Photographic bleach composition

A photographic bleach solution contains hydrogen peroxide, or a compound capable of releasing hydrogen peroxide, and two or more sequestering agents capable of complexing with a transition metal. The bleach solution ha a pH in the alkaline range. A method of using such a photographic bleach solution is also disclosed.

FIELD OF THE INVENTION 
This invention relates to photographic bleach compositions for use in 
photographic colour processing. 
BACKGROUND OF THE INVENTION 
There are a number of proposals in the art to use peroxy compounds, e.g. 
hydrogen peroxide or a compound capable of releasing hydrogen peroxide, in 
bleach compositions in conventional colour processes. Some peroxide bleach 
solutions must contain an organic metal complex salt e.g. U.S. Pat. No. 
4,301,236, while others must employ a bleach accelerator e.g. Japanese 
specifications 611250647A and 611261739A. 
In U.S. Pat. No. 4,277,556 there are described bleach solutions having a pH 
between 2.0 and 5.5 consisting of a hydrogen peroxide solution and a mono- 
or dicarboxyllc acid, or an alkylidene diphosphonic acid, typically 50 
ml/l 30% of hydrogen peroxide solution and 30 ml/l concentrated acetic 
acid. Such solutions however do not bleach the entire amount of silver 
present. U.S. Pat. No. 4,454,224 describes an improvement on this in which 
the hydrogen peroxide bleach solution is alkaline having a pH of 7 or more 
and contains a polyacetic acid such as diethylenetriaminepentaacetic acid 
or `DTPA`. This complexes with the silver ion formed by the oxidising 
action of the peroxy compound and the silver complex thus formed, being 
soluble in the photofixing composition, can then be readily removed. In 
spite of all these suggestions no such solution has ever been used 
commercially. There is further disclosed in European Patent Application 
Publication No. 0 428 101 A1 an alkaline bleach solution containing a 
water-soluble chloride for producing rapid desilvering to which an organic 
phosphonic acid may be added to eliminate blister formation. 
It has also been established that peroxide bleaches used for processes for 
low silver materials are unstable and decompose in a matter of days or 
even hours, becoming unusable. Complex metal ions, which are naturally 
present in the water used to make up the bleach or which leach out of the 
paper during processing, catalyse the decomposition of the peroxide in the 
bleach. In U.S. Pat. No. 4,454,224 it can be seen that `DTPA` has little 
or no stabilising effect on the bleaching agent at the pH used. 
SUMMARY OF THE INVENTION 
It has now been found, however, that the addition of even a tiny amount of 
a combination of two or more sequestering agents which can complex with a 
transition metal ion, can significantly enhance the stability of peroxide 
bleaches at a particular pH range. This preservation of the active 
ingredient of the bleach causes a reduction in waste and is inexpensive 
compared to replacement of the solution. 
According to the present invention therefore there is provided a 
photographic bleach solution, comprising hydrogen peroxide, or a compound 
capable of releasing hydrogen peroxide, and in combination two or more 
sequestering agents for complexing with a transition metal which will 
inhibit the decomposition of the bleach. The preferred pH range is from 
5.5 to 14.

DETAILED DESCRIPTION OF THE INVENTION 
Sequestering agents which may be suitable include two or more of the 
following: 
a polyalkylcarboxylic, phosphonic or sulphonic acid containing at least one 
amine group which has condensed with one or more alkyl hydrogens of the 
alkylcarboxylic, phosphonic or sulphonic acid, such as a compound of 
formula (I) as defined hereinbelow or a salt thereof; 
an alkylidene-1,1-diphosphonic acid, such as a compound of formula (II) as 
defined hereinbelow; a polyhydroxyphenyl such as a compound of formula 
(III) as defined hereinbelow; or 
a polyphosphate such as a compound of formula (IV), as defined hereinbelow, 
or an alkali metal salt thereof. 
If a single sequestering agent is used some stabilising effect occurs but 
this is surprisingly dependent upon the pH used. Thus although a compound 
of formula (I), such as `DTPA`, has negligible effect at an alkaline pH, 
as mentioned hereinbefore, at a mildly acidic pH, for example pH 6.0, it 
can be highly effective in stabilising bleach. 
A compound of formula (I) is represented by the structure. 
##STR1## 
wherein X.sup.1 and X.sup.2 may be the same or different and each is a 
hydrogen atom, a straight or branched chain alkyl group having 1 to 6 
carbon atoms, a hydroxyl, carboxyl, sulphonyl or phosphonyl group or the 
group Z, 
wherein Z= 
##STR2## 
wherein a, b, c and d are the same or different and each is a hydrogen 
atom, a straight or branched chain alkyl group having 1 to 6 carbon atoms, 
a hydroxyl, carboxyl, sulphonyl or phosphonyl group or either X.sup.1 
and/or X.sup.2 may be repeat units of A or B, 
Y is as defined for a, b, c and d or is the group Z or the group B, 
##STR3## 
wherein X.sup.3 and X.sup.4 are the same or different and are as defined 
for X.sup.1 and X.sup.2 
wherein the sum of m, n and r is an integer from 1 to 10 and 
wherein one or both of the hydrogen atoms in each of the 
(CH.sub.2).sub.m,(CH.sub.2).sub.n or (CH.sub.2).sub.r groups may be 
replaced by a straight or branched chain alkyl group having 1 to 6 carbon 
atoms, 
with the proviso that at least one of X.sup.1, X.sup.2, X.sup.3 and X.sup.4 
is or contains a carboxylic, sulphonyl or phosphonyl group, or a salt 
thereof. 
Examples of compounds of formula (I) that are particularly suitable are 
ethylenediaminetetraacetic acid `EDTA` propylenediaminetetraacetic acid 
`PDTA`, nitrilotriacetic acid `NTA` but most preferably 
diethylenetriaminepentaacetic acid `DTPA`, these compounds generally being 
used in the form of their corresponding tetra- or penta-sodium salts. 
In contrast, an alkylidene -1, 1-diphosphonic acid of formula (II) 
##STR4## 
wherein X is a hydrogen atom, a halogen atom 
or a hydroxyl group and n is from 0-12 whilst found to be suitable over a 
range of pH from 5.5 to pH 14 is more effective in the alkaline range. For 
example at pH 8.0 the preferred compound 
1-hydroxy-ethylidene-1,1-diphosphonic acid (X=OH, n=1) or `HEPA` has been 
found to have surprising stabilising effect in preserving the peroxy 
compound. 
Compounds of formula (III) and (IV) are both more effective in stabilising 
bleach at a mildly acidic pH, these compounds having the formulae 
represented hereinunder: 
##STR5## 
wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are the same or different and 
each is a hydrogen atom, a straight or branched chain alkyl group having 1 
to 6 carbon atoms, a hydroxyl, sulphonyl or carboxyl group, especially 
dihydroxyphenyl sulphonate (`catechol` disulphonate `CDS`) or `TIRON`.TM. 
and 
EQU HO(P(O)(OH)O--).sub.n H (IV) 
wherein n is from 4 to 12, preferably 4 to 8 or an alkali metal salt 
thereof, such as `QUADRAFOS`.TM. or `CALGON`.TM.. 
However, according to the invention and for optimum bleaching efficiency a 
combination of two or more sequestering agents is used. Thus, although 
`DTPA` has little effect at an alkaline pH, its combination with the 
effective `HEPA` produces a further enhanced or `synergistic` effect. 
A synergistic effect may be said to occur when at some time during the 
storage of the bleach solution the combined stabilising effect of two or 
more of the components is greater than the sum of the individual 
stabilising effects of the two or more components. 
In a preferred aspect of this invention such an effect may result by the 
combination of a polyalkylcarboxylic acid of formula (I) or a salt thereof 
with a diphosphonic acid of formula (II) or a polyphosphate of formula 
(IV) or an alkali metal salt thereof. 
In a most preferred aspect of this invention the combination comprises 
diethylenetriamine-pentaacetic acid, `DTPA` or a salt thereof and 
1-hydroxy-ethylidene-1,1-diphosphonic acid, `HEPA`. 
The components may be combined with the ratio of any two components being 
in the range 1:1 to 1:10 by volume wherein either component may be present 
in the greater amount. 
Each sequestering agent as a component in the synergistic combination, is 
added in an amount of from 0.005 to 5% by weight of bleach solution, 
preferably from 0.04 to 0.1% by weight. 
Compounds capable of releasing hydrogen peroxide include metal peroxides; 
compounds which include hydrogen peroxide in their crystal structure such 
as sodium percarbonate; other peroxy compounds such as sodium perborate 
and persulphate; or soluble organic peroxides, such as butyl peroxide or 
benzyl peroxide. 
The hydrogen peroxide or a compound such as mentioned above is added in a 
sufficient quantity to enable the bleach step to take place in a suitable 
length of time. Conveniently commercially available 30% hydrogen peroxide 
is used in an amount of from 20 to 100 ml/liter bleach solution, 
preferably about 50 ml/liter, but equivalent amounts of, for example, 8% 
or 3% hydrogen peroxide may also be used. 
The bleach solution preferably has one or more acids, alkalis or buffers to 
maintain the pH at the required level, such as an alkali metal carbonate, 
carbonate/hydrogen carbonate buffer, phosphate buffer, amine/borate 
buffer, boric acid, or a carboxylic acid, such as acetic acid, propionic 
acid or glycollic acid. 
The bleach solutions of the present invention may be used with any type of 
photographic silver halide colour material. Such materials and their 
possible constituents are described, for example, in Research Disclosure 
Item 308119, December 1989, published by Kenneth Mason Publications, 
Emsworth, Hants., United Kingdom. However materials based on predominantly 
silver chloride emulsions are preferred. 
The present invention also provides a method of photographic colour 
processing including a bleach step, which comprises the addition of 
hydrogen peroxide, or a compound capable of releasing hydrogen peroxide, 
and in combination two or more sequestering agents for complexing with a 
transition metal which will inhibit catalytic decomposition of the bleach. 
The bleach step may directly follow the developing or redox amplification 
stages or an intermediate stop bath may be employed. 
The photographic material to be processed preferably contains low levels of 
silver and is preferably based on emulsions which comprise at least 80%, 
preferably at least 90%, silver chloride and especially substantially pure 
silver chloride. 
The invention will now be described with reference to the following 
examples which in no way limit the scope of the invention. 
EXAMPLE 1 
A peroxide bleach, suitable for use with a low silver paper material, was 
made up with the following formula: 
______________________________________ 
hydrogen peroxide 30% 50 ml 
sodium hydrogen carbonate 
25 g 
potassium chloride 0.5 g 
tap water to 1 liter 
______________________________________ 
To this solution was added 1 ml of water containing 0.1% by weight of each 
of the transition metals copper (II), iron (III) and manganese (II) as 
their sulphates, giving a final concentration of each of these metal ions 
of 1 ppm. 
To a 100 ml sample of this bleach, 0.1 ml of 
1-hydroxyethylidene-1,1-diphosphonic acid (`HEPA`) was added. To another 
100 ml sample 0.1 ml 80% diethylenetriaminepentaacetic acid (`DTPA`) was 
added and to yet another 100 ml sample 0.1 ml of each of `HEPA` and `DTPA` 
was added. A control sample of 100 ml with no sequestering agent was also 
taken. The pH of each sample was adjusted to 8.0 with sodium hydroxide 
solution or dilute sulphuric acid as appropriate and the solutions 
transferred into clean brown glass bottles, which were kept in the dark at 
room temperature (about 20.degree. C.). 
At intervals the amount of residual hydrogen peroxide was determined using 
the method outlined on page 363 of the 3rd edition of `A Text-book of 
Quantitative Inorganic Analysis` by Vogel. The titration was made with 5 
g/l sodium thiosulphate. The residual peroxide was determined as the 
amount of this solution need to reach the end point. 
The results, which are shown graphically in FIG. 1, show that `HEPA` 
preserves the hydrogen peroxide to a substantial extent and an even 
greater improvement results by the addition of `DTPA`, indicating a 
synergistic effect between the sequestering agents, even though `DTPA` on 
its own has little or no effect. 
Thus, whereas the relative peroxide concentration of the control sample and 
`DTPA` falls to zero within 10 days, with both `HEPA` and the combination 
of `HEPA` and `DTPA` very little peroxide is destabilised until three 
weeks have elapsed and there is still a residual effect after six weeks. 
In order to try the bleaching ability of the stored bleach it was tested as 
follows: 
A piece of conventional silver-chloride-based photographic color paper 
(containing a total silver coverage of 700 mg/m.sup.2) was exposed to room 
light for 10 seconds, developed for 45 seconds in a conventional color 
developer and plunged into the bleach solution. The silver loss was 
followed by observing the transmitted infra-red density. The time of 
bleaching was taken as the time taken to reach a point at which the 
density no longer changed. The time for bleaching increased by no more 
than 100% after one month, whereas a solution with no addenda was 
bleach-inactive in considerably less than one week. 
EXAMPLE 2 
The above experiment was repeated with the pH, however, adjusted to 6.0 
with sodium hydroxide solution or dilute sulphuric acid as appropriate, 
using separate samples of `HEPA`, `DTPA`, catechol disulphonate 
(`TIRON`.TM.) and a polyphosphate (`QUADRAFOS`.TM.), and also a control 
sample with no sequestering agent present. 
The results, which are shown graphically in FIG. 2, show that each 
sequestering agent tested had a substantial peroxide stabilising effect 
when compared with the control, with `DTPA` and `TIRON`.TM. having 
negligible loss in effect even after 6 weeks, `QUADRAFOS`.TM. retaining a 
good level for this time and `HEPA` only falling to nil peroxide 
concentration after this 6 week period compared with 2 weeks for the 
control. 
The bleaching ability of the stored bleach was confirmed as described in 
Example 1.