Stop bath
From Silvergrain Research
Contents |
The role of stop bath
There are two major roles of stop bath:
1. Remove developer from the material (film or paper) so that the developer will not be carried over to the fixer.
2. Arrest the development reaction rapidly.
Removal of developer is always important in b&w processing, because when developer is brought in contact with fixer on the material, it may cause dichroic stains and other problems in image quality. Therefore, carryover of the developer to fixing bath needs to be minimized.
Rapid halting of the development reaction is very important in lithographic development, where the image quality can change very rapidly near the optimal development time. However, this requirement is much more relaxed with most pictorial b&w processing.
Acid stop bath
Traditionally, stop bath is a dilute weak acid solution. The most common formula is 1.5% acetic acid solution. Organic developing agents are active in alkaline pH and inactive in acidic environment. Therefore, rinsing off the developer in an acidic solution will arrest the development. Acetic acid is usually used for the stop bath because of its suitable dissociation constant (pKa) and low cost, although the odor is often objectionable.
One problem with the dilute 1.5% acid stop bath is that they are not very rapid in arresting development in lith development or rapid access development. Increasing the acid content will increase the acidity of the bath, which may have adverse effect. Therefore, the best solution to this problem is to use buffered acid stop bath which will very rapidly halt the development, and it also has greater processing capacity. This is because buffered acid stop bath can be made fairly strong (more concentrated) without increasing the acidity (without dropping the pH too low). Buffered acid stop bath can be made from acetic acid, citric acid, and many other organic acids, although acetic acid is the least expensive.
The advantage of buffered acid stop bath is further realized in large processing tanks, because buffered acid stop baths are usually replenishable or they can be designed to be replenishable. The replenisher is usually pure acetic acid (either glacial or 28%) in the case of buffered acetic acid stop baths.
Alternatives to acetic acid in the stop bath
Because of the objectionable odor of the acetic acid, there is some desire to formulate an odorless acid stop bath. A few odorless stop baths are proposed, which often replace acetic acid with nonvolatile organic acid such as citric acid. For example, Ilford manufactures an stop bath using citric acid, which is odorless.
Citric acid, especially when unbuffered, can cause some bleaching when it comes in contact with rapid fixer and silver image. Citric acid is also considerably more expensive than acetic acid, when the amount used is adjusted for equal processing capacity. There are few inexpensive, harmless and odorless acid compounds suitable for stop bath, and therefore a buffered acetate-citrate stop bath is a good technical compromise. Other suitable alpha- or beta-hydroxycarboxylic acids may be used, but they tend to be more expensive.
Bill Troop advocated the use of boric acid (50g/L) as a stop bath. While it is usable, the processing capacity is small, and it is undesirable to discharge borate in some areas due to plant toxicity. Boric acid is not commercially used as a stop bath agent.
Omitting stop bath
Despite its name, the most important role of the stop bath is removal of the developer, which can be achieved by mechanical squeegee. Modern processing machines usually omit a stop bath but they rather use a mechanical squeegee to remove adsorbed developer before the material is immersed in the fixing bath. This is generally adequate, especially in color processing, where dichroic stain, even if formed, would be thoroughly removed by the bleach and fix stage. B&W developers for roller transport processors almost always contain an antistaining agent to prevent any stain that may occur from the small amount of developer that is not removed by pinching rollers.
Plain water rinse as the stop bath
In manual b&w processing, acid stop bath may be replaced with water rinse. A running water rinse is ideal, but fresh water and continual agitation for 30 to 60 seconds is adequate. The amount of water should be enough to generously cover the material, and the water should be replaced fairly frequently. A 30 to 60 second rinsing is very effective in removing the developer, more so than mechanical squeegee method used in the processing machines.
Special-purpose stop baths
Hardening stop bath
- stop bath for tropical processing: to minimize swelling at a higher temperature, stop bath may contain 4% sodium sulfate.
- hardening stop bath may contain chrome alum, aluminium alum or other hardener to harden the emulsion.
However, these special stop baths are made largely obsolete because modern films are well hardened in the manufacturing process. The modern films and papers can withstand higher processing temperature and long wet time, and there is little need to use antiswelling or hardening agent in the processing chemicals.
Alkaline stop bath
Maffet and Peters (US Patent 3,284,199) disclosed buffered alkaline stop baths for lithographic and aerial processing. In particular, in the case of lithographic processing, the image quality is very sensitive to the developing time, and the correct time must be determined by inspection. Because the accurate determination of the optimal development time is very important in such processing, the stop bath must be able to halt the development rapidly. Furthermore, if the development time is insufficient, the film is put back in the developer. If the stop bath is too acidic, additional development will be impaired or too slow to come. Therefore, Maffet and Peters attempted to solve these conflicting requirements by making the weakly alkaline stop bath that is not alkaline enough to allow development, but strongly buffered to arrest development rapidly.
Although this is a somewhat unique attempt, there is no technical advantage in terms of image quality or image permanence to use stop baths like theirs, compared to water rinse bath or conventional buffered acid stop bath.
Recommendation
- Manual processing in small tanks and trays
Since it is replace the bath in these processing methods, water rinse is very often the most sensible option for the stop bath. It minimizes the chemical waste without any ill effect.
- In large group darkrooms with shared trays
Since replacing the rinse water may be too cumbersome, acid stop bath may be useful in this situation.
- Processing in deep tanks and slot processors
Since replacing the bath in these setup is a nontrivial task, acid stop bath is necessary. However, in order to maximize the processing capacity of the solution, a buffered stop bath is recommended, and it should be replenished to maintain its performance.
