Hi, I was wondering if there is a difference between sodium bisulfite and sodium metabisulfite. Some references on the web say that they are different and some say that they are the same and therefore interchangable in chemical formulae. I've looked up the chemical formula or name for each and they appear different, although I have come across a reference (on a Kodak site) that says that Sodium bisulfite is simply sodium metabisulfite in solution.

Where is this going, i hear you say. Well, I have all the makings of a PMK brew but have been supplied with sodium metabisulfite instead of sodium bisulfite. I dont know whether to just use the same quantity as in the recipe or whether I have to modify it. I don't really want to waste time and lf film in needless experiments if I'm not starting off on the right foot.

Thanks in advance for any advice.

Neil.

-- Neil Miller (nkd-miller@thefreeinternet.co.uk), March 11, 2002

Answers

They're actually different chemicals. Sodium bisulphite is NaHSO3, and Sodium metabisulphite is Na2S2O5. Both are available in powder form, and incidentally have the foodstuff additive numbers of E222, and E223 respectively.
However, as soon as you dissolve Sodium metabisulphite in water it becomes Sodium bisulphite. The addition product of Na2S2O5 + H2O is 2(NaHSO3).
As you may see, there's a difference in the molecular weights of the bisulphite and metabisulphite. Anhydrous Sodium metabisulphite has a molecular weight of 190 to the bisulphite's 104, but the metabisulphite will form two molecules of bisulphite with the addition of one molecule of water. Therefore 19 gms of anhydrous metabisulphite is equivalent to 20.8 gms of pure bisulphite.
Since pure Sodium bisulphite is almost impossible to obtain, it's easier and cheaper to substitute 91.3% of the weight of bisulphite with metabisulphite.

-- Pete Andrews (p.l.andrews@bham.ac.uk), March 11, 2002.

Pete's answer is perfect. For better clarity, regarding the last suggestion, I should say "you must use 91.3 grams of metabisulphite as substitute for 100 grams of bisulfite". Franco Rallo

-- Franco Rallo (rallo@ele.uniroma3.it), March 11, 2002.

If I recall correctly there are one or two applications where one can not be substituted for the other, but those are exceptions to the rule and I dont recall the exact recipes. I think its in one of the Cookbooks

-- Wayne (wsteffen@skypoint.com), March 11, 2002.

FWIW I use meta in the quantity called for in the formulae for pyro I mix myself and it works fine.

-- Jim Galli (jimgalli@lnett.com), March 11, 2002.

Steven Anchell says you can interchangably use sodium metabisulfite and sodium bisulfite. He even claims the two are often mispackaged, and sold one for the other. I've always used sodium bisulfite (from PF) with PMK and my own developer formulas.

Recently a friend went through his chemistry lab and discovered an unopened 5 lbs jar of sodium metabisulfite, which he gave to me. I've since tried it and it seems to work just fine.

Pete's response, however, seems a more precise treatment than Anchell gave in his book. I shall, in the future, follow Pete's suggestion.

-- Ted Kaufman (writercrmp@aol.com), March 11, 2002.

Thanks for the help, guys. I will stick with what Pete says.

-- Neil Miller (nkd-miller@thefreeinternet.co.uk), March 12, 2002.

>>If I recall correctly there are one or two applications where one can not be substituted for the other<<

I was thinking of the sodium/ potassium forms of one of these chemicals. Its those that arent always interchangeable. Found it in the film cookbook last night but now I lost it again!

-- Wayne (wsteffen@skypoint.com), March 14, 2002.