Re: Van Dyck and Kallitype
If your question is whether you can substitute Fe(III) oxalate with Fe(III) sulfate and alkali metal oxalate, I think the answer is positive, although I didn't test kallitype this way. Fe(III) oxalate is more stable than complex involving sulfate, and water should be the same as long as the pH is same... (but then you probably thought about it.) Sulfate is generally not reactive in silver chemistry. (Gelatin gets affected by it, though.) But was your point somewhere else?