PROCESS FOR THE REMOVAL OF METALLIC SILVER FROM PHOTOGRAPHIC MATERIAL
United States Patent 3716362
A process for removing metallic silver from photographic materials is provided, wherein any sparingly soluble silver salts present are not attacked. This process comprises treating the photographic material with an acid bath free from anions that form sparingly soluble silver salts and which bath contains an oxidant and a complexing agent. The oxidant has a standard redox potential of at most +0.8 Volts and the complexing agent has a silver complex stability constant of at most 109 (liter/mole)2. This process is of special importance in carrying out the silver dye bleach negative-positive process. The main advantage of the process is the fact that it uses only a small number of bathes and no strong oxidants.
US Patent References:
Method of lowering contrast of dye images
Young - December 1942 - 2304987
Removing silver and silver halide from photographic elements
Mader - May 1956 - 2748000
Method of producing photographic pictures in colors
Gaspar - November 1935 - 2020775
Color photography
Mannes et al. - April 1938 - 2113329
Method of producing dye images
Ehrenfried - June 1943 - 2322001
Method of lowering contrast of dye images
Young - December 1942 - 2304987
Removing silver and silver halide from photographic elements
Mader - May 1956 - 2748000
Method of producing photographic pictures in colors
Gaspar - November 1935 - 2020775
Color photography
Mannes et al. - April 1938 - 2113329
Method of producing dye images
Ehrenfried - June 1943 - 2322001
Application Number:
05/056945
Publication Date:
02/13/1973
Filing Date:
07/21/1970
View Patent Images:
Export Citation:
Assignee:
Ciba-Geigy AG (Basle, CH)
Primary Class:
Other Classes:
430/393, 430/390, 430/430, 430/407, 430/408
International Classes:
G03C5/44; G03C7/28; G03C7/42; G03C5/40; G03C7/00; G03C5/50; G03C5/32
Field of Search:
96/20,53,61,59,60
US Patent References:
Primary Examiner:
Brown, Travis J.
Assistant Examiner:
Kelley M. F.
Claims:
I claim
1. In a process for removing metallic silver without attacking any dyestuffs or sparingly soluble silver halides present from a photographic material comprising sparingly soluble silver halides and metallic silver distributed in at least one emulsion layer on a support by the steps of exposing, developing, reversal developing, dye bleaching, silver bleaching and fixing, the improvement which comprises treating said material after the first silver image developing bath with an acid bath free from anions that form sparingly soluble silver halides, said bath containing an oxidant selected from a copper (II) salt, a quinone or an iron (III) salt, and a complexing agent selected from allyl alcohol, a nitrile and a thio-ether.
2. A process as claimed in claim 1, wherein a water-soluble aliphatic monothioether is used as complexing agent.
3. A process as claimed in claim 1, wherein the complexing agent is a monothioether of the formula
4. A process as claimed in claim 1, wherein the complexing agent is a monothioether of the formula
5. A process as claimed in claim 1, wherein the silver halide is silver bromide.
6. A process as claimed in claim 1, wherein the pH value of the bath is below 6.
7. A process as claimed in claim 1, wherein the metallic silver and the silver halide are present in the same emulsion layer.
8. A process as claimed in claim 1, carried out in a reversal development process.
9. A process as claimed in claim 1, carried out in a silver dyestuff bleaching negative-positive process.
1. In a process for removing metallic silver without attacking any dyestuffs or sparingly soluble silver halides present from a photographic material comprising sparingly soluble silver halides and metallic silver distributed in at least one emulsion layer on a support by the steps of exposing, developing, reversal developing, dye bleaching, silver bleaching and fixing, the improvement which comprises treating said material after the first silver image developing bath with an acid bath free from anions that form sparingly soluble silver halides, said bath containing an oxidant selected from a copper (II) salt, a quinone or an iron (III) salt, and a complexing agent selected from allyl alcohol, a nitrile and a thio-ether.
2. A process as claimed in claim 1, wherein a water-soluble aliphatic monothioether is used as complexing agent.
3. A process as claimed in claim 1, wherein the complexing agent is a monothioether of the formula
4. A process as claimed in claim 1, wherein the complexing agent is a monothioether of the formula
5. A process as claimed in claim 1, wherein the silver halide is silver bromide.
6. A process as claimed in claim 1, wherein the pH value of the bath is below 6.
7. A process as claimed in claim 1, wherein the metallic silver and the silver halide are present in the same emulsion layer.
8. A process as claimed in claim 1, carried out in a reversal development process.
9. A process as claimed in claim 1, carried out in a silver dyestuff bleaching negative-positive process.
Description:
The present invention relates to a process for removing metallic silver from photographic materials.
A number of processes are known for removing silver and silver halides from photographic materials. The choice of the process to be applied depends on the requirements of the individual photographic process. Thus, in the conventional production of a silver image by developing exposed silver halide it is necessary to remove the residual, undeveloped silver halide, for example by means of a solution of thiosulphate.
In a direct positive development, on the other hand, the primarily developed silver must be removed without attacking any residual, undeveloped silver halide since in a further process step the latter is developed to the positive silver image. This can be done with oxidation baths which convert the silver into a soluble silver salt which can diffuse out of the photographic layer, for example, a sulphuric acid solution of potassium dichromate or potassium permanganate, for which purpose the standard redox potential of these solutions must be greater than +0.8 Volt, which is the standard potential of Ag + ➝Ag° .
In other photographic processes, especially color processes, both the silver and any silver halide must be removed; this can be done with so-called bleach fixing baths that contain an oxidant as well as a solvent for silver halide, for example Farmer's reducer or bleach fixing baths, described in German specification No. 1,146,363 and U.S. Pat. No. 2,748,000.
If desired, the oxidation of silver and the washing out of the silver halide may be carried out in separate baths by first oxidizing the silver and then dissolving the silver halide. In this case oxidation of the silver may also be carried out with a solution that converts the silver into a sparingly soluble silver salt, for example a hydrochloric acid solution of copper(II)chloride or a solution of potassium-iron(III)cyanide since the sparingly soluble silver salts formed remain in the layer and are removed during the subsequent fixing operation.
For certain reversal processes, however, oxidation baths, such as dichromate-sulphuric acid, cannot be used since these oxidants also decompose other components present in the photographic layer.
Thus, strong oxidants cannot be used in a silver reversal development within the framework of the silver dye bleach-negative-positive process since many of the azo dyestuffs used in this process may be destroyed by oxidation. While it is possible in such a case to adopt the procedure proposed in German specification No. 1,154,347, this process is cumbersome since it requires several additional treatment baths.
In addition, strongly acid oxidation baths are very corrosive, require great care in their handling and strongly attack the gelatin layers.
The present invention is based on the observation that these disadvantages can be overcome by using a silver bleach bath which contains in addition to an oxidant an appropriate complexing agent for silver.
The present invention provides a process for removing metallic silver, without attacking any sparingly soluble silver salts present, from photographic material which contains sparingly soluble silver salts and metallic silver on a support medium distributed over at least one emulsion layer, wherein the said material is treated with an acid bath which is free from anions that form sparingly soluble silver salts and contains
1. one or more oxidants having a standard redox potential of at most +0.8 Volt and
2. a complexing agent having a silver complex stability constant of at most 10 9 (liter/mole) 2 .
The standard redox potential of the oxidant should usually be within the range from +0.15 to +0.8 Volt, preferably from +0.4 to +0.8 Volt. Suitable oxidants are copper(II)salts or more especially quinones and iron(III)salts. Particularly good results are obtained with iron(III)salts, for example iron(III)perchlorate, iron(III)nitrate or ammonium iron(III) sulfate. According to this invention only anions that do not form sparingly soluble silver salts can be used. The oxidation potential of these oxidants must be below +0.8 Volt, the oxidation potential of Ag + ➝Ag° .
The standard redox potential of the following electro-chemical reactions is, for example: ##SPC1##
Preferably used complexing agents are those which display a silver complex stability constant of from 10 1 to 10 7 (liter/mole) 2 . Among the complexing agents having a silver complex stability constant within this range are water-soluble unsaturated compounds, for example allyl alcohol, nitriles, for example acetonitrile, certain heterocyclic amines, for example pyrazine, and thioethers. Especially suitable complexing agents are those having a silver complex stability constant of from 10 2 to 10 7 (liter/mole) 2 , especially water-soluble thioethers, and preferably monothiosethers. Of special value are water-soluble aliphatic monothioethers, which corresponds to the formula
(1) X 1 --R 1 --S--R 2 --X 2
in which R 1 and R 2 which may be the same or different each represent an alkyl group containing from one to three carbon atoms, and X 1 and X 2 each represent a hydrogen atom or a hydroxyl or carboxyl group.
Suitable water-soluble thioethers are, for example:
thiodiglycollic acid,
thiodipropionic acid,
2-hydroxyethyl-methylsulphide and especially thiodiglycol.
Apart from the aliphatic monothioethers of the formula (1) it is possible to use water-soluble thioethers in which R 1 and R 2 are aromatic or heterocyclic residues, for example the disodium salt of di-(para-sulphophenyl)-sulphide, or the free acid thereof. Further suitable are cyclic water-soluble thioethers, such as thiophene-2,5-dicarboxylic acids which correspond, for example, to the formula
in which X 1 and X 2 have the meanings defined above.
Very weak complexing agents, such as allyl alcohol and acetonitrile, must be used in very large quantities and are, therefore, less favorable than stronger complexing agents, such as thioethers, of which a lower concentration suffices. Some stronger complexing agents, for example cyanides, phosphines and certain bisthioethers, are also unsuitable since they dissolve the sparingly soluble silver salts.
The silver complex stability constants of many of the suitable complexing agents are known. For the stability constants of the complex of silver ion with thioethers see Ahrland and co-workers, J. Chem. Soc. (London) 54, pages 264 - 276 [1958] and E. Larsson in the book "The Svedberg," Uppsala 1944, pages 311 - 319. The stability constant of the complex of silver ion with acetonitrile: see F.G. Pawelka, Zeitschr. Elektrochemie 30, page 180 [ 1924]; the stability constant of the complex of silver ion with allyl alcohol: see S. Winstein and H.J. Lucas, J.Am.Chem.Soc.60, page 836 [1938].
The stability constant K of the reaction
where L represents the ligand concerned. For some suitable ligands the K-value in (Liter/mole) 2 is, for example, as follows:
Thiodiglycol 10 6 .1 2-hydroxyethyl-methylsulphide 10 6 .3 disodium salt of di-(p-sulphonyl)-sulphide 10 2 .3 thiodiglycollic acid 10 6 .3 thiodipropionic acid 10 6 .7 acetonitrile 10 1 .2 allyl alcohol 10 1 .6 pyrazine 10 2 .6
The pH value of the treatment bath should advantageously be below 6. Acids suitable for adjusting the pH value are, for example, sulphamic acid, sulphuric acid, oxalic acid, citric acid, perchloric acid, nitric acid and other acids capable of forming readily soluble silver salts. When the salts of the oxidant, for example copper sulphate, form acid solutions in water it is not necessary to add additional acid to the treatment bath.
The proportions of the individual constituents may be varied within wide limits. It is also possible to use mixtures containing more than one of each of the acids, oxidants and complexing agents. The bath may further contain conventional additives, such as surface-active substances and hardeners.
The solubility product of the sparingly soluble silver salts, which are not attacked by the bleaching baths of this invention, must be smaller than 10 -8 .5 mole 2 liter -2 ; these include silver halides, for example silver chloride, silver iodide and especially silver bromide, whose solubility product is smaller than 10 -10 mole 2 liter -2 .
The process of this invention may be used whenever metallic silver is to be removed without attacking any sparingly soluble silver salts present, especially when the metallic silver and the sparingly soluble silver salt are contained in the same silver emulsion layer. This is the case whenever the metallic silver is distributed to form an image, especially in reversal processes where a primarily developed silver image is dissolved and the residual silver salt image is converted into a black or colored image by diffuse light and second development or by development in the presence of a fogging agent, or by sulphidation.
The invention is of special importance in carrying out the silver dye bleach-negative-positive process. In the usual silver dye bleaching process a material is used that contains photosensitive silver halide and a dyestuff. On exposure and development a silver image is formed whose gradation is the opposite of that of the master (negative silver image). By decomposing the dyestuff on the silver a color gradation is achieved which is opposite to that of the silver image, that is to say equal to that of the master. Thus, a positIve color image of a positive master is obtained.
To produce a positive color image from a negative master by the silver dye bleach process, a reversal development is needed. Accordingly to this invention a positive color image is obtained from a negative master by development of a material that contains a photosensitive silver halide emulsion and a bleachable dyestuff after exposure, removing the primarily formed silver image, reducing the undeveloped residual silver halide to metallic silver and then decomposing the dyestuff depending on the amount of silver present in the layer, and removing the primary silver image by means of an acid bath which is free from anions that form sparingly soluble silver salts and which contains an oxidant and a complexing agent capable of forming with silver complexes of little stability.
When the acid and the oxidant are chosen according to this invention, the bleachable dyestuff is not attacked and this makes the present process superior to the dichromate process. The superiority over the process according to German specification No. 1,154,347 is based on the fact that the present process requires a smaller number of baths.
The temperature of the silver bleaching bath and the treatment time may be varied within wide limits without incurring a loss of undeveloped silver halide.
The present process is applicable to the treatment of both single-layer and multilayer color materials, of materials for viewing in incident light and transparencies, of mixed grain emulsion layers and of materials with incorporated capsules. Thus, for example, a rapid reversal process can be carried out with a metarial which contains acid, oxidant and complexing agents in crushable pods. Another variant starts from a material that contains oxidant and complexing agents in pods which disintegrate only when they come into contact with an acid bath.
The process according to the present invention is of special value in connection with reversal processes but the invention is also suitable for other processes in which metallic silver (such as developed image silver, latent image nuclei or colloidal silver) is dissolved without affecting any sparingly soluble silver salts present (which are present in the same grain, in the same layer or in another layer).
In the following Examples, which illustrate the invention, all percentages are percentages by weight.
EXAMPLE 1
Two strips of a blue-sensitized emulsion layer on a cellulose triacetate base, containing 0.7 g of silver bromide and 0.12 g of a yellow dyestuff of the formula ##SPC2##
per square meter, are exposed under a stepped wedge. The exposed strips are developed with a commercial para-methylaminophenol sulphate/hydroquinone developer and the development process is stopped with a 3 percent solution of acetic acid. After having been washed in water for 1 minute, one strip is treated for 3 minutes and the other strip for 10 minutes with a solution of the following composition to remove the metallic silver left in the layer after the first development:
Iron-III-perchlorate nonahydrate 52 g sulphamic acid 60 g thiodiglycol 12.5 g water, to pH value: 0.35. 1 liter.
After having been rinsed in water for 4 minutes, the strips are treated for 6 minutes with a commercial p-methyl-aminophenol-sulphate/hydroquinone developer to which 0.1 g of tributylaminoboran per liter have been added, to reduce the silver bromide left in the layer. The two strips are then rinsed for 2 minutes and dried.
Both strips carry a silver image which is positive with respect to the stepped wedge used for exposure. The two silver images are completely identical.
To bleach the dyestuff the strips are immersed in a solution of the following composition:
Hydrochloric acid, 37 % strength 70 ml potassium bromide 50 g thiourea 80 g 2-amino-3-hydroxyphenazine 5 mg water, to 1 liter.
After 6 minutes' immersion the strips are rinsed in water and treated with a silver bleaching bath of the following composition:
Potassium iron(III)cyanide 75 g potassium bromide 15 g primary sodium phosphate monohydrate 10 g sodium acetate trihydrate 5 g glacial acetic acid 10 ml water, to 1 liter.
The strips are then rinsed in water for 1 minute and fixed with an acid thiosulphate solution in the known manner. Both strips carry identical yellow images of the stepped wedge used for the exposure, which are negative with respect to the master.
EXAMPLE 2
The procedure used is as described in Example 1, except that the metallic silver left after the first development is removed with a bath composed as follows:
Iron(III)perchlorate nonahydrate 28 g 2N-nitric acid 50 ml 2-hydroxyethyl-methylsulphide 9.6 g water, to 1 liter. pH value: 0.65
The resulting images are identical to those obtained as described in Example 1.
EXAMPLE 3
Instead of the bath described in Example 2 a solution of the following composition is used:
Benzoquinone, dissolved in 200 ml of methanol 10.8 g 2N-sulphuric acid 50 ml 2-hydroxyethyl-methylsulphide 10 g water, to 1 liter pH value: 0.8
The resulting images are identical to those described in Example 1.
EXAMPLE 4
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g perchloric acid, 70% strength 150 g disodium salt of di-(p-sulphophenyl)-sulphide 80 g water, to 1 liter pH value: 0.6.
The resulting images are identical to those of Example 1.
EXAMPLE 5
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g thiodiglycolic acid 15 g sulphamic acid 60 g water, to 1 liter pH value: 0.35.
The resulting images are identical to those of Example 1.
EXAMPLE 6
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g thiodipropionic acid 18 g sulphamic acid 60 g water, to 1 liter pH value: 0.35.
The resulting pictures are identical to those described in Example 1.
EXAMPLE 7
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g water, to 1 liter thiophene-2,5-dicarboxylic acid to saturation pH value: 0.35
The resulting images are identical to those of Example 1.
EXAMPLE 8
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g acetonitrile 100 g water, to 1 liter pH value: 0.35.
The resulting images are identical to those described in Example 1.
EXAMPLE 9
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g allyl alcohol 140 g water, to 1 liter pH value: 0.35.
The resulting images are identical to those of Example 1.
EXAMPLE 10
A red-sensitized emulsion layer which contains per square meter of cellulose triacetate base 0.7 gm of silver bromide and 0.115 g of a blue-green dyestuff of the formula ##SPC3##
is exposed under a stepped wedge and then processed as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath composed as follows:
Iron(III)perchlorate nonahydrate 52 g citric acid dihydrate 52 g thiodiglycol 12.8 g water, to 1 liter pH value: 0.5.
A blue-green image of the stepped wedge used for the exposure is obtained which is negative with respect to the master. No differences can be noted by comparing the results of the 3-minutes test with those of the 10-minutes test.
EXAMPLE 11
A green-sensitized emulsion layer which contains per square meter of a triacetate base 0.7 g of silver bromide and 0.150 g of a magenta dyestuff of the formula ##SPC4##
is exposed under a grey wedge and then processed as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath of the following composition:
Iron(III)perchlorate nonahydrate 52 g oxalic acid dihydrate 40 g thiodiglycol 12.5 g water, to 1 liter pH value: 0.35.
The resulting magenta image of the grey wedge used for the exposure is a negative of the master. Identical results are obtained in the 3-minutes and the 10-minutes test.
EXAMPLE 12
A photographic tripack material which carried on a transparent base a red-sensitized silver bromide emulsion containing a cyan dyestuff as described in Example 10, covered by a green-sensitized emulsion layer containing a magenta dyestuff as described in Example 11, which layer in turn is covered by a blue-sensitized silver bromide emulsion layer containing a yellow azo dyestuff as described in Example 1, is exposed under a grey wedge. Processing is carried out as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath of the following composition:
Copper(II)sulphate pentahydrate 125 g thiodiglycol 12.5 g water, to 1 liter pH value:3.5.
A reproduction of the grey wedge used for the exposure is obtained which is a negative of the master.
EXAMPLE 13
A commercial enlarging paper, whose emulsion layer contains a silver halide consisting of 54 mol percent of silver chloride and 47 mol percent of silver bromide, is exposed under a grey wedge and then developed with a commercial p-methyl-aminophenol-sulphate/hydroquinone developer. After having been rinsed in water for 2 minutes the paper is immersed in a solution of the following composition:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g thiodiglycol 12.5 g water, to 1 liter pH value: 0.35.
After 8 minutes the paper is rinsed in water and then exposed in diffuse light.
The silver halide left in the layer is then reduced to metallic silver with the developer used in Example 1. The resulting silver image of the grey wedge used for the exposure is a positive reproduction of the master.
EXAMPLE 14
A strip of an emulsion layer on an opaque cellulose triacetate base, sensitive to red light, which carries per square meter of base 0.7 g of silver bromide and 0.115 g of the cyan dyestuff of the formula (4) is exposed under a stepped wedge and then processed as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath of the following composition:
Ammonium-iron(III)sulphate dodecahydrate 60 g 2N-sulphuric acid 100 ml pyrazine 50 g water, to 1 liter pH value: 0.8.
The resulting cyan-colored image is a negative of the stepped wedge used for the exposure.
EXAMPLE 15
The process described in Example 14 is used, except that the metallic silver formed after the first development is removed with a bath of the following composition:
Iron(III)nitrate nonahydrate 40 g 2N-nitric acid 100 ml pyrazine 50 g water, to 1 liter pH value: 0.8.
As in Example 14 a cyan-colored image is obtained which is a negative reproduction of the stepped wedge used for the exposure.
A number of processes are known for removing silver and silver halides from photographic materials. The choice of the process to be applied depends on the requirements of the individual photographic process. Thus, in the conventional production of a silver image by developing exposed silver halide it is necessary to remove the residual, undeveloped silver halide, for example by means of a solution of thiosulphate.
In a direct positive development, on the other hand, the primarily developed silver must be removed without attacking any residual, undeveloped silver halide since in a further process step the latter is developed to the positive silver image. This can be done with oxidation baths which convert the silver into a soluble silver salt which can diffuse out of the photographic layer, for example, a sulphuric acid solution of potassium dichromate or potassium permanganate, for which purpose the standard redox potential of these solutions must be greater than +0.8 Volt, which is the standard potential of Ag + ➝Ag° .
In other photographic processes, especially color processes, both the silver and any silver halide must be removed; this can be done with so-called bleach fixing baths that contain an oxidant as well as a solvent for silver halide, for example Farmer's reducer or bleach fixing baths, described in German specification No. 1,146,363 and U.S. Pat. No. 2,748,000.
If desired, the oxidation of silver and the washing out of the silver halide may be carried out in separate baths by first oxidizing the silver and then dissolving the silver halide. In this case oxidation of the silver may also be carried out with a solution that converts the silver into a sparingly soluble silver salt, for example a hydrochloric acid solution of copper(II)chloride or a solution of potassium-iron(III)cyanide since the sparingly soluble silver salts formed remain in the layer and are removed during the subsequent fixing operation.
For certain reversal processes, however, oxidation baths, such as dichromate-sulphuric acid, cannot be used since these oxidants also decompose other components present in the photographic layer.
Thus, strong oxidants cannot be used in a silver reversal development within the framework of the silver dye bleach-negative-positive process since many of the azo dyestuffs used in this process may be destroyed by oxidation. While it is possible in such a case to adopt the procedure proposed in German specification No. 1,154,347, this process is cumbersome since it requires several additional treatment baths.
In addition, strongly acid oxidation baths are very corrosive, require great care in their handling and strongly attack the gelatin layers.
The present invention is based on the observation that these disadvantages can be overcome by using a silver bleach bath which contains in addition to an oxidant an appropriate complexing agent for silver.
The present invention provides a process for removing metallic silver, without attacking any sparingly soluble silver salts present, from photographic material which contains sparingly soluble silver salts and metallic silver on a support medium distributed over at least one emulsion layer, wherein the said material is treated with an acid bath which is free from anions that form sparingly soluble silver salts and contains
1. one or more oxidants having a standard redox potential of at most +0.8 Volt and
2. a complexing agent having a silver complex stability constant of at most 10 9 (liter/mole) 2 .
The standard redox potential of the oxidant should usually be within the range from +0.15 to +0.8 Volt, preferably from +0.4 to +0.8 Volt. Suitable oxidants are copper(II)salts or more especially quinones and iron(III)salts. Particularly good results are obtained with iron(III)salts, for example iron(III)perchlorate, iron(III)nitrate or ammonium iron(III) sulfate. According to this invention only anions that do not form sparingly soluble silver salts can be used. The oxidation potential of these oxidants must be below +0.8 Volt, the oxidation potential of Ag + ➝Ag° .
The standard redox potential of the following electro-chemical reactions is, for example: ##SPC1##
Preferably used complexing agents are those which display a silver complex stability constant of from 10 1 to 10 7 (liter/mole) 2 . Among the complexing agents having a silver complex stability constant within this range are water-soluble unsaturated compounds, for example allyl alcohol, nitriles, for example acetonitrile, certain heterocyclic amines, for example pyrazine, and thioethers. Especially suitable complexing agents are those having a silver complex stability constant of from 10 2 to 10 7 (liter/mole) 2 , especially water-soluble thioethers, and preferably monothiosethers. Of special value are water-soluble aliphatic monothioethers, which corresponds to the formula
(1) X 1 --R 1 --S--R 2 --X 2
in which R 1 and R 2 which may be the same or different each represent an alkyl group containing from one to three carbon atoms, and X 1 and X 2 each represent a hydrogen atom or a hydroxyl or carboxyl group.
Suitable water-soluble thioethers are, for example:
thiodiglycollic acid,
thiodipropionic acid,
2-hydroxyethyl-methylsulphide and especially thiodiglycol.
Apart from the aliphatic monothioethers of the formula (1) it is possible to use water-soluble thioethers in which R 1 and R 2 are aromatic or heterocyclic residues, for example the disodium salt of di-(para-sulphophenyl)-sulphide, or the free acid thereof. Further suitable are cyclic water-soluble thioethers, such as thiophene-2,5-dicarboxylic acids which correspond, for example, to the formula
in which X 1 and X 2 have the meanings defined above.
Very weak complexing agents, such as allyl alcohol and acetonitrile, must be used in very large quantities and are, therefore, less favorable than stronger complexing agents, such as thioethers, of which a lower concentration suffices. Some stronger complexing agents, for example cyanides, phosphines and certain bisthioethers, are also unsuitable since they dissolve the sparingly soluble silver salts.
The silver complex stability constants of many of the suitable complexing agents are known. For the stability constants of the complex of silver ion with thioethers see Ahrland and co-workers, J. Chem. Soc. (London) 54, pages 264 - 276 [1958] and E. Larsson in the book "The Svedberg," Uppsala 1944, pages 311 - 319. The stability constant of the complex of silver ion with acetonitrile: see F.G. Pawelka, Zeitschr. Elektrochemie 30, page 180 [ 1924]; the stability constant of the complex of silver ion with allyl alcohol: see S. Winstein and H.J. Lucas, J.Am.Chem.Soc.60, page 836 [1938].
The stability constant K of the reaction
where L represents the ligand concerned. For some suitable ligands the K-value in (Liter/mole) 2 is, for example, as follows:
Thiodiglycol 10 6 .1 2-hydroxyethyl-methylsulphide 10 6 .3 disodium salt of di-(p-sulphonyl)-sulphide 10 2 .3 thiodiglycollic acid 10 6 .3 thiodipropionic acid 10 6 .7 acetonitrile 10 1 .2 allyl alcohol 10 1 .6 pyrazine 10 2 .6
The pH value of the treatment bath should advantageously be below 6. Acids suitable for adjusting the pH value are, for example, sulphamic acid, sulphuric acid, oxalic acid, citric acid, perchloric acid, nitric acid and other acids capable of forming readily soluble silver salts. When the salts of the oxidant, for example copper sulphate, form acid solutions in water it is not necessary to add additional acid to the treatment bath.
The proportions of the individual constituents may be varied within wide limits. It is also possible to use mixtures containing more than one of each of the acids, oxidants and complexing agents. The bath may further contain conventional additives, such as surface-active substances and hardeners.
The solubility product of the sparingly soluble silver salts, which are not attacked by the bleaching baths of this invention, must be smaller than 10 -8 .5 mole 2 liter -2 ; these include silver halides, for example silver chloride, silver iodide and especially silver bromide, whose solubility product is smaller than 10 -10 mole 2 liter -2 .
The process of this invention may be used whenever metallic silver is to be removed without attacking any sparingly soluble silver salts present, especially when the metallic silver and the sparingly soluble silver salt are contained in the same silver emulsion layer. This is the case whenever the metallic silver is distributed to form an image, especially in reversal processes where a primarily developed silver image is dissolved and the residual silver salt image is converted into a black or colored image by diffuse light and second development or by development in the presence of a fogging agent, or by sulphidation.
The invention is of special importance in carrying out the silver dye bleach-negative-positive process. In the usual silver dye bleaching process a material is used that contains photosensitive silver halide and a dyestuff. On exposure and development a silver image is formed whose gradation is the opposite of that of the master (negative silver image). By decomposing the dyestuff on the silver a color gradation is achieved which is opposite to that of the silver image, that is to say equal to that of the master. Thus, a positIve color image of a positive master is obtained.
To produce a positive color image from a negative master by the silver dye bleach process, a reversal development is needed. Accordingly to this invention a positive color image is obtained from a negative master by development of a material that contains a photosensitive silver halide emulsion and a bleachable dyestuff after exposure, removing the primarily formed silver image, reducing the undeveloped residual silver halide to metallic silver and then decomposing the dyestuff depending on the amount of silver present in the layer, and removing the primary silver image by means of an acid bath which is free from anions that form sparingly soluble silver salts and which contains an oxidant and a complexing agent capable of forming with silver complexes of little stability.
When the acid and the oxidant are chosen according to this invention, the bleachable dyestuff is not attacked and this makes the present process superior to the dichromate process. The superiority over the process according to German specification No. 1,154,347 is based on the fact that the present process requires a smaller number of baths.
The temperature of the silver bleaching bath and the treatment time may be varied within wide limits without incurring a loss of undeveloped silver halide.
The present process is applicable to the treatment of both single-layer and multilayer color materials, of materials for viewing in incident light and transparencies, of mixed grain emulsion layers and of materials with incorporated capsules. Thus, for example, a rapid reversal process can be carried out with a metarial which contains acid, oxidant and complexing agents in crushable pods. Another variant starts from a material that contains oxidant and complexing agents in pods which disintegrate only when they come into contact with an acid bath.
The process according to the present invention is of special value in connection with reversal processes but the invention is also suitable for other processes in which metallic silver (such as developed image silver, latent image nuclei or colloidal silver) is dissolved without affecting any sparingly soluble silver salts present (which are present in the same grain, in the same layer or in another layer).
In the following Examples, which illustrate the invention, all percentages are percentages by weight.
EXAMPLE 1
Two strips of a blue-sensitized emulsion layer on a cellulose triacetate base, containing 0.7 g of silver bromide and 0.12 g of a yellow dyestuff of the formula ##SPC2##
per square meter, are exposed under a stepped wedge. The exposed strips are developed with a commercial para-methylaminophenol sulphate/hydroquinone developer and the development process is stopped with a 3 percent solution of acetic acid. After having been washed in water for 1 minute, one strip is treated for 3 minutes and the other strip for 10 minutes with a solution of the following composition to remove the metallic silver left in the layer after the first development:
Iron-III-perchlorate nonahydrate 52 g sulphamic acid 60 g thiodiglycol 12.5 g water, to pH value: 0.35. 1 liter.
After having been rinsed in water for 4 minutes, the strips are treated for 6 minutes with a commercial p-methyl-aminophenol-sulphate/hydroquinone developer to which 0.1 g of tributylaminoboran per liter have been added, to reduce the silver bromide left in the layer. The two strips are then rinsed for 2 minutes and dried.
Both strips carry a silver image which is positive with respect to the stepped wedge used for exposure. The two silver images are completely identical.
To bleach the dyestuff the strips are immersed in a solution of the following composition:
Hydrochloric acid, 37 % strength 70 ml potassium bromide 50 g thiourea 80 g 2-amino-3-hydroxyphenazine 5 mg water, to 1 liter.
After 6 minutes' immersion the strips are rinsed in water and treated with a silver bleaching bath of the following composition:
Potassium iron(III)cyanide 75 g potassium bromide 15 g primary sodium phosphate monohydrate 10 g sodium acetate trihydrate 5 g glacial acetic acid 10 ml water, to 1 liter.
The strips are then rinsed in water for 1 minute and fixed with an acid thiosulphate solution in the known manner. Both strips carry identical yellow images of the stepped wedge used for the exposure, which are negative with respect to the master.
EXAMPLE 2
The procedure used is as described in Example 1, except that the metallic silver left after the first development is removed with a bath composed as follows:
Iron(III)perchlorate nonahydrate 28 g 2N-nitric acid 50 ml 2-hydroxyethyl-methylsulphide 9.6 g water, to 1 liter. pH value: 0.65
The resulting images are identical to those obtained as described in Example 1.
EXAMPLE 3
Instead of the bath described in Example 2 a solution of the following composition is used:
Benzoquinone, dissolved in 200 ml of methanol 10.8 g 2N-sulphuric acid 50 ml 2-hydroxyethyl-methylsulphide 10 g water, to 1 liter pH value: 0.8
The resulting images are identical to those described in Example 1.
EXAMPLE 4
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g perchloric acid, 70% strength 150 g disodium salt of di-(p-sulphophenyl)-sulphide 80 g water, to 1 liter pH value: 0.6.
The resulting images are identical to those of Example 1.
EXAMPLE 5
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g thiodiglycolic acid 15 g sulphamic acid 60 g water, to 1 liter pH value: 0.35.
The resulting images are identical to those of Example 1.
EXAMPLE 6
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g thiodipropionic acid 18 g sulphamic acid 60 g water, to 1 liter pH value: 0.35.
The resulting pictures are identical to those described in Example 1.
EXAMPLE 7
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g water, to 1 liter thiophene-2,5-dicarboxylic acid to saturation pH value: 0.35
The resulting images are identical to those of Example 1.
EXAMPLE 8
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g acetonitrile 100 g water, to 1 liter pH value: 0.35.
The resulting images are identical to those described in Example 1.
EXAMPLE 9
Instead of the bath described in Example 2 a solution of the following composition is used:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g allyl alcohol 140 g water, to 1 liter pH value: 0.35.
The resulting images are identical to those of Example 1.
EXAMPLE 10
A red-sensitized emulsion layer which contains per square meter of cellulose triacetate base 0.7 gm of silver bromide and 0.115 g of a blue-green dyestuff of the formula ##SPC3##
is exposed under a stepped wedge and then processed as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath composed as follows:
Iron(III)perchlorate nonahydrate 52 g citric acid dihydrate 52 g thiodiglycol 12.8 g water, to 1 liter pH value: 0.5.
A blue-green image of the stepped wedge used for the exposure is obtained which is negative with respect to the master. No differences can be noted by comparing the results of the 3-minutes test with those of the 10-minutes test.
EXAMPLE 11
A green-sensitized emulsion layer which contains per square meter of a triacetate base 0.7 g of silver bromide and 0.150 g of a magenta dyestuff of the formula ##SPC4##
is exposed under a grey wedge and then processed as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath of the following composition:
Iron(III)perchlorate nonahydrate 52 g oxalic acid dihydrate 40 g thiodiglycol 12.5 g water, to 1 liter pH value: 0.35.
The resulting magenta image of the grey wedge used for the exposure is a negative of the master. Identical results are obtained in the 3-minutes and the 10-minutes test.
EXAMPLE 12
A photographic tripack material which carried on a transparent base a red-sensitized silver bromide emulsion containing a cyan dyestuff as described in Example 10, covered by a green-sensitized emulsion layer containing a magenta dyestuff as described in Example 11, which layer in turn is covered by a blue-sensitized silver bromide emulsion layer containing a yellow azo dyestuff as described in Example 1, is exposed under a grey wedge. Processing is carried out as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath of the following composition:
Copper(II)sulphate pentahydrate 125 g thiodiglycol 12.5 g water, to 1 liter pH value:3.5.
A reproduction of the grey wedge used for the exposure is obtained which is a negative of the master.
EXAMPLE 13
A commercial enlarging paper, whose emulsion layer contains a silver halide consisting of 54 mol percent of silver chloride and 47 mol percent of silver bromide, is exposed under a grey wedge and then developed with a commercial p-methyl-aminophenol-sulphate/hydroquinone developer. After having been rinsed in water for 2 minutes the paper is immersed in a solution of the following composition:
Iron(III)perchlorate nonahydrate 52 g sulphamic acid 60 g thiodiglycol 12.5 g water, to 1 liter pH value: 0.35.
After 8 minutes the paper is rinsed in water and then exposed in diffuse light.
The silver halide left in the layer is then reduced to metallic silver with the developer used in Example 1. The resulting silver image of the grey wedge used for the exposure is a positive reproduction of the master.
EXAMPLE 14
A strip of an emulsion layer on an opaque cellulose triacetate base, sensitive to red light, which carries per square meter of base 0.7 g of silver bromide and 0.115 g of the cyan dyestuff of the formula (4) is exposed under a stepped wedge and then processed as described in Example 1, except that the metallic silver left in the layer after the first development is removed with a bath of the following composition:
Ammonium-iron(III)sulphate dodecahydrate 60 g 2N-sulphuric acid 100 ml pyrazine 50 g water, to 1 liter pH value: 0.8.
The resulting cyan-colored image is a negative of the stepped wedge used for the exposure.
EXAMPLE 15
The process described in Example 14 is used, except that the metallic silver formed after the first development is removed with a bath of the following composition:
Iron(III)nitrate nonahydrate 40 g 2N-nitric acid 100 ml pyrazine 50 g water, to 1 liter pH value: 0.8.
As in Example 14 a cyan-colored image is obtained which is a negative reproduction of the stepped wedge used for the exposure.