Film Stop Bath

[Gerald C Koch]

It's not the pH that is important as long as it is below 7 and not acid enough to chew up the emulsion or paper. A single drop of concentrated sulfuric acid to a liter of water would produce a low pH but would be worthless as a stop bath because the solution would have little capacity to neutralize alkalis. This is why we use a moderate amount of a weak acid instead of a small amount of a strong acid for stop baths. Buffer capacity is the single most misunderstood point in photographic chemistry.

[Photo Engineer]

Gerald, it would work one time!

But in any event, Grant Haist devotes quite a bit of space to stop baths, pH and buffer capacity. I use 1% - 2% Acetic Acid quite successfully and do it one shot as Acetic Acid is so inexpensive.

PE

[cmo]

Boric acid at that pH is well within the acceptable range for a stop. I have never tried it, nor do I have any formulas using it. So, other than being in the right pH range, it is all I can say.

PE

One thing that speak against boric acid: it's reprotoxic.

[RalphLambrecht]

Boric acid at that pH is well within the acceptable range for a stop. I have never tried it, nor do I have any formulas using it. So, other than being in the right pH range, it is all I can say.

PE

I guess, I don't understand this part of your post:

Any acid solution should not be lower than about 3.0 nor about about 5.0 for a good stop.

[Photo Engineer]

If the pH is > 5.0 then it does not stop well. If it is below abaout 3.0 it will swell gelatin too much or destroy the hardness.

PE

[RalphLambrecht]

Gerald

That makes sense. Thanks for the clarification.