Diluting C41 bleach(?)

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alanrockwood

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If one is doing single-shot processing where one discards the solutions after a single use is it OK to use diluted bleach, such as a 50/50 bleach/water mix?

Normally bleach is used several times before being discarded. It seems to me that if fresh bleach is used in a one-shot mode it might be fine to dilute the bleach. The rationale for this idea is that bleach is normally used several times before being discarded, so it is probably at least partially degraded by the time the last rolls of film are processed. Also, the C41 process is probably designed with a built in safety margin. This might give some wiggle room to use fresh solutions that are diluted and discarded after a single use instead of using full strength solutions that are re-used.

The context is that I have a Photo-Therm processor. My Photo-Therm does have an option to save fixer but not bleach. I am gearing up to do some C41 processing, and I would like to save some money by using a diluted bleach.

Has anyone actually tested this idea?

Let me acknowledge right off the bat that some will make the "false economy" argument. However, if diluted bleach would work then we can counter the "false economy" argument with the "unnecessary expense" argument.

Thanks in advance for your comments.
 

Mick Fagan

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I find it incredible that a processing machine will not allow you to retain one of the processing steps bath; either for re-use or discarding correctly later.

C41 Bleach is the most expensive part of the C41 process and it has approximately double the capacity of the developer part of C41, as does the fix solution.

C41 bleach has a specific pH requirement to work correctly, and although I'm not a chemist, I would think diluting the bleach bath with 50% of water would possibly change the bleach pH outside of the bleach pH requirements. Best would be to dilute some C41 bleach with 50% water and measure the pH compared to the standard C41 bleach pH.
 
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alanrockwood

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I find it incredible that a processing machine will not allow you to retain one of the processing steps bath; either for re-use or discarding correctly later.

C41 Bleach is the most expensive part of the C41 process and it has approximately double the capacity of the developer part of C41, as does the fix solution.

C41 bleach has a specific pH requirement to work correctly, and although I'm not a chemist, I would think diluting the bleach bath with 50% of water would possibly change the bleach pH outside of the bleach pH requirements. Best would be to dilute some C41 bleach with 50% water and measure the pH compared to the standard C41 bleach pH.

Good idea about checking the pH.

A comment or two about pH from a chemist (me). If the solution is buffered then diluting the solution will have almost no effect on the pH.

If the solution is unbuffered then it would change the pH by about 0.3 pH units, assuming that the pH is controlled by a strong acid or strong base. However, if an unbuffered solution pH is controlled by the presence of a weak acid or weak base then the dilution issue becomes more complicated.

An exception to the comment above about diluting unbuffered solutions: If the solution is unbuffered and of neutral pH (i.e. 7.0) then all bets are off. Theoretically, diluting it would leave the pH at 7.0, but only if the added water also has a pH of 7.0. However, because of various impurities that may be present, tap water is usually either slightly acidic or slightly basic, so diluting a pH 7.0 solution with tap water will probably pull the pH one way or the other.

Of course, if pH is important then good design would dictate the the solutions should be buffered, in which case dilution would have almost no effect on the pH.

I agree with your comment about saving solutions. My processor allows the saving of one solution, but it does not give a choice about what solution to save. The choice (fixer only) is hardwired into the firmware. There is (or was, since Phototherm no longer makes or sells photo processing equipment) a higher-level model that could save up to two solutions, and I believe it gave the user a choice of what two solutions to save. I wish I had that model, but I don't.
 
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alanrockwood

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One or two more thoughts. There are at least two parameters that could limit the ability of bleach to work. One is capacity, i.e. are there enough ferric ions (complexed with EDTA in Kodak's bleach) to oxidize the silver. The other is whether the reaction is fast enough to reach completion in the time (six and a half minutes) programmed for the bleach process. Dilution could affect either of those parameters, or neither one, depending on the details of the process.

For example, if there are more than the minimum required number of ferric ions in the diluted solution then that would not be a limiting factor. However, if the kinetics of the undiluted solution were such that there was barely enough time to oxidize all of the silver within six and a half minutes, then dilution the solution would be a problem because it would slow down the reaction enough that not all of the silver would be oxidized. However, if the kinetics of undiluted solution were fast enough to leave a lot of time to spare at completion then a diluted solution might work, even if its kinetics were somewhat slower.
 
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Mick Fagan

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The six and a half minutes is only there as it is a doubling of the developer time of three minutes fifteen seconds. This is (I believe) mainly brought about by the plethora of Dip N Dunk machines which dip the film into a developer bath for 3 minutes, then starts raising the rack up, then moving the whole shebang forward, then dropping the film rack into the next bath with completion approximating to 3'15" give or take depending whether the film is at the top or bottom of the rack. For super critical stuff we would always have sheet film in the centre of the racks

The bleach and fix baths are double length baths, so the film rack goes up on the next cycle and is then dropped back into the bleach (or fix bath) for a second cycle; hence the 6½ minutes.

C41 is the shortest commercial developing cycle I know of, time in the first bath is critical for film speed and many other factors. A plus or minus of 15 seconds is around ½ a stop pull or push. The temperature at 37.7ºC (carried) is also critical for nailing things down. Although there is supposedly a real lot of give or take with C41 processing; for really on the money processing with good films, nothing beats perfect processing. The results of careful film exposure combined with careful film processing control, can lift your end result from pretty good to stupendous in a home processing situation.

For quite some time I was involved with Dip N Dunk C41 and E6 processes using Kodak chemistry and monitoring film strips etc. Sometimes when things went awry, the film racks would be lifted up, they didn't move forward and were then dropped back into whatever bath they came out of. Providing it wasn't the developer bath, the film would survive and within reason it could be there for some time. This happened at least twice on my shift in my time with these machines.

What I 'm suggesting is that if you can change the cycle length of your bleach bath to offset the possible weaker bleach solution this may also help you somewhat with your completion issue, if in fact it is an issue. Thinking out loud here.

For many years I was re-halogenating my C41 bleach solution using a simple fish tank aerator, then adjusting the pH back. I was rotary processing in a Jobo and using 500ml of solution at a time for four rolls of 135 film or four sheets of 4x5" film. There is always some solution loss in the tanks, plus evaporation losses due to heat, so around every 10 developing runs I would decant down to around 250ml add 500ml of fresh bleach then top up with the old stuff to 1000ml, discarding the used solution. Roughly every 20 developing runs, I would mix up a fresh litre of bleach and start all over again.

Short of running a Kodak C41 control strip through your diluted bleach and then doing a readout of the control strip, I think you'll be winging it. It would probably be alright though. If you have anyone doing commercial C41 nearby, they should have some kind of control monitoring system, worth a shot anyway as you have nothing to lose by asking.
 

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@alanrockwood I have used bleach at a higher dilution a number of times. It works OK. Especially in a one-shot situation, a 1+1 diluted bleach will work just fine, especially since the bleach step has a safety margin in terms of its duration. Give it a try and see how it goes. If there's a problem, just re-bleach and re-fix the negatives. You could work out this way how far you can push the dilution. I bet you can get away with 1+3 or so in a one shot situation. Of course, at some point, the situation becomes marginal, so that's something you may want to conduct tests (preferably densitometry) for. If you're not willing/able to do such tests, I would venture that the planned 1+1 dilution is just fine & safe.

I agree with @Mick Fagan that the bleach really should be saved, replenished and reused.

I was re-halogenating my C41 bleach solution using a simple fish tank aerator

No, that's not re-halogenating it, since you're not adding back any used up halide this way! Proper replenishment requires actual rehalogenation. Your method would have worked quite well had you also added some potassium bromide from time to time. But in all honesty, just replenishing 'by the book' is much simpler and still very economical.
 

mshchem

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Cibachrome in tubes. Seems like I used 90mL for 8x10 prints. I would use 45mL of used and 45mL of unused for each step, developer, bleach and fixer. Like Mick says the bleach has huge capacity, I think the Kodak replenishment rate is something like 9mL per 36 exp roll.
 
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alanrockwood

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The six and a half minutes is only there as it is a doubling of the developer time of three minutes fifteen seconds. This is (I believe) mainly brought about by the plethora of Dip N Dunk machines which dip the film into a developer bath for 3 minutes, then starts raising the rack up, then moving the whole shebang forward, then dropping the film rack into the next bath with completion approximating to 3'15" give or take depending whether the film is at the top or bottom of the rack. For super critical stuff we would always have sheet film in the centre of the racks

The bleach and fix baths are double length baths, so the film rack goes up on the next cycle and is then dropped back into the bleach (or fix bath) for a second cycle; hence the 6½ minutes.

C41 is the shortest commercial developing cycle I know of, time in the first bath is critical for film speed and many other factors. A plus or minus of 15 seconds is around ½ a stop pull or push. The temperature at 37.7ºC (carried) is also critical for nailing things down. Although there is supposedly a real lot of give or take with C41 processing; for really on the money processing with good films, nothing beats perfect processing. The results of careful film exposure combined with careful film processing control, can lift your end result from pretty good to stupendous in a home processing situation.

For quite some time I was involved with Dip N Dunk C41 and E6 processes using Kodak chemistry and monitoring film strips etc. Sometimes when things went awry, the film racks would be lifted up, they didn't move forward and were then dropped back into whatever bath they came out of. Providing it wasn't the developer bath, the film would survive and within reason it could be there for some time. This happened at least twice on my shift in my time with these machines.

What I 'm suggesting is that if you can change the cycle length of your bleach bath to offset the possible weaker bleach solution this may also help you somewhat with your completion issue, if in fact it is an issue. Thinking out loud here.

For many years I was re-halogenating my C41 bleach solution using a simple fish tank aerator, then adjusting the pH back. I was rotary processing in a Jobo and using 500ml of solution at a time for four rolls of 135 film or four sheets of 4x5" film. There is always some solution loss in the tanks, plus evaporation losses due to heat, so around every 10 developing runs I would decant down to around 250ml add 500ml of fresh bleach then top up with the old stuff to 1000ml, discarding the used solution. Roughly every 20 developing runs, I would mix up a fresh litre of bleach and start all over again.

Short of running a Kodak C41 control strip through your diluted bleach and then doing a readout of the control strip, I think you'll be winging it. It would probably be alright though. If you have anyone doing commercial C41 nearby, they should have some kind of control monitoring system, worth a shot anyway as you have nothing to lose by asking.

That's excellent information. Thanks.

Also, koraks, thanks for your comments. They are encouraging.

I found some information on Kodak's document titled "Using KODAK FLEXICOLOR Chemicals" that seems relevant. On p. 3-6 it gives steps and conditions for rotary tube processing, and it lists 6:30 for the bleach time, but it give a temperature range of 24 C to 38 C (74 F to 100 F). Most chemical reactions are a lot slower at 24 C than at 38 C, and since the bleach will work at 24 C for a 6:30 bleach step that suggests that kinetics are probably not limiting. This issue of kinetics can actually get into some fairly complicate subtleties. My PhD research was on chemical kinetics, so I am aware of some of the potentially troubling subtleties that can some up in kinetics experiments, but I won't go into them here. (For the technically-inclined, this relates to whether it is a first or second order process in ferric ion concentration.)

On page 4-2 (table 4-5) it gives the process cycle for minilab processors, and it gives bleach time range of 4:15 to 6:30 at 100 F, which is another indicator that kinetics might not be limiting.

That leaves a possible issue of capacity. In single shot mode. I suspect that this would not be limiting, partly because of safety factors that the designers of the process probably built into the system, but there is another thing that might make the process very robust in this regard, and that is that there is a lot of agitation of the solution in a rotary tube processor. This would likely keep the bleach solution well-oxygenated while it is doing its work, which would tend to regenerate the depleted iron ions (Ferrous ions) back to the active (ferric) form. This is assuming that the kinetics of oxidation of the ferrous ions would be sufficiently fast to that it is not limiting and also that there would be sufficient oxygen in the tube so that it would not be greatly depleted by the ferrous --> ferric process.

Anyway, based on the comments so far I am at least somewhat encouraged about the possibility of using diluted bleach.

I do wish that my Photo-Therm processor would allow saving the bleach, but it does not, which is why I am investigating plan B to make the process economical, which is to use dilute bleach.


Also, I see that mshchem posted while I was writing this post, and koraks also posted another comment. Thanks for those comments, especially the comment by mshchem that bleach has huge capacity, which was based on a comment by Mick. Also the comment by koraks about having used dilute bleach in the past.
 
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alanrockwood

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Test strips were mentioned. It would be great if I could find a source for a small number of test strips. However, I think test strips cost about $100 for a box of 50. I wouldn't need 50 test strips, and I don't think I could spare $100 to get a full box.

I do have a densitometer. I don't have color density standards for it. (I do have black and white density standards.) However, for this application that might not be necessary to have color density standards because the measurements would be a comparison against a test strip standard, so absolute accuracy would not be required, only relative accuracy over narrow density ranges.
 

mshchem

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I use a Jobo. I use Developer and Fixer 1 shot, I replenish and save the bleach. The bleach lasts for years when properly replenished in my low use situation. I use the Flexicolor C-41RA bleach, very fast acting.
 

Mick Fagan

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@alanrockwood

No, that's not re-halogenating it, since you're not adding back any used up halide this way! Proper replenishment requires actual rehalogenation. Your method would have worked quite well had you also added some potassium bromide from time to time. But in all honesty, just replenishing 'by the book' is much simpler and still very economical.

Yes, not a good name for the process I described.

Essentially aerating the bleach by whatever method converts the ferrous EDTA to ferric EDTA, which is what I was doing. Then adjusting the pH to change the minor increase of alkalinity from that process.

I started doing this around 33 years ago when obtaining chemistry was prohibitively expensive in Australia outside of bulk purchasing. I continued with this C41 bleach aeration practice for around 23 something years then gradually moved back to B&W film and printing as I was sliding into retirement 11 years ago.

While it won't be relevant for Alanrockwood and his current processing machine, using a 2% acetic acid stop bath immediately after film development, stops the developing process instantly. Around 15 seconds is more than adequate, then two water rinses to remove the stop bath solution from the tank and the emulsion as much as possible before putting in the bleach bath, enhances the life of the bleach enormously.
 

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Back in the late 1970, early 1980s I ran a C412 dip & dunk processor. We collected the bleach waste and regenerated it with glacial acetic acid, using a hydrometer to calculate the amount and a top up with water.
The memory is not too great, but we kept a log of all chemical mixing but especially bleach for fear of retained silver.
 
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