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sodium sulfite v sodium bisulfite

Duane Schrag , May 20, 2005; 05:19 p.m.

Are the two interchangeable? I had thought not, but then I went and bought Hutchings "The Book of Pyro" and he has his pyro recipe in there a couple times -- once with sodium bisulfite, once with sodium sulfite.

What's the difference (if any)?


Bill C , May 20, 2005; 07:54 p.m.

>> Are the two interchangeable? <<

Hi Duane; no, they are not really. If you only needed sulfite ion in solution, you could make a weight adjustment and match up the molar amount (both of them will supply sulfite ion). However, the solution pH will be somewhat different and this often affects us in photography.

Generally, you would stick to sodium sulfite in a developer. If you had something where you needed to hold the pH lower (like a fixer or bleach-fix, etc), it might be convenient to get some or all of the sulfite ion from sodium bisulfite. This could save you from having to add a pH adjuster. A mix of the two could function as a pH buffer in the general range of pH 6.5 to 8 or so. A higher proportion of sodium bisulfite gives the lower pH, while a higher proportion of sodium sulfite gives the higher pH.

I haven't seen the book, but I suspect that if you look at those developer formulas more closely, you'll find that either the one with bisulfite has lower activity, or a pH adjustment is made in one of the developers.

Ryuji Suzuki , May 21, 2005; 02:40 a.m.

Sodium bisulfite as a dry chemical agent is really sodium metabisulfite. It's a common misnomer in darkroom literature. What's more confusing, some photographic chemical suppliers have both bisulfite and metabisulfite, often with different price. You can find more on this on my web site.

Sodium (meta)bisulfite is a *lot* more acidic when dissolved in water and is not directly substituable with sodium sulfite, and vice versa.

Duane Schrag , May 21, 2005; 09:48 a.m.

Thanks for the information. Bill, I think the use of sulfite/bisulfite is (unfortunately) simply a typo. Here's what the book says:

A liter of working solution of PMK contains the following chemical amounts: Pyro ... 1.0 g metol ... 0.1 g sodium sulfite ... 0.2 g sodium metaborate ... 6.0 g

But elsewhere, it provides this formula for stock solution: Solution A to make 1 liter stock metol ... 10 g sodium bisulfite ... 20 g pyro ... 100 g

Solution B to make 2 liter stock sodium metaborate ... 600 g

To make working, you use 1 part A, 2 parts B, 100 parts water.

Every other version of this formula that I've seen uses only sodium bisulfite in Solution A, and in the proportions shown. I think the reference to sodium sulfite is an error ... :(

Patrick Gainer , May 21, 2005; 03:45 p.m.

When PMK stock is mixed with the metaborate B solution, the pH is greater thn neutral. Those chemicale that went in as molecules are now available as ions. Sodium bisulfite or meta bisulfite can provide sodium, hydrogen and sulfite ions. The working solution might be made up from sodium sulfite, borax, sodium hydroxide, boric acid, pyrogallol and metol in proper proportions (did I leave out anything?) but without the convenience of a long lasting 2-part stock system.

If a single solution formula specifies sodium sulfite, use that. If it specifies sodium bisulfite, use either that or sodium metabisulfite as Ryuji explains.

Ryuji Suzuki , May 21, 2005; 07:40 p.m.

It's off topic but since I find an error.

Gainer said:

> Sodium bisulfite or meta bisulfite can provide sodium, hydrogen and sulfite ions.

Sodium bisulfite or metabisulfite provides mostly bisulfite ion and sodium ion in a dilute solution. Depending on the pH of the solution, which can be adjusted by any of the standard technique, the bisulfite ion is at equilibrium with sulfurous acid or sulfite.

Duane Schrag , May 22, 2005; 10:26 p.m.

I'm just disappointed that a $26 book ("The Book of Pyro") that is often held up to be the bible in its field can't seem to keep sodium sulfite and sodium bisulfite straight. Makes me wonder what other (less obvious) errors the book contains.

Mark Sage , May 25, 2007; 10:57 a.m.

Duane, reread what Patrick says. The book is probably not in error, rather it appears that one formula is a two-part version whereas the single-mix version contains only sulfite. For two- part formulas, one part is often kept acidic and the other alkaline to increase keeping properties or improve ease of mixing. When the two parts are mixed, the resulting solution is always alkaline and when you're making an alkaline solution for immediate use you don't need bisulfite (which is acidic), so the formulas can be different and yet yield the same pH solution (and same degree of activity) when mixed for use. If you substitute sulfite for bisulfite in a two-part formula, your working solution will be far more alkaline than intended.

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