"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