GB1575470A

GB1575470A - Method of forming silver halide grains by electrolysis

Info

Publication number: GB1575470A
Application number: GB13578/77A
Authority: GB
Original assignee: Polaroid Corp
Current assignee: Polaroid Corp
Priority date: 1976-04-01
Filing date: 1977-03-31
Publication date: 1980-09-24
Also published as: FR2346741B1; JPS6034744B2; CA1109822A; DE2714489A1; DE2714489C2; JPS52129519A; FR2346741A1

Description

(54) METHOD OF FORMING SILVER HALIDE GRAINS BY ELECTROLYSIS (71) We, POLAROID CORPOR ATION, a corporation organised under the laws of the State of Delaware, United States of America, of 549 Technology Square, Cambridge, Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- Photographic silver halide emulsions are generally prepared by precipitation of the silver halide in the presence of a carrier or binder, generally gelatin, wherein the silver halide grains are formed by the interaction of a water-soluble halide, such as potassium bromide. The formation of the silver halide grain is usually accompanied by the liberation of counterions which must be removed in order to render the surface of the silver halide grain available for efficient chemical sensitization and to coat the grains without the risk of formation of crystals of the counterions which would deform the emulsion layer and render it unsuitable for photographic use.

In order to avoid the deleterious effects of the counterions, extensive removal techniques well known to the art are employed. These washing operations are varied and extensive and occupy a large proportion of the time and equipment employed in emulsion manufacture. The removal of the counterions by a washing procedure is one of the most critical phases of emulsion manufacture since the quality or even the usefulness of the emulsion depends upon this precedure. The term "grain" as used herein refers to a crystalline particle of silver halide and should be understood to include particles of any composition of silver halide with any mixture of crystal habits.

From the foregoing, it will be noted that the formation of the silver halide grains and the sensitization takes place in the presence of a binder material. Gelatin is the most commonly used binder material for silver halide, but other materials such as synthetic polymers are also employed. It is a requirement of the binder material that it permit the growth of silver halide grains at a controllable rate. The binder material must also be capable of being noodled or flocculated to permit washing of the emulsions to remove unwanted counterions and excess salts. It is also a requirement that the binder material allow the various sensitization processes to take place. A further requirement is that the binder prevent agglomeration of the silver halide grains and- not be salted out by the counterions present. These requirements disqualify a large number of synthetic polymeric materials from being employed in silver halide emulsions when otherwise they may possess some properties desired in such employment. For example, some polymers are good for grain growing but not for the washing step, and vice versa. In addition, because the reaction forming the silver halide grains takes place in the presence of the binder, starting materials and reaction products other than silver halide are entrapped therein, which contribute to the necessity of the above-mentioned extensive washing procedures.

A method for forming emulsions has now been found which is not susceptible to the deficiencies of the prior art and which circumvents the above limitations by forming silver halide grains without substantially increasing the concentration of counterions and therefore eliminates wash steps and removes many restrictions on polymers.

The invention provides a method for the production of photosensitive silver halide, the method comprising forming silver halide grains by electrolytic generation of silver ions and halide ions in an electrolyte substantially free of counterions using a silver anode and a halide generating cathode and precipitation of the resultant ion pairs remote from the electrodes. The grains are then generally grown to the desired size, disposed in polymeric binder and coated on a support. They may be disposed in a polymeric binder by the inclusibn in the electrolyte of a solution of a binder or by addition of a binder after their formation. They may be sensitised in conventional manner and conventional sensitising agents and.. other conventional additives may be employed, for instance by inclusion in the electrolyte or by addition subsequent to grain fortnation.

It is preferred that the electrolyte should be a solution containing a conductive polymer, which may thus serve as a binder.

An electrolytic cell for the generation of silver ions and halide ions at precipitation of the ion pairs remote from the electrodes comprises a housing, a silver anode, a halide generating cathode and an electrolyte which comprises a solution of a conductive polymer. Suitable polymers are described in more detail below.

The anode preferably comprises particulate silver especially in the form of silver needles which may be adhesively secured to a support such as graphitic carbon. An electrolytic cell for the generation of silver ions and halide ions and precipitation of the ion pairs remote from the electrodes may thus comprise a housing a silver anode comprising particulate silver and a halide generating cathode and an electrolyte.

The cathode may be a halide salt insoluble in the electrolyte but preferably comprises elemental halogen and electrically conductive carbon. Accordingly an electrolytic cell for the generation of silver ions and halide ions and precipitation of the ion pairs remote from the electrodes may comprise a housing, a silver anode, a halide generating cathode comprising elemental halogen and electrically conductive carbon and an electrolyte.

The term "electrolyte" as used herein is intended to refer to a solution of a substance that dissociates into two or more ions, to some extent, in water and thus provides a solution which conducts electric current.

The term is also intended to embrace solutions of two or more such substances.

The term "remote from the electrodes" as used herein is intended to refer to the reaction of the ions in the electrolyte at a position spatially removed from the electrodes so that the silver salt is not plated on either of the electrodes.

The invention will now be described in more detail, including reference to the accompanying drawings which are photomicrographs of silver halide grains prepared in accordance with the invention.

The present invention avoids the use of the conventional sources for the silver and halide ions employed in the formation of silver halide grains, e.g., the silver nitrate and potassium halide salts. By utilizing the essential ionic reactants only, normally associated counterions, such as NO3- and K+, are not involved in the reaction and, therefore, need not be removed or otherwise taken into account in the process.

Because the silver and halide ions are preferably produced substantially stoichiometrically, an unwanted excess of either ion is avoided, thus eliminating a problem encountered in conventional precipitation of silver halides with respect to the control of excess halide concentration which influences the rate of growth, grain size and grain habit.

The term "substantially stoichiometric" is intended to refer to a sufficient equivalence of silver ions and halide ions to form silver halide grains so that there is not such an excess of either silver ions or halide ions at the end of the electrolysis as to be photographically undesirable. However the ions can "be generated nonstoichiometrically if desired.

The present invention contemplates the precipitation of silver salts from only the essential reactants, Ag+ and X-, wherein X- is halide, which are provided by the reaction of the respective electrodes comprising a relatively simple electrolytic cell, one electrode (anode) being silver, the other (cathode) being a source of a negative ion -which will form an insoluble halide with the silver. For convenience, the process of the present invention will be described primarily in terms of the formation of silver bromide grains; however, it will be understood that any halide ion may be employed in the electrolysis which will provide an insoluble, photographically active silver halide grain.

For example, the cell and the reactions may be represented as follows: The cell: Ag/Ag+//Br-/AgBr (solid)/Ag Anode reaction: Ag < Ag++e Cathode reaction: AgBr+e-tAgO+Br The silver and halide ions produced at the two respective electrodes diffuse into the electrolyte where they react to form a precipitate of silver halide: Ag++Br-eAgBr; In a preferred embodiment, a relatively small amount of polymeric binder may be employed, e.g., about 1 /" gelatin in the electrolyte. It should be noted that the selection of a binder is not limited to the types ordinarily characterized as optimum for growing grains since the binder, in the prior art, is principally present to prevent agglomeration of grains as they form.

Selection of a binder material need not be limited to those which can be flocculated or gelled and noodled, as in conventional emulsions, since these properties are associated with washing requirements which do not exist in the present invention.

Subsequent to or concomitant with the formation of the grains a ripening step may be employed to grow the particles to the desired size. Since substantially no counterions are present, a wash step generally is not required. The particles may be separated from the electrolyte, as by filtering or centrifuging before a ripening step.

Alternatively, ripening and the other processing operations may be carried out in the same medium without separation of the grains from the electrolyte. Ostwald ripening agents may be employed to grow grains to the desired size. Silver halide growing agents known to the art which may be employed include ammonia (ammonium hydroxide) thiocyanate, thioethers (e.g.

2,2' - thiodiethanol) and excess halide. The grains preferably have a substantially uniform grain size distribution.

Electrolysis may be continued after initial grain formation to provide grain growth under conditions of temperature and current density which do not provide new nucleation.

Subsequent to grain growth, binder polymer may be added, as well as chemical and/or spectral sensitizing agents coating aids, dispersing agents or other emulsion addenda conventionally employed in the art. Chemical sensitizing agent may be included in the solution of electrolyte.

While a silver anode in a flat sheet form may be used, silver in this form is relatively expensive and limited with respect to surface area. The employment of particulate silver provides an anode having high electrical conductivity, density, surface area and cost advantage.

The term "particulate silver" as used herein is intended to refer the discrete, particulate material generally 1-10 mm. in diameter. Preferred are silver needles which is a term applied to particulate silver obtained by isolating silver electrolytically from solution without any further reforming.

The anodes of the present invention preferably comprise particulate silver loose (retained by an electrolyte permeable container or other barrier, such as parchment) in a conductive or nonconductive binder or adhered to a conductive or non-conductive support.

Preferred supports include polyester, polymethyl methacrylate or graphite structures, such as plates. The particulate silver may be adhered by any suitable adhesive. Care should be taken, however, in the case of conductive supports, that good electrical contact be maintained between the silver needles and the support.

Thus, one of the advantages of the present invention comprise the ready employment of an anode of substantially any desired shape. Any shape support can be selected and then the particulate silver adhered thereto.

Any suitable electrolyte which is not detrimental to the photographic process and which does not require a removal step may be employed. Electrolytes may also be selected for other utility in addition to electrical conductance. Such electrolytes may also function as pH buffers, pAg buffers, redox buffers, developing agents, Ostwald ripening agents, quaternary salts, dispersants and surfactants. Chemical and spectral sensitizing agents may also be present in the electrolyte during electrolysis to provide sensitization of the grains as they are formed.

It has been found that a conductive polymer can be employed as the electrolyte, thus providing both the conductivity required and at least a portion of the binder material. A particularly useful conductive polymer comprises poly - 2 - acrylamido 2 - methylpropane sulfonic acid.

The cells of the present application may employ a non-polymeric electrolyte and a binder material, such as gelatin to prevent "clumping" or phase separation of the silver halide grains as they are formed. The amount of binder may be less than 50/, by weight of the electrolyte solution. However, by employing at least one conductive polymer of the present invention as a single substance to function both as the electrolyte and protective colloid or binder advantageous results are obtained. Utilizing one or more of such conductive polymers has resulted in superior protective colloid action as well as high conductivity.

The conductivity of the solution of polymer is preferably in excess of 10,000 Mmhos. A higher conductivity level is preferred as a higher conductivity will result in a lower cell resistance and attendant more efficient and faster silver halide formation.

In a preferred embodiment, a pAg buffer is employed. A particularly useful pAg buffer/electrolyte is the disodium salt of ethylene diamine tetraacetic acid (EDTA).

Other suitable pAg buffers include combinations of EDTA salts with the free acid and gluconic acid, alone or with EDTA.

In the process of the present invention pAg may be controlled. pAg may be set initially by addition of dilute KBr or AgNO3 and maintained as there is no dilution in the course of the reaction and the reactant ions are generally generated in stoichiometric balance. Since pAg may be controlled and maintained consistently low throughout the precipitation, the present invention is particularly suited for the preparation of homogeneous emulsions with preferred and predetermined crystal habit and uniform grain size distribution. Alternatively, the pAg may be programmed by setting conditions, such as cell voltage, so that ions are generated in a nonstoichiometric ratio.

It should also be understood that conventional, nonpolymeric electrolytes may be employed in conjunction with the conductive polymers.

As examples of suitable conductive polymers, mention may be made of the following: Poly - 2 - acrylamido - 2 - methylpropane sulfonic acid Copolymers of 2- acrylamido- 2methylpropane sulfonic acid and trimethylamine acrylimide Copolymers of 2- acrylamido- 2methylpropane sulfonic acid and acrylamide Copolymers of 2- acrylamido- 2methylpropane sulfonic acid and trimethyl gamine - N - acryloyl - methylalaninimide It should be understood that the conductive polymer may constitute the sole binder or may be used in conjunction with another polymer which may be conductive or it may be non-conductive, e.g. gelatin or deacetylated chitin.

Any electrode that can generate a desired halide ion by electrolytic action can be employed as a cathode in the present invention. The cathode may comprise an insoluble halide salt, preferably silver halide. Other suitable cathode materials may comprise, for example, thallium halide, gold (Au+') halide, lead halide or elemental bromine.

A cathode comprising silver halide may be generated by a preliminary electrolysis in a cell by methods known to the art.

A single pair of electrodes may be employed or a plurality of electrodes of various combinations. When a plurality of cathodes and/or anodes are used, they may be employed sequentially in the generation of the desired ion pairs.

Alternatively, the electrode may be composed of an insoluble salt or a mixture of salts. In still another embodiment, the ion source may comprise a halogen absorbed onto an inert carrier contained in a porous container, such as bromine on starch granules contained in a porous aluminum oxide thimble which allows ions to flow but retains the particles therein. Still another embodiment employs bromine in a conductive polymer. In still a further embodiment, solid silver halide may be disposed in a conductive, permeable binder to provide the electrode.

The desired distribution halide may be achieved by employing chloride, iodide and bromide electrodes in any ratio, sequentially or concomitantly.

Alternatively, a silver halide electrode may be prepared with one halide and then converted at least in part to a second halide.

Thus, an AgBr electrode can be prepared and then converted to AgBrI or AgI by providing I- to the electrolyte.

In a particularly preferred embodiment a cell of the present invention employs, as a cathode, elemental halogen absorbed into electrically conductive carbon, preferably in the form of carbon black origraphite. The halogen may be bromine, iodine or chlorine.

Retention by carbon thus minimizes the corrosive effects, and particularly in the case of chlorine and bromine, the volatile property of the halogens, while at the same time making them readily available in the quantities desired during cell operation.

The use of a halogen-carbon black powder cathode preferably includes a physical barrier in the cell to retain the carbon black powder and avoid contamination of the silver halide grains with carbon black particles.

Preferably, the carbon is employed in the form of graphitic carbon or pressed carbon black plates or rods. Graphite or pressed carbon black in these forms are particularly suitable since no physical barrier is required; and graphite, in plate or rod, as thin as one-quarter of an inch is mechanically strong, self-supporting and can be fabricated to the size and shape desired. In addition, graphite possesses sufficiently high conductivity to permit the direct electrical contact.

There are references in the literature to the effect that definite lamellar compounds are formed by the addition of hologen to the graphite. (Ubbelohde, A. R. and Lewis, F.A., Graphite and Its Crystal Compounds, Oxford (1960), pp 118-130 and Kagan, H.B., Chemtech 6, 510 (1976).) As used herein, the expressions "having halogen absorbed thereon" or "absorbed into" are intended to include both the physical and/or chemical retention of halogen by the carbon substrate.

In order to illustrate the retentive properties of a graphite. plate, liquid bromine was added to a dry graphite plate (4cmx4cmxO.7cm) about one minute allowed for absorption, then 25 ml. of water was added. After two hours the plate was removed and the liquid titrated with silver nitrate. The amount of silver nitrate needed to reach the endpoint, as detected with a silver sulfide specific ion electrode, was compared with the amount of bromine originally added. The data indicated that about 85% of the bromine applied was retained. It has also been found that about 90 /" bromine retention can be achieved by a heat treatment of the graphite in an oven; for example, 2 hours at 7000 C, before bromine addition.

As a comparison, the above procedure was repeated except that an equimolar amount of potassium bromide (KBr in water) was substituted for the liquid bromine. It was found that 65 /n of the potassium bromide was eluted.

The halogen may be absorbed into the carbon from liquid or vapour. In the case of iodine, the carbon can be exposed to sublimating iodine crystals.

It will be readily seen that the process of the present invention is economical, versatile, adapted to continuous operation, does not require flow controls or systems and has practically no waste effluent.

The following nonlimiting examples illustrate the present invention; EXAMPLE I A silver btomide electrode was prepared using a cell comprising a platinum cathode, a silver anode comprising a flat sheet of commercially obtained silver lin.x7in. in size and, as an electrolyte, 600 ml. of 2% lithium bromide in water. A potential of 2 volts as measured at the power supply was applied to the system for one hour, resulting in a current flow of 0.4 amps. After 30 minutes, 12 g. of lithium bromide were added. At the end of the hour, the silver/silver bromide electrode formed from the silver anode was rinsed with water and immersed in a 1.5 /" potassium iodide solution until it took on about 3 mg of iodide.

The thus-formed Ag/AgBrI electrode was then employed as a cathode with a cell comprising, in addition, a silver anode and, as an electrolyte, 250 ml. of 1% EDTA (2.3:1 disodium salt to free acid) and 2.5 g of dry gelatin.

The cell comprised a 4in.x8in.x6in.

Plexiglas (a Trade Mark of Rohm and Haas Co., Philadelphia, Pa. for acrylic resin plastic sheet) tank. The anode was a flat sheet (lin.x7in.) of silver obtained commercially and was positioned along one wall of the container. The cathode was positioned at the opposite wall of the tank, approximately 2 inches from the anode.

The cell was run for 2 hours at 2 volts (measured at the power supply) and 0.04 amps. At the end of that time, the grains were placed in a 250 ml. beaker and ammonium hydroxide solution was added until a pH of 7.8 was obtained. The mixture was heated at 50 degrees C. and held at that temperature for one hour. The grains had grown to a mean diameter of about 0.8 um.

The solution pH was adjusted to 5.4 with sulfuric acid and the grains separated by centrifuging.

1 gram of the 0.8 um grains were mixed with 1 gram of inert deionized gelatin, 6 ml.

distilled water, 0.01 ml. of 0.1 hypo. 0.02 ml. gold as the thiocyanate complex (526 ppm gold) and digested for 60 minutes at 50"C. To the grains was added 1 ml. of a 2% solution of Triton X-100, (an octylphenoxy polyethoxy ethanol sold by Rohm & Haas Company, Philadelphia, Pennsylvania) and the mixture was coated on a plastic support at a coverage of about 100 mg./ft2, exposed at equivalent ASA 18 and processed with a Type 42 receiving sheet and processing composition (Polaroid Corporation, Cambridge, Mass.). A continuous tone positive silver transfer image was obtained with a Dmin of 0 and a Dmax of 1.5. Figure I is a photomicrograph of grains prepared according to the procedure of Example I.

An analysis of the grains by X-ray fluorescence showed the grains to comprise silver iodobromide with about 2 mole percent iodide.

EXAMPLE II Silver iodobromide grains were prepared according to the procedure of Example I up to the step of separation by centrifuging and were then grown according to the following procedure. To 100 ml. of the fine-grained silver iodobromide grains produced by the electroloysis was added 10 ml. of 20% freshly distilled 2,2'- thiodiethanol. The mixture was stirred for 15 minutes at 60 degrees C. The remainder of the grains were added in 4 aliquots at 15 minute intervals.

The grain size ranged from 0.1 to 1.5 um.

The grains were separated by centrifuging.

2 grams of the thus-formed grains were mixed with 6 g of inert deionized gelatin, 105 mls. of a 2% solution of the sodium salt of the dioctyl ester of sulfo-succinic acid.

0.01 ml. of 0.1 hypo, 0.2 ml. of gold as the thiocyanate complex (526 ppm gold) and digested for 210 minutes at 54 degrees C.

During this time the diffusion transfer speed (Type 42 processing) increased 3 stops. To the grains was then added 0.5 ml. of a 1 mg.

per ml. of water of a cyanine sensitizing dye of the formula:

The mixture was coated on a plastic support at a silver coverage of about 100 mg./ft.2, exposed, and processed with a Polaroid Land Type 42 receiving sheet and processing composition. A continuous tone silver transfer image with panchromatic response was obtained.

Figure 2 is a photomicrograph of grains prepared according to the procedure of Example II.

EXAMPLE III A cell was constructed employing a 400 ml. beaker, a bromine cathode comprising a Cellophane (Trade Mark) dialysis tube I 1/2 in. in diameter and 2 in. long enclosing 40 ml. of a 40 /n solution of an 80/20 2acrylamido - 2- methylpropane sulfonic acid/trimethylamine - N - acryloyl methylalaninimide copolymer and 20 ml. of methanol, a platinum electrode (commercially available platinum basket) and 5 ml. of 20% bromine in methanol; as an anode, a silver sheet 2in.x8in. wrapped around the inside wall of the beaker; and 250 ml. of a 5% solution of an 80/20 2 acrylamido 2 - methylpropane sulfonic acid/trimethylamine - N - acryloyl methylalaninimide copolymer. Additional bromine solution was added inside the tubing during electrolysis when the amperage dropped below 0.6. The cell was run for 5 hours at 2 volts (measured at the power supply) and at 0.6 amps. The thus-formed emulsion was then analyzed and was found to contain 5.2 /n silver. The silver bromide grains averaged about 0.5 um in diameter.

The grains were coated directly, without additional growing. Figure 3 is a photomicrograph of grains prepared according to the procedure of Example III.

EXAMPLE IV A silver iodobromide emulsion was prepared, coatedand tested according to the procedure of Example II with the following changes: the electrolyte solution contained 1.25 g of dry gelatin; the cell was run at 2 volts (measured at the power supply), 0.04 amps; ammonium hydroxide was added to a pH of 8.5 and the ammonium hydroxidegrain mixture was heated to 600C and held at that temperature for one hour.

The grains were found to have a mean diameter of about 0.7 um with 90% of the grains having diameters within + 25 /n of the mean diameter. The pH was adjusted to 5.5 with sulfuric acid and the pAg adjusted to 8.8 Following the coating, exposure and development procedure of Example I, a continuous tone positive silver transfer image was obtained. Figure 4 is a photomicrograph of grains prepared by the procedure of Example IV.

EXAMPLE V A cell was constructed comprised of a flat Teflon (Trade Mark) tank assembly 6"x6"x 1/2"; an anode comprising a paste of 300 g of silver needles in 100 ml. of poly 2- acrylamido - 2- methylpropane sulfonic acid water (2.5% solids) with a platinum wire contact; a parchment separator; and a cathode comprising 65 g. of carbon black, 5 ml. of elemental bromine and 100 ml. of poly - 2 - acrylamido - 2 methylpropane sulfonic acid in water (2.5% solids) with a platinum screen contact; and, as an electrolyte, 350 ml. of poly - 2 acrylamido - 2- methylpropan sulfonic acid in water (5.5% solids). The cell was operated for one hour with a current efficiency (amps/liter) of 1.1. A vibrating stirrer was employed during the cell operation. Turbidity increased with time indicating thus grains were forming.

EXAMPLE VI A cell was constructed composed of a circular, 6 inch diameter container, fitted with a paddle stirrer; an anode comprising silver needles (100 g.) adhesively bonded with a pyroxylin based cement sold under the trade name DUCO by E. I. duPont de Nemours & Co., Wilmington Delaware, to a 1/8 inch thick graphite disc 5 3/4 inch in diameter; and, as an electrolyte 400 ml. of poly - 2 - acrylamido - 2 - methylpropane sulfonic acid in water (5.5% solids). The cell was operated for 4 hours with a current efficiency of 1.3. Turbidity increased as grains were formed and analysis showed the emulsion to contain 1.2% silver.

EXAMPLE VII A cell was constructed composed of a 600 ml. beaker, an anode comprising a circular sheet of silver; a cathode comprising a sintered glass thimble containing 30 g of carbon black, 10 ml. of elemental bromine and 200 ml. of poly - 2 - acrylamido - 2 methylpropane sulfonic acid in water (25% solids), and, as an electrolyte 350 ml. of poly - 2 - acrylamido - 2 - methylpropane sulfonic acid in water (5.5% solids). The cell was operated for 3 hours with a current efficiency of 1.0. Turbidity increased as grains were formed and analysis showed the emulsion to contain 1.4% silver.

EXAMPLE VIII A cell was constructed composed of the Teflon tank assembly of Example V, an anode comprising 66 g. of silver needles adhesively secured to a polymethyl methacrylate support with DUCO cement; a cathode comprising 10 g. deacetylated chitin; 5 ml sensitizing agents suitable for employment in the present invention, mention may be made of U. S. Patents Nos. 1,574,944; 1,623,499; 2,410,689; 2,597,856; 2,597,915; 2,487,850; 2,518,698; 2,521,926; as well as Neblette, C. B., Photography, Its Materials and Processes, 6th Ed., 1962.

Spectral sensitization of the silver halide grains may be accomplished by contact of the grain composition with an effective concentration of the selected spectral sensitizing dyes dissolved in an appropriate dispersing solvent such as methanol, ethanol, acetone or water; all according to the traditional procedures of the art, as described in Hamer, F.M., The Cyanine Dyes And Related Compounds, as well as the above-mentioned disposition of the sensitizers in the electrolyte solution prior to or during grain formation.

Reduction sensitization of the grains prior to or subsequent to the addition of the binder may also be accomplished employing conventional materials known to the art, such as stannous chloride.

Sensitizers of the solid semiconductor type, such as lead oxide, may also be employed.

Additional optional additives, such as coating aids, hardeners, viscosity-increasing agents, stabilizers and preservatives, also may be incorporated in the emulsion formulation, according to the conventional procedures known in the photographic emulsion manufacturing art.

While the use of emulsions of the present invention have been described primarily in terms of diffusion transfer processes, it should be understood that substantially any type of photographic process can be employed.

WHAT WE CLAIM IS: 1. A method for the production ofphotosensitive silver halide, the method comprising forming silver halide grains by electrolytic generation of silver ions and halide ions in an electrolyte substantially free of counterions using a silver anode and a halide generating cathode and precipitation of the resultant ion pairs remote from the electrodes.

2. A method according to claim 1 in which the electrolyte comprises a solution of a polymeric binder.

3. A method according to claim 1 in which the electrolyte comprises a solution of a conductive polymeric binder.

4. A method according to claim 3 in which the conductive polymeric binder is selected from poly - 2 - acrylamido - 2 - methylpropane sulphonic acid, copolymers of 2 acrylamido - 2 - methylpropane sulphonic acid and trimethylamine acrylimide, copolymers of 2 - acrylamido - 2 - methylpropane sulphonic acid and acrylamide and copolymers of 2 - acrylamido - 2 - methylpropane sulphonic acid and trimethylamine - N - acryloyl - methylalaninimide.

5. A method according to any of claims 1 to 4 in which the anode comprises particulate silver.

6. A method according to claim 5 in which the anode comprises silver needles.

7. A method according to claim 6 in which the needles are adhesively secured to a support.

8. A method according to claim 7 in which the support is graphitic carbon.

9. A method according to any preceding claim in which the cathode comprises elemental halogen and electrically conductive carbon.

10. A method according to any preceding claim in which the precipitated grains are grown to the desired size, photographically sensitised and disposed in a polymeric binder and the binder and grains are then coated on a support.

11. A method according to claim 1 which includes the step of coating the grains on a support.

12. A method according to claim 1 in which the ions are generated substantially stoichiometrically.

13. A method according to claim 1 in which the ions are generated nonstoichiometrically.

14. A method according to claim 1 which includes the step of growing the grains to a desired size.

15. A method according to claim I in which the silver halide grains have a substantially uniform grain size distribution.

16. A method according to claim 1 in which the halide ions comprise a plurality of halides.

17. A method according to claim 1 in which the silver ions and the halide ions are generated by a plurality of electrodes.

18. A method according to claim 17 in which a plurality of cathodes and/or anodes are employed sequentially in the generation of the ion pairs.

19. A method according to claim 11 which includes the step of disposing the grains in a polymeric binder prior to coating the binder and grains on a support.

20. A method according to claim 2 in which the polymeric binder is gelatin.

21. A method according to claim 2 in which the binder is present in an amount of less than 5%.

22. A method according to claim 1 in which the silver ions and halide ions are generated substantially simultaneously in an electrolyte solution from a silver anode and a halide salt cathode insoluble in the electrolyte solution.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. sensitizing agents suitable for employment in the present invention, mention may be made of U. S. Patents Nos. 1,574,944; 1,623,499; 2,410,689; 2,597,856; 2,597,915; 2,487,850; 2,518,698; 2,521,926; as well as Neblette, C. B., Photography, Its Materials and Processes, 6th Ed., 1962. Spectral sensitization of the silver halide grains may be accomplished by contact of the grain composition with an effective concentration of the selected spectral sensitizing dyes dissolved in an appropriate dispersing solvent such as methanol, ethanol, acetone or water; all according to the traditional procedures of the art, as described in Hamer, F.M., The Cyanine Dyes And Related Compounds, as well as the above-mentioned disposition of the sensitizers in the electrolyte solution prior to or during grain formation. Reduction sensitization of the grains prior to or subsequent to the addition of the binder may also be accomplished employing conventional materials known to the art, such as stannous chloride. Sensitizers of the solid semiconductor type, such as lead oxide, may also be employed. Additional optional additives, such as coating aids, hardeners, viscosity-increasing agents, stabilizers and preservatives, also may be incorporated in the emulsion formulation, according to the conventional procedures known in the photographic emulsion manufacturing art. While the use of emulsions of the present invention have been described primarily in terms of diffusion transfer processes, it should be understood that substantially any type of photographic process can be employed. WHAT WE CLAIM IS:

1. A method for the production ofphotosensitive silver halide, the method comprising forming silver halide grains by electrolytic generation of silver ions and halide ions in an electrolyte substantially free of counterions using a silver anode and a halide generating cathode and precipitation of the resultant ion pairs remote from the electrodes.

6. A method according to claim 5 in which the anode comprises silver needles.

8. A method according to claim 7 in which the support is graphitic carbon.

20. A method according to claim 2 in which the polymeric binder is gelatin.

23. A method according to claim 22 in

which the cathode comprises silver halide.

24 A method according to claim 23 in which the cathode comprises silver bromide.

25. A method according to claim 23 in which the cathode comprises silver iodobromide.

26. A method according to claim I in which the cathode comprises elemental halogen.

27. A method according to claim 9 or claim 26 in which the halogen is bromine.

28. A method according to claim I in which the electrolyte comprises a sodium salt of ethylene diamine tetraacetic.

29. A method according to claim 1 which includes the step of Ostwald ripening the grains.

30. A method according to claim 29 in which the electrolyte includes Ostwald ripening agent.

31. A method according to claim 29 in which the Ostwald ripening of the grains is conducted subsequent to grain formation.

32. A method according to claim 31 in which the ripening is carried out in the presence of ammonium hydroxide.

33. A method according to claim 31 in which the ripening is carried out in the presence of 2,2' - thiodiethanol.

34. A method according to claim 1 in which the electrolyte includes spectral sensitising agent.

35. A method according to claim I in which the electrolyte includes chemical sensitising agent.

36. A method according to claim 29 in which the grains are chemically sensitised after ripening.

37. A method according to claim 29 in which the grains are spectrally sensitised after ripening.

38. A method according to claim I substantially as herein described with reference to any of the Examples.

39. An electrolytic cell for the generation of silver ions and halide ions and precipitation of the ion pairs remote from the electrodes and which comprises a housing, a silver anode comprising particulate silver and a halide generating cathode and an electrolyte.

40. A cell according to claim 39 in which the particulate silver comprises silver needles.

41. A cell according to claim 40 in which the needles are retained by an electrolytepermeable barrier.

42. A cell according to claim 40 in which the needles are disposed in a binder.

43. A cell according to claim 42 in which the binder is a conductive polymer.

44. A cell according to claim 40 in which the needles are adhesively secured to a support.

45. A cell according to claim 44 in which the support is non-conductive.

46. A cell according to claim 44 in which the support is conductive.

47. A cell according to claim 46 in which the support is graphitic carbon.

48. An electrolytic cell for the generation of silver ions and halide ions and precipitation of the ion pairs remote from the electrodes and which comprises a housing, a silver anode, a halide generating cathode comprising elemental halogen and electrically conductive carbon, and an electrolyte.

49. A cell according to claim 48 in which the carbon is in the form of carbon black.

50. A cell according to claim 48 in which the carbon is in the form of graphitic carbon.

51. A cell according to claim 50 in which the carbon is in the form of a graphite plate;

52. A cell according to claim 51 in which the carbon is in the form of a graphite rod.

53. A cell according to claim 48 in which the halogen is bromine.

54. A cell according to claim 48 in which the halogen is chlorine.

55. A cell according to claim 48 in which the halogen is iodine.

56. A cell according to claim 48 in which the silver anode comprises silver needles.

57. A cell according to claim 48 in which the electrolyte comprises a conductive polymer.

58. An electrolyte cell for the generation of silver ions and halide ions and precipitation of the ion pairs remote from the electrodes and which comprises a housing, a silver anode, a halide generating cathode and an electrolyte which comprises a solution of a conductive polymer.

59. A cell according to claim 58 in which the conductive polymer is the sole electrolytic component of the electrolyte.

60. A cell according to claim 58 in which the solution includes a second polymer.

61. A cell according to claim 60 in which the second polymer is gelatin.

62. A cell according to claim 60 in which the second polymer is a conductive polymer.

63. A cell according to claim 58 in which the electrolyte solution includes a nonpolymeric electrolyte.

64. A cell according to claim 58 in which the conductive polymer is selected from poly - 2 - acrylamido - 2 - methylpropane sulphonic acid, copolymers of 2 acrylamido - 2 - methylpropane sulphonic acid and trimethylamine acrylimide, copolymers of 2 - acrylamido - 2 - methylpropane sulphonic acid and acrylamide and copolymers of 2 - acrylamido - 2 - methyl propane sulphonic acid and trimethylamine - N - acryloyl - methylalaninimide.

65. An electrolytic cell according to any of claims 39, 48 or 58 substantially as herein described.