GB2350436A - Preparing silver halide emulsions using iodine - Google Patents

Abstract

The invention relates to a method of forming a silver halide emulsion comprising precipitating silver chloride grains, with the proviso that during precipitation elemental iodine is added to the precipitation in between 1 and 50 seconds after at least 90 mol percent of the total silver chloride in said grains has been precipitated. The method may also comprise adding osmium as a run additive up to 90 mol percent of silver precipitation and ruthenium as a run additive up to 85 mol percent of silver precipitation.

Description

2350436 IODINE FOR THE PREPARATION OF SILVER CHLORIDE EMULSIONS

FIELD OF THE INVENTION

The invention relates to a process of preparing iodide containing radiation-sensitive silver halide emulsions useful in photography.

DEFINITION OF TERMS In referring to grains and emulsions containing two or more halides, the halides are named in order of ascending concentrations.

The term "silver iodohalide" in referring to grains or emulsions indicates a grain structure in which silver chloride and/or bromide provide a face centered cubic rock salt crystal lattice structure containing iodide ions.

The term "iodine" refers to the diatomic and neutral elemental compound. I The term "'iodide" refers to the negatively charged ionic monoatomic species.

The term "high chloride" in referring to grains and emulsions indicates that chloride is present in a concentration of greater than 50 mole percent, based on total silver.

Research Disclosure is published by Kenneth Mason Pulilications, Ltd., Dudley House, 12 North St., Ernsworth, Hampshire PO 10 7DQ, England.

BACKGROUND OF THE INVENTION

In the most widely employed form of photography, images are captured by a photographic element comprised of a support and at least one emulsion layer comprised of radiation-sensitive silver halide grains. The radiation-sensitive grains are prepared by reacting halide ions with silver ions in a dispersing medium. Silver chloride, silver bromide, and silver iodide are known to be useful, alone or in combination to form the radiation-sensitive grains.

Silver iodide grains exhibit 0 or y phase crystal lattice structures that can accommodate only minor amounts of silver bromide and/or chloride.

Difficulties with development have severely limited the use of these grains for latent image capture in photography.

Silver chloride and silver bromide each forms a face centered cubic rock salt crystal lattice structure. All relative proportions of chloride and bromide ions can be accommodated in this crystal lattice structure. Iodide ion can be accommodated up to its saturation limit, which is approximately 40 mole percent, based on total silver in a silver bromide crystal lattice structure, and up to about 13 mole percent, based on silver in a silver chloride crystal lattice structure, the exact limit varying within a few percent, based on temperature.

A large proportion of photographic emulsions contain silver iodohalide grains--that is, grains in which a significant, performance modifying concentration of iodide is contained in a face centered cubic rock salt crystal lattice structure formed by one or both of!he silver chloride and bromide. The highest levels of photographic sensitivity are typically realized by providing high bromide grains containing a minor amount of iodide, such as silver iodobromide grains. The presence of minor amounts of iodide ion can also enhance the sensitivity of high chloride grains. It is disclosed in U.S. Patents 5, 547,827; 5,726,005; 5,736,3 10; and 5,728,516 that iodochloride emulsions maybe. formed that have improved speed. These emulsions have the iodide incorporated at or below the surface of the grains.

To appreciate the techniques and difficulties for preparing mixed halide grains that contain iodide, it is necessary to appreciate the relative solubilities of the different photographically useful silver halides.

Although the majority of the silver and halide ions are confined to the grains, at equilibrium a small fraction of the silver and halide ions is also present in the dispersing medium, as illustrated by the following relationship:

Ag+ + X- - AgX (I) where X represents halide. From relationship (1) it is apparent that most of the silver and halide ions at equilibrium are in an insoluble form, while the concentration of soluble silver ions (Ag') and halide ions (X-) is limited. However, it is important to note that equilibrium is a dynamic relationship--that is, a specific halide ion is not fixed in either the right-hand or left-hand position in relationship (I). Rather a constant interchange of halide ion between the left- and right-hand positions is occurring.

At any given temperature the activity product of Ag' and X- is at equilibrium a constant and satisfies the relationship:

Ksp = [Ag+][X-1 010 where Ksp is the solubility product constant of the silver halide. To avoid working with small fractions, the following relationship is also widely employed:

-log Ksp = pAg + pX (IM where pAg represents the negative logari!hm of the equilibrium silver ion activity and pX represents the negative logarithm of the equilibrium halide ion activity. From relationship (M) it is apparent that the larger the value of the -log Ksp for a given halide, the lower is its solubility. The relative solubilities of the photographic halides (Cl, Br, and I) can be appreciated by reference to Table I:

TABLEI

Temp. 'C AgO AgBr AgI -log Ksp ' -log Ksp -log Ksp 9.2 11.6 15.2 8.9 11.2 14.6 8.6 10.8 14.1 8.1 10.1 13.2 From Table I it is apparent that at 400C the solubility of AgCI is one million times higher than that of AgI, while the solubility of AgBr ranges from about one thousand to ten thousand times that of "AgI.

When silver ion and two or more halide ions are concurrently introduced into a dispersing medium, the silver ion precipitates disproportionately with the halide ion that forms the least soluble silver halide. It is therefore appreciated that the presence of local iodide ion concentration variances in the dispersing medium in the course of precipitation of silver iodohalide grains results in iodide ion non-unifon-nities in the grains precipitated. When the limited ability of a face centered cubic rock salt crystal lattice structure to accommodate iodide ions is taken into account, it is readily appreciated that if iodide ion non uniformities in the dispersing medium are sufficiently large, a separate, unwanted high iodide (P or y phase) grain population can be produced.

In the large scale precipitation of iodochloride emulsions, a mixing sensitivity problem arises. This occuFs wben KI is used as the source of iodide in precipitating the iodochloride emulsion. The rate of reaction between iodide ion and silver ion is much faster than the rate of dispersion of the potassium iodide reactant. The latter rate is dependent on the amount of the KI dispersed, the rate of blending, and the kettle volume. This results in the uneven distribution of the iodide ion from grain to grain and from batch to batch depending on the rate of mixing and thus the rate of dispersion. The resulting silver iodochloride- emulsion thus varies in photographic performance and lacks manufacturability control.

As a technique for better controlling the uniformity of iodide ion availability within the dispersing medium, it has been recently suggested (see Takada et al U.S. Patent 5,389,508; Suga et al U.S. Patent 5,418,124-, Maruyarna et al U.S. Patent 5,525,460; and Kikuchi et al U.S. Patent 5,527,664) that the uniforn-dty of iodide ion within the dispersing medium can be better controlled by introducing iodide in the form of a compound satisfying the formula:

R-1 (IV) wherein R represents a monovalent organic residue which releases iodide ion upon reacting with a nucleophilic reagent, such as hydroxide, sulfite ion, or ammonia.

Hydroxide ion and arnmonia are basic species that are known to cause a rise in pH. An increase in pH has been demonstrated to produce fog in emulsion making. Such fog formation is non-discriminatory and gives rise to poor image in the art of silver halide photography. Additionally, formation of sulfite anion, a silver halide grain ripening agent, may lead to changes in grain morphology.

U.S Patent, 5,726,005 describes photographic elements containing cubical grain silver iodochloride emulsions. U.S. Patent 5,736,3 10 teaches the preparation of cubical grain silver iodochloride emulsions and processes. U.S. Patent 5,792,601 of Edwards et al discloses a process for the preparation of iodochloride emulsions with incorporated iridium dopant. U.S. Patent 5, 736,312 of Jagannathan et al discloses a process for introducing iodide ion into the crystal lattice of silver halide grains by reacting an iodate (103-) anion with a sulfite anion, a known silver halide grain ripening agent.

The organic ligand release (see forrnula IV above) approach for introducing iodide into silver halide grain,crystal lattice structures, as well as the Jagannathan et al approach of employing iodate (103-) anion, has significant disadvantages. In order to release iodide ion by these methods, either a strong grain ripening agent, such as sulfite ion, or an elevated pH is required. Elevated pH conditions risk undesirably elevating fog levels in the emulsions. This occurs because the conditions are favorable for a portion of the silver ions, Ag+ ,.being reduced to Ag 0. When a few Ag 0 atoms are located in close proximity,'the grain can spontaneously develop, independent of its exposure. This is sometimes referred to as reduction fog or R-typing.

The requirement of a sulfite anion is particularly undesirable, since sulfite is known to act as a grain ripening agent. That is, it tends to speed the ripening out of smaller grains onto larger grains and the preferential solubilization of grain edge and comer structures. This can have an undesirable effect of changing the shape of the grains. For example, where it is desired to maximize a particular class of external crystal faces, such as I 1111 or ( 100) faces, ripening can have the effect of rounding edges and comers to decrease the proportion of clearly (I I I I or 1100) grain faces. This same edge and comer rounding can also degrade grain shapes, such as well-defined cubic, octahedral, or tabular grains, causing regression toward spherical forms as a function of the degree of ripening that has occurred.

Finally, the use of iodate ion to release iodide anion, as taught by Jagannathan et al, is relatively inefficient, since three sulfite anions are required to release a single iodide anion, as illustrated by the following equation:

103 + 3SO3 -4 1 + 3SO4 (V) Thus, to arrive at a 3 mole percent iodide concentration in the grains by the process of Jagannathan et al, it is necessary to introduce nearly 10 mole percent sulfite ion, based on silver. This is a high proportion of sulfite ion.

PROBLEM TO BE SOLVED BY THE INVENTION There is a need for a method of introducing iodide in the silver chloride grain without using materials that have the disadvantage of causing deleterious photographic effects. I SUMMARY OF THE INVENTION

It is an object of the invention to overcome disadvantages of prior methods of forming iodochloride grains.

It is another object to precipitate iodochloride grains that have improved photographic characteristics.

It is a further object to provide iodochloride grains that have improved speed.

These and other objects of the invention generally are accomplished by a method of forming a silver halide emulsion comprising precipitating silver chloride grains, with the proviso that during precipitation iodine is added to the precipitation in between I and 50 seconds after at least 90 mol percent of the total silver chloride in said grains has been precipitated.

ADVANTAGEOUS EFFECT OF THE INVENTION Thus, the use of iodine as a source of iodide ion makes more efficient use of materials, starting with a readily available material and elin- driating iodide compound components that require the use of deleterious materials for iodide release. To this siguificant advantage is added the further advantage that iodine provides a source of iodide ion under mild conditions that avoid both the risks of reduction fog and grain ripening, with their known attendant disadvantages to grain characteristics and performance.

DETAILED DESCRIPTION OF THE INVENTION

The invention has numerous advantages over prior practices in the art. The invention provides a silver iodochloride emulsion that does not require the use of chemicals for release of iodide ion. Some of these materials may have potentially deleterious photographic effect such as indiscriminate fog formation and/or grain morphology changes. With the addition of iodine rather than an organic iodo compound, the formation of organic by- products is avoided. This has the advantage of eliminating the possibility of such by-products having undesirable photographic effects.

By the addition of iodine rather than a salt or complex of iodine, there ard fewer by-products formed in the placement of the iodine into the grain. This has the advantage that there is less possibility of such byproducts having harmful photographic effects. It is known that byproducts of the addition of prior iodides may have the effect of lowering the Dmin of the emulsions. The by-product would cause R-typing in the grains and thereby lower their performance.

These and other advantages will be apparent from the detailed description below.

In one aspect this invention is directed to a process of preparing a photographically useful emulsion, containing gelatin as the dispersing medium and radiation-sensitive silver iodochloride grains. The process comprises simply dissolving iodine in a suitable solvent such as an alcohol, methanol, or ethanol and introducing iodine into the growth kettle, while maintaining the dispersing medium within a pH range of from 5 to to release r for incorporation into the crystal lattice structure.

Iodine, as a source of iodide ion, shares with formula IV R-I compounds and iodate the Advantage of avoiding excessive local iodide ion concentrations at the point of addition into the dispersing medium within the reaction vessel.

A fundamental advantage of introducing iodine rather than a formula IV R-I compound, as noted above, is that introduction of the R- moiety is eliminated along with its reaction by-product. Therefore, the potential for by-product unwanted interactions with other ingredients in the dispersing medium present during precipitation and added after precipitation is either eliminated or minimized.

A further advantage is that no reducing agent or uncommon starch peptizing agent is required to release F for incorporation into the grains. As demonstrated in the Examples, the employment of hydrophilic colloids such as starch used as peptizers for the purpose of reacting with iodine is avoided. Iodine is fairly inexpensive and is readily available.

Conventionally grain precipitation is initiated by adding to.the dispersing medium within the reaction vessel a small amount of a bromide or chloride salt, such as alkali, alkaline earth, or ammonium halide salt, contemplated to be later introduced during precipitation. This assures a stoichiometric excess of halide ion with respect to silver ion at the initiation of precipitation.

Subsequently in a preferred process a soluble silver salt, such as silver nitrate, is introduced through a first jet. A soluble iodide salt, such as an alkali, alkaline earth, or ammonium iodide salt, is introduced through a second jet.

Chloride and/or bromide ions can be introduced through the second jet with the iodide or introduced through one or more separate jets. If sufficient chloride and/or bromide salt is initially placed in the reaction vessel, it is possible to dispense with further chloride and/or bromide addition. In most instances chloride and/or bromide ions are introduced into the reaction vessel concurrently with the introduction of silver ion (double jet).

The presence of iodide in the reaction vessel is limited in.relation to the chloride present in the reaction vessel so that silver iodochloride grains are precipitated exhibiting a face centered cubic rock salt crystal lattice structure. This is achieved by limiting iodide addition to less than the saturation level of iodide ion in the silver chloride and/or bromide crystal lattice being formed by precipitation.

While iodide ion constitutes only a minor component of the silver iodochloride grains, its concentration and distribution can significantly influence photographic performance. While iodide concentrations can range up to saturation levels in the face centered cubic rock salt crystal lattice structure, for most photographic applications iodide levels are limited to low iodide levels, typically ranging from about 0. 1 to 10 mole percent, based on silver.

Both uniform and non-uniform iodide distributions are common, as illustrated by Research Disclosure, Item 38957, cited above, L Emulsion grains and their preparation, A. Grain halide composition, paragraph (4). Typically low surface iodide concentrations are desired, although Chaffee et al U.S. Patent 5,358,840 illustrates advantageous photographic properties with a maximum iodide concentration at the surface of the grains.

The silver iodochloride grains produced by the process of -the invention can take any conventional shape. Illustrations of varied forms of silver iodohalide grains are provided by Research Disclosure, Item 38957, cited above,

1. Emulsion grains and their preparation, B. Grain morphology.

The process of the present invention can be practiced by modifying conventional silver iodohalide emulsion precipitations of the type described above by substituting iodine addition for a or any portion of the soluble iodide salt conventionally introduced in aqueous solution during grain precipitation, including halide conversion.

To maximize the localization of crystal lattice variances produced by iodide incorporation, it is preferred that the iodide ion be introduced as rapidly as possible. That is, the iodide ion forming the maximum iodide concentration region of the grains is preferably introduced in I to 50 seconds. Preferably, it is added in between I and 30 seconds. The optimum. time is between 1 and 10 seconds for best creation of lattice defects without crystal rearrangement. When the iodide is introduced more slowly, somewhat higher amounts of iodide (but still within the ranges set out aove) are required to achieve speed increases equal to those obtained by more rapid iodide introduction and minimum density levels are somewhat higher.

Iodine is preferably dissolved in a water miscible photographically inert solvent, such as low molecular weight alcohols, such as methanol, ethanol, or low molecular weight amides such as dimethylformarnide. When iodine is introduced in the presence of gelatin peptizer, the reactions may be quite complex because of the multitude of reactive components present in the peptide chain. It may be speculated that the methionine group reacts with iodine in the following manner:

M2 + 2RSMe + 3H20 - 61- + RS(O)Me + RS(0)2Me + 6H+ (VI) where R is the residue of the methionine group in the peptide chain of the gelatin.

Whatever the reactions of iodine with gelatin may be, the reaction goes to completion, efficiently converting iodine introduced to iodide ion. However, the reactions are not instantaneous. A finite amount of time is required for the tri-molecular reaction to take place. The constant removal of the iodide ion from the dispersing medium by incorporation in the grains drives the reactions. In conventional precipitation of silver iodochloride emulsion in which potassium iodide is used as the iodide source, grains that happen to impinge upon the point of iodide ion introduction encounter higher iodide ion concentrations than the remainder of the grains, resulting in grain-to- grain variances in iodide levels and, often, variations in the structural form and photographic performance of the grains. Delaying iodide ion release during iodine introduction, thereby allowing distribution of iodine within the dispersing medium, local grain-to-grain and unintended intragrain variances in iodide content are entirely avoided.

- I I - Similarly, in the large scale precipitation of iodochloride emulsions, the delayed formation of iodide ion allows a more uniform distribution of the iodine before the iodide combines with the silver ion. The result is a silver iodochloride emulsion that has a much less degree of variability in terms of iodide distribution within the grain and intergrain. Overall, a more robust emulsion results with improved photographic performance From formula (VI) it is apparent that the conversion of iodine to iodide ion results in the formation of hydrogen ion (H+) as a by-product. In the art of silver halide precipitation, formation of atomic silver species from the reduction of silver ion may lead to the undesirable formation of fog. Such formation, however, is retarded when the pH of the emulsion medium is lowered, that is, a more acidic medium retards the formation of atomic silver as expressed in (VII).

2 Ag" + 1/2 02 +2H+ = 2Ag+ + H20 (VID It is appreciated that when iodine is used as the source of iodide, the propensity to fog formation is also reduced as a result of generation of hydrogen ion.

Although the invention is described in terms of substituting iodine for a water soluble iodide salt in preparing a silver iodochloride emulsion, it is appreciated that the iodine can be alternatively substituted for an organic iodide compound (R-1) employed without the need for an additional reducing reagent or in combination with a starch peptizer that is costly and difficult to manufacture.

To maximize the localization of crystal lattice variances produced by iodide incorporation, it is preferred that the iodine be introduced as rapidly as possible. That is, in order to form the maximum iodide concentration in the desired region of the grains, iodine is preferably introduced in I to 50 seconds.

Preferably, it is added in between I and 30 seconds. The optimum time is between I and 10 seconds for best creation of lattice defects without crystal rearrangement. When the iodine is introduced more slowly, somewhat higher amounts of incorporated iodide (but still within the ranges set out above) are required to achieve speed increases equal to those obtained by more rapid iodide introduction and minimum density levels are also higher.

Instead of introducing iodide into the grains as they are being formed, it is recognized that iodide can be used to form silver iodohalide grains by halideconversion. During halide conversion the iodine is reacted to release iodide ion in a dispersing medium containing silver halide grains having a face centered cubic rock salt crystal lattice structure while withholding the addition of silver. Thus, the process of the irIvention can be readily adapted to any conventional halide conversion process. Conventional techniques for halide conversion are illustrated by Research Disclosure, Item 38957, cited above, I. Emulsion grains and their preparation, A. Grain halide composition, paragraph (8).

Apart from the features that have been specifically discussed, the high chloride grain emulsions can contain selections of dopants, peptizers, vehicles, and hardeners. Once prepared, the emulsions can be chemically sensitized, spectrally sensitized, combined with antifoggants, and stabilizers, image dye providing components, and other conventional photographic addenda. Such conventional features are illustrated by Research Disclosure, Vol. 389, September

1996, Item 38957.

The following examples illustrate the practice of this invention.

They are not intended to be exhaustive of all possible variations of the invention.

Parts and percentages are by weight unless otherwise indicated.

EXAMPLES

PART I: EMULSION MAKING Emulsion A (control cubic grain AgC1 emulsion) A stirred tank reactor containing 6.65 Kg of distilled water, 201 g of bone gelatin was heated to 68.3 'C and the pAg maintained at 7.15 with a 2.0 M NaCl solution. 1,8-Dihydroxy-3,6-dithiaoctane in the amount of 1.65 g was added to the reactor 30 seconds before the double jet addition of AgN03 (3.722 M) at 39.9 ml, per min and NaCl (3.8 M) at a rate such that a constant pAg of 7.15 was maintained. The silver jet addition rate remained at 39.9 mL per min for 5.25 min then accelerated to 80.3 mL per min over a period of 7.5 min while the salt stream was adjusted such that the pAg was held constant at 7.15. The silverjet addition rate remained at 80.3mL per min for another 25.3 min while the pAg was maintained at 7.15. A total of 10 moles of AgCl was precipitated in the form of a monodispersed cubic grain emulsion having a mean grain size of 0.76 Rm.

Emulsion B (example of AgCI emulsion with H902) The emulsion was prepared similar to Emulsion A except that the AgN03 solution used in the double jet precipitation contained H902 in the amount of 0.29 gmol per silver mole. A total of 10 moles of AgCI was precipitated in'the form of a monodispersed cubic grain emulsion having a mean grain size of 0.79 gm.

Emulsion C (example of AgCI emulsion with HgC12) The emulsion was prepared similar to Emulsion B except that the AgN03 solution used in the double jet precipitation contained HgC12 in the amount of 0.58 gmole per silver mole. A total of 10-moles of AgCI was precipitated in the form of a monodispersed cubic grain emulsion having a mean grain size of 0.77 Rm.

Emulsion D (example of AgCI emulsion with HgC12) The emulsion was prepared similar to Emulsion B except that the_,gN03 solution used in the double jet precipitation contained HgC12 in the amount of 1. 16 gmole per silver mole. A total of 10 moles of AgCI was precipitated in the form of a monodispersed cubic grain emulsion having a mean grain size of 0.75 Am.

Emulsion E (example of AgCI/I emulsion, 0.3 M% I after 93% Ag) The emulsion was prepared similar to Emulsion A, except with the following changes: After the accelerated flow rate of 80.3 mL per min was established, the silver jet addition was held at this rate for 22.9 min with pAg held at 7.15, resulting in the precipitation of 93 percent of the total silver to be -14 introduced. At this point 200 mL of KI solution containing 4.98 g KI was dumped into the reactor. The silver and chloride salt additions following the dump were continued as before the dump for another 2.33 min to provide a total of 10 moles of AgCH emulsion. The emulsion contained monodispersed tetradecahedral grains with. an average grain size of 0.74 pim and an iodide content of 0. 3 mole percent. 1 Emulsion F (example of AgCH emulsion, 0.6 M% I after 93% of Ag) The emulsion was prepared similarly as Emulsion E, except that 9.96 g of KI was dumped into the reactor. A total of 10 moles of AgCIII emulsion with an iodide content of 0.6 mole percent was precipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.77 gm.

Emulsion G (example of AgClll emulsion, 0.9 M% I after 93% of Ag) The emulsion was prepared siTilar, to Emulsion G, except that 14.94 g of KI was dumped into the reactor. A total of 10 moles of AgGlI emulsion with an iodide content of 0.9 mole percent was precipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.77 gm.

Emulsion H (example of AgCI/I emulsion with H9C12. 0.3 M% I after 93% of Ag) The emulsion was prepared similar to Emulsion E, except 0.29 gmole per silver mole of H9C12 was contained in the AgN03 solution. A total of 10 moles of AgCL/I emulsion with an iodide content of 0.3 mole percent was precipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.77 gm.

Emulsion I (example of AgCH emulsion with HgC12,03 M% I after 93% of Ag) The emulsion was precipitated similar to Emulsion H, except that 0.58 gmole per silver mole of H9C12 was contained in the AgN03 solution. A total of 10 moles of AgGA emulsion with an iodide content of 0.3 mole percent was precipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.75 gm. Emulsion J (example of AgCH emulsion with H9C12. 0.3 M% I after 93% of Ag) 5 The emulsion was precipitated similar to Emulsion I, except 1. 16 PLmole per silver mole of HgC12 was contained in the AgN03 solution. A total of 10 moles of AgCH emulsion with an iodide content of 0.3 mole percent wasprecipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.75 gm. 10 Emulsion K (example of AgGA emulsion, 0.3 M% I after 93% of Ag) The emulsion was precipitated similar to Emulsion E, except the 200 mL KI solution was replaced with 3.81 g iodine dissolved in 102.5 mL of ethanol and 98.33 g distilled water to form 205 mI 12 solution. A total of 10 moles of AgGA 15 emulsion with an iodide content of 0.3 mole percent was precipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.77 gm. Emulsion L (example of AgCH emulsion, 0.6 M% I after 93% of Ag) 20 The emulsion was precipitated similar to Emulsion K, except that 7.62 g iodine was dissolved in the 205 mL ethanollwater solution. A total of 10 moles of AgCH emulsion with an iodide content of 0.6 mole percent was precipitated. The emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.77 gm. 25 Emulsion M (example of AgChl emulsion, 0.9 M% 1 after 93% of Ag) The emulsion was precipitated similar to Emulsion L, except 11.42 g iodine was dissolved in the 205 mI ethanollwater solution. A total 10 moles of AgGlI emulsion with an iodide content of 0.9 mole percent was precipitated. The - 16emulsion contained monodispersed tetradecahedral grains with an average grain size of 0.77 gm.

PART 11: EMULSION SENSITMATION In accordance with the present invention, a 0.30 mole each of emulsions A through M was sensitized with a colloidal suspension of aurous sulfide (4.6 mg/Ag mol) mole for 6 min at 40'C. The emulsion was heated to 60'C at which time, the blue spectral sensitizing dye, anhydro-5-chloro-3, Y-di(3 sulf6propyl) naphtho[1,2-d] thiazolothiacyanine hydroxide triethylammonium salt (220 mg/Ag mol), and 1-(3-acetamidophenyl)-5-mercaptotetrazole (103 mg1Ag mol) were added to the emulsion which was held at 60"C for 27 minutes. The emulsion further contained a yellow dye-forming coupler alpha-(4-(4- benzyloxy phenyl-sulfonyl)phenoxy)-alpha(pivalyl)-2-chloro-5-(gamma-(2,4-di-5amylphenoxy)butyramido)acetanilide (1.()o g/m2) in di-n-butylphthalate coupler solvent (0.27 g/m2), gelatin (1.51 g/r2). he emulsion (0.26 g Ag/m2) was coated on a resin coated paper support and 1.76 g/m2 gel overcoat was applied as a protective layer along with the hardener bis (vinylsulfonyl) methyl ether in an amount of 1.8% of the total gelatin weight.

The coatings were given a 0. 1 second exposure, using a 0-3 step tablet (0. 15 increments) with a tungsten lamp designed to stimulate a color negative print exposure source. This lamp had a color temperature of 3000 K, log lux 2,95, and the coatings were exposed through a combination of magenta and yellow filters, a 0.3 ND (Neutral Density), and a UV filter. The processing consisted of a color development (45 sec, 3 5 0 Q, bleach-fix (45 sec, 35 0 Q, and stabilization or water wash (90 sec, 3 5 0 Q followed by drying (60 sec, 60 0 Q.

The chemistry used in the Colenta processor consisted of the following solutions:

17- Develope Lithium salt of sulfonated polystyrene 0.25 mL Triethanolamine 11.0 ML N,N-diethylhydroxylamine (85% by wt.) 6.0 mL Potassium sulfite (45% by wt.) 0.5 mL Color developing agent (4-,(N-ethyl-N-2-methanesulfonyI aminoethyl)-2-methyl-phenylenediaminesesquisulfate monohydrate 5.0 g Stilbene compound stain reducing agent 2.3 g Lithium sulfate 2.7 g Acetic acid 9.0 ML Water to total 1 liter, pH adjusted to 6.2 Potassium chloride 2.3 g Potassium bromide 0.025 g Sequestering agent 0.8 mL Potassium carbonate 25.0 g Water to total of I liter, pH adjusted to 10. 12 Bleach-fix Ammonium suffite 58 g Sodium thiosulfate 8.7 g Ethylenediaminetetracetic acid ferric ammonium salt 40 g Stabilize Sodium citrate I g Water to total I liter, pH adjusted to 7.2.

Example I

Data in Table II show the speed and fog density of the blue sensitized coatings for the pure chloride emulsion and the silver iodochloride emulsions using iodine as the iodide source. The speed taken at the 1.0 density point of the D log E curve is taken as a measure of the sensitivity of the emulsion.

D-min is measured as the minimum density above zero.

TABLE II

Saale Emulsion Halide I source I,mol% Speed Fog !Xpe I (comparison) AgCl none 0 130 0.070 2 (invention) K AgICI iodine 0.3 150 0.050 3 (invention) M AgICI iodine 0.9 161 0.06 It can be seen from data in Table 11 that samples of the present invention (samples 2-3) have significantly higher speed and lower fog than the comparison emulsion (sample 1) that has no iodide in the silver chloride grain.

Example 2

This example compares the speed and fog parameters of the coatings of emulsions that are made in the presence of a conventional antifoggant, mercuric chloride versus the silver ioaochloride emulsions made in the presence of iodine.

It can be seen from data in Table III that despite the use of mercuric chloride, the fogging propensity of the coatings of the silver chloride emulsions (samples 4-6) are no better than the emulsions of the present invention (samples 2, 3) that are made in the presence of iodine.

TABLE 1111

Sample Emulsion Halide I,mol% HgC12 Speed Fog !Xpe 4 (comparison) B AgCl 0 X 130 0.070 comparison) C AgCl 0 2X 143 0.070 6 (comparison) D AgC1 0 4X 144 0.070 2 (invention) K AgICI 0.3 0 150 0.050 3 (invention) M AgICI 0.9 0 161 0.060 x--0.29 umol/Ag-mol.

19- Example 3

This example compares the speed and fog parameters of the coatings of emulsions that are made using potassiuM iodide as the iodide source versus the silver iodochloride emulsions made in the presence of iodine.

TABLE IV

Sample Emulsion Halide I source I,mol% S eedd Foz type 7 (comparison) E AgICI KI 0.3 190 0.27 8 (comparison) F AgICI KI 0.6 188 0.10 9 (comparison) G AgICI KI 0.9 189 0.13 (invention) L AgICI iodine 0.6 190 0.08 It can be seen from Table IV that no matter what mole percent iodide is in the grain of the silver iodochloride emulsion that is made using KI as the iodide source (samples 7-9), the fog position of these emulsions are still higher than that of the emulsion made with iodine as the iodide source of the present invention (sample 10).

Example 4

This example compares the speed and fog parameters of the coatings of silver iodochloride emulsions that are made with KI as the iodide source and in the presence of mercuric chloride versus a silver iodochloride emulsion made in the presence of iodine.

TABLE V

Sample Emulsion Halide I source I,mol% HgCl Fog type I I (comparison) H AgICI KI 0.3 X 195 0.12 n 12 (comparison) I AgICI KI 0.3 2X 190 0.11 13 (comparison) I AgICI KI 0.3 4X 185 0.15 (invention) L AgICI iodine 0.6 0 190 0.08 x=0.29 umol/Ag-mol.

The invention has been described in detail with particular reference to the preferred e mbodiments thereof, but it will be understood that variations and 25 modifications can be effected within the scope of the invention.

Claims (16)

T IS CLAIMED IS: 1. A method of forming a silver halide emulsion comprising precipitating silver chloride grains, with the proviso that during precipitation iodine is added to the precipitation in between 1 and 50 seconds after at least 90 mol percent of the total silver chloride in said grains has been precipitated.

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2. The method of Claim 1 wherein greater than 90 mol percent of said grains are AgCl.

3. The method of Claim 1 wherein Ir is added after 90 mol percent of the total silver chloride has been precipitated.

4. The method of Claim 3 further comprising adding osmium as a run additive for up to 90 mol percent of silver precipitation and ruthenium as a run additive for up to 85 mol percent of silver addition to precipitation.

5. The method of Claim 4 wherein said silver chloride grains comprise at least 99 percent silver chloride with the remainder being substantially silver iodide.

6. The method of Claim 3 wherein iridium is added between after 90 mol percent of the total silver chloride as precipitated and 99 percent of the total silver chloride.

7. The method of Claim 1 wherein said precipitating is carried out at a pH of between 5 and 6.

8. The method of Claim 1 wherein the vAg is between about 100 and 120 rnillivolts.

9. The method of Claim 1 wherein precipitating is carried out at a temperature of between 50 and 700C.

10. The method of Claim 1 wherein precipitating is carried out at a temperature of between 60 and 660 C.

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11. The method of Claim 1 wherein iodine is added to the precipitation in a period of between 1 and 30 seconds.

12. The method of Claim 1 wherein said iodine is added to the precipitation in a time of between 1 and 10 seconds.

13. The method of Claim 2 wherein said silver chloride grains coMPirse at least 99 percent silver chloride with the remainder being substantially silver iodide.

14. The method of Claim 1 wherein iodine is added at about 0.05 percent to 3 percent of the total silver chloride precipitated.

15. The method of Claim 1 wherein iodine is added at about 0. 1 percent to 1.0 percent of the total silver chloride precipitated.

16. The method of Claim 13 wherein iodine is added between about 90 mol percent to 97 mol percent of the total silver chloride.