Dear Rudeofus, I have a question about the order of dissolving the ingredients. Your formula does contain sodium sulfite as a preservative and antioxidant, but also as a buffer.
I wonder if it would not be more appropriate to add sodium sulfite as the first ingredient to the solution.
Why?
1. Sodium thiosulfate oxidizes quickly and its fixing power decreases. Water contains large amounts of oxygen and if we dissolve sodium thiosulfate as the first ingredient, the oxygen from the water binds with sodium thiosulfate.
If we add an antioxidant to the water as the first ingredient, then this antioxidant will consume the oxygen from the water, stabilizing the environment and protecting sodium thiosulfate from oxidation.
2. In chemical practice (laboratory and photographic), the rule is often used:
"First the stabilizing component of the environment, then the active reagent"
- to achieve the correct pH,
- to prevent decomposition,
- to avoid undesirable side reactions.
This certainly works in the case of fixers based on sodium thiosulfate with the addition of sodium sulfite as an alkalizing buffer and antioxidant. However, everywhere in recipes I see sodium thiosulfate always as the first on the list, i.e. the first in the order to be dissolved.
3. Sodium thiosulfate, when dissolved in water, cools the solution strongly. Often the temperature of the solution drops dramatically and makes it difficult to dissolve the subsequent components (especially if we dissolve in cold water). Therefore, it is better to dissolve the "smaller" components first, which require less mixing, and then add sodium thiosulfate in batches.
Do I understand correctly? Forgive me, I don't have much experience, but I read a lot and then experiment with chemistry.
Kind regards,
Marcin