While having similar characteristics, Selectol and D-52 are not identical.
Selectol is formulated for long term storage as a powder or liquid and
contains other ingredients.
>Sodium Sulfite, anhydrous 21.2 grams (not 21.3, nor 21.1)
The "Cookbook" is just being picky, and this is an old formulation as the
compound of choice today is sodium sulfite monohydrate. The anhydrous,
after a little time in storage, absorbs water, so is of indeterminate
composition (which is why I say the book asks for more precision than can
be obtained without testing). The monohydrate, on the other hand, is very
stable, and most of the carbonate being sold today is of this composition.
(If anyone is interested - after all, this is the "alternate" list - I can
furnish formulas.)
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net