The DARKROOM COOKBOOK, Third Edition

188 THE DARKROOM COOKBOOK
POTASSIUM DICHROMATE
Synonyms: Bichromate of potash, Bichromate of potassa, Potassium dichromate, Red chromate
of potash, Red chromate of potassa.
Appearance: Bright, orange-red crystals.
Uses: Bleaching ingredient in tray cleaners and chromium intensifi ers.
Notes: Bichromate is the older word for dichromate.
Caution: Potassium dichromate is corrosive and can cause burns. Avoid contact with eyes,
skin, and clothing. In case of contact, fl ush eyes and skin with water. Potassium dichromate
can also cause allergic skin reactions; the dust can cause irritation. Mixing should be performed
in a well-ventilated area using gloves, goggles, and aprons. Potassium dichromate is a
suspected carcinogen.
POTASSIUM FERRICYANIDE
Synonyms: Ferricyanide, Ferricyanide of potash, Red praised of potash.
Appearance: Ruby red, lustrous crystals.
Uses: Principal use is in bleaching baths; iron printing processes; and as a bleach before
sulfi de toning. Also used in dye-toning formulas.
Notes: Keep protected from light.
POTASSIUM FERROCYANIDE
Synonyms: Ferrocyanide of potash, Yellow prussiate of potash.
Appearance: Lemon yellow crystals or powder.
Uses: A small amount in pyro and hydroquinone developers tends to lower fog and give
greater density.
Notes: Light-sensitive in solution.
POTASSIUM HYDROXIDE
Synonyms: Caustic potash, Caustic potassa, Hydrate of potassa, Potassium hydrate.
Appearance: White, deliquescent lumps or sticks.
Uses: Caustic alkali accelerator in development.
Caution: Poisonous; attacks the skin, infl icting severe burns. Extremely corrosive. Keep in a
tightly sealed bottle and wear gloves. It is good practice to dissolve potassium hydroxide separately
in a small amount of cold water, then add the solution after the developing agent has
been dissolved, stirring vigorously.
POTASSIUM IODIDE
Synonyms: Iodide of potash.
Appearance: White crystals, granules, or powder.
Uses: Has the property of reducing emulsion fog. Can be used as a restrainer although this
application has not been adequately explored.
POTASSIUM METABISULFITE
Synonyms: Metabisulfi te of potash, Potassium pyrosulfi te.
Appearance: Transparent needle crystals or crystalline powder.