Mixing Photo Chemicals can be bought in liquid form, which only needs diluting, or powder form, which needs dissolving and diluting.
Hazards
- Developer solutions and powders are often highly alkaline, and glacial acetic acid, used in making the stop bath, is corrosive by skin contact, inhalation and ingestion.
- Developer powders are a potential skin and inhalation hazard, due to the alkali and developers themselves (see Developing Baths below).
Precautions
- Use liquid chemistry whenever possible, rather than mixing developing powders. Pregnant women, in particular, should avoid exposure to powdered developers.
- When mixing powdered developers, use a glove box or local exhaust ventilation. If needed, wear a NIOSH-approved particulate respirator.
- Wear gloves, goggles and a protective apron when mixing concentrated photochemicals. Always add acid to water, never the reverse.
- In case of skin contact, rinse with copious amounts of water. In case of eye contact, rinse for at least 15-20 minutes using an eyewash station and seek medical attention.
- Store concentrated acids and other corrosive chemicals on low shelves to reduce the chance of exposure of burns to the face or eye in case of breakage and splashing.
- Do not store photographic solutions in glass containers.
Developing Baths
The most commonly used developers are hydroquinone, monomethyl para-amino phenol sulfate, and phenidone. Several other developers are used for special purposes. Other common components of developing baths include an accelerator, often sodium carbonate or borax, sodium sulfite as a preservative, and potassium bromide as a restrainer or anti fogging agent.
Hazards
- Developers are skin and eye irritants, and in many cases strong sensitizers. Monomethyl-paminophenol sulfate creates allergies to it (although this is thought to be due to the presence of para-phenylene diamine). Hydroquinone can cause depigmentation and is a mutagen.
- Most developers are toxic and potentially fatal if ingested.
- Para-phenylene diamine and some of its derivatives are a skin, ingestion and inhalation hazard.
- Sodium hydroxide, sodium carbonate, and other alkalis used as accelerators are corrosive to the skin and can cause chemical burns.
- Potassium bromide is a potential skin, ingestion and inhalation hazard.
- Sodium sulfite is a potential ingestion and inhalation hazard.
Precautions
- See the section on Mixing Photo chemicals for mixing precautions.
- Do not put your bare hands in developer baths. Use tongs instead. If developer solution splashes on your skin or eyes immediately rinse with copious amounts of water. For eye splashes, continue rinsing for 15-20 minutes and seek medical attention.
- Do not use para-phenylene diamine or its derivatives.
Stop Baths and Fixer
Stop baths are usually weak solutions of acetic acid. Acetic acid is commonly available as pure glacial acetic acid or 28% acetic acid. Some stop baths contain potassium chrome alum as a hardener. Fixing baths contain sodium thiosulfate (“hypo”) as the fixing agent, and sodium sulfite and sodium bisulfite as a preservative. Fixing baths also may also contain alum (potassium aluminum sulfate) as a hardener and boric acid as a buffer.
Hazards
- Acetic acid, in concentrated solutions, is a skin, ingestion and inhalation hazard. It can cause dermatitis and irritate the mucous membranes. The final stop bath is a skin irritant when in direct contact.
- Potassium chrome alum or chrome alum (potassium chromium sulfate) is a skin, ingestion and inhalation hazard. Upon heating or long-standing in solution, it can decompose to form sulfur dioxide, which can be a respiratory irritant. Many asthmatics are particularly sensitive to sulfur dioxide.
- Sodium bisulfite decomposes to form sulfur dioxide if the fixing bath contains boric acid, or if acetic acid is transferred to the fixing bath on the surface of the print.
- Alum (potassium aluminum sulfate) may cause skin allergies or irritation.
Precautions
- All darkrooms require good ventilation to control the level of acetic acid vapors and sulfur dioxide gas produced in photography.
- Wear gloves and goggles.
- Cover all baths when not in use to prevent evaporation or release of vapors and gases.
Intensifiers and Reducers
A common after-treatment of negatives (and occasionally prints) is either intensification or reduction. Common intensifiers include hydrochloric acid and potassium dichromate, or potassium chlorochromate. Mercuric chloride followed by ammonia or sodium sulfite, Monckhoven’s intensifier consisting of a mercuric salt bleach followed by a silver nitrate/potassium cyanide solution, mercuric iodide/sodium sulfite, and uranium nitrate are older, now discarded, intensifiers. Reduction of negatives is usually done with Farmer’s reducer, consisting of potassium ferricyanide and hypo. Reduction has also been done historically with iodine/potassium cyanide, ammonium persulfate, and potassium permanganate/sulfuric acid.
Hazards
- Potassium dichromate and potassium chlorochromate are probable human carcinogens and can cause skin allergies and ulceration. Potassium chlorochromate can release chlorine gas if heated or if acid is added.
- Concentrated hydrochloric acid is corrosive; the diluted acid is a skin and eye irritant.
- Mercury compounds are moderately toxic by skin absorption/contact. They are also toxic by inhalation and ingestion. Uranium intensifiers are radioactive, and are especially hazardous.
- Sodium or potassium cyanide is extremely toxic by inhalation and ingestion, and moderately toxic by skin contact. Adding acid to cyanide forms extremely toxic hydrogen cyanide gas, which can be rapidly fatal.
- Potassium ferricyanide, although only slightly toxic by itself, will release hydrogen cyanide gas if heated, acid is added, or if exposed to strong ultraviolet light (e.g., carbon arcs). Cases of cyanide poisoning have occurred through treating Farmer’s reducer with acid.
- Potassium permanganate and ammonium persulfate are strong oxidizers and may cause fires or explosions in contact with solvents and other organic materials.
Precautions
- Chromium intensifiers are probably the least toxic intensifiers, even though they are probable human carcinogens. Gloves and goggles should be worn when preparing and using these intensifiers. Mix the powders using local exhaust ventilation or in a glove box. Do not expose potassium chlorochromate to acid or heat.
- Do not use mercury, cyanide or uranium intensifiers, or cyanide reducers because of their high or extreme toxicity.
- The safest reducer to use is Farmer’s reducer. Do not expose Farmer’s Reducer to acids, ultraviolet light, or heat.
Toners
Toning a print usually involves replacement of silver by another metal, for example, gold, selenium, uranium, platinum, or iron. In some cases, the toning involves replacement of silver metal by brown silver sulfide. A variety of other chemicals are also used in the toning solutions.
Hazards
- Sulfides release hydrogen sulfide gas during toning, or when treated with acid.
- Selenium is a skin and eye irritant and can cause kidney damage. Treatment of selenium salts with acid may release hydrogen selenide gas. Selenium toners also give off large amounts of sulfur dioxide gas.
- Gold and platinum salts are strong sensitizers and can produce allergic skin reactions and asthma.
- Thiourea is a probable human carcinogen.
Precautions
- Carry out normal precautions for chemicals as described in the previous sections. In particular, wear gloves and goggles. See also the section on mixing photo chemicals.
- Toning solutions must be used with local exhaust ventilation.
- Take precautions to make sure that sulfide or selenium toners are not contaminated with acids. For example, with two bath sulfide toners, make sure you rinse the print well after bleaching in acid solution before dipping it in the sulfide developer.
- Avoid thiourea whenever possible because of its probable cancer status.
Other Hazards
Many other chemicals are used in black and white processing, including, formaldehyde as a pre-hardener, a variety of oxidizing agents as hypo eliminators (e.g., hydrogen peroxide and ammonia, potassium permanganate, bleaches, and potassium persulfate), sodium sulfide to test for residual silver, silver nitrate to test for residual hypo; solvents such as methyl chloroform and freons for film and print cleaning, and concentrated acids to clean trays.
Electrical outlets and equipment can present electrical hazards in darkrooms due to the risk of splashing water.
Hazards
- Concentrated sulfuric acid, mixed with potassium permanganate or potassium dichromate, produces highly corrosive permanganic and chromic acids.
- Hypochlorite bleaches can release chlorine gas when acid is added, or if heated.
- Potassium persulfate and other oxidizing agents used as hypo eliminators may cause fires when in contact with easily oxidizable materials, such as solvents and other combustible materials. Most are also skin and eye irritants.
Precautions
- See previous sections for precautions in handling photographic chemicals.
- Cleaning acids should be handled with great care. Wear gloves, goggles and acid-proof, protective apron. Always add acid to the water when diluting.
- Do not add acid to, or heat, hypochlorite bleaches.
- Keep potassium persulfate and other strong oxidizing agents separate from flammable and easily oxidizable substances.
- Install ground fault interrupters (GFCIs) whenever electrical outlets or electrical equipment (e.g. enlargers) are within six feet of the risk of water splashes.
