Stop baths - why Acetic or Citric?

[NedL]

Speaking of all this, I have a plastic bottle of glacial acetic acid. I've kept it inside on a desk in my study because it freezes at not much below room temperature. Other chemicals are mostly stored in the garage, in a place that has some benefits if there was ever an earthquake. But I was worried that the acetic acid might constantly be freezing and thawing out there, and not sure it it might make the bottle weak over time....
Do others have special considerations for storing acetic acid?

 

[Rudeofus]

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".

Emulsion swelling depends on how far away from the isoelectric point your soup is, and according to PE typical isoelectric points are around 4-5. If your pH is too low, your emulsion can swell too much and suffer damage. Xmas has already elaborated on this issue.

 

[pdeeh]

Thank you Rudeofus.

As xmas has previously told me in another thread that stop baths don't stop development at all, I prefer to give credence to statements by the likes of yourself, PE, Gerald and so on.

 

[DREW WILEY]

Storing acid inside a desk sounds like a great way to rust everything else in there that is in fact susceptible to rust. Tiny bits of vapor can
escape even seemingly tight bottle caps over time. Get a plastic-lined picnic cooler if you need an insulated surrounding.

 

[Photo Engineer]

Acetic Acid has a high vapor pressure, and that is why you can smell it. Storing the concentrate in a room other than one used for photographic chemical storage is not appropriate! It is verging on being dangerous.

And, any acid can stop development virtually instantly. It depends on whether it can do it with no photographic effects and at low cost. I can name a half dozen or more organic acids that can do the job but cost too much or leave a residue in the film and etc........

PE

 

[Rudeofus]

As xmas has previously told me in another thread that stop baths don't stop development at all, I prefer to give credence to statements by the likes of yourself, PE, Gerald and so on.

There has been some research at Kodak that was heavily quoted by Bill Troop, and the conclusion was that dilute stop bathes take a long time to stop development action. The reason for this is limited diffusion, i.e. you have a concentrated, alkaline and well buffered developer in your emulsion, and not much acid enters at first. Remember, you're not mixing two liquids here, you immerse a gelatine emulsion soaked with developer into an acidic liquid. I don't know whether the issue persists with today's thin emulsions.

While all the conclusions from this research may be valid, they may not matter for our practical purposes. In fact barely anybody cares (or knows) whether our film effectively develops for five or six minutes, but everybody would be upset about unevenly developed negs. And we have every reason to believe that standard stop bathes (which are quite dilute) give us evenly developed negatives even in large formats.

Since you mentioned Xmas, his warnings about emulsion swell and destroyed film applies to some emulsions, but AFAIK not to Kodak's, Fuji's and Ilford's. These three companies properly preharden their films, and that's why ECN-2 can use Sulfuric Acid as stop bath without suffering damage.

 

[pdeeh]

Excellent, some really nitty-gritty interesting stuff in the last half dozen posts, and some more missing bits of knowledge filled in for me!

So at this point, I feel I understand the following things about why Acetic acid (and to a lesser extent Citric acid) might be preferred as the basis of a stop bath for normal b&w processing for the home user - in no particular order:

• 
  Good buffering (important in paper processing so it has good tray life)
  pH quite near a theoretical "ideal"
  No residues or other damage likely to emulsions
  Health & Safety
  General availability
  Cost

Other acids might meet some but not all of these criteria, or they might fail badly at some while still meeting others.

Thus on balance, Acetic is an excellent choice, with Citric somewhat less so but much better than possible alternatives.

 

[Photo Engineer]

As for the "quick neutralization of developer", I participated in one R&D experiment where we coated an indicator dye on film and then a thick emulsion was coated on top. The neutralization was almost instantaneous in every condition and the reason is - the very fast diffusion rate of the proton which is the smallest of all ions, the Hydrogen Ion! This is what makes an acid acidic. In fact, this work was pursued with other methods of "indication" for other ions such as sulfite, sulfate, halides and etc. Some was published with Karl Tong or Kai Liang as one of the authors, I can't remember OTOMH. I once had a summary here of a few of the diffusion rates and it was quite astounding to see how fast things moved.

PE

 

[Mainecoonmaniac]

I use citric acid because it's low odor and low toxcity. I tablespoon per liter of food grade citric acid does the trick for me. But a chemical question. Isn't vinegar is acetic acid? No one has ever died eating a salad.:confused:

 

[pdeeh]

No one has ever died eating a salad.

depends what's in it ... try and avoid too many Merciless Peppers of Quetzalacatenango ...

 

[NedL]

Acetic Acid has a high vapor pressure, and that is why you can smell it. Storing the concentrate in a room other than one used for photographic chemical storage is not appropriate! It is verging on being dangerous.

You have my complete attention! Right now I have a place in the garage where I keep photographic chemicals. The garage is on a concrete slab, so even if there was a spill it should be possible to clean it. They are stored in plastic lined boxes and the bottles are also wrapped in plastic, so that they will not jostle against each other and break in an earthquake. All bottles are clearly labelled with date and contents. The boxes are under a heavy workbench, so even if the entire structure collapsed in a quake, they would most likely be protected. There really isn't much dangerous out there, mostly bottles of stock developers and fixer. I do have a bottle of straight KRST and a bottle working strength KRST, which is probably the most toxic if it spilled and dried.

The only reason the glacial acetic acid is not out there is that I worried about it freezing. The temperature in the garage does not vary much from day to day but it stays in the 50s during the winter and the 60s or low 70s during summer. I think it would freeze if I put it out there right now. Maybe that is preferable to keeping it indoors ( it is sitting in a plastic-lined box at the back of a desk and yes it is true that when you open the box there is a slight aroma of acetic acid, so some tiny amount of vapor is escaping inside the closed box, even from the capped plastic 500ml bottle. )

I certainly want to do whatever is safest. I am willing to consider diluting all of it down to the strongest concentration I might need.

 

[Kobin]

I have some NAACO Odorless Indicating Stop Bath, hydroxyethanoic (glycolic) acid. And it is almost odorless.

K.

 

[Photo Engineer]

Kobin; Glycollic Acid is an excellent skin penetrator and is thus a bit dangerous. It can metabolize to oxalic acid a powerful poison. So wear your rubber gloves!

NedL; Acetic acid only freezes below about 20C. or about 63F so therefore it might freeze in the winter in your area. Unlike water though it does not expand when it freezes IIRC. At least I have never seen it burst a container when water burst pipes!

PE

 

[NedL]

NedL; Acetic acid only freezes below about 20C. or about 63F so therefore it might freeze in the winter in your area. Unlike water though it does not expand when it freezes IIRC. At least I have never seen it burst a container when water burst pipes!

Yes the container was my main concern. I've read that it does not hurt the acetic acid to freeze and thaw. It is now safely in the garage with my other chemicals, inside a secondary sealed tupperware container just in case. Thank you!

( I'll probably squeeze the bottle slightly this evening to see if it is hard, curious to see something frozen at such a high temperature. It is 57 degrees in the garage right now. )

 

[Kobin]

So wear your rubber gloves!


PE

Always.

K.

 

[alanrockwood]

A quick note on chemical safety. Always store acids and bases separately.

The same goes for oxidizers and flammable compounds (mostly organic compounds.)

 

[railwayman3]

NedL; Acetic acid only freezes below about 20C. or about 63F so therefore it might freeze in the winter in your area. Unlike water though it does not expand when it freezes IIRC. At least I have never seen it burst a container when water burst pipes!

PE

About 15 years ago I cleared out my late Father's shed where he had stored various photo-chemicals.....a full sealed glass bottle of glacial acetic acid had completely cracked in half and the contents totally vanished, presumably by evaporation. The shed hadn't been opened, so presumably frost must have caused it (in the UK, so certainly freezing at times during the winter, but not usually "arctic" conditions) ?

 

[NedL]

I just had a look and it is not frozen. The concrete slab has a huge thermal mass, possibly it will not freeze even with air temperatures below 62F. About ten years ago I buried a thermal probe about 2 or 3 feet underground and monitored it for a year. Less than 3 degrees Fahrenheit variation over the whole year.

( To see how much variation a homemade fluxgate magnetometer would have to cope with... in the end it didn't work because I had crosstalk problems with the line that brought the signals to and from the house and I didn't pursue it further. )

 

[pdeeh]

Ned I had to google "fluxgate magnetometer" and I am now worried that you think you may be under threat of attack from submarines beneath your house ...

 

[NedL]

I like to tinker with electronics and it was a fun challenge to see if I could build a working fluxgate from scratch. Gotta watch out for those subterranean submarines though, they are way more scary than solargraphers.