Chemical Question

**John Cahill** · 25-Mar-2007, 22:35

I keep seeing formulae calling for sodium metabisulfite. All I have is a drum of Kodak sodium bisulfite. Are they the same thing? Or "close enough for gummint work?"

**Greg Lockrey** · 25-Mar-2007, 22:57

Originally Posted by John Cahill

I keep seeing formulae calling for sodium metabisulfite. All I have is a drum of Kodak sodium bisulfite. Are they the same thing? Or "close enough for gummint work?"

No. Sodium Bisulfite is NaSO3 and Sodium Metebisulfite is Na2S2O5. There are industrial mixtures of these two compounds that some call Sodium Bisulfite. That's where some confusion comes in. Look closely on your drum for the actual compond or compouds.

**j.e.simmons** · 26-Mar-2007, 05:53

My understanding is that in photo formulas, they are interchangeable.
juan

**Michael Gudzinowicz** · 26-Mar-2007, 07:03

Most commercial grade "sodium bisulfite" is actually bisulfite with a small amount of metabisulfite and water. Sodium metabisulfite may be regarded as an anhydride or "dehydrated" form of bisulfite. The formula is Na2S2O5 and formula weight, 190.13. In solution, a molecule of water reacts with it, forming two molecules of sodium bisulfite 2 x HNaSO3. The formula weight is 104.07, but weight equivalent to one molecule of metabisulfite is twice that, or 208.14. The difference is the weight of one molecule of water, 18.

So if you really have sodium bisulfite, you would use 1.095g when 1.00 g of sodium metabisulfite is specified. Photographic grade Kodak sodium bisulfite is approximately 98.5% bisulfite, with the remainder as metabisulfite and water. It may be treated as 100% bisulfite using the conversion factor above, or in many cases, the conversion may be ignored. Usually one is concerned with the exact amount when using it as an analytical reagent, not as a developer or buffer component.

**John Cahill** · 26-Mar-2007, 15:41

Thanks All,
I shall use the "Sodium Bisulfite" as equal to sodium metabisulfite for my fixer mixes, and use Mike's figures if I need anything more critical. I suppose if I were ever to mix up Eastmen D-25 developer, I would make sure to use the proper ratio just to be on the safe side.