Dumb question about “pinch” of Sulfite mixing convention
Metol doesn’t dissolve in Sodium Sulfite solutions, which is why the Sulfite goes in after the Metol when scratch-mixing. But a common convention is to dissolve a small amount (“pinch”) of the Sulfite before the Metol, which is supposed to scavenge dissolved Oxygen and therefore result in better preservation of the Metol while it is being dissolved.
Haist says nothing about the pinch of Sulfite, using two examples, D-76 and D-72.
So, why does the pinch of Sulfite (in say 750ml water) not cause any precipitation of Metol? The two possible explanations I can come up with are:
1)The concentration of the Sulfite solution must be significantly higher to cause precipitation of Metol
2)The entire pinch of Sodium Sulfite combines with dissolved Oxygen in the water to form Sodium Sulfate. My understanding (again from Haist) is this reaction is one of the ways in which Sodium Sulfite acts as a preservative in developers. And I’m assuming that Sulfate in solution doesn’t cause precipitation of Metol. That’s just a guess though.