MQ and PQ developers are two closely related categories of photographic developing solution, each named after the combination of chemical reducing agents - also known as developing agents - that they contain. MQ developers contain a combination of metol and hydroquinone as their active developing agents, while PQ developers substitute phenidone in place of metol, combining it with hydroquinone to achieve a similar but subtly different developing action. Both types of developer exploit the well established phenomenon of superadditivity - also known as synergism - in which the combined developing activity of two complementary developing agents working together is significantly greater than the sum of their individual activities when used separately.
Metol, also known by various trade names including Elon, is a developing agent that acts quickly and produces good shadow detail and fine gradation across the mid-tones of the image, but has relatively limited capacity and is prone to causing skin sensitisation in some individuals with repeated exposure. Hydroquinone is a slower acting developing agent that builds density in the highlight areas of the image and contributes contrast, but has relatively poor shadow detail rendering when used alone and is sensitive to low temperatures, becoming increasingly inactive below approximately 15 degrees Celsius. When combined in an MQ developer, metol and hydroquinone complement each other's characteristics through superadditivity, with the metol rapidly initiating development and generating the intermediate oxidation products that regenerate and activate the hydroquinone, resulting in a developer of greater overall activity, better tonal gradation, and more balanced shadow to highlight rendering than either agent could achieve independently.
PQ developers replace metol with phenidone, a developing agent introduced by Ilford in the 1940s that offers several practical advantages over metol. Phenidone is effective at very low concentrations - typically ten to twenty times less phenidone is required to achieve the same developing activity as metol - making PQ developers more economical in their use of the more expensive developing agent. Phenidone also exhibits a stronger superadditive effect with hydroquinone than metol does, producing a very high level of combined developing activity from relatively modest concentrations of both agents. Additionally, phenidone causes significantly fewer skin sensitisation reactions than metol, making PQ developers generally safer and more comfortable to handle in darkroom practice.
Both MQ and PQ developers have found widespread application in a broad range of photographic developing formulations, including film developers, paper developers, and general purpose developers intended for use with both materials. Many of the most well known and widely used commercial photographic developers are MQ or PQ formulations, including Kodak D-76 and Ilford ID-11, which are MQ developers, and Ilford Ilfosol and various other Ilford and third party developers which are PQ based. The versatility, reliability, and well understood behaviour of MQ and PQ developing systems have made them the foundation of the majority of conventional photographic developing chemistry for many decades.