Abstract:
A fixing composition for processing a silver halide photographic material comprises a fixing agent, a sulfite stabilizer, an acid and a buffer. The composition has a pH greater than 5.4 and the buffer comprises at least 2.0 mol/l ammonium acetate.

Description:
FIELD OF THE INVENTION 
     The invention relates to a fixing composition for processing a silver halide photographic material. 
     BACKGROUND OF THE INVENTION 
     Exposed photographic materials, for example black-and-white photographic materials, are developed in a solution which reduces the exposed silver halide. The unexposed silver halide is then removed by treating the film or paper with a second solution known as a fixer which comprises a silver halide complexing agent, e.g., a thiosulfate. The fixer solution is acidic to ensure that any developer which is carried over from the development stage does not remain active. If the fixer solution is insufficiently acidic, the silver complexes formed on fixing can be reduced by the active developer to form dichroic fog. 
     It is essential that the pH of the photographic material is reduced rapidly throughout the emulsion layer when fixing takes place. In order to achieve this, a fixer which is buffered between pH 4 and 5 may be used. If the fixer is not buffered then either it will not be able to neutralize the alkali quickly in the emulsion or it will require a very low pH making it unsafe and unstable. 
     It is known to use an organic aliphatic acid as a buffer at a concentration between 1 and 3% in the working strength fixer solution as it will buffer in the region of pH 3 to 5. The acid most commonly used is acetic acid but other weak acids have also been used, particularly when they have less odor, e.g., tartaric, succinic and citric acids. 
     JP-A-5127323 describes a fixing composition having reduced odor comprising a fixing agent, e.g., ammonium thiosulfate, a stabilizer, e.g., sodium sulfite, a hardening agent, e.g., aluminum sulfate and acids including succinic or maleic acid. 
     The odor associated with fixers also appears to be due to the evolution of sulfur dioxide produced by the disproportionation of sulfite and the fixing agent in acid. The odor problem caused by the evolution of sulfur dioxide needs to be overcome. 
     Although it has been found that sufficiently raising the pH of the fixer can substantially reduce or eliminate the evolution of sulfur dioxide, the fixer loses its ability to effectively neutralize the alkaline solution carried over by the photographic material during processing. A typical fixer concentrate has a pH drop of about 0.1 unit on dilution with water to the working strength fixer. If the odor problem of the fixer concentrate is to be overcome by increasing pH, a much greater drop in pH is required on dilution with water. 
     SUMMARY OF THE INVENTION 
     The invention provides a fixing composition for processing a silver halide photographic material comprising a fixing agent, a sulfite stabilizer, an acid, and a buffer, wherein the composition has a pH greater than 5.4 and comprises at least 2.0 mol/l ammonium acetate as the buffer. 
     The release of sulfur dioxide from the fixing composition is inhibited and the resulting odor eliminated by using this invention. The high concentration of ammonium acetate produces an unexpectedly large drop in pH on dilution of the fixing composition with water to the working strength solution. The large increase in the concentration of ammonium acetate in the fixer causes the fixer, when stored in a concentrated form, to have a higher pH which improves keeping. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 4 are graphs showing the effect on pH of diluting fixer concentrates referred to in the Examples below. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferably, the buffer in the composition of this invention, is from 2.0 to 6.0, and more preferably from 2.5 to 3.5 mol/l, of ammonium acetate. 
     Preferably, the fixing agent is present in an amount from 0.4 to 4.0, and more preferably from 2.5 to 3.5 mol/l. 
     Fixing agents are water-soluble solvents for silver halide such as a thiosulfate (e.g., sodium thiosulfate, ammonium thiosulfate, and potassium thiosulfate), a thiocyanate (e.g., sodium thiocyanate, potassium thiocyanate and ammonium thiocyanate), a thioether compound (e.g., ethylenebisthioglycolic acid and 3,6-dithia-1,8-octanediol), a thioglycolic acid or a thiourea, an organic thiol, an organic phosphine, a high concentration of halide, such as bromide or iodide, a mesoionic thiolate compound, and sulfite. These fixing agents can be used singly or in combination. Thiosulfate is preferably used and ammonium thiosulfate, in particular, is used most commonly owing to the high solubility. Alternative counter-ions such as potassium, sodium, lithium, cesium as well as mixtures of two or more cations may be used. 
     Preferably, the sulfite stabilizer is present in an amount from 0.1 to 2.0, and more preferably from 0.2 to 0.5 mol/l. 
     Suitable sulfite stabilizers include sodium sulfite, potassium sulfite, ammonium sulfite, sodium bisulfite, potassium bisulfite, ammonium bisulfite, sodium metabisulfite, potassium metabisulfite, and ammonium metabisulfite. 
     Preferably, the acid in the composition is an organic aliphatic acid, e.g., acetic, tartaric, succinic, maleic and citric acids. In a particularly preferred embodiment, the acid comprises acetic acid. 
     Other components of the fixing composition may be preservatives and sequestering agents. 
     The fixing composition may have a pH from 5.4 to 6.0, and preferably from 5.4 to 5.6. 
     A detailed description of fixing compositions and their use is found, for example, in Research Disclosure, Number 365, September 1994, Item 36544 published by Kenneth Mason Publications Ltd. (hereinafter referred to as Research Disclosure) pages 539-540. 
     Dilution of a fixing composition in accordance with the invention to normal working strength results in a significant drop in the pH of the solution, for example, a drop of between 0.2 and 0.5 units. 
     The invention is further illustrated by way of example as follows. 
     EXAMPLE 1 
     A fixer concentrate in accordance with the invention was prepared having the following composition. 
     Photographic Fixer of the invention: 
     
         ______________________________________Water                    113    gAcetic acid (90% solution)                    105    gAmmonium acetate         250    gAmmonium thiosulfate/sulfite solution                    750    gSodium sulfite           10     gBoric acid               25     g______________________________________ 
    
     The pH of the concentrate was 5.5. Diluting the concentrate to provide a working strength solution reduced the pH to 5. The results of dilution are shown in FIG. 1 which is a plot of dilution versus pH. 
     For comparison, a conventional fixer concentrate was prepared having the following composition. 
     Conventional Black-and-White Photographic Fixer Formula: 
     
         ______________________________________Ammonium thiosulfate/sulfite solution                    1000   gWater                    95     gSodium metabisulfite     25     gBoric acid               40     gSodium acetate           120    gEDTA                     0.1    g______________________________________ 
    
     The pH of the concentrate was 5.1. Diluting the concentrate to provide a working strength solution reduced the pH to 5. The results of dilution are shown in FIG. 2 which is a plot of dilution versus pH. 
     EXAMPLE 2 
     Ammonium acetate was added to the conventional black-and-white fixer described in Example 1 at a level of 500 g/l. The solution was diluted with water and the pH of the solution was measured as it was diluted. The resulting curve is shown in FIG. 3. 
     A significant drop in pH occurred on dilution. 
     EXAMPLE 3 
     Ammonium acetate was added to the conventional black-and-white fixer described in Example 1 at a level of 500 g/l. The pH of the solution was adjusted to 5.5 from 6.8 with sulfuric acid. The solution was diluted with water and the pH of the solution was measured as it was diluted. The resulting curve is shown in FIG. 4. 
     A significant drop in pH occurred on dilution. 
     EXAMPLE 4 
     A fixing composition in accordance with the invention was prepared having the following composition 
     
         ______________________________________Acetic acid (90% solution)                    105    gAmmonium acetate         250    gAmmonium thiosulfate/sulfite solution                    600    gSodium sulfite           10     gBoric acid               25     gAmmonium thiocyanate     50     gWater to make 1 liter.______________________________________ 
    
     The pH of the composition was 5.71. On dilution with water (1 part fixer and two parts water) The pH dropped to 5.36, i.e., a drop of 0.35 units. 
     EXAMPLE 5 
     A fixing composition in accordance with the invention was prepared having the following composition 
     
         ______________________________________Acetic acid (90% solution)                    105    gAmmonium acetate         250    gAmmonium thiosulfate/sulfite solution                    600    gSodium sulfite           10     gBoric acid               25     gAmmonium thiosulfate     50     gWater to make 1 liter.______________________________________ 
    
     The pH of the composition was 5.5. On dilution with water (1 part fixer and two parts water) The pH dropped to 5.14, i.e., a drop of 0.36 units. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.