How much indicator in stop bath?

Donald Qualls · May 15, 2020

Okay, we all know that there's considerable controversy over whether stop bath is needed, or beneficial, or best avoided in favor of a plain water wash (for film -- for paper, there seems to be no objection).

Regardless of that argument, if you're not using a commercially formulated indicator stop bath, you either need to one-shot the bath (wasteful, as in most cases it has enough acidity reserve to stop development dozens of times -- and even then, if you're printing, you don't know where you stand during a long printing session) -- or you need to add an indicator.

It turns out bromocresol purple, the indicator used in most commercial indicator stop bath concentrates, isn't hard to come by, but harder is knowing how much you need. It clearly needs to color the solution enough that you can easily see, in a white tray or clear bottle, when it starts to turn from yellow to purple as the pH rises above 5.2 -- but how much is that, in grams per liter of working solution? I'm guessing not much, but is that milligrams or micrograms, and how many?

Andrew O'Neill · May 15, 2020

Would this work?

Kodak and others have published stop bath formula:
SB-1 for Paper
Water 750ml
Acetic acid 80% 17ml
Add water to top off to 1000ml
SB-5 for Film
Water 500ml
Acetic acid 80% 11ml
Sodium sulfite anhydrous 45g
Add water to top off to 1000ml
The most common indicator used are bromocresol purple.
The indicator:
Bromcresol purple, 10% solution, 9.25 g
Sodium hydroxide, 1.0 g
Distilled water to make 250.0 ml
Add this concentrated indicator solution to a working strength fresh stop bath solution. Continue adding until the solution changes from clear to distinctly yellow. The yellow tinted stop bath will appear clear under safelight conditions. The solution will change to purple when the stop bath is exhausted.

Donald Qualls · May 15, 2020

That should work, but it seems to boil down to "by eye". I wouldn't have thought the indicator should be in a lye solution before adding to the stop bath, though. Buy the stop bath in the bottle and the indicator's already in it, so Kodak, Ilford, and Foma (at least) seem to know how much is needed per liter of concentrate -- they apparently just aren't telling,

GBS · Sep 6, 2022

Curious, I saw the same information posted here:

Stop bath, pH and indicators

Some time ago, when I couldn't get hold of indicating stop, I decided to make my own. I bought a litre of 80% acetic acid to dilute 1+39 for working...

www.photo.net

Could you break down how to mix this part?

Bromcresol purple, 10% solution, 9.25 g

Thanks!

AgX · Sep 7, 2022

Donald Qualls said:
If you're not using a commercially formulated indicator stop bath, you either need to one-shot the bath ... -- or you need to add an indicator.

Or you need to take samples and test them. Likely the cheapest way.

Petrochemist · Sep 7, 2022

The normal approach when doing titrations is to add 1-2 drops of indicator. I don't see any reason that should change for film processing.

koraks · Sep 7, 2022

GBS said:
Could you break down how to mix this part?

I'd wonder about that, too. I think there's some issues with that formula.

For starters, it should be 'bromocresol purple', CAS 115-40-2.

To make a 10% solution, dissolve 10g of the material in 80ml 2-methoxyethanol, then top up to 100ml.
Weigh 9.25g of this solution for use in above formula.

I think there's an error in the formula. 9.25g per liter of a 10% solution would result in roughly a 0.1% solution of bromocresol. That sounds high to me, given that it's normally used as a 0.04% solution. You could probably cut back the concentration of the 10% solution to 5%, or use around 5g of the stock solution instead of 9.25g. Probably won't make all too much of a difference.

If you don't have any 2-methoxyethanol / methyl cellosolve on hand, try making a 5% solution of the bromocresol in ethanol. use 10-20g of this solution in the stop bath formula; 10-20ml would work just as well. If you can't get the 5% bromocresol to dissolve in the ethanol, try 2.5% and increase the amount of stock solution you use.

Or....

* just buy indicator stop bath. How expensive is it anyway? Is it worth the fuss of ordering bromocresol powder?
* buy ready-madde bromocresol indicator solution and do as @Petrochemist says and add it to the stop bath until the color looks distinctive enough for your taste.
* just make a stop bath without indicator (use citric acid or acetic acid; if you want to go crazy various other acids will do fine too) and replace frequently.

Donald Qualls · Sep 7, 2022

AgX said:
Or you need to take samples and test them. Likely the cheapest way.

Given the cost of acetic (hint: you can get 75% strength from Amazon for much lower cost than glacial) or citric acid, especially for printing I'd be prone to just one-shot the tray solution if I don't have an indicator. Then again, as noted in another post, indicator stop bath isn't that expensive -- cheaper than buying and diluting vinegar at the supermarket, for the same acid strength in the tray/tank.

pentaxuser · Sep 7, 2022

Andrew O'Neill said:
The yellow tinted stop bath will appear clear under safelight conditions. The solution will change to purple when the stop bath is exhausted.

Andrew, can we assume that commercial indicator stop bath will change colour at the point of exhaustion as well or at least that at that point at which there is so small a safety margin as to render any attempt to continue with the purple bath senseless/ worthless?

Perhaps from a practical point of view the more relevant question is how many prints/ films can be done in an indicator stop bath as it is this that determines whether it makes any sense even to ask my above question when you consider how cheap indicator stop bath is

pentaxuser

koraks · Sep 7, 2022

pentaxuser said:
can we assume that commercial indicator stop bath will change colour at the point of exhaustion as well

The same indicator dye will turn color at the same pH. It doesn't care about things like who it was sold to or by, what kind of label it was stuck on the bottle it spent some time in, etc.

Donald Qualls · Sep 7, 2022

Practically speaking, fiber paper kills stop bath fastest, then RC, then film. This is directly related to the amount of (alkaline) developer that carries over into the stop tray.

bernard_L · Sep 7, 2022

Probably the minimum order of bromocresol purple from a chemical supplier will last you several photographer lifetimes. And cost a pretty sum.
Even the indicator stop bath concentrate (Indicet in my case) goes a long way. I monitor its pH and extend its lifetime with household vinegar; freshly prepared is ph=2.1.

grain elevator · Sep 7, 2022

Mixing yourself doesn't seem like a bad idea to me, the advantage being precisely that you can adjust the amount of indicator dye. The Ilford stop bath I have has too little indicator dye for me to clearly see the change. Maybe a fluke batch, I've only ever bought the one bottle, otherwise use distilled vinegar or citric acid.

Michel Hardy-Vallée · Sep 7, 2022

I decided to try Anchell and Troop's buffered stop bath TS-7 a while ago. It's basically just dilute acetic acid with sodium acetate to act as a pH buffer, so that way you maintain constancy over use. It's quite stable and cheap, and also long-lasting enough.

I was tempted to add an indicator dye. They recommend maintaining the pH between 4 and 5, so that Bromocresol green is the preferred indicator (a few drops from a 0.5% alcoholic solution). However, I can't find the stuff in Canada unless you have an account with chemical companies. I decided instead to follow A&T's recommendation for calculating exhaustion, and so I track usage and make spot-checks with my electronic pH meter. I bought a tally counter for printing sessions, and click every time I dip something.

I do the same for fixer, sticking to Ilford's recommendations and using FT-1 solution to make spot checks when I'm close to expiry date or to maximum capacity.

In both cases for stop and fix, I found the guidelines to be reliable and easy to follow.

Donald Qualls · Sep 7, 2022

bernard_L said:
Probably the minimum order of bromocresol purple from a chemical supplier will last you several photographer lifetimes. And cost a pretty sum.

I bought a couple ounces of it in solution through Amazon for under $20 -- might have been as little as $10 or so. Haven't used it; my Indicator Stop Bath turned up about the time it arrived.

AgX · Sep 7, 2022

AgX said:
Or you need to take samples and test them. Likely the cheapest way.

I mean, you only need a few millimeters of indicator paper strip, held by a fine tongs.

gone · Sep 7, 2022

I like citric acid/vinegar for stop bath w/ prints, only because it's one less thing to store and have to ck up on. Those big plastic bottles of vinegar from Walmart are very inexpensive.

Donald Qualls · Sep 8, 2022

momus said:
Those big plastic bottles of vinegar from Walmart are very inexpensive.

Sure. Until you figure out how much of the same strength you can make from a pint bottle of 75% acetic acid. That's a 37+1 dilution to get the 2% strength commonly used for stop bath -- or put another way, that little bottle makes a bit more than 4.5 gallons of stop bath (for about $12).

koraks · Sep 8, 2022

Donald Qualls said:
that little bottle makes a bit more than 4.5 gallons of stop bath (for about $12).

The local equivalent here of Wallmart sells 5l jugs of cleaning vinegar; this is acetic acid at a concentration of around 8%. So 1+3 makes a 2% solution. One of those jugs costs € 3.29 currently and hence makes roughly 4.5 ~ 5 gallons (20 liters after all) of stop bath at a grand total of € 3.29.
I generally mix it a little weaker and use one shot. For film 0.5% works totally fine; for paper a little stronger is nice because there's more carry-over.

Cost really isn't a factor whichever way you turn the 'problem'.

Of course, we could have a heated debate on the pros and cons of keeping a jug of cleaning vinegar at hand; on the one hand, it's much more voluminous than a more concentrated solution, then again, the fumes aren't half as bad whenever you open it, and it's convenient for cleaning as well...oh, all these considerations...

Doremus Scudder · Sep 12, 2022

Let's see... Kodak Indicator Stop Bath at B&H is $6.49 for enough to make 8 gallons. Seems pretty reasonable to me, especially if you use it at a bit less than the recommended dilution.

Also, before indicator was common, practitioners simply noted if the stop bath quickly removed the slippery feeling from fingers first dipped into the developer. If it took more than a few seconds, the stop bath was replaced.

Doremus

How much indicator in stop bath?

Donald Qualls

Andrew O'Neill

Moderator

Donald Qualls

GBS

Stop bath, pH and indicators

AgX

Petrochemist

koraks

Moderator

Donald Qualls

pentaxuser

koraks

Moderator

Donald Qualls

bernard_L

grain elevator

Michel Hardy-Vallée

Membership Council

Donald Qualls

AgX

gone

Donald Qualls

koraks

Moderator

Doremus Scudder

How much indicator in stop bath?

Moderator

Moderator

Moderator

Membership Council

Moderator

Privacy & Transparency

Privacy & Transparency