions containing this fixing agent show a higher rate of
fixation than sodium thiosulfate solutions.
The other fixing agent should be used in an amount
sufficient to solubilize any remaining inactivated silver halide.
More specifically, the other fixing agent may be present in an
amount from about 20 percent to about 30 percent, and more
preferably, from about 22 percent to about 26 percent based on
total weight of the concentrate.
It should be noted, that some of the additives encompassed
by this invention have been previously used in full working
strength solutions. However, heretofore, working solutions were
not made from a one-part bleach-fix liquid concentrate, but
instead were prepared immediately before use in the bleaching,
fixing, or bleach-fix process. Moreover, it was unknown,
heretofore, that a specific ratio of a thiocyanate salt to an
additional fixing agent combined in a one-part bleach-fix liquid
concentrate could provide such unexpected results as those
demonstrated in the present invention.
The one-part bleach-fix liquid concentrates may contain
bleaching agents and fixing agents, including a thiocyanate salt.
They may further contain preservatives, acids, alkaline agents,
buffering agents, anti-staining agents, and minor amounts of
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bleaching accelerators, brightening agents, defoaming agents,
surfactants, fungicides, anticorrosion agents and organic
solvents.
The concentrates of the present invention may contain an
effective amount of a preservative for stabilization of a
thiosulfate salt fixing agent to reduce any decomposition that
may occur due to the oxidizing components in the concentrate.
Especially useful for this purpose are sulfurous acid ion
releasing compounds such as a sulfite. Sulfites which can be
employed as preservatives include such representative examples
as sodium sulfite, lithium sulfite, potassium sulfite, ammonium
sulfite, sodium bisulfate, ammonium bisulfate, potassium
metabisulfite, sodium formaldehyde bisulfate and the like.
Other preservatives that can be used include hydroxylamine,
hydrazine, a bisulfate addition product of an aldehyde compound,
such as sodiumacetaldehyde bisulfate.
Generally, the preservative should be used in a sufficient
amount to be effective in protecting the fixing agents from
decomposition. More specifically, the preservative may be
present in an amount from about 5 percent to about 20 percent,
and more preferably, from about 10 percent to about 15 percent
based on total weight of the concentrate.
The one-part liquid bleach-fix concentrates of the present
invention may further comprise an alkaline agent which becomes
basic or alkaline when the concentrates are diluted to a working
solution with water. The alkaline agent provides the desired pH
level for photographic performance. Generally, the alkaline
agents used are ammonium, potassium, and sodium hydroxide and
preferably, ammonium hydroxide.
The alkaline agent should be introduced into the bleach-fix
concentrates in an amount sufficient to maintain the desired pH
in the dilute working strength solution. Typically, an alkaline
agent is added in an amount from 0.05 percent to about 3 percent
based on total weight of the concentrate.
When photographic materials are transferred from the
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development bath to the bleach-fix solution it is necessary to
neutralize the alkali in the developer to inhibit further
development of the image. If the alkali is not neutralized, the
bleach-fix solution can become contaminated with reduced
effectiveness. Thus understood, the use of an acid will help to
buffer the solution arid prevent stains when development of the
latent image is not arrested. Practically any acid will function
as an anti-staining agent, however, organic acids are more
suitable than inorganic acids because they are less dissociated,
and therefore, have a correspondingly smaller tendency to
precipitate sulfur from the bleach-fix concentrate or the working
solutions made from the concentrates. Well-known acids,
including organic and inorganic acids, useful for preventing
staining and maintaining an acidic pH in the working solutions
include formic acid, acetic acid, monochloroacetic acid, glycolic
acid, propionic acid, phosphoric acid, malefic acid, succinic
acid, and the like. Among these, preferred acids have a carboxyl
group, and most preferred is acetic acid. A full explanation for
the superior results of the one-part bleach-fix liquid
concentrates has not yet been arrived at. By way of theory only
and without limitation, it is suggested that with the further
inclusion of acetic acid/acetate ions in the concentrates of the
present invention, the ferric/ferrous complex is further
stabilized thereby extending the crystal free period within the
concentrates.
It is beneficial to add the acid in an amount to stop
further oxidation of the developer and prevent staining of the
photographic materials. Preferably, the acid can be added in an
amount from about 1 percent to about 5 percent based on total
weight of the concentrate.
In addition to modifying the pH of the concentrate and
preventing staining, it has been discovered that the use of
phosphoric acid also results in a stable concentrate that remains
free from Fe2+ precipitates for extended time periods.
Typically, reducing agents such as sulfites in the bleach-fix
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concentrate reduce the Fe3* of the bleaching agents to the F~+
state. Normally, Fe3+ compounds are less soluble compared to
those containing Fe2+. However, when iron is part of a bleaching
agent compound, the solubility of ferrous and ferric iron
compounds reverses. This results in precipitation of the ferrous
iron containing bleaching agents. Any precipitate formation in
a concentrate immediately ends the effectiveness and shelf life
of the concentrate.
It has been observed that when phosphoric acid, or any other
source of phosphate ion, is used in the concentrate, the
phosphate ion acts to keep the Fe2+ bleaching agent in solution.
This prevents precipitates from forming, thus, increasing both
the effectiveness of the concentrate and its shelf life.
Furthermore, it has been observed that phosphoric acid
prevents ferrous iron precipitation to such an extent that it
can, by itself, stabilize the one-part bleach-fix concentrate to
provide a concentrate with an increased shelf life capacity.
When phosphoric acid is used as a source of phosphate ion,
it serves to control the pH of the concentrate. If a phosphate
salt, rather than phosphoric acid, is used as the source of
phosphate ion, then the addition of acid may be needed to prevent
staining and to maintain the pH of the concentrate to values
between about 4 and about 7, and more specifically, from values
between about 5 and about 6.
If a phosphate salt is used instead of the phosphoric acid,
additional acid may be required to maintain the desired pH and
prevent staining. Representative phosphate salts may include
trisodium phosphate, disodium monohydrogen phosphate, monosodium
dihydrogen phosphate, tripotassium phosphate, dipotassium
monohydrogen phosphate, monopotassium dihydrogen phosphate,
triammonium phosphate, diammonium monohydrogen phosphate, and
monammonium dihydrogen phosphate, to name but a few.
Further, it has been observed that the shelf life of the
concentrate increases as the amount of phosphate ion is increased
(as shown by Examples 5 and 6, below). The amount of phosphate
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that is used is generally from about 0.5 to about 8.0 percent
wt/wt.
The bleaching agent may be selected from the group
consisting of compounds of polyvalent metals such as iron, (III),
cobalt (III), chromium (VI), and copper (II), peracids, and
mixtures thereof. Preferably, the compounds of the polyvalent
metals would include iron (III) compounds such as ferric
chloride; ferricyanide; bichromates; and organic complexes of
iron (III) with aminopolycarboxylic acids, aminopolyphosphonic
acids and organic phosphonic acids. From the above, the ferric
aminopolycarboxylic acid is preferred, and more preferably,
ferric ethylenediamine tetraacetic acid. The bleaching agent is
typically used in an amount of from about 1.0 to about 35 percent
wt/wt.
The fixing agent is selected from the group that includes
thiosulfate salts, thiocyanate salts, thioethers, thiourea,
thioglycolic acid, and the like, where thiosulfate is preferred.
The fixing agent is generally present in an amount of from 10
percent to about 55 percent wt/wt.
The present invention may further provide a re-halogenating
agent, for example, bromides, such as potassium bromide, sodium
bromide, and ammonium bromide; and chlorides, such as potassium
chloride, sodium chloride, and ammonium chloride.
The stable one-part liquid bleach-fix concentrates of the
present invention have excellent bleach-fixing power, and hence,
a bleaching accelerator is not necessary. Notwithstanding, if
desired, a bleaching accelerator may be incorporated.
Representative examples of useful bleaching accelerators include
organic sulfur compounds, quaternary ammonium compounds and
selenium compounds.
Usually, it is desirable that working strength bleach-fix
solutions be used immediately after color development. Dilution
of the one-part bleach-fix liquid concentrates of the present
invention may be accomplished using tap, deionized, or sterilized
water. Upon dilution to working strength solutions, the one-part
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bleach-fix liquid concentrates may be used effectively for
bleaching and fixing photographic materials
In the bleach-fixing process agitation of the working
solution ensures that more of the bleach-fix agents are available
to the photographic mater=als being treated. This results in an
increased bleaching and fixing rate. Agitation can be achieved
by various techniques including injection of a jet flow of
processing solution against the surface of the photosensitive
material or increasing the rate of circulation flow through the
entire processing solution. The photographic material may be
treated in bleach-fix working solution for generally up to 150
seconds, and preferably up to 60 seconds.
The one-part bleach-fix liquid concentrates may be used in
automatic processors. In general, automatic processors perform
continuous processing in either a "replenishment" or "batch"
mode. In the replenishment mode, replenisher is made up in
amounts proportional to the quantity of color photosensitive
material processed in order to keep the performance of processing
solutions constant. In the batch mode all or part of the used
processing solution is replaced with a fresh solution whenever
the quality of color photo sensitive material processed reaches
a predetermined level.
When regeneration mode is used, silver recovery is necessary
to regenerate spent bleach-fix solutions to permit their reuse.
As part of the regeneration, it is necessary to remove the
silver from the spent bleach-fix solution, and this is most
advantageously accomplished by the use of electrolytic silver
recovery systems. Such systems function to plate out the
dissolved silver as metallic silver on the cathode of the
electrolytic apparatus. After a sufficient amount of silver has
been collected on the cathode, it is removed from the cathode and
subjected to appropriate refining processes to permit its reuse.
Conditions used in carrying out the electrolytic silver recovery
are varied but higher temperatures and increased current density
can cause the formation cf toxic fumes when the fixing agent is
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a thiocyanate salt. However, by combining a thiocyanate salt
with an additional fixing agent the concentration of thiocyanate
is low enough to reduce the production of any significant amount
of hydrogen cyanide gas.
In the following examples, the amounts given in units of
grams refer to the associated named pure chemical.
EXAMPLE 1
One liter of a one-part bleach-fix liquid concentrate was
prepared using the following basic formulation as shown below:
Ammonium thiosulfate 310.0 g
Ammonium bisulfate 150.0 g
Aqua ammonia 9.5 g
Ferric ammonium EDTA 220.0 g
Acetic Acid 37.0 g
Ammonium thiocyanate 30.0 g
pH 5.70 - 5.80
The components were added sequentially in a single vessel
providing approximately 1 liter of the one-part concentrate of
the present invention. The vessel was placed in storage at
ambient pressure and at a temperature of approximately 20°C. The
one-part liquid concentrate prepared according to the above
formulation remained crystal free even after 14 months of
storage.
COMPARATIVE EXAMPLE 2
To illustrate the importance of including at least some
thiocyanate salt to prevent crystallized precipitates forming in
the concentrates of the present invention, one liter of a one-
part concentrate was prepared without the addition of any
thiocyanate salt according to the following formula:
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Ammonium thiosulfate 387.5
g
Ammonium bisulfate 150.0
g
Aqua ammonia 9.5 g
Ferric ammonium EDTA 220.0
g
Acetic Acid 37.0 g
pH 5.70 - 5.80
The components were added to a single vessel providing
approximately 1 liter of a one-part concentrate. It should be
noted that the concentration of the thiosulfate salt was
increased because an additional fixing agent was not included in
the concentrate. The vessel was placed in storage at ambient
pressure and at a temperature of approximately 20°C.
Results: The one-part liquid concentrate prepared without
the addition of a thiocyanate salt produced crystallized
precipitates in the concentrate in approximately one month
indicating the importance of including a sufficient amount of a
thiocyanate salt to provide a one-part concentrate that is
essentially free of crystallized precipitates.
EXAMPLE 3
Several different concentrations of a thiocyanate salt were
added to the formulation of Example 1 to illustrate the
increasing shelf life of the concentrates with the addition of
increasing amounts of the thiocyanate salt in the concentrates
of the present invention.
TABLE 1
Amount of Amount of Time period
Ammonium Ammonium free of
Thiocyanate thiosulfate crystals
10 310.0 5 months
...............
...............................................................................
.....................................
................................................; 310.0 g ; 10 months
20 g
....................................................................
..............................................................
. ...
.
.........................3 ~
g 10 a
30 : 0 g : 4
: ....'...................................months
. .................................................
~
.............................................................................>
14 months
......... ..
40 g 310.0 g
g . 0 ;....
.................................................
300
..............................................................> 24 months
The above results illustrate that the shelf life of the one-
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part bleach-fix liquid concentrates may exceed one year by the
addition of at least 30 g of a thiocyanate salt. When a
thiocyanate salt was added without the addition of another fixing
agent the concentrate did not form crystals in two years.
However, it should be noted that high concentrations of
thiocyanate salt may form toxic hydrogen cyanide gas during
electrolytic recovery of silver from a spent bleach-fix solution.
EXAMPLE 4
The formulation of Example 1 prepared without the addition
of acetic acid to the one-part bleach-fix concentrate:
Ammonium thiosulfate 310.0
g
Ammonium bisulfate 150.0
g
Ferric ammonium EDTA 220.0
g
Ammonium thiocyanate 30.0 g
pH 5.70 - 5.80
Results: The lack of acetic acid in the concentrate obviated the
need for an alkaline agent to help buffer the acid. It was found
that this formulation produced crystals in about 6-7 months.
Clearly, the acetic acid plays a role in the stabilization of the
one-part concentrates. While not wishing to be bound by any
specific mechanism of action, it is believed that the acetic acid
either further stabilizes the iron complex or simply increases
the buffer capacity of the concentrate.
EXAMPLE 5
To demonstrate the efficacy of phosphate ions in inhibiting
crystal formation and increasing shelf-life expectancy of one-
part bleach-fix concentrates, the following experiment was
conducted. One liter of a one-part bleach-fix concentrate using
phosphoric acid as a source of phosphate ion was prepared with
the following formulation:
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Ammonium thiosulfate 450 g
Ammonium bisulfate 100 g
Ammonium hydroxide 30 g
Ferric ammonium EDTA 3008
Phosphoric acid 64 g
The components were added to a single vessel in the order
presented to provide one liter of stable one-part bleach-fix
concentrate.
Additionally, a second concentrate sample using the same
ingredients and concentrations was prepared, but without
phosphoric acid as a control.
The stability of both samples was tested using the following
accelerated test protocol. A 200 ml aliquot of each of the
samples was placed in a 250 ml beaker. The beakers were left
uncovered and maintained at ambient conditions on a laboratory
table. The samples were monitored for precipitate formation by
visual inspection. The first precipitate to form was observed
as a small crystallite floating on the surface of the
concentrate. The two concentrates were then allowed to age for
two more days under ambient conditions. At the completion of day
two, the concentrates were quickly poured out of their respective
250 ml beakers so that the bottom of the beakers could be
inspected for the presence of crystals. During the two extra
days of aging, crystal growth was enough for precipitates to
settle to the bottom of the beaker where they were readily
observed. The time between preparation of the formulations and
the observation of crystals on the bottom of the beakers was
noted and used to estimate the shelf life of samples prepared in
the same manner but kept in closed, larger (commercial size)
containers. Samples that are stored in closed, commercial sized
containers remain free from crystallized precipitates for much
longer periods of time as was observed when similar experiments
were conducted using the thiocyanate concentrate (see Example 1),
as described below.
During the past two years, accelerated stability tests were
performed on a series of samples made using thiocyanate instead
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of phosphoric acid (see Example 3, for example). Using the
results of these experiments, a table was constructed showing the
relationship between the time required for precipitates to form
in the concentrate samples that were stored in small, open
containers and the time required for precipitates to form in
separate aliquots of the same samples when stored in closed,
larger commercial sized containers (see Table 2). Generally,
samples are monitored for an estimated shelf life of only up to
a year as other ingredients in bleach-fix formulations will have
degraded by that time.
TABLE 2
ACCELERATED STABILITY TEST DATA
PART A: OBSERVED SHELF LIFE PART B: ESTIMATED SHELF
L I FE
Periods of time from concentrate
preparation to formation of crystals Estimated shelf life of the
in concentrates. Concentrates were concentrates of Part A when
prepared using varying amounts of stored in a large, closed
thiocyanate and stored in an open 250 containers. The estimates are
2 0 ml beaker under ambient conditions. based on the known shelf life
of the thiocyanate concentrate
as prepared in Example 1.
55 days » 1 year
> 20 days > 1 year
days 1 year
11 days 6 months
5 days 3 months
2 days - Control no shelf life
Accelerated testing of the phosphate containing concentrate
of Example 5 showed that this formulation, when stored in a small
open container, remained free from Fe2+ precipitates for over six
months. Using the data given in Table 2, it can be estimated
that the phosphate containing concentrate, when stored in a
large, closed container will have a stable shelf life of greater
than one year. Accelerated testing of the control, without
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phosphate, showed precipitate forming within two days. Table 2
shows that if crystals form within two days the concentrate has
no practical shelf life. This result clearly illustrates the
advantage gained by using a source of phosphate ion in the one
s part bleach-fix concentrate.
EXAMPLE 6
To demonstrate that phosphate ions, even in reduced amounts,
are effective in inhibiting crystal formation and increasing
shelf-life expectancy of one-part bleach-fix concentrates, the
following experiment was conducted. One liter of a one-part
bleach-fix concentrate was prepared having the formulation:
Ammonium thiosulfate 230 g
Ammonium bisulfate 50 g
Ammonium hydroxide 20 g
Ferric ammonium EDT 120 g
Phosphoric acid 8 g
This concentrate was prepared in the same manner as Example
5 and was subjected to accelerated testing following the protocol
described above.
When a 200 ml portion of this concentrate was stored in an
open, 250 ml container under ambient conditions, it remained free
from precipitate for 11 days. This observation translates,
according to Table 2, to an estimated shelf life of about 6
months for a sample prepared in the same manner and stored in a
closed, larger container.
Examples 1 - 6 are presented as illustrations of
different embodiments of the present invention, however, the
scope of the invention is not limited to the examples.
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