Fixing baths

Abstract: There is described a fixing bath for developed silver halide photographic material which comprises as the active silver halide solvents a mixture of sodium thiosulphate and potassium thiosulphate, the ratio of sodium to potassium being from 40% to 60% by molar ratio. A useful concentration of thiosulphate salts in the solution is from 0.5 to 1.5 moles/litre. Preferably the concentration is about 1 mole/litre. Preferably an equal molar ratio of sodium and potassium thiosulphate is present.

Description: This invention relates to fixing baths for photographic silver halide materials. After silver halide material has been exposed and developed, it is necessary to remove the unexposed and hence undeveloped silver halide remaining in the material to render the material light stable. The most commonly used silver halide solvents used in fixing baths to remove or 'fix-out' the undeveloped silver are thiosulphates, either alkali metal thiosulphates or ammonium thiosulphate. The thiosulphate first used was sodium thiosulphate, the hypo of the old photographic books. More recently ammonium thiosulphate has been used as it removes the undeveloped silver very much faster than sodium thiosulphate. Potassium thiosulphate has also been proposed to be used but as it is more expensive than sodium thiosulphate and not much if any faster at fixing out, it has not been used much commercially. Most often the dissolved or fixed out silver is removed from the fixing solution by electrolysis, ion exchange or other means and the used desilvered fixing solution is discharged into the effluent system. However, in recent years the control of effluent has become very strict. Sodium or potassium salts which are very wide-spread in nature do not significantly affect the environment into which the effluent is discharged but ammonium salts tend to act as a fertiliser for the micro-organisms in the aqueous effluent environment and if such micro-organisms proliferate greatly they reduce the oxygen content of the aqueous effluent environment. This is in detriment to the higher species found in such effluent environment such as fish. Further when such organisms die this further consumes oxygen and renders the water stagnant and unfit to support fish or higher-plant life. Thus it is desirable to find a fixing solution which contains no ammonium thiosulphate but which fixes out silver faster than sodium thiosulphate. In the past, various other salts have been used as fixing agents such as alkali metal thiocyanates but in general these are less environmentally friendly than ammonium thiosulphate. we have found a fixing solution which is sufficiently fast-acting to be of use commercially but which is environmentally friendly. Therefore, according to the present invention, there is provided a fixing bath for developed silver halide photographic material which comprises as the active silver halide solvents a mixture of sodium thiosulphate and potassium thiosulphate, the molar ratio of sodium to potassium being from 2:3 to 3:2. A useful concentration of thiosulphate salts in the solution is from 0.5 to 1.5 moles/litre. Preferably the concentration is about 1 mole/litre. Preferably sodium and potassium thiosulphate is present at a molar ratio of 1:1 of the fixing solution . Such a fixing bath fixes undeveloped silver at an appreciably faster rate than when sodium thiosulphate or potassium thiosulphate are used alone and not in admixture. This is shown in the Example which follows. This was an entirely unexpected result which could not be predicted. It is not clear why this admixture of sodium and potassium thiosulphate provides this useful result. The admixture of sodium and pctassium thiosulphate in the fixing bath of the present invention is not as fast as when ammonium thiosulphate is used alone, but a very worth-while improvement is obtained than by using a bath containing sodium thiosulphate alone or potassium thiosulphate alone. Any of the usual ingredients often found in fixing baths may be present except for ammonium thiosulphate or ammonium salts of any description. Ingredients which may be present include alkali metal sulphites or bisulphites as anti-oxidants, acetic acid as a buffer and potassium sulphite (alum) as a hardening agent. Preferably the pH of the fixing solution is from 4 to 6 and most preferably about 5. The speed of the fixing process is normally determined as the so-called clearing time. That is to say unexposed silver halide film material is treated under standardised conditions with the fixing solution and the time after which the optical turbidity caused by the disperse silver halide disappears and the film material becomes transparent is determined. Optical inspection is usually used to determine the disappearance of turbidity and this method was used in the Example which follows. Example Three types of film were used in this Example. Type A was a fast film with a high coating weight of silver halide of which 5% by weight was silver iodide. Type B was a fast film with a high coating weight of silver halide but only 1% of silver iodide. Type C was a medium speed film with a very much lower coating weight of silver halide than type A or B. Four fixing solutions were prepared. Each comprised 40g/litre sulphite and each had a pH of 5. F.S.1. comprised 129.7g/litre of ammonium thiosulphate F.S.2. comprised 138.4g/litre of sodium thiosulphate F.S.3. comprised 193.4g/litre of potassium thiosulphate F.S.4. comprised 69.2g/litre of sodium thiosulphate and 96.7g/litre of potassium thiosulphate Thus each solution comprised 1 mole/litre of thiosulphate. Equal area strips of the three films were overall exposed to fully fog them. Then strips of the three films were immersed in the four fixing solutions at 25°C and the clearing time for each film in each fixing solution was recorded. These results are set forth in the Table below. The clearing times are in seconds. Fixing Solution 1 2 3 4 Film type A 42 110 105 65 Film type B 27 109 94 70 Film type C 23 93 68 57 Thus in all cases the clearing time in F.S.1. which comprises ammonium thiosulphate is the fastest. But in all cases the clearing time in F.S.4. which comprises an equal amount of sodium and potassium thiosulphate is faster than in F.S.2. which is sodium alone or in F.S.3. which is potassium alone. Furthermore in the case of film type A which comprises a relatively high proportion of silver iodide the clearing time in F.S.4. approaches that in F.S.1.

Claims: A fixing bath for developed silver halide photographic material which is characterised in that it comprises as the active silver halide solvents a mixture of sodium thiosulphate and potassium thiosulphate, the molar ratio of sodium to potassium being from 2:3 to 3:2. A fixing bath according to claim 1 characterised in that it comprises sodium and potassium thiosulphate at a molar ratio of 1:1. A fixing bath according to either claim 1 or claim 2 which is characterised in that it comprises from 0.5 to 1.5 moles/litre of thiosulphate salts. A fixing bath according to either claim 1 or claim 2 which is characterised in that it comprises 1 mole/litre of thiosulphate salts. A fixing bath according to claim 1 characterised in that its pH is from 4 to 6.

Inventors: FIELDING ROSS, MILLINGTON ROBERT STEPHEN, FIELDING, ROSS, MILLINGTON, ROBERT STEPHEN
Applicants: ILFORD LTD, ILFORD LIMITED
Application year: 1993
Main classification: G03C5
IPC codes: G03C5