From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 09/26/01-12:18:45 AM Z
At 07:19 PM 09/25/2001 -0600, you wrote:
>Here goes: (from the chemically impaired--me--to the chemically gifted on
>this list)
>
> I have these following chemicals that I have questions about; let me
>list them, their prices, and then ask.
>
>potassium bromide: (all in 1 lb) $11.25 (1.16 x potassium = sodium bromide)
>sodium bromide: $33.95
>
>potassium hydroxide: $13.89
>sodium hydroxide: $5.95 (1.4 x sodium = potassium hydroxide)
>
>potassium metabisulfite: $10.95
>sodium metabisulfite: $5.25
>
>potassium thiocyanate: $89.95
>sodium thiocyanate: $49.95.
>
>Are all sodium and potassium compounds interchangeable? Why the huge price
>difference? Do they, in substitutions, truly work the same? And, what
>makes catechol so stinky?
> And what, in layman's terms, is the difference between a sulfate and
>sulfite and I assume they are not interchangeable? Someone already answered
>this a while ago but I lost my notes.
>Chris
>
>
The only one I am going to try is the last.
Sodium Sulfite is Na2SO3 and Sodium Sulfate is Na2SO4 The sulfite tends
to absorb oxygen very readily, becoming sulfate in the process. Sulfate
does not absorb oxygen. Sulfite is used in developing and fixing formulas
to prevent oxidation of the other components since it is oxidized
preferentially. It also serves several other functions in developers.
Sulfate is used mostly as an anti-swelling salt for high temperature
processing. About 45grams/liter can be added to most developers. The
sulfate both slows down the development and helps prevent swelling of the
gelatin. It can also be added to stop baths in the same amount to prevent
swelling.
Most formulas call for Sodium Sulfite, dessicated or anhydrous. They are
interchangible, the difference is in the process of making them. Some old
formulas may call for sulfite in crystaline form.
Potassium sulfite performs the same duties.
In wash aids the sulfite ion is quite specific as an ion exchanger,
displacing thiosulfate and silver complex ions. Sulfate has some wash aid
properties but is not nearly as effective as sulfite. Since a sulfite
solution becomes sulfate in the presence of air the rather dilute wash aid
solution has very limited life. It does no harm once the sulfite has become
sulfate, but it no longer is effective as a wash aid.
For carbonates the potassium form is more soluble but is delequesent
(absorbs moisture). The sodium form, particularly monohydrated is more
stable. However, potassium carbonate has slightly different activity in
developers tending to make them slightly more warm toned.
---- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
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