Although a plain water rinse bath is commonly used between development and fixation, a better procedure is to use an acid stop bath. The function of a stop bath is not only to remove the developer that is clinging to the surface of the material but to also neutralize the developer in the swollen emulsion to stop development completely.

The acid stop bath stops the action of the developer because developing agents cannot work in an acid solution. An acid stop bath also helps protect or prolong the life of the fixer by neutralizing developer carry-over.

An acid stop bath should meet the following requirements:

lThe pH must be low enough to neutralize the action of the developer carried over.

The acidity should be limited so the small amount carried over into the fixing bath does not increase the free-acid content of the fixing bath and cause sulfurization.

It must not contain enough acid to produce blister formations in an emulsion.

You should use only a weak acid stop bath between development and fixation. Strong acid and the acid in the fixing bath have a tendency to form carbon dioxide gas bubbles in the emulsion. When the film is taken from the developer and placed directly into a strong acid or fixing bath, these bubbles may break and cause small, round holes in the emulsion. These bubbles are sometimes mistaken for pinholes like those caused by dust particles settling on the emulsion before camera exposure.

When you are using an acid stop bath, remember that some of the stop bath is carried into the fixer when materials pass through it. Therefore, you cannot use a strong acid (such as sulfuric acid) because it can cause precipitation of sulphur in the fixer. Acetic acid is the type of acid used for stop baths. In its pure form as glacial acetic acid (99.5 percent), it freezes at a temperature of about 61F. Its freezing tendency gives it the name glacial. For use as a stop bath, 99.5 percent glacial acetic acid is diluted with water to make a 28 percent working solution. Approximately 1/2 ounce of 28 percent acetic (not glacial) acid is added to 32 ounces of water. The process of determining the concentration of a liquid is discussed in chapter 8.

FIXING

When a light-sensitive material is removed from the developing solution, the emulsion contains a large amount of silver salts (halides) that has not been affected (developed) by the developing agents. This silver salt is still sensitive to light, and if it remains in the emulsion, light ultimately darkens and discolors the salt which obscures the image. Obviously, when this action occurs, the negative (or print) is useless.

The fixing bath is used to prevent this discoloration and to make the developed image permanent. It accomplishes this by removing the undeveloped silver halides by making them water soluble. Therefore, to make an image permanent, you must fix the light-sensitive material by removing all of the unaffected silver salt from the emulsion.

The fixing bath contains five basic ingredients: the fixing agent, preservative, neutralizer or acidifier, hardening agent, and an antisludge agent.

Fixing Agent

All fixing baths must contain a silver halide (salt) solvent. This solvent is known as a fixer or fixing agent. The two most commonly used in photography are sodium and ammonium thiosulfate, commonly termed hypo (taken from their other chemical name hyposulfite). Ammonium thiosulfate is used in rapid fixers that are stronger and require less fixing time.

Sodium and ammonium thiosulfate changes undeveloped silver halide to soluble silver sodium thiosulfate. It removes this compound from the emulsion and refills the space it occupied with nonexhausted fixing solution. Therefore, the function of the fixing agent is to convert the silver salts remaining in the emulsion after development to soluble compounds and to remove these soluble compounds by constantly diluting and replacing them in the emulsion. The number of substances capable of functioning as fixing agents is small because a good fixer must meet the following requirements:

It must dissolve silver salts without affecting the metallic silver image.

The compounds it forms must be soluble so they can be removed from the emulsion.

The fixer should neither swell excessively nor soften the gelatin.

Preservative

A preservative prevents oxidation of the developing agents that are carried over into the fixing bath by the film. It also prevents decomposition of the fixer. Oxidized developer in a fixing bath produces stains.

Strong acids may cause a fixing agent to decompose (sulfurize). You must add preservative (sodium sulfite) in the fixer to prevent sulfurization. The preservative prevents the acid from decomposing hypo into free sulfur, prevents discoloration of the solution because of oxidation, and aids in preventing stains.

Neutralizer

After development, the pores of the swollen emulsion retain a portion of the developer. If allowed to remain, the developer continues its activity. Even when the emulsion is thoroughly rinsed in a water bath before being placed in the fixer, some developer activity remains. This causes uneven stains in the gelatin of the emulsion and makes the negative unusable. To stop development and prevent stains, you must add an acid neutralizer to the fixer. The most frequently used neutralizer is acetic acid.

Hardening Agent

During development, the gelatin becomes softened and swells. Frilling, reticulation, scratches, and other undesirable effects may result when processing is continued without hardening the emulsion. A hardening agent is included in the fixer to harden the emulsion in the fixing bath. The most common hardening agent is potassium alum. The hardening and toughening of the gelatin by the alum stops the tendency of the emulsion to swell but leaves it expanded and rigid enough for the washing process.