Everything I've been able to find online indicates that the monohydrate is the most stable form---the others tend to change into it over time in normal storage conditions. You can get it in the anhydrous form (e.g., from the Formulary), but in a cardboard washing-soda box it would pick up water molecules from the air and turn into monohydrate even if it didn't start that way.
Disclaimer: I'm repeating what I've read, so I'm only as sure as my trust in my sources (which, however, include some of the Actual Chemists here on APUG).
i suppose you could force out some of the water
by heating it up in your oven on a cookie sheet
the same way you would "convert" baking soda into
sodium carbonate.
It is the monohydrate form and is essentially technical grade and contains a small amount of sodium chloride and a smaller amount of iron. It is good enough for paper developers but not for ascorbate developers where the iron may be a problem.
A purer form can be obtained by heating Arm & Hammer Bicarbonate of Soda which is USP grade. I would recommend doing this for developers such as Xtol substitutes and for the Beutler formula.
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