What happened to my KRST working solution?

[Steve Goldstein]

Something odd occurred in my half-gallon of working-strength (about 1+9) Rapid Selenium Toner and before I mix up more I hope to find out what happened. This was mixed using steam-distilled water - I have a counter-top distiller and make my own.

The solution was fine 4 days ago when I last used it. As per usual, I had added a bit of "fresh" (purchased in May 2005, but this bottle is now nearly used up) stock from the Kodak bottle before starting my session, and at the end I coffee-filtered the solution back into its clear half-gallon glass bottle. My liquid chems are stored in the basement at a pretty constant ~68F. It's dark in that room except when I turn on the incandescent ceiling light to fetch things.

Yesterday, 3 days after the last use, I noticed a thin translucent white coating on the bottom and lower sides, about 1/4 way up from the bottom, of the bottle. Today I dumped the contents and washed it out. The white film wouldn't come off with hot water and shaking, but I could easily remove it using a small piece of paper towel pushed into the bottle and moved around with a dowel.

This bottle has been stored in the same location and used several times in the past few weeks, and has been used for diluted (always with distilled water) KRST exclusively since I purchased it (the glass bottle) new. I've filtered out the usual black flecks at each use, but have had no other issues until this time.

When toning I start with previously well-washed dry prints, rewet for a few minutes in tap water, drain and place into the toner, drain and place into 2% sodium sulfite for a 5-10 minutes, then into the washer. I'm careful to avoid back-contamination. My tap water has very low mineral content. I've never had issues with print staining so I believe my processing regimen (two baths of Ryuji Suzuki's neutral fix, 2% sodium sulfite for 5-10min, wash for ~30 min) is good.

Does anyone have any idea what might have happened? I have unopened bottles of stock from 2008 and 2009, along with newer ones, and hope to use it all. I'm not aware that KRST stock has a shelf life, but that could be lack of knowledge on my part. But remember that this stuff worked just fine over the past few weeks and suddenly went weird...

 

[Rudeofus]

Selenium toner contains lots of Ammonium Thiosulfate, and when it goes bad, it will show up as yellow/white precipitate (Sulfur). Stock solution should be more stable than working solution.

 

[Steve Goldstein]

This wasn't a precipitate or suspension, it was a film on the bottle - the liquid that came out was clear. But that's interesting.

Any idea what could cause this to happen so seemingly rapidly?

 

[john_s]

Selenium toner contains lots of Ammonium Thiosulfate, and when it goes bad, it will show up as yellow/white precipitate (Sulfur). Stock solution should be more stable than working solution.

Sulfur precipitate would be my first guess too. When fixer deteriorates, this is what we see. It normally happens with acidic fixer. I have never seen it with alkaline or near neutral fixer. KRST stock has a pH of 9 so the diluted KRST is going to be something like that too (i.e. not acid). There aren't many ingredients in KRST to go wrong:

Weight % Components - (CAS-No.)
55 - 60 Water (7732-18-5)
25 - 30 Ammonium thiosulphate (7783-18-8)
10 - 15 Sodium sulphite (7757-83-7)
1 - 5 Selenious acid, disodium salt (10102-18-8)

Acid in KRST is well known to cause the black precipitate of selenium, but to precipitate sulfur? Could it be that the sulfite became oxidised and then the thiosulfate was no longer protected? If the OP wants to experiment with the same batch, perhaps a bit of extra sulfite could be tried.

 

[Rudeofus]

This wasn't a precipitate or suspension, it was a film on the bottle - the liquid that came out was clear. But that's interesting.

Very odd, and doesn't sound like precipitation of Sulfur. Was that film some sticky goo, or can you give some details about its appearance?

Any idea what could cause this to happen so seemingly rapidly?

If you look at (there was a url link here which no longer exists) that claim to be "like KRST", there isn't really that much in KRST that can cause trouble. Besides the compounds listed in the formula I would assume there is some sequestering agent for water hardness in KRST.

 

[Steve Goldstein]

Very odd, and doesn't sound like precipitation of Sulfur. Was that film some sticky goo, or can you give some details about its appearance?

It didn't seem gooey, but I didn't get any out intact to see. It seemed to kind of disintegrate as I worked it off the bottle, and came off quite easily. It was translucent whitish, kind of like an HDPE milk container. Much thinner, of course.

There's only a few ounces left in that 10-year-old bottle of KRST stock, the rest is air. Maybe I'll just pitch it an open the next newer one, purchased in December 2008.

Would there be any advantage to storing the stock in glass rather than in the original poly bottles?

 

[Jim Noel]

It didn't seem gooey, but I didn't get any out intact to see. It seemed to kind of disintegrate as I worked it off the bottle, and came off quite easily. It was translucent whitish, kind of like an HDPE milk container. Much thinner, of course.

There's only a few ounces left in that 10-year-old bottle of KRST stock, the rest is air. Maybe I'll just pitch it an open the next newer one, purchased in December 2008.

Would there be any advantage to storing the stock in glass rather than in the original poly bottles?

To answer your question - Yes, by all means store important chemicals in glass only. Poly bottles allow oxygen to infiltrate via osmosis.

 

[Rudeofus]

While glass bottles may give you some advantage, I would generally not expect photo chemistry to stay intact for 5+ years. There are a few exceptions (some powders, Rodinal, HC-110, most stop bath concentrates), but most others will suffer from aerial oxidation or hydrolysis sooner or later. Glass may not let as much oxygen through, but glass bottles still have plastic lids most of the time, and hydrolysis may not even need oxygen.

PS: if you have access to Carbon Disulfide and suitable facilities, you can try dissolving the precipitate. If it dissolves, it's most likely Sulfur.

 

[adelorenzo]

My Kodak rapid selenium toner is old enough that it still came in a brown glass bottle. I don't know the date although it is stamped 7701 so maybe 1977? I'm the second or possibly third owner and it's still working just fine. That is only one data point but I'd be skeptical that your much newer batch suffered from aging.

 

[Rudeofus]

My Kodak rapid selenium toner is old enough that it still came in a brown glass bottle. I don't know the date although it is stamped 7701 so maybe 1977?

There was a (there was a url link here which no longer exists) of Kodak's Selenium Toner which did not use Thiosulfate. There is a good chance that such a mix lasts a lot longer than modern KRST.