Chemicals

Description

The printing industry employs graphic arts photography in the reproduction of artwork and copy, using materials similar to those in other fields of photography. Wastes associated with this part of the prepress operation include developed film, developer, stop bath, fixer, solvents in plate coatings and developers, outdated materials, waste fixer containing silver compounds, and rinse water.

These are the basic steps involved in the black and white photographic process.

The photographic film, which is coated with a silver halide emulsion* (the "film" on the plastic base, consisting of many tiny grains of silver halide evenly spread throughout the emulsion) is exposed to capture the image. (Silver halides are silver salts, which includes silver bromide and silver chloride).

Then the film is developed. The developer reacts with the emulsion in proportion to the amount of light exposure it received, and where the most light strikes the emulsion it leaves the highest concentration of elemental silver, creating the densest areas.

Typically, the development is stopped through a change in pH by using an acidic stop bath solution such as acetic acid, though sometimes this step is skipped or plain water is used.

Then a fixer solution is used to "fix" the silver in the emulsion. The fixer is usually comprised of sodium thiosulfate (sometimes called sodium hyposulfite or hypo for short), though other chemicals can be used.

Since the unexposed grains of silver halide crystals are dissolved by the fixer, leaving only the elemental silver behind, this creates a negative image (or "negatives"). If this negative is projected onto photographic paper, then it becomes a positve (normal) image because the same principles apply to photographic paper as to film.

Sometimes the stop bath is mixed with the fixer so both of these steps are done at once. In addition, sometimes a "hypo-clearing" agent is used to reduce wash times and water used in rinsing the remaining chemicals out of the film. Thorough washing is necessary to remove the soluble silver compounds and fixing agent, in order to prevent discoloration over time.

Typical solutions and chemicals that comprise them:

  • Developer Solution - Alkaline material that contains accelerator, preservatives and restrainers. May contain sodium hydroxide, sodium carbonate, sodium tetraborate (borax), and/or hydroquinone.
  • Rejuvenator Solution (used to rejuvenate the developer) - Typically contains hydroquinone.
  • Stop bath - Typically is acetic acid.
  • Fixer Solution - Sodium thiosulfite (known also as sodium hyposulfite or hypo), or ammonium thiosulfate.

*In modern films, technically the reactive coating is not an emulsion, since an emulsion is a mixture of non-polar (for example, oil) and polar (for example, water) chemicals. However, this terminology is still used in common practice. (The coating is actually a colloid suspension, which typically includes gelatin).

Best Management Practices & Pollution Prevention

A proper photographic chemical management plan should include

  • Proper handling of chemicals
  • Proper storage of chemicals
  • Using chemicals which are less hazardous (example: hydroquinone-free developer)
  • Silver recovery
  • Water use reduction

Other tips

  • Use rejuvenator solution in developer solution to extend the life of the developer
  • If using traditional prepress, make the switch to electronic prepress technology
  • Neutralize acidic or alkaline waste film processing solutions on-site to make them acceptable to discharge into the sanitary sewer system. Simple neutralization treatment activities are exempt from the U.S. EPA RCRA on-site waste treatments regulations

Since there are a variety of wastes associated with this part of the prepress operation (developed film, developer, stop bath, fixer, solvents in plate coatings and developers, outdated materials, waste fixer containing silver compounds, and rinse water), conservation and reclamation should be the first concern.

Many manufacturers of photographic chemicals have products specifically designed to neutralize the acidic and basic chemicals used in photoprocessing. See your vendor for details.

Equipment for silver recovery can be installed which recovers the silver compounds that would otherwise go down the drain. Details can be found at the Kodak FAQ on Silver Management, and at the PRO-ACT Factsheet on Silver Recovery from Photographic and Imaging Wastes.

For access to vendors who may supply alternative materials and equipment, see the PNEAC Vendor Directory.

Environmental Regulations

Silver-laden wastewater is regulated by local discharge limits for silver. The local POTW establishes this limit based on the number of organizations within the community that discharge silver laden waste water and their U.S. EPA National Pollution Discharge Elimination System (NPDES) permit limits.

The photographic process uses both highly acidic and basic chemicals. These most be disposed of properly. Acid wastes and alkaline wastes should be neutralized prior to being discharged into the sanitary sewer system. The local POTW establishes the range of pH values of the waste water allowed to be discharged based on their U.S. EPA National Pollution Discharge Elimination System (NPDES) permit limits.

Many companies which produce photographic chemicals and/or equipment may also have pH-neutralizing and chemical reclaiming products available. Contact your vendor for details.

Additional Details

Health & Safety

Film processing chemistry is not necessarily toxic, however care should be take when handling chemicals due to the low and high pH's present in the fixer and developer respectively.

The act of neutralizing acidic or caustic processing bath can result in an exothermic chemical reaction (gives off heat). Proper precautions, including wearing personal protective equipment, should be taken if solutions are treated on site.