The DARKROOM COOKBOOK, Third Edition

Pharmacopoeia 187
Uses: To precipitate silver from photographic waste. Used in toning formulas.
Notes: Decomposes upon exposure to air. Potassa sulfurated often becomes coated with a
hard crust having no value. This crust should be broken away as much as possible before
weighing out the inner part.
POTASSIUM ALUM (see Alum, Potassium)
POTASSIUM BICHROMATE (see Potassium Dichromate)
POTASSIUM BROMIDE
Synonyms: Bromide, Bromide of potash, Bromide of potassium.
Appearance: White crystalline granules or powder.
Uses: Fog preventer and restrainer in developing solutions.
Notes: Potassium bromide is the most commonly used restrainer in developing formulas,
especially those using hydroquinone, pyrocatechin, or pyrogallol. The addition of potassium
bromide usually results in a reduction of contrast and can have a warming effect on image
tone when used in paper developers. It is also used as an ingredient in intensifying, reducing,
toning, and many other photographic solutions.
POTASSIUM CARBONATE
Synonyms: Carbonate of potash, Pearl ash, Potash, Salt of tartar, Salt of wormwood,
Subcarbonate of potash.
Appearance: White, deliquescent, granular powder.
Uses: Since potassium carbonate is more soluble than sodium carbonate it is used in concentrated
solutions where an alkali is needed.
Notes: Potassium carbonate can be substituted for sodium carbonate as an alkali accelerator
in developers. As its solubility is much greater, developers with greater concentration can be
formulated. Thirteen parts by weight are equal to 10 parts by weight of sodium carbonate.
Substituting potassium carbonate for sodium carbonate will help to obtain warmer tones in
paper development.
Substitution: To substitute potassium carbonate anhydrous, multiply the amount of sodium
carbonate monohydrate by 0.90.
(see Sodium carbonate)
POTASSIUM CHLORIDE
Synonyms: Chloride of potash, Muriate of potash.
Appearance: Colorless or white crystals or powder.
Uses: In special developing formulas.
POTASSIUM CITRATE
Synonyms: Tribasic citrate of potash.
Appearance: White granular powder.
Uses: As a restrainer in alkaline development; used in several copper toning baths.