In aqueous solution you can convert the Sodium Metabisulfite to Sodium Sulfite by adding 2 moles of NaOH per mole of Na2S2O5.
What looks very simple on paper will be a mess when you are dealing with pound quantities of this chemical. A lot of heat will br given off and the solution may splatter when the hydroxide is added. Sodium sulfite is not that expensive. Besides you have to pay for the sodium hydroxide.
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