Citric Acid Stop Bath

[thefizz]

Having always used an acetic acid stop bath, I'm thinking of switching to a citric acid stop. Will this cause any problems or necessitate any changes to my workflow using fibre paper?

 

[RalphLambrecht]

It's unlike to cause issues with paper (well, you won't see it), but it can cause issues with film. The reason is that citric acid has a much lower pH than acetic acid (out of memory 2.5 vs 4.0). This can cause an issue with developers high in sodium carbonate due to carbon dioxide gas bubbles forming, which may leave tiny pinholes in the emulsion. I did not experience these issues at citric acid solutions of 1.5% or lower in combination with D76 or ID11 1+1.

 

[Photo Engineer]

If you stick FB paper into an acetic acid stop directly from Dektol, you will hear fizzing and popping of Carbon Dioxide being released. It causes no problem for me with either film or paper. The speed of the release of the gas is another matter. In the case of low pH Citric Acid baths, it may be violent enough. IDK. But, I have never heard of a problem with stop baths. I have been warned away from using Citric Acid stops with color materials. I have forgotten the reason over the years.

PE

 

[Ian Grant]

Citric acid has been found to aid the efficiency of fixing according to some research by Fuji who have used it in some fixers, as have Kodak. A benefit can be lower residual silver levels.

A downside of Citric acid is that with some older fixer formulae it can cause cloudiness and sulphur precipitation, you'd need a lot of stopbath carry over, at it's worst causing a very fine scum on the surface of a glossy print. This does not occur with modern commercial B&W fixers.

Ian

 

[Martin Aislabie]

If you stick FB paper into an acetic acid stop directly from Dektol, you will hear fizzing and popping of Carbon Dioxide being released. It causes no problem for me with either film or paper. The speed of the release of the gas is another matter. In the case of low pH Citric Acid baths, it may be violent enough. IDK. But, I have never heard of a problem with stop baths. I have been warned away from using Citric Acid stops with color materials. I have forgotten the reason over the years.

PE

I have often wondered why my FB Paper "squealed" in the Stop Bath

Thanks

Martin

 

[Ole]

citric acid will quickly become "buffered" but carryover from the developer. Which in fact is a good thing, as it moves the pH to a more appropriate level.

Acetic acid is "hard" - it has one acid group, and that's it. Citric acid has two acid groups, and gives a more gradual deterioration of acidity.

Note: These are darkroom results, not science.

On another note, I have never liked the smell of acetic acid - or even vinegar. So my "results" may be biased: If I do need an acidic stop, I will only use citric acid.

 

[Ian Grant]

Ole, the best Citric acid stop baths are buffered anyway as they use the Citric acid/Sodium Citrate buffer.

Ian

 

[Photo Engineer]

The best Acetic Acid stop baths are buffered Acetic Acid and Sodium Acetate (with indicator), but most people prefer to just use the acetic acid as-is due to the higher pH obtainable right off. "Hardness" or lack thereof has nothing to do with it. After all, Sulfuric Acid has two acid groups (if you look at it that way) and it is pretty "hard".

PE

 

[olleorama]

I used household (i.e. from the grocery store) citric acid as stop bath for B&W papers for a while. It developed some slimy residue in it. Actual gobs floating around. Looked like small amounts of egg white (or maybe more like semen) floating around. It didn't really effect the processing though, but it looked disgusting.

Does anybody know what that could have been? Sulphur precipitation?

 

[Photo Engineer]

Not Sulfur. It can precipitate some other salts though. IIRC, it can also sludge up in a fixer. There are more details in other posts here.

PE

 

[Ian Grant]

Any stop bath or fixer can get mould growths as it gets neutralised, so can dev if left too long in an open tray. (The dev will have oxidised anyway).

Yellow Sulphur will precipitate from even new (past it's sell by date) fixer over time, and also dilute fixer but I don't think this is your problem

Ian

 

[hpulley]

I use Ilford indicator Iflostop. Have noticed a chalky film on some glossy fiber prints on some occasions but thought that was from the Heico Permawash.

 

[Gerald C Koch]

Acetic acid is "hard" - it has one acid group, and that's it. Citric acid has two acid groups, and gives a more gradual deterioration of acidity.

Note: These are darkroom results, not science.

Actually citric acid has 3 carboxylic (organic acid) groups not 2. It is slightly more acidic than acetic acid.

I do not understand your use of the word "hard" in relation to acids. Some acids are weak and others are strong. Most organic acids are considered to be weak to moderate acids. Certainly not in the class with an inorganic acid such as sulfuric.

Citric acid stop baths should not be saved for more than a day or two due to the formation of mold which can adhere to emulsions. When stored in a refrigerator this can be increased to about a week.

Another odorless stopbath for film and papers would be a 3% solution of boric acid.

 

[RalphLambrecht]

... Another odorless stopbath for film and papers would be a 3% solution of boric acid.

Gerald, I'm concerned that the pH of 3% boric acid is too low (4.8).
A 1.5% solution of acetic and citric acid has pH of 2.7 and 2.1, respectively.

That's quite a difference.

 

[Smudger]

I use a hybrid stop : citric acid with a capful of Kodak Indicator Stop added,per 2 liters.
This seems to keep the solution clear for months,no growths or slime.
If I mix just citric acid,it gets nasty after about a week.

 

[Gerald C Koch]

Gerald, I'm concerned that the pH of 3% boric acid is too low (4.8).
A 1.5% solution of acetic and citric acid has pH of 2.7 and 2.1, respectively.

That's quite a difference.

That interesting, my reference gives 3.8 as the pH. Boric acid has an advantage in that it forms a buffer with the alkali introduced by the developer and resists further pH increase. Too low a pH can also cause excessive swelling of the emulsion as the isoelectric point is pH = 5.

 

[Photo Engineer]

The isoelectric point of bone gelatin, used in most products today, is 4.5. The isoelectric point of Pig gelatin used in a few products even today is about 9.5. I would have to look up the exact figure.

Minimum swell is at the isoelectric point.

We settled the pH and IE points in a huge previous thread on this subject IIRC.

PE

 

[RalphLambrecht]

That interesting, my reference gives 3.8 as the pH. Boric acid has an advantage in that it forms a buffer with the alkali introduced by the developer and resists further pH increase. Too low a pH can also cause excessive swelling of the emulsion as the isoelectric point is pH = 5.

Gerald

This was my source of information. Maybe it's wrong, or I used it wrong.

http://www.sensorex.com/support/more/ph_calculator

 

[Gerald C Koch]

Gerald

This was my source of information. Maybe it's wrong, or I used it wrong.

http://www.sensorex.com/support/more/ph_calculator

Ralph,

I don't think you are necessariiy wrong. I was looking at several sites and they give pH values that vary considerably for the same concentration. Why this is so I don't know.

 

[Gerald C Koch]

The pH of a stop bath is not as important as its buffer capacity. While a single drop of a strong acid like sulfuric acid in a liter of water will provide a low pH, the resulting bath might be good for one print and no more. This is because such a bath has no buffer capacity. Remember the definition of a buffer as a solution that resists the addition of acid or bases with only a small change in pH. A dilute solution of acetic acid, citric acid or boric acid provides such a buffer. Considering the possibility of emulsion swelling the best stop bath for film may be a well buffered solution with a pH in the range of 3.5 to 5.5.

Some people are sensitive to the odor of sulfur dioxide but a 3% solution of sodium bisulfite can also make an effective stop bath.

The only time that an indicator stop bath is effective is when there is the possibility of exhausting it in a single darkroom session. It is never a good idea to save used stop and attempt to use it again. It is a false economy as dust, lint and other crud will accumulate in it and cause problems.

 

[RalphLambrecht]

Gerald

If you have access to Haist, please look it up. I won't get to my books until next week. But, I think there is a paragraph on boric acid in his chapter on stop baths.

 

[Photo Engineer]

I have access to Haist, just down the road about 1 mile. Oh, different access! I see. I have to look that stuff up all over again unless some of these people look up that previous, long thread.

When I get a chance, I'll look up and post a bit of it.

BTW, AFAIK, Grant is still in Okemos.

PE

 

[Gerald C Koch]

Ralph,

Sorry I don't have access to Haist. My recomndation was based on a post by Ryuji Suzuki. Looking at my notes I find that the actual concentration that he mentioned was 5% boric acid. He was concerned about spilling acetic acid in a small confined space. Since boric acid is an odorless powder it would be easier to clean up. There are no shipping restrictions on boric acid as there are on acetic acid. The chemistry store lists 7 pounds for $13.44 plus shipping. Its toxicity is said to be about the same as that for table salt.

 

[Photo Engineer]

I've checked Haist again and he lists just about every acidic mixture possible and gives the best range for pH. The pH range is about 4 - 5 with some possible danger taking place below 3 if the emulsion is not hardened sufficiently, and some possible danger of nonuniformity about pH 5.5 due to uneven development and stopping, particularly with active developers. This is a rough synopsis of about 10 pages.

One compound he does not mention is Boric Acid, but he does mention the use of Borax + Acetic Acid.

PE

 

[RalphLambrecht]

... My recomndation was based on a post by Ryuji Suzuki. Looking at my notes I find that the actual concentration that he mentioned was 5% boric acid. ...

The pH does not drop significantly from 3 to 5% (4.75 to 4.64 pH). I think this is too high for a stop bath. I'll get to my books next week and can look up Haist's suggestion again.