How do I make citric acid stop bath?

dcy

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This should be incredibly easy, but I'm getting conflicting information. Digging through the Photrio archives, I've seen:

1) One person who uses 20 g/L.

2) One person who uses 4 tsp/L --- so about the same as (1).

3) An old post by Photo Engineer who said that the pH should be around 4-5. I just grabbed a beaker and diluted 1/8 tsp into 500 mL of tap water and that already gave me a pH slightly below 4. --- If correct, this would translate to around 1 g/L.

4) The MSDS of the Ilford "IlfoStop Citric Acid Stop Bath" says that it's 10-30% citric acid. Let's say it's 20%. The instructions say to dilute it 1+19. So the final solution would be 1% citric acid --- so about the same as (3).


In other words, I have four data points that cluster around two values --- 20 g/L vs 1 g/L --- that differ by more than an order of magnitude.

I don't know enough chemistry to calculate the pH of citric acid solutions from first principles.

Help?
 

Paul Howell

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From Chatbot

A common formula for a citric acid stop bath involves using 15 grams of citric acid powder per liter of water. Alternatively, you can use 1 teaspoon of citric acid per liter of water. This is a simple and effective solution for halting the development of photographic film or paper.
 

MattKing

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I'll let the chemists here speak with better precision then I'm able to.
But I can tell you that you don't need high precision.
Over a fairly wide range, a variation in the concentration won't change how well the stop bath works, it will only change how much it will work - i.e. the capacity in prints per litre you will end up with.
IlfoStop and other commercial concoctions will have the advantage of the included Indicator, which adds a visual indicator that reveals when the working strength solution is exhausted.
Photo Engineer's approach will also inevitably reflect a commercial element, because it was that environment he worked in, but Ron (RIP) would have been the first to say that you should use what is practical and works for you. Ron would also have had a good idea how much buffering, if any, would have been available to help keep the pH sufficiently stable.
My mixing instructions are in the bin downstairs with my printing stuff - written in felt pen on the bottle of citric acid I keep refilling - but I think it is about 2 tsp per litre.
 

Paul Howell

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BTW, I have a few packets of critic acid stop bath that came with MCM developer from Photographers Formulary and it is 15grams for 1Lt.
 

Yezishu

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According to Haist, a stop bath with a pH of 4–5 prevents overly acidic residues that could decompose some fixers or cause carbonate in the developer release CO₂, resulting in bubbles in the emulsion. And solutions of stronger acids is suitable for high activity (high alkalinity) developer.
 

gordonrgw

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Posted yesterday on YT by John Finch
the second half covers citric acid stop.
 
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koraks

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dcy said:
This should be incredibly easy
Click to expand...

It is. Take some water. Add some citric acid.
How much? Doesn't really matter. A tablespoon to a liter, some teaspoons to a jug - whatever.

For film, use it once. It can be very dilute.
For paper, use it until the paper stops feeling rubbery on the emulsion side after a few seconds in the stop bath. At that point, replace the stop bath or add some new citric acid.

The concentration of citric acid in a stop bath is absolutely non-critical.
 
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dcy

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Thanks!

The post from PE says that there is "some possible danger taking place below 3 if the emulsion is not hardened sufficiently". I was trying to avoid getting a pH below 3. But nobody else seems to be worried about that.
 

koraks

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All modern camera films intended for regular still photography will withstand brief exposure to extremely acidic conditions without any problem.
With citric acid it's hard to get far below pH=3.0 anyway.
 

lamerko

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Commercial kits contain about 1.5% citric acid. I don't think it's particularly important to follow it exactly. But anyway, indicator kits are cheap enough...
 

koraks

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To add to the above - there's one particular reason why some may want to avoid too acidic a stop bath. It has nothing to do with the film, and it's actually not even very relevant when processing (small amounts of) film and more relevant to paper/print processing. When using a developer that contains a lot of sulfite (many/most print developers), a highly acidic stop bath (pH<4.0) will result in the release of some sulfur dioxide gas as a result of carryover of developer into the stop bath. The amounts involved are generally far too small to constitute any danger, even in a poorly ventilated darkroom. However, sulfur dioxide has a rather pungent smell easily irritates the mucous membranes, so many people find it uncomfortable. The cause is in the pH-dependent equilibrium of sulfite:


As you can see, this effect is virtually absent at pH>4.0, it's only very slight and may go unnoticed at 3.0<pH<4.0, but it tends to be quite noticeable at pH<3.0.
If this bothers you, you could consider using a buffered stop bath. See here: https://www.photrio.com/forum/threads/buffered-citric-acid-stop-bath.201215/
Again, it's only relevant really for processing prints in trays.
 

Milpool

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dcy said:
Thanks!

The post from PE says that there is "some possible danger taking place below 3 if the emulsion is not hardened sufficiently". I was trying to avoid getting a pH below 3. But nobody else seems to be worried about that.
Click to expand...

It’s not a problem.

If you are worried, just get Sprint’s stop bath which is an off the shelf “ideal” stop bath (buffered acetic acid / sodium acetate) pH~4 with high capacity, low odor.
 
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dcy

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Milpool said:
It’s not a problem.

If you are worried, just get Sprint’s stop bath which is an off the shelf “ideal” stop bath (buffered acetic acid / sodium acetate) pH~4 with high capacity, low odor.
Click to expand...

Oh, I think this thread has put to rest any concern I had over making the bath too acidic.

Out of curiosity, I did look up Sprint's stop bath... It's vanilla scented. Honestly, the smell of acetic acid doesn't bother me, but the idea of having my darkroom smell like vanilla amused me.
 
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