Kodak indicator stop bath dilution

[Gerald C Koch]

Its easy to calculate the pH of an acetic acid solution by using the following equation.

[H⁺] [Ac^-] = K_a = 1.8 x 10^-5

For a dilute acid and not a buffer [H⁺] = [Ac^-]

The pH is the log₁₀[H⁺].

An indicator cannot be used to determine a pH if the solution is outside of its range. So if bromcresol purple yields a yellow color we only know that the pH is equal or less than 5.2.

If the pH of a stop bath is 2.8 and we double its volume with water the pH increases by the log₁₀2 = 0.3. So the resultant pH is 3.1. The capacity of such a bath is half what the original bath had.

[Michael R 1974]

What happens to the purple indicator dye at a pH somewhere between 5.2 and 6.8?

So it seems dilution in this case is not going to raise the pH enough unless dilution is rather extreme, which could render the resultant stop bath useless from a capacity perspective. It might not even work for one-shot use.

[Gerald C Koch]

What happens to the purple indicator dye at a pH somewhere between 5.2 and 6.8?

Obviously the color changes from yellow to purple. At a pH of say 5.9 what you might see would be a tan or muddy color perhaps even a gray color. The color during the transition depends on the indicator. For this reason we want indicators that change color over as small a range as possible. Typically in the lab the color within the range is not very useful. You would have to have a color chart worked out using a set of buffers that are within the range. This is sometime done but not all that frequently.

If you were titrating acetic acid with sodium hydroxide you would note the end point when the solution first turns purple.

[Michael R 1974]

Thanks. I wish I remembered more of my Chemistry. This is pretty sad.

So I guess if in the end I wanted a normal capacity stop bath with a pH around 4 to 4.5 without resorting to scratch mixing I'm pretty much out of luck.

[Gerald C Koch]

You could use a solution of boric acid which would get you in the range of a pH of 4 to 5. pH 5.1 (1.0% solution); 3.7 (4.7% solution) Its cheap and readily available. Powdered boric acid is hard to dissolve in cold water as water doesn't wet the powder and it tends to float. Use the crystalline form or the powder with hot water.

[Michael R 1974]

Interesting option. What about a Sodium Bisulfite stop bath?

[Gerald C Koch]

Solutions of sodium bisulfite will be in the pH range 4.8 to 5.2 and this is for a 25% solution. This chemical releases sulfure dioxide which is very irritating and can cause asthma. For this reason I personally avoid it.

[ParkerSmithPhoto]

To rephrase Michael's question slightly ...

If one dilutes Kodak Indicator Stop Bath more than recommended, does the indicator continue to be a reliable test for appropriate activity - i.e. is the stop bath still good if it hasn't turned blue?

Yes. Even after the purple indicator shows, you can get a little developer on your fingers, dip them in the stop, rub them together, and in a few seconds that "slippery" base feeling will go away, which shows that the stop is still working.

I dilute mine to keep the smell down. Once it turns, I might run a few more sheets through and give it the finger test.

[Gerald C Koch]

I dilute mine to keep the smell down. Once it turns, I might run a few more sheets through and give it the finger test.

Once the SB turns purple the bath is at a pH of 6.8 or higher. Pure water has a pH of 7. When the stop bath is purple it is no longer a stop bath but a water rinse. If you wouldn't use a water rinse in your processing then you shouldn't do this. What you describe is a false economy.

[Michael R 1974]

I've never used it until it even had a chance to change from its initial bright color. The odor is a poor reason for not following Kodak's dilution instructions. There are odorless stop baths if one finds the odor objectionable.

Gerald, I re-read the chapter on stop baths in Haist. Some interesting things I had forgotten and a good refresher on the importance not only of pH but total acidity. In fact the data from Henn/Crabtree would probably be quite surprising to most photographers, particularly the effect of variations in total acidity on the amount of time needed to stop development, not just capacity. I'm also wondering why Kodak doesn't put Bromocresol Green in rather than Purple.

On the other hand some of the information found in Haist regarding acceptable stop bath pH ranges, effects of total acidity etc may be out of date in the context of current films.

Bill Troop's "ideal" stop bath appears in line with Haist - a buffered acetic acid stop bath with a pH around 4.5. Seems like that would work fine for just about any kind of developer with an acidic or neutral fixer.

I assume a citric acid stop bath is ok as long as there's no hardener in the fixer?