Sodium metabisulfite = sodium bisulfite

[haryanto]

i hard to find sodium bisulfite, is there someone knew that's sodium metabisulfite is sodium bisulfite, many thanks

[gainer]

The Darkroom Cookbook says one can be substituted for the other weight by weight. My issue of the CRC Handbook of Chemistry and Physics does not show a metabisulfite that I can find. It only shows Sodium Hydrogen Sulfite, and no common name. My 81 year old brain doesn't remember why this is so. Most cases of it's use are not so critical, and IIRC there is some reason why it all tends to be the same thing soon after the atmosphere gets to it anyway.

[haryanto]

thanks , your 81 year old brain still great, to share lot of knowledge here, many thanks for that, i learn a lot from you

[jochen]

Hello harryanto,
sodium bisulfite is an old name for sodiumhydrogensulfite NaHSO3 and does not exist in solid form. In the solid form the formula is Na2S2O5 which is Sodium disulfite and this has the old name sodium metabisulfite. So if in your formula sodiumbisulfite (in solid form) is required, take the metabisulfite. In aqueous solution (this form is readily commercially available, in German the name is "Bisulfitlauge") there exists the NaHSO3. In old formulae you will more often find the potassium salt K2S2O5.

[Ian Grant]

Jochen, bisulphite & disulphite are the same, the bi- or di- just means two. Similarly Sodium Bichromate and Sodiun Dichromate are identical.

Sodium Bisulphite/Disulphite is not the same as Sodium Metabisulphite, the Metabisulphite is a far better anti-oxidant so is preferred as a preservative in developers. In fact it's also used to preserve peeled potatoes and stop them going brown, and used in the wine industry to kill wild yeasts

However in practice once dissolved Metabisulphite decomposes to the Bisulphite. The Bisulphite is rarely sold in solid form now, but I do have some on my chemical shelves.

Many formulae require Metabisulphite because it gives off SO2 in solution which forms a weak acid.

Ian

[haryanto]

thanks Jochen
Ian how to decompose it metabisulfite to bisulfite?

[Ian Grant]

It decomposes in water, most people just use the same weight of metabisulphite.

Ian

[jochen]

Hello Ian,
you are absolutely right "bi"means the same as "di". But the substance you have in the shelf in solid form under the name "bisulphite" is Na2S2O5 and that is the same as "metabisulfite".

[Ian Grant]

No Jochen, I have Sodium Bisulphite, as well as Sodium & Potassium Metabisulphite. I'm well aware of the chemical differences.

Usually what is sold as solid Bisulphite isn't pure and contains some Metabisulphite. but fresh Metabisulphite gives off far more SO2 in solution and has better preservative properties.

Ian

[Ian Grant]

To clarify, a manufacturers MSDS sheet states:

Sodium Metabisulphite

Composition/Information on Ingredients
Ingredient CAS No Percent Hazardous
Sodium Metabisulfite 7681-57-4 100% Yes
Sodium Bisulfite 7631-90-5 < 0.001% No

while:

Sodium Bisulphite

Composition/Information on Ingredients
Ingredient CAS No Percent Hazardous
Sodium Bisulfite 7631-90-5 58 - 99% Yes
Sodium Metabisulfite 7681-57-4 1 - 42% Yes

So if a Formula specifies Metabisulphite then you shouldn't use Bisulphite.

Ian