What is stop bath for?

It might be of interest to point that EKC's stop bath formula SB-5 uses sodium sulfate in addition to acetic acid to minimize swelling.

If odor of acetic acid is not a problem, here's some inexpensive suggestions:

60 ml acetic acid (90%) and 25g sodium hydroxide, water to make 1 liter.

30 ml acetic acid (90%) and 80g sodium acetate trihydrate, water to make 1 liter.

These are some back-of-the-envelope calculation after failing to find a published formula. Both of these should work equally well, and you can mix in some sulfate as in SB-5 if you want to minimize swelling. (i.e. higher temperature processing etc.)

These buffered formulations can take a considerable amount of developer carryover and they continue to halt development.

Another of my favorite stop bath is that proposed by Troop: 50g boric acid in 1 liter of water. Its pH range is in a good window, and it has some buffering and anti-swelling effect. Its pH is acutally a bit higher than those suggested above (about 5) but it's resistant to rise further. Unfortunately, this stop bath is difficult to make in a concentrate form, and therefore is for people who don't mind mixing from dry chemical. However, it is very simple, safe and effective. (I'm happier to keep a few pounds of boric acid than a quart of concentrated acetic acid in my room :-)

It's only stop bath, but I thought to add a bit more info because its role and formulae are much less frequently discussed than fixer, which is much less frequently discussed than developer.

Just because I don't think many people care about stop baths :-) here's a correction to my own typo: I mistyped 30ml instead of 20ml for the volume of 90% acetic acid in acetic acid + sodium acetate bath. Not a big difference, though :-) After correction, both should result in initial pH of 5.05 to 5.15 range, which is considerably higher than plain acetic acid bath.

I appreciate your input here - it gives me a lot to consider.

In light of these recent threads, and my own use of simple diluted distilled white vinegar as a stop bath, I checked the labels for Ilford and Kodak stop baths yesterday.

Granted, I didn't have my reading glasses with me and could have overlooked something, but I didn't see any ingredient that might have served as a buffer. These stop baths appear to contain only acetic acid and a color change indicator. Perhaps I missed something.

Another concern, possibly unfounded: a buffer would be alkaline, correct? How would a buffer and a stop bath agent coexist? Wouldn't one neutralize the other?

Sodium hydroxide is lye right? One of the reasons I like a simple water/vinegar bath is it's all safe. The vinegar normally sits in the kitchen. I just looked at the MSDS for Kodak SB-1A and SB-5. The first one only lists water and acetic acid. The second one adds Sodium sulfite.

Now if using something different will produce better films/prints then that's well worth looking at. I've been doing a water wash after the acid stop before fixing film. My fixer is akaline and I'm under the impression it doesn't like acid stops. It doesn't take a lot of extra time to fill the tank twice.

BTW I like the smell of vinegar-))

First of all, one might ask, if plain water rinse or plain 1% acetic acid bath works, why the heck do I worry about stop bath?

There are a few different ways to answer that. One is convenience. If you are tank processing films and tray processing prints, there's little difference. But since when I started using Nova slot processor, changing solution became much less convenient, and I strongly prefer formulae that provide stable performance over its processing capacity. Indicator dye is practically useless in slot processors, because it is hard to see, and even if it tells me to change the bath it's another source of trouble.

If you are selling chemicals as a business, and there are two formulae, which one do you choose to sell? If one costs a dime more per liter but provides longer life and more gentle to emulsion. Both are usable. Everyone knows acetic acid, only some know what exactly "buffer" is. Again, both work with difference in details. You gotta be competitive and you may not be able to afford to pay that extra dime which I can happily pay if I'm mixing a batch for my own use.

Sprint BLOCK stop bath is reasonably buffered... to the pH of about 4. Though its pH is a bit low to my taste, this range has been kinda standard for this kind of stop bath, and there are products like this. (MSDS lists acetic acid and sodium acetate) SB-1/SB-1a and SB-5 are not buffered. I noticed that MSDS for SB-5 lists sodium sulfIte instead of published formula's sulfAte, and this may raise the pH a bit.

pH buffer is a system of chemicals that acts to maintain stable, nearly constant pH by resisting against addition of acid, base and water, and also often against change in overall concentration. Buffer can be designed for any target pH, acid or base, within some practical limitations. Acetic acid + sodium acetate buffer is a classic example and you'll find several examples and exercise problems in virtually any freshman chemistry text. (I deserve -1 for the typo above.)

In the first formula above, acetic acid concentration is about 0.9M (moles per liter) and sodium hydroxide 0.63M. This neutralizes 0.63M of acetic acid to make sodium acetate, leaving 0.27M of acetic acid untouched. The end result is very close to the second formula. Depending on what you have access to, you can mix either one and the result is the same. Sodium acetate is much safer than sodium hydroxide, but can be more expensive. Once these two are mixed, the stop bath is not very toxic or anything, just like most other common photo chemicals.

What's nice about buffered stop bath is that it is perfectly safe to make it a bit stronger (multiply all the ingredients by the same factor) to increase the capacity. Of course, you can do this with Sprint BLOCK if you like. (There's nothing special about Sprint BLOCK, it's just what I used to use.) With plain acetic acid bath, I would not do that.

So what's not ideal about unbuffered, plain, acetic acid stop? It's pH is very low when mixed fresh, but the pH rises rather quickly as you use the bath, increasing developer carryover. If you make it strong in an attempt to make it last longer, the initial pH will be way low, and I worry about stress to gelatin. If you make it weak, stress might be reduced, but there will be little difference from plain water rinse. Buffered stop bath is more like forcing the emulsion to a specified pH even if there is some developer carryover. And the specified pH is rather independent of concentration, you can vary it in some range with no appreciable effect in resulting pH.

Whenever you hear buffer, whether in developer or in stop bath, what's intended is the same. Maintain the designed pH against external disturbances. For a fixed specified pH, more concentrate buffer is more stable, that is, can take more external disturbance and still perform like there was nothing. Obviously, if you want to make a perfectly ideal buffer it would require a lot of chemical and it may very well be a waste of chemical. It's a trade-off between stability, cost, and often convenience. It's so nice if I can dump developer, stop bath, and fixer at the same time, and all have roughly the same processing capacity.

Thanks for the info!

I'm really enjoying this thread. I've never used a stop bath for film, only a water rinse. Its just so simple and eliminates at least one chemical and I see no way that it effects the result, I have 25 year old film that never felt an acid stop and is still fine. I've been looking at ways to eliminate the acetic acid stop I use for prints since I'm using TF4 fix which is alkaline and I think that yo-yoing the PH back and forth is not a good idea. What I wonder is - does it matter in terms of print longevity if I use a water rinse instead of an acid stop bath? So I repeat and add to the question in title of this thread - what is stop bath for - and does it matter that we use one at all in terms of quality of results?

I was expecting a bit more of pragmatic responses like "stop worrying about things that make little visible difference" but...

> does it matter in terms of print longevity if I use a water rinse instead of an acid stop bath?

Probably not, or perhaps even better for your prints, if you don't have stain problem or overdeveloping in water, which is unlikely. There's nothing in water to prevent swelling but that's the same for plain acetic acid stop bath. If I were tray processing prints in a big sink, I would place a water rinse tray between developer and fixer, and use that as a stop and a preliminary rinse, before washing aid. Use one of those inexpensive print washer or something similar so that water flows *slowly* but continually. (Slowly, because I don't want to make an instant damage :-) This is not my option because my water access is limited to final wash area, and I have to use stop bath for my print processing. Naturally, I use one of buffered stop baths. (In my darkroom, the dividing line for "dry side" and "wet side" is drawn a bit differently... if you consider Nova slot processor semi-dry.)

Frankly, I don't know of any hard evidence showing which print didn't last long because of bad stop bath, or something like that. It would be very hard to control various other factors, and it'll take time to show difference, and while you are waiting, the manufacturing of the emulsion material changes and the result may not apply. It is also true, print developer is harsh on gelatin so why do I worry about a small difference? But, but, I just thought it's nice to know what stop baths are supposed to do and how they work.

I don't know how many people here are full-time darkroom workers but to me, a bit of convenience features count a lot because if the system is inconvenient I know I will be lazy and my wet processing won't be as it is supposed be. I personally count anti-laziness feature as a positive safety factor, and I'll do that if the cost difference is comparable to a sheet of paper.

I am...my job is split between shooting in the studio & location and doing the b&w labwork and some E6. I have to maintain both the replenished deeptank line (b&w) and the b&w print processor...before my ten years here, I had about another 5 on top of that working in newspaper labs. In the newspapaer darkrooms we either did tray processing, at one we ran all singleweight graded fiber actually, or we used either Kodak Dektomatics or Royalprint processors....the biggest paper I was at, had the best darkroom I've ever been in, and in addition to a great wetline, had 2 paper processors--one Kodak and one Ilford--and actually stocked both manufacturers papers for staff use. At all these labs, we used stop baths on the traylines...usually good old indicator stop. We used that here for the first two or so years I worked before we got our roller processor....every day, 40 hrs plus a week I tray processed b&w and used a stop....the only time we didn't for prints was when we mural processed--and with that we used 2 nine foot Kreonite sinks as our "trays"--we filled each up with over 25 gallons of chemistry & walked the fiber paper through for agitation...for a rinse between developer & fix, one sink was filled up with running water--these were lab sinks and had built in chillers & water panels and water jets--they would circulate water really well....as long as I've been here we've run a weak (half strength) Kodak stop bath for our deeptank line...why Kodak? Well, it works for one thing, the other is we get Kodak products dirt cheap on contracts.....I don't rely on the indicator but use the log for the replenishment....we rarely run the max through the tank in a month--with floating lids & tank covers, everything is fine. A busy month will be running about 250-300 sheets 4x5 b&w and about 25 tops roll film....However, a lab next door has a much larger tankline & uses a quick dump wash tank as the rinse between the dev & fix...to each their own....

as for processors, they rarely have stop tanks, but use configurations of hard, soft or both rollers in the racks to squeegee the RC prints off almost bone-dry as they exit a tank...on ours, if the tension springs in the rack get relaxed ( happens), the carryover developer causes all sorts of streaks & such....I have an old thermophot processor myself, and it's regeared for RA4/b&w from EP2 originally...it's original configuration was with a small stop tank that has a roller rack that actually has a hard roller on top, and one like a big sponge on the bottom...I use a weak stop bath in this for b&w...I usually use Kodak Ind stop, sometimes Ilford, just whatever I have around....

in all these places it was never a big deal...no fuss, no problem..I even was at one place that mixed the stop up in huge quantities using those 25 gallon chem mixer & transfer units Kreonite makes...the stop was mixed from a diltion of glacial acetic acid--a much hated task--in the end, the chemistry for the b&w film rooms & print rooms was all on tap....like being at a soda fountain or something...you had your little film room--tankline and all---and all chemistry was on tap, ready to go....

Henry, I use alkaline fixer for everything. Agfa FX-Universal is primarily sold as colour fixer but it's perfect for B&W. Its pH when diluted is about 7.5 so it's only barely alkaline. I use running water instead of stop bath.

It is said that the water rinse does not stop the development quickly and I'm sure that is true. It is also said that when the print goes into the alkaline fixer the traces of developer that are not washed out will continue to develop.

My choices of paper developer and fixer i think will eliminate this problem: The fixer is only minimally alkaline. My print developer is Agfa Neutol-WA in which the only developing agent is hydroquinone which needs a quite alkaline environment in which to work. If I want to use a different developer for some special reason, my water rinse is longer.

An acid free darkroom doesn't smell much at all.

Though DK's story is impressive and full of insights, here's a quick comment on fixer.

> Its pH when diluted is about 7.5 so it's only barely alkaline.

For films, papers and fixers, if you stay well above 5, the major purpose and the sales point of "alkaline fixer" is achieved. This is for the same reason -- if the fixer's pH is below 5, then gelatin is positively charged and thiosulfate is negatively charged, so they attract each other and it takes a lot of effort to wash thiosulfate ions out the gelatin. If the fixer's pH is above 5, the gelatin is negatively charged and it is much easier to wash fixer out of the emulsion.

Alum hardening fixers must be below pH of 5 (otherwise the hardening is ineffective). If a fixer states *incompatible* with such hardner additives, it is very likely that the pH is above 5. Ilford Multigrade fixer (which they call Ilford Rapid fixer now?) is about 6, and this is what I used to use before started mixing fixers from scratch. Unfortunatelly I have no experience with Agfa FX-Universal...

At pH of 7.5, even metol and phenidone would be VERY slow acting, and because partially used fixer is full of restrainers (chloride, bromide, and iodide in fixer that processed films) I wouldn't worry too much about tiny developer residue through water rinse, as long as water rinse is decent (good agitation while rinsing).

Hydroquinone in exhausted print developer smells pretty bad... though it's ok when fresh.

If you do lith printing, it is possible that you may benefit from nonswelling, buffered, acid stop bath and alkaline rapid fixer. This is because highlight of lith printing is so delicate and the way it looks changes so much as the print enters fixing bath. I just started lith printing not long ago, so I leave this as an open, speculative statement.

Another interesting effect of a non-acid process (I use a water rinse and TF-4 of late) is that it shortens toning time. I was toning 3 minutes in Rapid Selenium Toner (1:15) after an acid fix and a thorough wash to enhance Dmax with Azo, and with TF-4, I get the same degree of change in less than one minute.

Don't get me wrong here, it's just that when I read your original post, it's hard for me to understand what your point is....I see it as looking back on my experiences and what I was taught both in school and on the job, and think if ain't broke, why fix it? It's like re-inventing the wheel, which if you want to tinker with chemistry & all that...fine, but don't discount what alot of people have been doing for years....I have been messing around in darkrooms since I was a kid, and have negs & prints that I made almost 30 years ago, using off the shelf Kodak chemistry, film & papers..and they look fine all things considered. I work in a system that has been around for 100 years, and has had various darkrooms & depts....that have either been together as a unit or split or both over the years...the negs and some prints have been kept as public records however...I can look back through the years, as I print these for work, and I don't see any radical change or damage from using stop baths...I don't worry about pinholes because I can honestly say I've neverhad one happen in film development--I've had agitation problems from gas burst lines, or overworked repl developers & bromide drag, I've had dust & crap imaged onto sheet film & such, but no pinholes...why I should worry about something that's never happened to me with a process I use practically every day? (knock on wood...hahaha)

The thing I learned looong ago was that two things mattered the most in doing darkroom & shooting everyday--consistency of materials, BUT more importantly consistency of method....find something that works for you and stick with it. Everyone has their own working methods, you can do whatever you want...more power to ya...but before thinking of me as pedestrian or some lazy lab guy...get a job working in a lab everyday printing other's negs and trying to make them look decent--while being judged on your performance & materials usage--it's good training believe it or not, not always fun, but good experience....Opinions expressed in this message may not represent the policy of my agency.

No DK, I'm afraid I'm not saying all those stuff you are talking about. I never said better stop bath will increase the life of the print or anything like that. I did not talk about pinhole in this thread either. Unless you use Dektol and Tech Pan or something, pinhole is just a risk factor to avoid. (If you look at regular films under microscope, even correctly processed films have tiny holes anyway.) And I explicitly said a few times that people with different practice set up need not change their system just because of what I said. I explicitly said that this thread was intended for your information only in different words.

If you are set up like those labs then you probably don't realize that a small difference in chemistry can make a difference in terms of convenience, without sacrificing the quality or without increasing any risk, stain or damage.

When it comes to developers, there are a lot of information around, but stop bath is probably the least important but sometimes more confusing topic, and I just tried to clarify a bit. If you look around for past discussion, there are radical opinions like "one doesn't need to stop" and "stop bath is cheap anyway, make it strong and use one shot" many many other opinions. I just wanted to provide some information for people to understand some people are taking risk, some are doing wasteful practice, etc. There's a reason people use stop bath and there's a reason some cost a buck more than others. Especially because some people said they use alkaline fixer here, there is a good reason to re-think stop baths, although I said pretty much all I consider about stop bath in this thread.

Some people said (not recently, but I've seen it a long time ago) there's no need for stop bath because more recently developed color processes omit one. But with the information I put out here, it is clear why b&w needs stop baths and C-41 doesn't. Removing these sources of confusion isn't a bad thing for curious people.

Lith printing is a different story. There's no "proper" set of chemistry for such a strange printing practice, and I don't think chemicals people use is optimized for it. This is because lith printing extremely softens the emulsion during development, and image forming silver particles are soooooo tiny that they are very susceptible to metalic silver dissolution in fixer, especially when stop bath or fixing bath is very acidic. There is no re-invention here, the process hasn't been perfected to the same extent as regular b&w printing.

My apologies then...all these chemistry posts are running together, maybe I crossed up responses or something....yes, we are set up this way...but I have my own darkroom and such as well and spend a good bit of time in there at night...some days I work in the dark all day, go home and work all night...years ago I used to mix my own developers & screw around with that stuff a bit, but after awhile I just got tired of it...for me, the more I did this stuff at work , it became second nature. I could do just fine with good old, standard chemistry...the most exotic thing I do it seems is to modify LPD with either sodium carbonate or potassium bromide...mostly I just replenish the developers and if I feel technical (usally not) I'll run control strips & plots...after dealing with the woes of running an E6 machine at work & trying to run control on that every day, believe me when I say this--b&w ain't color.....*enjoy* it....

But lith priting? Yeah, I do that too--just not often--I use the old kodalith A/B liquid and I do use a stop bath....in the past I've used Luminos Classic paper, but I have a box of TP5 I have yet to try....funny you should mention lith developers though, because we used to do quite a bit of work here with graphics films--we made all the film pos & film negs for our silkscreening operation--now this is done by digital output. When I first started we did a totally idiotic method of bumping up the shots from 4x5 Kodalith first to big sheets using a horizontal enlarger and those big long rolls of Kodalith...if it was a pos or neg, depending on the original, you might have two or three generations to work through...talk about a PIA. Then we bought this monstrous stat camera, and got into PMT transfer processes...but still had to do Kodalith for the really big stuff--the old kodalith, not the powdered RT stuff, was nasty...yuck...caustic, had formaldehyde in it, it was foul to use....it would make your skin feel like wax and after a while, I would often get a pressure on my chest even under a hood using this crap...I used a full strength (per Kodak) stop bath with this....lith printing is similar to using graphics films....with the films, you watch them under the safelight & learn when to yank them out as they develop--before they develop completely...you never time them, you work by inspection....I agree that it's a bizarro process & highly unpredictable, but anyone who has worked with offset printing (pre digital) or silkscreening, will have used this stuff--I knew how to use stat cameras & such because as an impoverished freelance photographer, I wound up running them for about 2 years in a printshop.....worst job I've ever had. The chemistry used in photolabs doesn't even hold a candle to that used in offset printing or silkscreen--our old darkroom (we have a newer one now) was like a toxic waste site with the silkscreen room next to it--toulene and all this Kodalith stockpiled....not to mention old antiquated carbon-arc plate burners....this is one area where digital can kill the old process and gno tears would be shed on my part....MY opinions only folks.....

Even though a photographer for twenty years I have to confess I didn't realise there was so much to say about stop bath! I have always used a general commercially available stop-bath (Kodak Indicating Stop). Agfa used to recommend a water wash at 20 C but I don't know why they eschewed an acid stop. Very concentrated stop can reticulate the emulsion but I have never had this experience myself. It is necessary to kill the dev in order to prevent dichroic fog on the negs.