Film developer chemistry part 2

[alanrockwood]

My next question overlaps partially with the first, but there may be enough unique aspects to the question to justify a separate discussion.

Here is the question: What role does pH play in film development?

To help frame the question, one might divide it into three categories: 1) What does pH do to the developer? 2) What does pH do to the thing being developed (e.g. silver halide) ? 3) How does pH affect the interaction between developer and the thing being developed?

I realize that these three categories are not mutually exclusive, but I think they are different enough to at least call them out. Also, there may be some things that do not fit neatly into the three categories. For example, if pH affects the robustness of the gelatin this might be relevant to the discussion, even if it doesn't fit neatly into the three categories.

Some specifics in the discussion would be highly useful. For example, a comment like "high pH activates the developer" may be true, but it is not very enlightening. More enlightening would be some comments on why this is so.

Thanks for your patience in helping me with my explorations on film developer chemistry, and also thanks for the references that some of you supplied in the first thread in this series.

 

[dE fENDER]

Hydroquinone example. Equations here:
https://ru.wikipedia.org/w/index.ph....82.D0.BE.D0.B3.D1.80.D0.B0.D1.84.D0.B8.D0.B8

First one is total process equation. Second - hydroquinone dissociation.

1) High pH (9.5 in the case of hydroquinone) starts dissoсiation reaction. Undissocitated hydroquinone will not go into the reaction described of the third equation;
2) Nothing important;
3) High pH moves equilibrium of the second equation to the right. Lower pH moves it to the left. Note that quantity of the HBr increasing during the developing and it shifts the equilibrium back to the left.