Sodium sulfite precipitate
Nothing earthshaking, just interesting.
I keep a bottle of sodium sulfite concentrate that I use for hypo clearing agent. Checked the bottle yesterday and was surprised to find big chunks in the bottom. *Really* big chunks. I guess they're big Na2SO3 crystals. They're clear, like ice, not like salt. They're hard. One was so big I had to shake and break it to get it out of the bottle. Weird. They all redissolved easily after heating in a pan on the stove. -- You were wrong, and I'm man enough to admit it. - a Usenet "apology" |
Sodium sulfite precipitate
On 1/13/2010 5:53 PM Richard Knoppow spake thus:
"David Nebenzahl" wrote in message s.com... Nothing earthshaking, just interesting. I keep a bottle of sodium sulfite concentrate that I use for hypo clearing agent. Checked the bottle yesterday and was surprised to find big chunks in the bottom. *Really* big chunks. I guess they're big Na2SO3 crystals. They're clear, like ice, not like salt. They're hard. One was so big I had to shake and break it to get it out of the bottle. Weird. They all redissolved easily after heating in a pan on the stove. It would be interesting to know if this is crystalized sulfite or if it changed to sulfate. Sulfite absorbs oxygen from the air and eventually becomes sulfate. The more diluted it is the shorter the life. A pretty concentrated solution of sulfite should last quite a while but, of course, it will depend on the exposure to the air including the permeability of the container. I guess it's time to toss that batch and mix some fresh. So what is the effect of sodium sulFATE on film and paper, if any? -- You were wrong, and I'm man enough to admit it. - a Usenet "apology" |
Sodium sulfite precipitate
"David Nebenzahl" wrote in message s.com... On 1/13/2010 5:53 PM Richard Knoppow spake thus: "David Nebenzahl" wrote in message s.com... Nothing earthshaking, just interesting. I keep a bottle of sodium sulfite concentrate that I use for hypo clearing agent. Checked the bottle yesterday and was surprised to find big chunks in the bottom. *Really* big chunks. I guess they're big Na2SO3 crystals. They're clear, like ice, not like salt. They're hard. One was so big I had to shake and break it to get it out of the bottle. Weird. They all redissolved easily after heating in a pan on the stove. It would be interesting to know if this is crystalized sulfite or if it changed to sulfate. Sulfite absorbs oxygen from the air and eventually becomes sulfate. The more diluted it is the shorter the life. A pretty concentrated solution of sulfite should last quite a while but, of course, it will depend on the exposure to the air including the permeability of the container. I guess it's time to toss that batch and mix some fresh. So what is the effect of sodium sulFATE on film and paper, if any? The main use of sulfate in photography is as an anti-swelling agent in developers. Mostly found in "tropical" developers of the past formulated at a time when film emulsions were not hardened as much as they are now. Sulfate also has the effect of retarding developing rate, a desirable property where solution temperatures are high. Typically, a tropical formula is about the same as the standard formula but with the addition of about 15 grams/liter of sulfate. The problem with sulfite is that it becomes sulfate when it absorbs oxygen. Since one of the main purposes of sulfite in developers and fixing baths is to absorb oxygen to protect other chemicals it looses its effectiveness with time and the protective properties are lost. Sulfites also have a specific ion-exchange property for thiosulfate. This is one of the main reasons its used as a wash aid. Sulfate does no damage but is not as effective as a wash aid and AFAIK, does not have the ion-exchange property, which actively displaces thiosulfate and thiosulfate-silver complexes. The rate at which a sulfite solution becomes sulfate from exposure to the air depends on its concentration: the stronger it is the longer it will last. Of course, the amount of exposure is also important, even a strong solution will not last more than a few hours in an open tray while it will have a life of several months in a closed bottle. The soluability of dessicated sulfite is a couple of hundred grams per liter (I've forgotten the exact amount) at room temperature but its generally not found at concentrations much above about 150 grams/per liter in practical solutions to prevent crystalization at low temperatures. In short, sulfate does no harm but is not effective as an oxygen absorber or preservative nor as a wash aid. -- Richard Knoppow Los Angeles, CA, USA |
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