Re: sodium bisulfite

romek@ozemail.com.au
Sat, 19 Aug 1995 00:07:55 +1000

>We of course use sodium bisulphite for clearing gum prints of residual
>dichromate stain. So while the spirit was upon me I checked every source I
>could find on that. The only one who mentioned more than "use this or
>that" was Scopick, who said,
>
>"sodium metabisulphite and sodium bisulphite ....are not as soluble as
>potassium metabisulphite in water and thus require a long wash time."
>
Hey, Jude,
no magic. Sulfites are reducers and dichromates are oxidizers. When you mix
them, a reaction takes place. Depending on pH different end products will be
produced. But it is irrelevant. When you apply sulfite or bisulfite to a
stain (dichromate) , you end up with sulfates or thionate and chromium 3+
salt. We chemists call it a redox reaction. All the above compounds are
soluble in water and can be washed out easily. Washing of an emulsion is
matter of a diffusion process rather than solubility. It means a diffusion
will control exchange rate, not dilution by washing water (well in excess in
comparison to the amount reatained in the emulsion. Some other compounds
will be attracted to gelatine and difficult to remove (spent fixer for
example). Funny, to get rid of that stucked fixer, we knowledgeable
chemists/photographers use sodium sulfite solution. Sorry, I am too lazy to
grab my CRC Handbook, but from memory K2S2O5 dissolves 44.9 g/100 g of water
@20 deg C, while sodium equivalent 65.3. We do not bother with the higher
temperatures, do we?
cu
Roman

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Roman Kielich, Lane Cove, Australia, ph. 61-2-418-6971
###And now let's get drunk and play ping-pong.###
romek@ozemail.com.au
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