Potassium Dichromate

(See Potassium Bichromate.)

Potassium Ferric Oxalate

Fe2(C2O4)3 + 3K2C2O4 + 6H2O. Emerald-green crystals. Soluble in water. Used in conjunction with sodium sulphite and hyposulphite as a reducer.

Potassium Ferricyanide

K3Fe(CN)6 Red Prussiate of Potash. Large, red prisms or red powder. Soluble in water. Used with hypo for reducing density of gelatin negatives; also in the manufacture of ferro-prussiate paper.

Potassium Ferrocyanide

K4Fe(CN)6 + 3H2O - II. Yellow Prussiate of Potash. Lemon-yellow, tabular crystals, or prisms; mild, saline taste; efflorescent on exposure to air. Soluble in 4 parts cold water and 2 parts boiling water. Insoluble in alcohol. Not poisonous alone, but acids will change it into a very violent POISON. Used in some of the blue-print processes; sometimes used in the developer to prevent fog. Assists in giving a clear, black image and a strong, brilliant negative.

Potassium Hydrate

(See Potassium Hydroxide.)

Potassium Hydroxide

KOH + H2O.

Potassium Hydrate; Caustic Potash. White, deliquescent pieces showing a crystalline structure on fracture. Soluble in both water and alcohol. Absorbs carbon dioxide from air. Generally obtainable in cylindrical-shaped sticks. Used sometimes in alkaline development.

Potassium Iodide

KI.

Colorless, transparent, or translucent, cubical crystals, or white, granular powder; pungent, saline, bitter taste. Very soluble in water and in 40 parts of absolute alcohol. Used principally in the manufacture of some gelatin emulsions to form silver iodide.

Potassium Metabisulphite

K2S2O6. Potassium Pyrosulphite. Large, colorless, transparent crystals. Soluble in cold water and has an odor of sulphuric acid, and a slight acid reaction. Not soluble in alcohol. Used in some developing formula; as a preservative of the developing agent, and also to acidify the sodium hyposulphite fixing bath.

Potassium Nitrate

KNO3. Saltpetre. Colorless, transparent, prismatic crystals, or crystalline powder, permanent in air. Soluble in 4 parts of cold, and in less than 0.5 parts of boiling, water. Almost insoluble in alcohol. In solution it is neutral to litmus paper. Used in the manufacture of nitric acid and pyroxylin (gun cotton).

Potassium Oxalate

K2C2O4 + H2O. Neutral Oxalate of Potash. Colorless, transparent crystals. Soluble in 3 parts water. Used in photography as a solvent for ferrous oxalate, therefore is the developing agent for platinum papers; also used as the developer for silver bromide and silver chloride plates and papers.

Potassium Percarbonate

K2C2O8 + H2O.

White crystals. Soluble in water with evolution of oxygen. Used principally for removing last traces of sodium hyposulphite from negatives and prints. (See Hypo Eliminator.)

Potassium Permanganate

KMnO4.

Dark-purple, slender, opaque prisms; blue metallic reflection. Sweet, with astringent after-taste. Soluble in 16 parts cold water and in 3 parts boiling water. Insoluble in alcohol. Used for intensifying wet-plate collodion negatives, as a test for hypo, and as a hypo eliminator. Also as a negative reducer.