Stop baths - why Acetic or Citric?

[Gerald C Koch]

Phosphoric acid is a weak acid and is used as an acidulant in soft drinks. It cannot be used as a stopbath because it would precipitate any aluminum salt from hardening fixers.

There are several other options for stopbaths such as boric acid or sodium bisulfite. However acetic and citric are readily available and convenient.

 

[Truzi]

Okay, one of my patented stupid questions as a follow-up.

I have bottles of Blu-Lite cleaner (for toilets, etc.) It has 20.50% Phosphoric Acid, 0.25% Didecyl Dimethyl Ammonium Chloride, and 79.25% undisclosed "other" ingredients.

No, I am NOT going to use it on film or paper, plus the other ingredients may harm something. Besides, if I used this as stop, I'd have to use my glacial acetic acid to clean the bathroom (though it does do wonders on soap scum).

Anyway, in theory, at the right dilution, this particular toilet cleaner could work as a stop, correct? (Potential damage and affect on fixers aside.)

 

[Xmas]

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

The reason for using acid stop is some developers can stain prints if they interact with fixer.

The people who make acid stop would like you to use it with film cause it increases their sales.

Two tempered water baths and lots of agitation will remove most of the developer from film with less stress to the emulsion. But it takes a whole lot longer.

The cine people needed their film 'rushes' at end of the days shoot to see if they had good takes. They would have thousand of feet of film to process, especially when on location, they ran their processing machines as fast as possible.

 

[Photo Engineer]

Truzi, it depends on what the other 79% of that cleaner is and whether it might leave an oily residue which is NOT good for film.

Xmas, you forgot that as format size goes up, stop baths increase image uniformity, and with the right mix a stop can increase the life of a fixer. There are other reasons as well, but if you think a stop is useless, you have not considered the problem from the "other side", as an engineer who must be aware of all that can go wrong.

PE

 

[Sirius Glass]

Only a mild acid is needed to stop the development. Why kill the negative completely when we have digisnappers and fauxographers to do that for us?

 

[Steve Smith]

Okay, one of my patented stupid questions as a follow-up.

I have bottles of Blu-Lite cleaner (for toilets, etc.) It has 20.50% Phosphoric Acid, 0.25% Didecyl Dimethyl Ammonium Chloride, and 79.25% undisclosed "other" ingredients.

You might be surprised at how similar some developers are to kitchen/bathroom cleaning substances.


Steve.

 

[RattyMouse]

Okay, one of my patented stupid questions as a follow-up.

I have bottles of Blu-Lite cleaner (for toilets, etc.) It has 20.50% Phosphoric Acid, 0.25% Didecyl Dimethyl Ammonium Chloride, and 79.25% undisclosed "other" ingredients.

No, I am NOT going to use it on film or paper, plus the other ingredients may harm something. Besides, if I used this as stop, I'd have to use my glacial acetic acid to clean the bathroom (though it does do wonders on soap scum).

Anyway, in theory, at the right dilution, this particular toilet cleaner could work as a stop, correct? (Potential damage and affect on fixers aside.)

Didecyl Dimethyl Ammonium Chloride is a VERY reactive cationic surfactant. This has the ability to react with and usually precipitate out of solution most anionic compounds.

 

[Athiril]

In my experience, if you dilute a colour developer without sufficiently lowering the pH you get fog from the anti-oxidant being diluted below a useful working strength and oxidised developer being free to do it's thing. Similar to if you mix up a colour developer without any anti-oxidant.

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

Sulphuric is used as a stop in ECN-2 iirc.

Kodak SB-14, is 50mL of Sulphuric Acid (7.0N) in a litre of water, or a bit under 2% iirc. pH range 0.8 to 1.5 according to the docs (as it gets used and replenished etc).

All 3 are too dangerous, also far too acidic.

Ian

It's about dilution.

 

[cmacd123]

Acetic is also a typical constituent of most fixers, so the idea may be to avoid upsetting the fixer too much, while citric acid is substituted by some users to avoid the Vinegar smell of acetic acid. there are probably many acids that would "Work" But for example fixer engineers are expecting those two in the carry-over so they are ready for them.

Citric is also a "food grade" acid so is naturally low on the toxicity scale. and acetic is often eaten with fried Potatoes and related foods so likewise is considered of low toxicity.

 

[Andrew O'Neill]

I use citric acid. I can source it easily from a DIY wine store near my house. I also use it for alt printing. Whenever I used acetic acid, I would develop a craving for fish n chips. I get no such craving with citric acid!

 

[ic-racer]

Do please read my OP again.

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

...and neither does any one else. Thus the answer to your question.

 

[NedL]

I use citric acid. I can source it easily from a DIY wine store near my house. I also use it for alt printing. Whenever I used acetic acid, I would develop a craving for fish n chips. I get no such craving with citric acid!

I use vinegar diluted 1+9. Now... I've discovered that the brand makes a difference. My wife bought me a bottle of white distilled Heinz vinegar, and it has the sharp vinegar smell. The bottle before that was a store brand ( sunnyside? Not sure ) and it had this really nice fruity smell. When I open my bottle of stop and no longer smell the vinegar, I know it's time to mix a new batch, which happened today. So this afternoon I was thinking "got to get the sunnywhatever brand next time"!

 

[Photo Engineer]

The fruity smell may be apple or wine vinegar. Do NOT use them. Use white or distilled vinegar.

PE

 

[Photo Engineer]

It seems to me that armchair chemists may do more harm to analog than digital will. Be careful out there people.

PE

 

[Arklatexian]

When I developed my first rolls of film in the late 1930s, I used diluted vinegar along with the MQ developer and sodium thiosulfate that came in glass tubes. Tri-chem packs had some kind of acetic acid in their packages. I have been using acetic acid in some form ever since and have not had some or any of the problems that I read about. So why change? Kodak (and others) had people like PE working on questions like this and I have always relied on their instructions to my advantage.....Regards

 

[Sirius Glass]

It seems to me that armchair chemists may do more harm to analog than digital will. Be careful out there people.

PE

Especially when they make no effort to learn what you have written. Now come the posts recommending using dish soap for PhotoFlo and Borax for proper photographic reagents.

 

[Xmas]

Xmas, you forgot that as format size goes up, stop baths increase image uniformity, and with the right mix a stop can increase the life of a fixer. There are other reasons as well, but if you think a stop is useless, you have not considered the problem from the "other side", as an engineer who must be aware of all that can go wrong.

PE

Hi Ron

You are correct I only ever process 120 or smaller formats
But the data sheet says only <2% acetic or water.
And I discard bath A when it slows don't care if it does not last.
Always use 2% with prints.

 

[Xmas]

Especially when they make no effort to learn what you have written. Now come the posts recommending using dish soap for PhotoFlo and Borax for proper photographic reagents.

The scratch mix formula for D76 and ID68 says Borax and you can only get reagent quality here it is banned for a wash aid in UK!

 

[NedL]

The fruity smell may be apple or wine vinegar. Do NOT use them. Use white or distilled vinegar.

PE

No it was distilled white vinegar... I was surprised but different brands smell different. Not like cider vinegar, but a hint of a sweet smell ( I know... sweet is a taste and not a smell, but it's hard to describe. )

 

[silverhalides]

One more reason to avoid sulphuric acid stop baths.

A dilute solution of sulphuric acid will concentrate as the water evaporates. Acetic acid is volatile and some evaporates with the water; so it' less likely to cause burns or eat holes in clothing as it concentrates. I worked in motion picture laboratories for twenty five years, and ruined at least that number of shirts from contact with the ECN or ECP stop baths. I never got a burn from them, but an unnoticed drop on clothing would show up as a hole after the next laundry.

 

[pdeeh]

Do you think you would be able to explain that again, but unpacking the meaning and significance of pKa for people who have no real understanding of chemistry?

(a link to the wikipedia page about the acid dissociation constant won't help me, by the way, as it might as well be written in Mandarin for all the sense it makes to me )

 

[Photo Engineer]

Since pH, pKa, pKb, pKsp and the other chemical math notations require quite a bit of background to understand completely, why not just take the advice of experts. Citric and Acetic are the only and best acids for Stop Baths. Others can be used and are being used, but are not very friendly to the casual user.

And, BTW, these constants relate to the amount of Acid, Base and Soluble materials that may be in your solution at any one time, but the actual calculation is quite complex.

PE

 

[pdeeh]

why not just take the advice of experts. Citric and Acetic are the only and best acids for Stop Baths. Others can be used and are being used, but are not very friendly to the casual user.

PE, I quite understand that, and I thought I had made it clear in my OP and subsequent posts that I have no interest in using anything other than those two, but at the same time I was interested to understand the photographic (rather than H&S) reasons why they might be preferred.

When there are experts such as yourself and GCK around to learn from, the reply "because I say so" seems a bit short shrift when I'm trying to increase my store of knowledge.

I am still unclear, for instance, why the "middling" pH of Acetic and Citric acid is preferred for photographic purposes to the very low pH of (say) Sulfuric, beyond the fact that they are "more friendly".

So, is that the only reason? Or are there other sound reasons not to use (say) Sulfuric to stop development of ordinary black and white materials?

 

[Photo Engineer]

Did you read the post above about the "burned shirt"? Sulfuric acid will eat a hole right through your clothes and skin if you do not remove it promptly and properly, but it gives superior uniformity to stopping development. So, it is used in a high speed professional process but not in commercial processes where people such as most APUG members might use it.

I could recommend any number of organic and inorganic acids for stopping, but they have either environmental or photographic problems. After all, Nitric Acid is quite usable, but cannot be purchased in many places or shipped due to its very nature. It can be used to make explosives and leaks can become flammable.

Hydrochloric Acid is quite usable but very very volatile as is Hydrofluoric, both of which can eat away at your lungs as you use them. Eventually, you are just corroded away. Many byproducts dissolve in the acids or are volatile thus allowing more and more corrosion as the surface is etched away.

All of these acids except Acetic and Citric require super lab cleanliness for use.

And thus endeth the lesson for the day. You see, pages and pages of typing and some additional knowledge of chemistry are needed for me to go on.

Sorry. But my knowledge is the result of 3 years of intensive course work and lab work, and to impart more than the above would be very very typing intensive.

PE

 

[DREW WILEY]

Sulfuric is nasty stuff and hard on plumbing. As every plumber knows, you don't want to use it very often, and then only directly into main drain lines, never in subsidary ones. Corrosive, even when used dilute over a period of time. I used sulfuric in the Cibachrome process, but instantly neutralized it with bakings soda when I drained the process drum. Acetic acid is cheap. I buy glacial a dilute it under a fume hood. All it takes is a tiny amount, really quite a bit less than the recommended 2%, which I toss at the end of each session, though I did use a bit of leftover to neutralize some darkroom floor efflorescence prior to repainting it. Wiser than stealing my wife's kitchen vinegar, or ruining my lungs with muriatic acid, which is the traditional product for this kind of application. I don't why anyone in their right mind, outside of very special industrial applications like glass etching, would ever go near hydrofluoric acid - it's like that "molecular acid" in the
saliva of the monster in the old Alien movies. I only use citric as a preservative in amidol developers. Acetic is way cheaper.