Positive Lens

(See Lens, Positive.)

Potash Caustic

(See Potassium Hydroxide.)



A mineral. Fresh cut surface has silvery luster, rapidly passing to bluish or gray. Caution. Keep covered with benzene or other liquid free from oxygen. Salts of potassium in combination with carbonate are often used as an accelerator in various developing formulae.

Potassium Bicarbonate


Colorless, transparent or translucent crystals, or white powder; slightly alkaline; saline taste. Soluble in about 3 parts water, almost insoluble in alcohol. Often sold as potassium carbonate, but is less soluble in water and less alkaline.

Potassium Bichromate

K,Cr2O7 - V.

Bichromate of Potash, Potassium Dichromate, Red Chromate of Potash. Large, orange-red, translucent crystals; bitter, metallic taste. Soluble in 10 parts water. Use. When in contact with gelatin or other organic matter it is decomposed by light and renders the gelatin insoluble and incapable of absorbing water. For this reason it is the foundation of a number of photographic printing processes, such as the carbon process.

Potassium Bromide

KBr - I; II; IV.

White, granular powder, or medium-size crystals; pungent, saline taste. Soluble in about 1.5 parts water, in 180 parts alcohol, in less than 1 part boiling water, in 16 parts boiling alcohol. Used largely in the manufacture of bromide emulsion; also as a restrainer in alkaline developer for both plates and paper. A 10% solution is usually employed for this latter purpose.

Potassium Caustic

(See Potassium Hydroxide.)

Potassium Carbonate

K2CO3 - II; IV. Salts of Tartar. White, deliquescent, granular powder; very soluble in water. Used as an accelerator in various developing formulae.

Potassium Chlorate


Colorless, lusterless, tabular crystals. Soluble in 16 parts of cold, and 12 parts of boiling, water, and in 13 parts of 85% alcohol; practically insoluble in absolute alcohol and in ether. Used in the platinum process for increasing the depth of contrast; also used in flashlight compounds.

Potassium Chloride


Colorless, cubical crystals, or white, crystalline powder. Soluble in 3 parts of cold, but not soluble in boiling, water. Insoluble in alcohol or ether. Use. Added to the ferrous oxalate developer as a means of obtaining sepia tones with bromide paper prints.

Potassium Chloroplatinite


Platinum and Potassium Chloride (Platinous). Ruby-red crystals. Soluble in water. Used in sensitizing platinum paper, also in acid toning bath.

Potassium Citrate

K3C6H5O7 + H2O.

Colorless crystals, or white, deliquescent powder; cooling, saline taste. Very soluble in water; slightly soluble in alcohol. Used occasionally in developing solutions.

Potassium Cyanide


White, amorphous, deliquescent pieces. Soluble in 2 parts of water, and readily in boiling 65% alcohol. POISON. Used largely as a fixing agent in the collodion process. Unsuitable for gelatin plates, as it tends to dissolve gelatin.