stop bath?: water vs. stuff-you-pay-for

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Hi everyone,

In my photo class (which I just started)- man is it fun!!- we're using water washes between the DEV and FIX steps. I guess this would be where you'd put Stop Bath, right? How does water work as a stop bath, and why would you use the commercial (kodak, etc.) stop baths. There's gotta be a reason....

Thanks for your help/input! Justin

-- Justin Indyk (indyk@pharm.sunysb.edu), July 11, 2001

Answers

Most developing agents require an alkaline environment to be able to operate (amidol is the one exception that proves the above rule). A stop bath like acetic acid changes the pH in the print to acidic very quickly and basically arrests development very rapidly. In addition, by neutralizing the developer, it is supposed to extend the life of the fixer (since the fixer does not have to neutralize the developer). Having said that, a water rinse works very well too - its just not a sudden stop the way an acidic stop bath is. Water dilutes the developer a lot pretty quickly but since the pH is probably neutral or mildly alkaline, there probably is some additional activity which will continue to take place. There are reasons you might prefer one or the other. Typically, a lot more folks seem to use a plain water stop in film development. With paper development, most people seem to prefer the acid stop. However, there are some folks who use a water stop even for paper (as long as you are consistent, you won't go too wrong since the marginal additional activity in the water stop bath can be adjusted by pulling the print from the dev just a smidgin earlier). Note that the most important criterion is how quickly you need to stop development. In lith printing, for example, you are developing by inspection and need to arrest development very rapidly (since we are talking about infectious development there). So, there's no way around an acid stop bath there. Good luck, DJ.

-- N Dhananjay (ndhanu@Umich.edu), July 11, 2001.

Developer residue carried over into fixer can cause staining, so the water must be changed frequently. With film that's easy: just use the water stop one shot. With printing it can be inconvenient, especially if your water faucet is in a seperate room.

-- Tim Brown (brownt@flash.net), July 11, 2001.

Agreeing with the others... also, with some film developers (Diafine, Divided D76 and a few others) you shouldn't use an acid stop. I prefer to use water (less stress on the film) but at the most, a simple solution of Photograde Citric Acid. I really don't like the smell of the acetic acid so for me it is a personal thing. Everyone has a different way of doing things and as DJ states... as long as your consistent you can't go wrong.

-- Scott Walton (f64sw@hotmail.com), July 12, 2001.

Yes, you can just rinse the film or paper thoroughly in water. However, I would recommend using stop bath. Stop bath performs several functions and should be used with all types of film and paper. Developer that is carried over into

Developer is alkaline (pH over 7), while fixer is acidic (pH under 7). When the developer is carried over into the fixer it exhausts the fixer rapidly. However, the cost of replacing the fixer is not the issue. As the fixer becomes loaded with silver from use, the change in pH caused by the developer carried into it can create dichroic fog on the film or paper. Worse, than that, some of the developer carried in the emulsion is oxidized, and this can react with the silver in the fixer and staining can occur. If the paper or film becomes stained, you will not be able to wash the stain out as it is composed of insoluable by-products. The stop bath neutralizes the developer and oxidized developer in the emulsion and prevents dichroic fog and staining.

Acetic acid is used because it has a unique property. Acetic acid stays at a very stable pH over a long period of time as it is used up. There is a limit to the amount of ions that will go into solution at any one time in acetic acid. As the ions are used up, more will become available keeping the solution working at a stable pH. In effect, it is "self buffering."

-- steve (s.swinehart@worldnet.att.net), July 19, 2001.


There was an error when my response was inserted, and not all of it got inserted correctly. Therefore, I am trying this post again.

Yes, you can just rinse the film or paper thoroughly in water. However, I would recommend using stop bath. Stop bath performs several functions and should be used with all types of film and paper. Developer that is carried over into fixer can greatly effect the final film or print image.

Developer is alkaline (pH over 7), while fixer is acidic (pH under 7). When the developer is carried over into the fixer it exhausts the fixer rapidly. However, the cost of replacing the fixer is not the issue. As the fixer becomes loaded with silver from use, the change in pH caused by the developer carried into it can create dichroic fog on the film or paper. Worse, than that, some of the developer carried in the emulsion is oxidized, and this can react with the silver in the fixer and staining can occur. If the paper or film becomes stained, you will not be able to wash the stain out as it is composed of insoluable by-products. The stop bath neutralizes the developer and oxidized developer in the emulsion and prevents dichroic fog and staining.

Acetic acid is used because it has a unique property. Acetic acid stays at a very stable pH over a long period of time as it is used up. There is a limit to the amount of ions that will go into solution at any one time in acetic acid. As the ions are used up, more will become available keeping the solution working at a stable pH. In effect, it is "self buffering."

-- steve (s.swinehart@worldnet.att.net), July 19, 2001.



Although the point is moving farther from the original queation, I don't quite agree with Steve. pH of acetic acid stop bath increases quite sharply during its use. Considering the strength of sodium carbonate typically used in paper developer, self-buffering effect of acetic acid is nothing. I recommend adding some sodium acetate to form a buffer if that's what you prefer. This will also make the stop bath act faster and last longer if mixed appropriatelly.

One could also buffer the fixer to reduce or remove those problems, whether it is made mildly alkaline, neutral or acidic. Buffering agents are cheaper than ammonium thiosulfate and it may be worthwhile experimenting to get maximum stable use out of same thiosulfate content. (I vaguely feel that amount of water is usually the limiting factor in fixer's processing capacity whereas the concentration of thiosulfate affects the processing time.)

-- Ryuji Suzuki (rsuzuki@rs.cncdsl.com), July 30, 2001.


Water does not act fast enough to freeze development

-- martin tai (martin.tai@capcanada.com), July 30, 2001.

The key with using water for a stop bath is to triple-rinse (completely filling the tank, agitating, and then emptying) the tanks with water before fixing. I work in the chemical industry and triple- rinsing is standard for everything we do to remove chemical residue from tanks, drums, and whatever.

Assume, for example, that a single rinse will conservatively remove 99.9% of the chemical from the tank. After three rinses, then, you have reduced the carry-over residue to 0.1% x 0.1% x 0.1% = 0.001% (one-thousandth of a percent). This is more than sufficient to prevent any appreciable cross-contamination.

-- Mike Klemmer (msklemmer@bouldersci.com), February 07, 2002.


By the way, one thousandth of a percent is 10 ppm (parts per million).

-- Mike Klemmer (msklemmer@bouldersci.com), February 07, 2002.

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