Sodium Sulfite / Sodium Metabisulfite

Discussion in 'B&W: Film, Paper, Chemistry' started by Jarinb, Aug 17, 2011.

  1. Jarinb

    Jarinb Member

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    Hello:

    I'm concocting a variant of Pyrocat HD, but have no Sodium Metabisulfite in the house. Can I use plain Sodium Sulfite if I am mixing from scratch just before use? When does one prefer to use Sod. Metabisulfite as a preservaive?

    Thanks!

    Jarin
     
  2. nworth

    nworth Subscriber

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    Not directly. Sodium metabisulfite is a weak acid, often mistakenly called sodium bisulfite, NaHSO3. It would probably be possible to use sodium sulfite and an acid in the correct proportions, but you will have to figure out what those are.
     
  3. Ian Grant

    Ian Grant Subscriber

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    If you were going to use it immediately then yes you could use sulphite. Metabisulphite in an alkali solution forms sulphite anyway but it's needed in Part A to prevent oxidaion during storage.

    What's noticeable is that when Part A collapses the Metabisulphite has stopped giving off SO2 which is the anti-oxidant and broken down to sulphites. It's important to use reasonably fresh Matabisulphite rather than Bisulphite which can be a mixture of Metabisulphite & Sulphite.

    Metabisulphite is very easy to find as it's used in the food industry and winemaking etc, so anywhere selling supplies for home wine making/brewing will have it and it's inexpensive.

    Ian