Life expectancy for bleach?

How long can potassium ferricynide bleach reasonably be expected to last--full strength & diluted?

I make a 10% full stength solution (100 g potassium ferricynide + 100 g potassium bromide per litre water) and dilute it 100 ml + 900 ml water for use. I made some step wedge strips recently. I varied both the time in the bleach (30", 1 ', 2', & 4') and the thiocarbamide toner (3 variations of thiocarbamide/sodium hydroxide mixture). The differences across the 12 strips were negligable with only a couple of exceptions. Not what I expected. Both the bleach and toners were made quite some time ago and used a few times, but not enough to exhaust the toners. My first conclusion was to suspect the toners because I understood that bleaches lasted a long time, certainly longer than toners. Before I make up fresh toners, I thought I should consult the wise folks of APUG.

The bleaching step is performed to completion. As long as the time/concentration are "enough" there should be no associated variations in the result. More surprising is the lack of variation from thiocarbamide/NaOH mixing ratio.

I am myself interested to learn wht is the life expectancy of the bleach stock solution.

bernard_L said:

The bleaching step is performed to completion. As long as the time/concentration are "enough" there should be no associated variations in the result. More surprising is the lack of variation from thiocarbamide/NaOH mixing ratio.

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I made a similar test run yesterday with Fomaspeed Variant 312 and a great variation of Thiourea based toners, and they all yielded the exact same (and not very pleasing) image tone and tonality. I varied Thiourea concentration from 2.8 g/l all the way down to trace amounts, and alkali varied from 5 g/l NaOH down to about 10 g/l Sodium Bicarbonate. I saw big changes in toning speed, but since intermediate results showed very uneven toning (agitation or not), I had to tone to completion, which gave me the same dark brown tone all the time. My bleach recipe pretty much resembled yours. I don't think that a switch from Potassium Bromide to Chloride or Iodide will change much, but I plan on testing this next week.

Conclusion: Unless there is some important parameter that I forgot to vary, some papers may simply not allow much variation in image tone.

Potassium ferricyanide solutions are not particularly stable toward light and heat. For this reason solutions are best made up before use.

I have been using MG IV RC & FB papers and getting favourable results until recently. That's what prompted me to make a set of step wedges. I'll mix up some new solutions and make some new step wedges. Since I didn't check the effectiveness of the bleaching stage, I'll put a wedge in a tray of the old bleach and compare with the new bleach, just out of interest. And, I'll store the bleach in brown glass bottles. Stay tuned

NEVER was concerned with the life of potassium ferricyanide in water (with, or without the bromide). When (if) it is mixed with fixer, then its life is measured in minutes. But, alone? Years, centuries, perhaps? It might NOT help to store in bright light, however. - David Lyga

David Lyga said:

NEVER was concerned with the life of potassium ferricyanide in water (with, or without the bromide). When (if) it is mixed with fixer, then its life is measured in minutes. But, alone? Years, centuries, perhaps? It might NOT help to store in bright light, however. - David Lyga

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There is a tendency to think of inorganic compounds as being very stable. We certainly do not worry about the sodium chloride in our salt shakers from day to day. But like all generalities this idea is not completely true. Boil a solution of sodium bicarbonate for a few minutes and what you have is sodium carbonate remaining. The iron cyanide complexes are stable but they are not completely so. Since the bleaching process goes to completion probably not enough ferricyanide ion is lost during storage to effect the bleaching action. However, light will reduce some of the iron III to iron II. You may notice the formation of a dark blue sediment forming in old solutions, this could cause staining with prints should any adhere to the emulsion. Remember this photosensitivity is the basis of the cyanotype process.

Well, I was basing my comment upon experience. I have never had a problem with a solution containing potassium ferricyanide. - David Lyga

Wouldn't aerial oxygen convert the Ferrocyanide back to Ferricyanide over time? After a session of bleaching, where the Ferricyanide would be exposed to room light, there is usually a long time frame when the bleach sits in a dark bottle somewhere away from light ...

Simple iron salts like ferrous sulfate are very easily oxidized. However when the iron is in the form of a complex such as ferrous ammonium sulfate or potassium ferrocyanide it is much more resistant. That is why the energy of light photons are needed to effect a change in electron charge.

It was not my intent to put undue emphasis on the problem but I would not recommend that the diluted bleach be saved.

Jury is in...potassium ferricyanide bleach has a limited life; how limited is the next question. I mixed up all new bleach and toner variations. The new bleach (1% working solution) bleached more steps than the old bleach. I repeated the bleach/toner experiment and got much better results. I'm going to make some bleached step wedges for future reference. When the bleach can't match the reference wedges, it will be replaced.

BTW, fellow APUGer PE set the record straight in another (there was a url link here which no longer exists) about potassium ferricyanide & light. It is not light sensitive.

Now please excuse me. I have some prints to tone.

FWIW,

I have a small bottle of rather strong rehalogenating bleach (potassium ferricyanide and potassium bromide) that I use in bleach/redevelopment. I used it yesterday and it did its job just fine. The solution is several years old.

Weaker dilution that I use printing seem to go bad quickly. This is always accompanied by a color change from yellow to greenish.

Perhaps the longevity depends a lot on the dilution; weaker solutions being more prone to oxidation. Certainly, any contamination with fixer will kill a bleach very fast.

Best,

Doremus

Good point Doremus. I'll monitor the colour. Making a diluted working solution is inexpensive enough. No need to throw the baby out with the bath water