This invention relates to a method of stabilising photographic silver halide developer solutions and, in particular, to stabilising them against pH changes brought about by aerial oxidation.

Photographic silver halide developer solutions are usually alkaline and contain a developing agent (which is a reducing agent) which reduces photoexposed silver halide to silver. Inherently therefore developing agents can react with oxygen in the air to become oxidised and thus ineffective for development purposes. Most developing solutions, however, contain considerable amounts of sulphite whose primary purpose is to protect the developing agent against aerial oxidation. This is effective but will not stabilise a developer over long periods of exposure to the air, eg a day or two if there is a significant surface area exposed to air. Certain developer solutions in the graphic arts area (not widely used now) produce images of extremely high contrast by a process known as "infectious development". Infectious development can only take place where there is a very low concentration of sulphite ions, equivalent to some 3 to 5 grams anhydrous sodium sulphite per litre. Under such circumstances, the developing agent is very prone to aerial oxidation making the solution unstable. Infectious developers contain a single developing agent, usually hydroquinone or similar dihydroxybenzene compounds. US Patent Specification 3 512 981 describes the use of ascorbic acid or an analogue thereof in such developing solutions to protect them against aerial oxidation ("tray life") .

Other developing solutions containing more sulphite do not suffer this extreme instability but nevertheless not only suffer the normal depletion of developing agent under conditions of use when developing an image but also under conditions of standing idle when a small but measurable amount of aerial oxidation occurs. Under conditions of silver halide development the consumption of dihydroxybenzene developing agent causes the pH of the developer to go down. Under conditions of aerial oxidation and in the presence of sulphite ions, however, the pH tends to go up. It is well known, of course, that the activity of a developing agent is dependant on pH and as this rises, the developing agent becomes more active. Variable pH will therefore lead to variable sensitometric results.

Under typical conditions of normal use any fall in the pH due to silver halide development occurring can be corrected by the normal replenishment which occurs to replace developing agent and other components in response to the volume of material processed. However, the rise in pH caused by aerial oxidation when idle is difficult to deal with especially over long periods of down time, for example, over a weekend.

Problems therefore arise with developers containing normal levels (ie not infectious development levels) of sulphite due to pH fluctuations caused both by developing activity and by aerial oxidation.

US Patent 3,865,591 describes developer compositions containing a dihydroxybenzene developing agent, a 3-pyrazolidone developing agent, an ene-diol developing agent including ascorbic acid and sodium or potassium thiocyanate as a silver halide solvent. The

advantage is said to be that the developer is capable of increasing the exposure index (speed) of the film that is developed.

US Patent 3,022,168 describes ascorbic acid based developers having a pH of from 8 to 9. Example 3 describes such a developer also containing Metol and hydroquinone.

US Patent 3,938,997 describes developers comprising an iron chelate developer, an ascorbic acid and a third developing agent which are said to form a ternary superadditive combination at pH 6. In Example 1 it is shown that the activity of the developer becomes less with ageing and ascorbic acid is said to stabilise it. This is clearly a protection against aerial oxidation of the developing agents which are subject thereto.

The present invention has as its object the solution to problems associated with dihydroxybenzene- containing developer solutions and their pH fluctuations due to aerial oxidation.

According to the present invention there is provided a method of controlling the pH of photographic silver halide developer solutions which contain a dihydroxybenzene developing agent and a secondary developing agent which together form a super-additive combination and an amount of sulphite ions sufficient to protect the developing agents from aerial oxidation by producing in the working solution a concentration of at least lOg/litre (as anhydrous sodium sulphite) wherein there is added to the developer solution a reducing agent which releases hydrogen ions during aerial oxidation of the developer solution and which is different from any developing agent of the superadditive combination.

Preferably the reducing agent which releases hydrogen ions is a compound of the formula:

)

wherein n is 0, 1, 2, 3 or 4 and

R is -OH or H but is -OH when n=0, or salts thereof.

The preferred reducing agents are also silver halide developing agents. Examples of compounds of formula (I) include (when n=l) 1-ascorbic acid, d- ascorbic acid, isoascorbic acid, and water-soluble salts thereof, eg potassium or sodium ascorbates. Preferably the developer solution contains hydroquinone as the polyhydroxybenzene developing agent.

The superadditive developing agents for use with the dihydroxybenzene developing agent may be either N- methyl-p-aminoρhenol or a pyrazolidone, for example 1- phenyl-3-pyrazolidone, 4-hydroxymethyl-4-methyl-l- phenyl-3-pyrazolidone, or 4,4-dimethyl-l-phenyl-3- pyrazolidone.

The developer preferably contains sulphite in the working strength solution at above 20 g/1, preferably above 25 g/1 and more preferanly above 40 g/1 (as anhydrous sodium sulphite) .

The present invention also provides silver halide developer composition having a pH above 9 and which is substantially free from a thiosulphate or thiocyanate silver halide solvent, contains a dihydroxybenzene developing agent and a secondary developing agent

which together form a super-additive combination and an amount of sulphite ions sufficient to protect the developing agents from aerial oxidation by producing in the working solution a concentration of at least lOg/litre (as anhydrous sodium sulphite) wherein the developer solution contains a reducing agent, different from any developing agent of the super¬ additive combination, which releases hydrogen ions during aerial oxidation of the developer solution. The preferred developers have a pH in the range from 9 to 13, preferably from 10 to 12.

The developer solutions may be suitable for use in the graphic arts area, especially to obtain very high contrast images with developers which are designed for use with silver halide materials containing an incorporated hydrazide as nucleating agent. Such developer solutions may also contain an amine which helps to provide the desired very high contrast as described in European Patent 0 032 456. Such materials often form a so-called "pepper fog" which is due to the spontaneous development of unexposed grains. The production of "pepper fog" is increased dramatically when the developer is exposed to the air and is often particularly bad in processors where the cross-over from the developer to the fixer is of a long duration. This effect is controlled by the method of the present invention, in particular by the action of the ascorbate ion which releases hydrogen ions on oxidation by silver ions or air. Thus in a developer which comprises a mixture of hydroquinone and ascorbate ions the pH can be kept constant. Ascorbic acid is also a developing agent hence it will not have gross effects on the sensitometry.

The following Examples are included for a better understanding of the invention.

EXAMPLE 1 The following basic developer formulation was used:

Sodium sulphite (anhy) 40.0 g/1

Sodium bicarbonate (anhy) 20.0

Hydroquinone 18.0 l-phenyl-4-methyl-4-hydroxymethyl-

3-pyrazolidone 1.0

Sodium bromide 2.5

5-Methylbenzotriazole 0.15

Potassium Hydroxide to pH=10.3

250 ml portions of the developer were placed in 250 ml beakers and the following compounds added at varying levels:

(a) Sodium metabisulphite (b) Sodium sulphate (c) Ascorbic acid.

The pH levels of all the solutions were made equal to the pH of the original solution (pH=10.37) . The beakers were then exposed to air for 6 days and the pH levels measured. The resulting pH's are recorded below in Table 1. The surface area per ml was 0.27 c 2.

Table 1

Addition grams Start pH Day 6 pH

Sodium Metabisulphite

Sodium Sulphate

Ascorbic Acid

The results show that the pH rises with time even with a reducing salt (metabisulphite) while the amount of ascorbic acid controls the degree of rise or fall of the pH.

EXAMPLE 2 Example 1 was repeated using 100 ml of developer in a 250 ml beaker thus creating a higher suface to volume ratio of 0.44 cm ² per ml. This time ascorbic acid additions are compared to polyethylene glycol (PEG) or sugar. The results are shown in Table 2 below.

Table 2

Addition grams Start pH Day 6 pH

Control PEG

Sucrose Ascorbic Acid

Results similar to Example 1 are obtained but the rise in pH of the comparative solutions is exaggerated by the greater surface to volume ratio.

EXAMPLE 3

A batch of Kodak UGF film was cut into 35mm strips and then developed in the Developer of the formula:

Sodium sulphite (anhy) 75.0 g/1

Sodium bicarbonate (anhy) 7.0

Hydroquinone 40.0 l-phenyl-4,4'-diethyl-3-pyrazolidone 0.4

Sodium bromide 3.0

5-Methylbenzotriazole 0.8

Disodium salt of EDTA 1.0

3-diethylamino-l-propanol 45.0

Potassium Hydroxide to pH=11.3

at 35°C with the following additions of ascorbic acid 0, 5, 10, 15, 20, 30 and 40 g per litre. The addition of extra sodium metabisulphite was also made to other solutions for a comparison with another antioxidant and the levels used were 10, 20 and 30 g per litre. The pH of all solutions was then adjusted back to that of the control.

The strips were developed for 30 seconds or 60 seconds and then held in the air to simulate long "cross-overs". They were then placed in a stop bath of 4% acetic acid and then fixed in Kodak Fixer 3000 (1+3) . The developed strips were examined for the presence of "pepper fog" and rated on a scale of 1-10 (10 worst) . The results are shown in Table 3 below.

Table 3

The results show that fresh developer with a level of 5g/l of ascorbic acid reduces the defect and that 15 g/1 eliminates the problem.

EXAMPLE 4

Example 3 was repeated using 0, 5, 10, 15, 20 and 40 g/1 of ascorbic acid and adjusting the pH of the solution back to that of the control. The samples were developed for 30 or 60 seconds at 35°C and then held in the air for 15, 30 or 60 seconds before stopping in 4% acetic acid. The samples were developed in 250ml of solution in a 250ml measuring cylinder which was then left exposed to air. The processing was repeated after 2, 3, 4 and 7 days. The results are shown below in Table 4 below. C means that the sample was clear of pepper fog.

The results show that a level of lOg/1 of ascorbic acid was sufficient to overcome the defect under all the conditions used. The control's improvement after 7 days is due to the decreased level of developing agent caused by the excessive amount of aerial oxidation.

EXAMPLE 5 Example 3 was repeated but in this case the processing was done in an INTERPRO AR10 machine using solutions at 35°C. The speed of the processor was adjusted so that the cross-over from the developer to the fixer varied from 10 to 20 to 34 seconds while the development time also increased from 30 to 60 to 102 seconds. These are referred to as fast, medium and slow in Table 5 below. Two solutions were used, these were the developer of Example 3 with 0 and then 10 g/1 ascorbic acid both at the same pH. Samples were processed on days 1, 2 and 3 and then evaluated for pepper fog. The results are shown in Tables 5 and 6 below.

Table 5 - Control Developer

Table 6 - 10 g/1 Ascorbic Acid added

Speed Day 1 Day 2 Day 3

<2 3-4 <2 3-4

C C C C c C c C

The results again show that the pepper fog formed in the control solution is overcome by the addition of ascorbic acid.