3,741,767 United States Patent Office Patented June 26, 1973 2 Preferably the alkali metal ferricyanide is added to the 3,741,767 emulsion after emulsification step. PHOTOGRAPHIC EMULSIONS CONTAINING The preferred alkali metal ferricyanide is potassium ALKAL METAL FERRCYANDES ferricyanide. Most preferably the alkali metal ferricyanide John Harold Baylis and Anthony John Bond, Ilford, Eng is added to the emulsion as an aqueous solution. land, assignors to Ilford Limited, Ilford, England 5 No Drawing. Filed June 22, 1971, Ser. No. 155,611 It is thought that the step of adding the alkali metal Claims priority, application Great Britain, June 26, 1970, ferricyanide to the emulsion has the effect of reducing 31,166/70 the number of internal sensitivity centres which are formed at C. G03c 1/02 during the physical ripening stage. Thus, the process of U.S. C. 96-94 7 Claims the present invention leads to emulsions having reduced O internal sensitivity and in some cases yields emulsions having improved properties in regard to speed and/or contrast. However, in photographic emulsions wherein the ABSTRACT OF THE DISCLOSURE ratio of internal sensitivity to external sensitivity is normal According to the present invention an alkali metal ly very low, it is very difficult to show any improvement ferricyanide is added to a gelatin silver halide emulsion 15 because the reduction in the internal sensitivity of the prior to flocculation. Emulsions having reduced internal emulsion is of little practical importance. sensitivity are obtained. Therefore, in a preferred embodiment of the present invention, there is provided a process for the production 20 of a silver halide photographic emulsion which has a high In the production of most colloid silver halide emul surface sensitivity relative to internal sensitivity by a sions the silver halide is precipitated in the presence of a process of which comprises the steps of emulsification, medium which comprises a protective colloid, usually physical ripening flocculation and digestion, which com gelatin, this is called the emulsification step. Then the prises modifying the aforesaid method of production of a silver halide crystals are caused to increase in size by 25 silver halide photographic emulsion which when so manu maintaining the medium at a raised temperature, there factured will have a surface sensitivity not substantially being present in the medium a silver halide solvent, for greater than its internal sensitivity, by adding to the example an alkali metal halide. During this stage in the emulsion, at a stage prior to flocculation, an alkali metal emulsion making process which is usually called the ferricyanide. physical ripening stage and hereinafter is so referred, the 30 Preferably in this embodiment of the invention as well, Smaller silver halide crystals increase in size. When the the alkali metal ferricyanide is potassium ferricyanide and crystals of the silver halide have achieved an optimum the alkali metal ferricyanide is added to the emulsion as crystal size for the type of emulsion being prepared the an aqueous solution. emulsion is flocculated. This flocculate may be washed The preferred amount of alkali metal ferricyanide to be to remove soluble by-products if required. The flocculate 35 added to the emulsion prior to the flocculation thereof is thereafter dispersed in an aqueous medium and a diges is from 0.01 to 1.0 g. per 1.50 moles of silver present tion or chemical sensitisation stage is carried out wherein in the emulsion and most preferably 0.2 to 0.5 g. It has the emulsion is digested at an elevated temperature in been found that when alkali metal ferricyanide is added the presence of chemical sensitisers, often sulphur com to a photographic emulsion prior to the flocculation there pounds. Sometimes this stage is referred to as the chemical 40 of, the chemical digestion stage usually has to be extended ripening stage but in order to differentiate it from the to reach the optimum sensitivity for the emulsion. When physical ripening stage referred to above, this stage is much more than 1.0 g. of alkali metal ferricyanide per hereinafter referred to as the digestion stage. 1.50 moles of silver is added, the length of time of The silver halide grains in silver halide emulsions have the chemical digestion stage has to be extended to an internal sensitivity centres and surface sensitivity centres. 45 unacceptable level. In general the internal sensitivity centres are formed The following examples will serve to illustrate the in during the physical ripening stage of the emulsion making vention: process and the external sensitivity centres are formed EXAMPLE 1. during the chemical digestion stage of the emulsion mak In order to show the difference between the internal sen ing process. Photographic emulsions can be prepared 50 sitivity of an emulsion made by the process of the present which have a very large number of internal sensitivity invention and an emulsion wherein the step of adding an centres and they are said to have a high internal sensi alkali ferricyanide has been omitted, a photographic tivity and a low surface sensitivity. emulsion was prepared as follows: Other photographic emulsions have a relatively high A solution of silver nitrate was added to an aqueous Surface sensitivity and a low internal sensitivity while 55 solution of gelatin, potassium bromide, sodium chloride a third type of emulsion may be formed wherein the and hydrochloric acid to give a precipitate of 1.5 moles ratio between the internal sensitivity and the surface of silver chloro-bromide consisting of 74% bromide. Fol sensitivity is such that its surface sensitivity is not sub lowing precipitation, a solution of the double salt of so stantially greater than its internal sensitivity. dium chloride-rhodium trichloride was added. At this Under certain conditions of exposure and development 60 stage the gelatin concentration was 1.6% the pH was 3.9 of photographic emulsions of this latter type, there is and the chloride ion concentration was 0.5 N. The emul competition between the surface sensitivity centres and sion was physically ripened for 20 minutes at 54 C. The the internal sensitivity centres and a certain loss of emulsion was then flocculated and redispersed in extra contrast and/or photographic speed is observable in the gelatin. Sulphur sensitiser in the form of sodium thiosul emulsion after exposure and development. 65 phate was added and the emulsion was digested for a time According to the present invention, in a process for the chosen to give optimum speed and contrast. The emulsion production of a gelatino silver halide emulsion which com was stabilized with 1-phenyl-1H-tetrazole-5-thiol and 4. prises the steps of emulsification, physical ripening, floccu hydroxy-6-methyl-2-methylthio-1,3,3a,7 - tetraazaindene. lation and digestion, there is provided the step of adding 70 The emulsion was optically sensitised and coated on a to the emulsion at a stage in the emulsion making, prior photographic quality paper base at a silver coating weight to flocculation, an alkali metal ferricyanide. of 1.5gns, per Square metre. 3,741,767 3 4. A similar emulsion was prepared to which 0.25 g. of Following precipitation sodium chloro-rhodite solution potassium ferricyanide were added as an aqueous solution and a solution of ammonium chloride and ammonia were to a portion of the emulsion which comprises 1.5 moles added to give a chloride ion normality of 1.0 N and an of silver. The ferricyanide solution was added during the ammonia concentration of 0.7 N potassium ferricyanide physical ripening stage of the emulsion making. The two 5 was then added as 25 mls. of a 1% aqueous solution. The emulsions so made were coated onto strips and exposed gelatin concentration was at this stage 0.7%. The emul in an intensity scale sensitometer to a tungsten light source sion was then physically ripened for 30 minutes at 52 C. for 10 seconds. The emulsion obtained was then flocculated and re-dis The surface image which is a measure of the surface persed in gelatin. Sulphur, sensitiser in the form of sodium sensitivity of the grains of the silver halide in the two 10 thiosulphate was added and the emulsion chemical di strips was determined by developing sample strips of the gested for a time chosen to give optimum speed and con two emulsions in a developer consisting of 30 parts of trast. glycin, 44 parts of anhydrous sodium carbonate and water The emulsion was stabilised with a tetraazaindene sta to 1000 parts by volume. The internal image which is a biliser, optically sensitised and then coated on a photo measure of the internal sensitivity of the grains of the 15 graphic quality paper base at a silver coating weight of 2 two samples of emulsion was determined by bleaching the gms. per Sq. metre. surface image by immersing the strips for two minutes at A control emulsion without the addition of potassium 20° C. in a 0.1% solution of potassium ferricyanide. The ferricyanide was also prepared. strips were washed for five minutes and then developed The results are shown in Table 3 which follows: for three minutes at 20° C. in a general purpose photo- 20 graphic developer of the following formula TABLE 3 G. Processed 2 minutes P.Q. universal developer at 20°C. Sodium sulphite ------------------------------ 130 Digestion Hydroquinone ------------------------------- 30 Exposure, Speed time, 1-phenyl-3-pyrazolidinone (phenidone) ----------- 1.5 25 seconds d = 0.7 Gamma Fog minutes Potassium carbonate -------------------------- 100 1X10-5 1.58 1.70 0.2 40 1X10- 2.06 1, 85 0.00 70 Sodium hydroxide ---------------------------- 5.5 1X10-2 2.27 2,30 0.21 40 Potassium bromide --------------------------- 5 1X10-2 2.55 2, 40 0.00 70 5-nitrobenzimidazole -------------------------- 0.2 0 2.82 1.32 0.2 40 Ethylene diamine tetracetic acid ----------------- 15 30 10 3, 16 1.85 0.00 70 Water to 1 litre.