Stop baths - why Acetic or Citric?

pdeeh · Jan 31, 2015

Stop baths are so far as I can see universally either based on Acetic acid or Citric acid.

So ... why only those two particular acids?

Why not Sulfuric or Phosphoric or Hydrofluoric or Whateveric?

(I'm only joking about Hydrofluoric)

Will acids other than those firstmentioned cause (e.g.) staining, or "poison" the fixer or cause some other problem?

This is a question just for information and learning for me - I've no intention or felt need to try anything other than citric or acetic

Ian Grant · Jan 31, 2015

All 3 are too dangerous, also far too acidic.

Ian

bsdunek · Jan 31, 2015

Right. It only takes a very mild acid to kill the developer, and the common ones are safe, cheap and work well.
As an aside, when I discovered Hydrofluoric acid in high school chemistry, it fascinated me. Couldn't even keep it in a glass bottle. Fortunately, I never hurt myself.

pdeeh · Jan 31, 2015

Ian Grant said:
All 3 are too dangerous, also far too acidic.

Ian

I certainly understand that concentrated solutions of Sulfuric or Hydrochloric are dangerous, but it would be helpful for me to have a rather fuller explanation.

So, for instance, why would (say) a 2% solution of Sulfuric acid be "too acidic"? I'm assuming that the pH of a 2% solution of Sulfuric is significantly lower than a 2% solution of Acetic.

In terms of photochemistry, what are the expected deleterious effects ?

Will the carryover cause a problematic change in the pH of the fixer? (assuming an ordinary acidic rapic fixer, say)

Is there danger to the gelatin (in either film or paper) from a very low pH stop? Or to the base?

Or what?

Xmas · Jan 31, 2015

Hi

There is a danger to the emulsion from the change of pH from high pH of developer to low pH of acid stop, the gelatine swells.

You would not use an acid stop readily with some film types e.g. PET base or non prehardened as there are risks of damage.

The weak acid at 2% is still a 'shock'.

Film does not really need an acidic stop, where as paper does with some developers, as there can be stain if the developer and fix interact.

This is received wisdom I use two plain faucet tempered baths to minimize shock cause some of the film I use is not Ilford or Kodak.

Noel

ic-racer · Jan 31, 2015

Use whatever you want. If you want to work with very strong acids and dilute them, have at it.

Jim Jones · Jan 31, 2015

Pure acetic acid is too nasty for me.

Xmas · Jan 31, 2015

Jim Jones said:
Pure acetic acid is too nasty for me.

Pure acetic acid is indeed rather dangerous, but we put dilute acid on our national dish, of fish and chips. (french fires in USA).

You could dilute our (ie some of our) culinary vinegar dressing for stop if you wanted, and if you are using prehardened emulsions...

Noel

Jeff Bradford · Jan 31, 2015

Acetic and citric acids can be diluted to the necessary pH and work safely without eating you, your film, or your equipment.

Sulfuric, hydrochloric, hydrofluoric, fuming nitric, etc. could fulfill the pH requirement, but would burn your skin and anything else they drip, splatter, or spill on, even if you are careful. Even at very low concentrations, they will still eat things, just more slowly.

Diluted phosphoric acid may be an option. It is the "magic ingredient" in Coca Cola that eats rust and cleans jewelry. Oxalic acid might also be an interesting experiment. It would leave your equipment clean and shiny.

pdeeh · Jan 31, 2015

ic-racer said:
Use whatever you want. If you want to work with very strong acids and dilute them, have at it.

Do please read my OP again.

I have no desire to do this and said so clearly.

I am, to repeat, seeking to understand the whats and whys of using acetic/citric stops rather than sulfuric/etc, not to try and find justifications to use sulfuric or whatever.

Rudeofus · Jan 31, 2015

Sulfuric Acid is used and recommended for ECN-2 process, so pdeeh's suggestion is not nearly as crazy as it has been treated as. The big advantage of Acetic Acid is that it gives moderate pH even at high concentrations, i.e. it is a good buffer around pH 4-5, whereas Sulfuric Acid would buffer between pH 0-2, which is unnecessarily low. Citric Acid is quite inferior to Acetic Acid in terms of buffering, but it is cheap and odorless, so it's actually the most common acid used for stop bathes. Other acids, like Boric Acid and Sulfurous Acid (typically provided as Metabisulfite) buffer at too high a pH, some developers would not completely stop their action.

Ian Grant · Jan 31, 2015

There's a big difference between using Sulphuric acid as a stop-bat in what's essentially a commercial mechanised process system (ECN-2) and in a normal darkroom. The carry over would rapidly acidify the fixer.

Ian

MartinP · Jan 31, 2015

1.) Cost, both raw material and handling+preparation costs.

2.) Effectiveness, buffering ability is important as well as the pH.

3.) Availability, indirectly a result of the handling and preparation problems.

RPC · Jan 31, 2015

I have read that sulfuric acid should be used when adjusting the pH of color developers and never acetic acid. Can anyone tell me why this is.

Ian Grant · Jan 31, 2015

4.) Safety, you don't want to use an acid that is hazardous and needs more careful handling even when dilute.

Ian

railwayman3 · Jan 31, 2015

Noting that the formula for basic stop-bath published by Kodak (SB-1), with the Ilford and Agfa equivalents, are simply glacial acetic acid at a correct dilution, I imagine that the use of this and citric acid has been fully researched as the best combination of properties for quick neutralisation of the developer, minimum carry-over to the fixer, no side-effects on the image or the gelatine, cost and reasonable user-friendliness.
I'm not a trained chemist, but I would think that detailed research would be contained in published papers or patents at some time?

Xmas · Jan 31, 2015

Rudeofus said:
Sulfuric Acid is used and recommended for ECN-2 process, so pdeeh's suggestion is not nearly as crazy as it has been treated as. The big advantage of Acetic Acid is that it gives moderate pH even at high concentrations, i.e. it is a good buffer around pH 4-5, whereas Sulfuric Acid would buffer between pH 0-2, which is unnecessarily low. Citric Acid is quite inferior to Acetic Acid in terms of buffering, but it is cheap and odorless, so it's actually the most common acid used for stop bathes. Other acids, like Boric Acid and Sulfurous Acid (typically provided as Metabisulfite) buffer at too high a pH, some developers would not completely stop their action.

ECN-2 stop is about 1.0 pH and is for a pre-hardened film that can take it, maybe cause they want to run the processor fast? and they then water bath the film, before the next stage
Vinegar is about 2.0 pH and we put it on food.
Some mono film specifies 2% or less acetic acid or water to minimize risk of emulsion damage.

So the answer is potential emulsion damage - read what the film supplier says on datasheet or inside box, or be sorry.

Lots of people 'slagged off' EFKE film as being rubbish...

Rudeofus · Jan 31, 2015

Ian Grant said:
There's a big difference between using Sulphuric acid as a stop-bat in what's essentially a commercial mechanised process system (ECN-2) and in a normal darkroom. The carry over would rapidly acidify the fixer.

I would expect more effect from 2% Acetic Acid than from 0.5% Sulfuric Acid, and the latter is much lower in pH. I would expect most people to use a water rinse between stop bath and fixer anyway. I do agree with your safety concerns, though, Sulfuric Acid can be nasty.

RPC said:
I have read that sulfuric acid should be used when adjusting the pH of color developers and never acetic acid. Can anyone tell me why this is.

The people doing these adjustments are trained workers, not random dark room hobbyists. Sulfate ion in low concentrations does not interfere with photographic processes, and does not change buffering above pH 3, which is where most photographic processes operate. Many recipes suggest Acetic Acid, which is less dangerous but has the same properties as Sulfuric Acid above pH 6.

Xmas said:
ECN-2 stop is about 1.0 pH and is for a pre-hardened film that can take it, maybe cause they want to run the processor fast? and they then water bath the film, before the next stage
Vinegar is about 2.0 pH and we put it on food.
Some mono film specifies 2% or less acetic acid or water to minimize risk of emulsion damage.

So the answer is potential emulsion damage - read what the film supplier says on datasheet or inside box, or be sorry.

Add a few droplets of developer to 2% Acetic Acid and watch its pH go up to 4, and that's exactly what you get when you have from dev to stop. Sulfuric Acid would likely stay below pH 2. But you do bring up a valid point: emulsion damage. If you market a stop bath, it has to work with all emulsions ot there, and some are unhardened and more prone to softening at very low pH.

Steve Smith · Jan 31, 2015

Xmas said:
but we put dilute acid on our national dish, of fish and chips. (french fires in USA).

Nothing like!!

Steve.

Xmas · Jan 31, 2015

Rudeofus said:
Add a few droplets of developer to 2% Acetic Acid and watch its pH go up to 4, and that's exactly what you get when you have from dev to stop. Sulfuric Acid would likely stay below pH 2. But you do bring up a valid point: emulsion damage. If you market a stop bath, it has to work with all emulsions ot there, and some are unhardened and more prone to softening at very low pH.

The ECN-2 bath does have to survive film running through it at speed so holding pH is important, although the processing staff would monitor with a meter and replenish.

If you market a film and it needs a water stop then people who think an acid stop is necessary are going to be sorry... lots of us don't read the manual...

The emulsion swells on the swing of pH and/or change of temperature and may bubble or separate from backing.

Photo Engineer · Jan 31, 2015

ECN, ECP, Ektar, Portra, Tri-X and all other Kodak films are hardened the same way with the same hardener. Thus they could conceivably survive the same process of 100F with Sulfuric Acid. But, it is not recommended. The MP films are treated to a fast stop to absolutely prevent any unevenness among other things, which shows up on projection.

Citric Acid stops are not recommended for color. I have forgotten the reason but it was discussed about 2 years ago here on APUG. Sorry.

PE

Xmas · Jan 31, 2015

Steve Smith said:
Nothing like!!

Steve.

As long as we ignore foreign imports like Chana, Rogan Josh, Jalfrezi,...

Xmas · Jan 31, 2015

Photo Engineer said:
Citric Acid stops are not recommended for color. I have forgotten the reason but it was discussed about 2 years ago here on APUG.

A lemon or orange colour caste?

Truzi · Jan 31, 2015

Very interesting thread.

I was never good at chemistry (or biology). For my own edification to see if I understand this correctly - in theory, not potential practice - I will try to reiterate what I've read so far. Please correct my statements/questions:

The reason for an acid stop is to neutralize the developer. Can nearly any acid theoretically do this (at proper dilution)?

The reasons to use certain designated acids, are safety, economy, and ability to not harm the emulsion, film/paper base, equipment, etc.?

Rudeofus · Jan 31, 2015

Xmas said:
The ECN-2 bath does have to survive film running through it at speed so holding pH is important, although the processing staff would monitor with a meter and replenish.

As soon as developers go below pH 6, they are inactive, but they themselves are quite alkaline and sometimes well buffered. If you want a fast, reliable stop, you don't use strong dilute acid, but something moderate but very well buffered. Think along the lines of 10 % Acetic Acid and enough Lye to bring pH back to 4.5.

Whatever Kodak intended with a Sulfuric Acid stop bath in ECN-2, it must be something more complicated. They provide four different bleach formulas, three different versions of their prebath, but only a Sulfuric Acid based stop bath ...

@Truzi: yes, you pretty much sum it up, but forget the most important aspect: reliably stop development

Stop baths - why Acetic or Citric?

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