Potassium hydroxide, widely known by its common name caustic potash and represented by the chemical formula KOH, is a strong inorganic base that dissociates completely in water to produce a highly alkaline solution with a pH approaching 14 at working concentrations. It is one of the most powerfully alkaline substances used in photographic chemistry, and its exceptional alkalinity makes it particularly well suited as the accelerating component in high contrast and high activity developing solutions where a strongly alkaline environment is required to drive rapid, vigorous, and complete development of the photographic emulsion.
In photographic developer formulations, potassium hydroxide serves the same fundamental function as milder alkaline accelerators such as sodium carbonate and potassium carbonate - raising and maintaining the pH of the developer solution to the level required for the developing agents to function effectively - but does so at a far more extreme level of alkalinity. While sodium carbonate typically produces developer solutions in the pH range of 10 to 11, and potassium carbonate produces solutions in the range of 11 to 12, potassium hydroxide can raise the developer pH to 13 or above, producing a level of alkalinity that dramatically accelerates the activity of the developing agents present in the solution and drives development at a rate and intensity quite different from that achievable with milder accelerators.
This extreme alkalinity makes potassium hydroxide the preferred accelerator for high contrast developer formulations - most notably the lithographic or lith developers used in graphic arts photography and in the specialist darkroom printing technique known as lith printing - where the combination of a highly alkaline environment with hydroquinone as the sole developing agent produces the characteristic infectious development effect that gives lith printing its distinctive visual qualities. In infectious development, the strongly alkaline potassium hydroxide environment enables hydroquinone - which is normally a slow acting, high contrast developing agent - to exhibit an autocatalytic behaviour in which the development products of already developing crystals dramatically accelerate the development of adjacent crystals, producing the infectious spread of development from exposed to lightly exposed areas that creates the characteristic sharp tonal transitions, deep shadows, and delicate highlight gradation of lith printed images.
Potassium hydroxide is also used in certain high energy film developers intended for push processing applications, where the extreme alkalinity drives rapid, energetic development that compensates for significant underexposure by developing the latent image centres in lightly exposed crystals more completely than a milder developer would, at the cost of increased grain and contrast. In some specialised technical and scientific developer formulations, potassium hydroxide provides the high pH needed for developers working with low sensitivity or specialised emulsions that require a highly active chemical environment for adequate development.
The handling of potassium hydroxide demands the utmost caution and strict adherence to appropriate safety precautions, as it is among the most caustic and potentially dangerous chemicals encountered in photographic chemistry. In solid form and in concentrated solution, potassium hydroxide is intensely corrosive to all biological tissue, causing severe and deep chemical burns on contact with skin, eyes, or mucous membranes that can penetrate rapidly and cause serious, potentially permanent injury. Eye contact with potassium hydroxide solution is a particular hazard, as even brief exposure can cause severe corneal damage and permanent vision impairment or blindness. The dissolving of potassium hydroxide in water is a strongly exothermic reaction that generates considerable heat, and the solid must always be added slowly and carefully to water - never the reverse - to prevent splashing of the hot, caustic solution.
Appropriate protective equipment including chemical resistant gloves, eye protection or a full face shield, and protective clothing must be worn at all times when handling potassium hydroxide in any form. All work should be carried out in a well ventilated area, and emergency eyewash facilities and copious clean running water should be immediately accessible for first aid in the event of accidental contact. Disposal of potassium hydroxide solutions must be carried out in accordance with applicable local regulations, typically requiring neutralization to a near neutral pH before disposal through drainage.