Stop baths

The relatively low pX values seen for the benzoyl acetanilides, especiaHy as two-equivalent couplers, minimize concerns over slow ionization rates and contribute to the couplers overaH reactivity. But this same property often results in slow reprotonation in the acidic bleach, where developer carried over from the previous step can be oxidized and react with the stiH ionized coupler to produce unwanted dye in a nonimage related fashion. This problem can be eliminated by an acidic stop bath between the developer and the bleach steps or minimized by careful choice of coupling-off group, coupler solvent, or dispersion additives. 

Wash the paper in a tray containing stop-bath solution for 30 seconds. 

Standard stop bath is 28% acetic acid. A pH indicator is added, usually bromcresol purple, which is yellow below pH 5.2 and blue above pH 6.8. When the stop bath is blue, the hydrogen ion concentration is too low to be effective. Dilute acetic acid is an irritant to the skin and can cause allergic reactions and chronic bronchitis. 

Adequate ventilation in the darkroom will eliminate acid fumes. The photographer should wear goggles, an apron, and plastic gloves, and should use tongs when handling negatives and prints in a stop bath. 

As with developers and the stop bath, having adequate ventilation wearing goggles, aprons, and gloves and avoiding inhalation or ingestion of fixer solutions will minimize health risks. 

Developers should be kept in brown amber glass bottles, particularly if stored for more than a few weeks. This is because developers easily oxidize and lose their potency when exposed to air and/or light. Plastic bottles breathe and will speed the rate of oxidation. High-density polyethylene storage bottles are suitable for solutions of fixer, stop bath, bromide, carbonate, and just about anything except developers. 

Balanced Alkali is a proprietary chemical of Eastman Kodak, and until recently it was known as Kodalk . Balanced Alkali is more alkaline than borax and more easily soluble, but less alkaline than carbonate. As Balanced Alkali contains no free carbonate, there is no danger of carbonic gas bubbles being formed when an acid stop bath is used (see the next section, Moderate Alkalis). Balanced Alkali can be substituted for carbonate (Conversion Tables Alkali Substitutions), and for almost all purposes it is identical to sodium metaborate. 

Carbonate has one important drawback. When a large amount is used carbonic gas bubbles can form in the emulsion of either film or paper when transferred to an acetic stop bath, resulting in pinholes and/or reticulation. Using a less acid stop bath, such as Kodak SB-1 Nonhardening Stop Bath or a running water bath, should prevent this from occurring. 

The amount of time required for a thorough wash depends on the type of fixer and whether or not a hypo clearing agent is used. This will be discussed in Chapter 9 Stop Baths and Fixers. 

It is recommended to use a 1 minute running water bath in place of a stop bath with this method. 

Stop Bath for Tropical Development Films should be immersed for three minutes in either Kodak SB-4 Tropical Hardener Bath or Kodak SB-5 Nonswelling Acid Rinse Bath after development and before fixation. Agitate the negative in the stop bath frequently. 

Because of pyro s self-hardening properties it is not necessary to use fixers containing hardener. If you wish to use an acid stop bath after the water rinse a good fixer to use is Kodak F-24 Nonhardening Acid Fixer. Otherwise, use plain running water and Photographers Formulary TF-4 Fixer (recommended). 

Whether or not to use an acid stop bath or a plain running-water bath has been a longstanding controversy among photographers. There are two reasons cited for using an acid stop bath. First, it arrests development in the shortest possible time. The second is, That s the way I was taught. ... 

Even so, the disadvantages to using acid stop bath are ... 

Use a 3% to 5% solution of sodium metabisulfite, a 3% solution of chrome alum, or a mildly acid stop bath, such as Kodak SB-1 Nonhardening Stop Bath. Or use a very gentle-acting stop bath, suggested by Jay Dusard, comprised of a solution of 10.0 grams of sodium bisulfite in 1.0 liter of water. 

The fumes which emanate from acetic acid stop baths are perhaps the single greatest health hazard in the darkroom. Over time, these fumes can cause damage to the delicate membrane of the sinuses, resulting in respiratory problems. For this there are also two solutions. 

Use an odorless citric acid stop bath such as Kodak SB-8. 

Stop using stop baths altogether. Instead, substitute a one minute running water bath between the developer and fixer, for both film and paper. If running water is not available fill the tray or tank with fresh water, agitate for 20 to 30 seconds, dump, and repeat three times. 

If you still prefer to use an acid stop bath instead of water the most widely used formulas call for acetic acid in a 1% to 2% working solution. The only other ingredient sometimes added is a pH indicator dye (Formulas Stop and Hardening Baths Indicator for Stop Baths). 

Other chemicals that can be used to make stop baths are boric acid, citric acid, and sodium bisulfite. Chemicals that should not be used are chrome alum and potassium salts, such as potassium metabisulfite or potassium sulfite, either of which can cause green stains on some enlarging papers. In addition, the introduction of potassium salts into a fixing bath can convert the bath to potassium thiosulfate, which is nearly inactive as a fixing agent.1... 

Prior to WWII, German companies formulated many stop baths using potassium salts, as they were less expensive than sodium (Germany was in a depression, as was the rest of the world). Subsequent research revealed the problems associated with potassium salts in stop baths and its use was discontinued. Unfortunately, there are still some old pre-WWII formulas floating around. 

No acid stop bath is required when using alkali fixer. In order to preserve the alkalinity of the fixer, acid stop bath should not be used. 

Do not use an exhausted or over-concentrated stop bath. Stop bath left in the tray overnight will evaporate and become concentrated. This can cause mottling in the base of a toned print. Replace the stop bath frequently or use an indicator stop bath. 

Insufficient agitation of prints, especially during the stop bath, can also cause mottling. The mottling will not be evident until the print is toned with a selenium or sulfide toner. 

NOTE Prints should be well washed between toning baths and in particular acidic prints (for example, from direct gold toner, acid stop baths, etc.) can stain if transferred straight to selenium without being well washed, or put through alkaline hypo-clearing agent. 

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