What happened to my KRST?

I dilute KRST stock 1+19 with steam-distilled water and keep it in a glass bottle in between uses. I occasionally add a bit of stock in an uncalibrated attempt at replenishment. It's stored in a dark and fairly temperature-stable place in my basement - according to the thermometer I keep there I've observed variations between 65F and 72F over the years.

The other day I went to get my KRST jug, last used a few months ago, and found it had developed a milky film on the bottom. The film could be detached by swirling the bottle, but it just fragmented and wouldn't go back into solution so I sadly had to dump the contents and start a new batch. I didn't even try to use the old batch, and yes, I thoroughly cleaned the bottle.

This has never happened to me before, and I'm baffled. Does anybody know what happened, and how I can prevent it from recurring? I really don't like having to dispose of KRST. I store KRST stock in its original plastic bottles on the same shelf and none of them, even a mostly-full one purchased in 2009, have this problem.

KRST, like many other selenium toners, contains Ammonium Thiosulfate. By mixing a 1+19 working solution, you have essentially created an alkaline rapid fixer in 1+39 dilution with some selenite ions. Give it enough time and Oxygen, and it will spit out Sulfur.

Since Thiosulfate concentration is not that high in your brew, you could monitor pH and replenish with Ammonium Thiosulfate and Ammonia to restore original pH. Depending on what kind of darkroom equipment you have, this may turn out more expensive than disposing of expired selenium toner every so often.

Thank you.

I feel sure I have read here on Photrio about members who are still using the same selenium toner for years which has been replenished. Some filtration is required as I understand it but the stuff lasts a lot longer than the time periods the OP has given.

I need to add, if it is not obvious already, that I do not use KRST myself but I don't think I have misunderstood what was said in other threads.

pentaxuser

pentaxuser said:

I feel sure I have read here on Photrio about members who are still using the same selenium toner for years which has been replenished. Some filtration is required as I understand it but the stuff lasts a lot longer than the time periods the OP has given...
pentaxuser

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That's me.

I have jugs of KRST that have been going for 10+ years. No problems. I add a bit of concentrate to the working solution when times get too long and filter before and after use. That's it.

I have to disagree with Rudi on his assessment of the problem this time. In the past I have also had a slimy, milky film develop in the toner working solution. I believe that it is bacterial slime, not sulfur. It is definitely not a precipitate in any case. A good filtering gets rid of it. Strangely, after filtering out the slime, it doesn't seem to return.

As for adding thiosulfate: I don't think it is at all necessary. My working solutions get replenished with concentrate, but only at rather lengthy intervals. Whatever ammonium thiosufate was in the original toning bath is likely long gone before that; the activity of the solution does not seem to be affected. The annoying ammonia odor, however, is gone (along with the ammonium thiosulfate, I presume), which makes toning a lot more pleasant.

FWIW, I just toned a run of prints yesterday with a jug of toner that is at least 10 years old. Worked just great. I haven't replenished it for months.

@OP,
Next time, don't toss your selenium toner (who needs heavy metals in the sewage sludge anyway...), just filter it and proceed as usual. If your toning times get too long, then add a dash of concentrate and you're back in business.

A word to those who use very weak selenium toner for a given time in order to protect their prints:
First, unless you see a change in image tone, there's no protection. Second, full protection is only achieved by full toning; an image tone which most of us don't like at all (bright reddish brown with reduced contrast). The partial toning most of us do is to change the image tone slightly to get rid of the olive cast many paper/developer combinations have and to increase d-max and contrast a bit; it provides very little archival protection. Third, every print you run through a toning bath takes a bit of the toner, and therefore, activity from the toning bath. In order to get the same degree of toning on print 10 as print one, you'll have to tone a tiny bit longer. If you're just mechanically dipping prints in weak selenium toner for x minutes and seeing no change in image tone, you're doing nothing.

I tone visually, watching each print closely and comparing it to an untoned print kept close at hand. I transfer the print from the toner to the next bath when the desired degree of toning is reached (keeping in mind that toning continues for a while in the next bath). Each print is toned individually and to the amount that suits the image, paper and feeling I'm trying to achieve. In my case, I prefer a slight eggplant undertone for image tone; more or less depending on the image.

I've written about this a lot here and over on the LF forum if anyone is interested.

Hope this helps,

Doremus

Just an addendum to my above post:

Selenium toner replenished and reused as I described above does throw off a black precipitate (likely silver selenide, but I'm not sure). This is easy to filter out. It's not the same as the milky slime that the OP reports, which is a different issue. It can be filtered out as well.

Best,

Doremus

Thanks for your replies Doremus.

I've seen the black precipitate and filter with every use, but the white slime was a new thing. I'll experiment with less dilute toner, there are plenty of scrap prints around here to work with!

Doremus is most likely correct, that slime is not Sulfur. I have seen fixer growing mold/bacteria/whatever, so it may be something like that. I wonder whether a dash for formalin would prevent this.

Formalin or some other anti-bacterial would likely do the job as long as it didn't affect toning and would last as long as the working solution. It may, however, simply not be necessary, since filtration seems to do the job; for me at least.

Best,

Doremus

I mix with hypo clear, I don't know if this makes a difference. I also use stronger solution 1&9 and even, 1&3. The stronger solution was mentioned in a later (1980s? )?Kodak manual on fine printing. That strong mix will drive you out of the darkroom from the ammonia fumes, and will soften the gelatin, but it only takes about 45 seconds,

I have concentrate in glass bottles that is 50 years old still good. One man's toxic waste is another's treasure.

I've used the 25% solution for years without doing anything but a little replenishment. 1+9 lasts as well.

mshchem said:

I mix with hypo clear, I don't know if this makes a difference. I also use stronger solution 1&9 and even, 1&3. The stronger solution was mentioned in a later (1980s? )?Kodak manual on fine printing. That strong mix will drive you out of the darkroom from the ammonia fumes, and will soften the gelatin, but it only takes about 45 seconds,

I have concentrate in glass bottles that is 50 years old still good. One man's toxic waste is another's treasure.

I've used the 25% solution for years without doing anything but a little replenishment. 1+9 lasts as well.

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Kodak, AA and others recommended mixing selenium toner with Hypo Clearing Agent in their literature. That's fine if you plan on tossing the selenium toner when the Hypo Clear gives out. It seems supremely wasteful, and even irresponsible to me to discard active selenium toner just to save a step (It's a good way to sell toner, though...). If you use your toner beyond the lifespan/capacity of the Hypo Clear, it will continue working just fine; you just don't get the wash-aid effect. I mix a separate tray of wash aid and transfer the prints directly to that from the toning bath.

Hypo Clear and most other wash aids are comprised of sodium sulfite and bisulfite as their primary ingredients. A working solution of Hypo Clear in an open tray will become useless due to oxidation in a short time (around 24 hrs.) and will need replacing.

Maybe you want to consider adding a separate wash-aid step to your workflow?

Best,

Doremus

Doremus Scudder said:

Kodak, AA and others recommended mixing selenium toner with Hypo Clearing Agent in their literature. That's fine if you plan on tossing the selenium toner when the Hypo Clear gives out. It seems supremely wasteful, and even irresponsible to me to discard active selenium toner just to save a step (It's a good way to sell toner, though...). If you use your toner beyond the lifespan/capacity of the Hypo Clear, it will continue working just fine; you just don't get the wash-aid effect. I mix a separate tray of wash aid and transfer the prints directly to that from the toning bath.

Hypo Clear and most other wash aids are comprised of sodium sulfite and bisulfite as their primary ingredients. A working solution of Hypo Clear in an open tray will become useless due to oxidation in a short time (around 24 hrs.) and will need replacing.

Maybe you want to consider adding a separate wash-aid step to your workflow?

Best,

Doremus

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I use fresh hypo clear separate from the toner. I make 1 gallon of HCA put it into 260 mL bottles (8 oz fllled to the rim) every printing session with fiber base paper, I mix working solution, use and discard.