when combined with hydroquinone. The combina-
tion of Phenidone and hydroquinone is called the
PQ system. It was also less susceptible to accumu-
lation of free bromide ions, a development reaction
byproduct that retards further development when
used in typical fine grain film developers. These
advantages of Phenidone improved several aspects
of the development process, especially in volume
processing plants, once this compound became
commercially available in 1953. Also, since Pheni-
done is used in such a small quantity, it enabled
formulation of packaged developer concentrates,
which are ready to use by simply diluting with
water. However, original Phenidone (also called
Phenidone A) was relatively unstable in alkaline
solutions. Thus Phenidone B (4-methyl-1-phenyl-
3-pyrazolidone) and Dimezone (4,4-dimethyl-1-
phenyl-3-pyrazolidone) were introduced. These
compounds solved the instability problem but
they had a new shortcoming of not being very
soluble in water. Ultimately, Dimezone S (4-hydro-
xymethyl-4-methyl-1-phenyl-3-pyrazolidone) was
introduced, which solved both the stability prob-
lem in alkaline solutions and the solubility problem
in aqueous solutions. In today’s commercial devel-
opers supplied as liquid concentrates, Dimezone S
is commonly used in a superadditive combination
with hydroquinone, with PQ and MQ developer
systems covering the majority of black-and-white
film and print developers in contemporary use.
Another developing agent from the 1880s that is
of current importance is p-phenylenediamine.
Chemical derivatives of this agent are used in
developers for color photography. When exposed
silver halide crystals are developed with these
agents, the developing agent is oxidized. An oxi-
dized developing agent reacts with a dye coupler
to form a dyein situ.Dye couplers are usually
colorless, but when reacted with oxidized forms
ofp-phenylenediamines, they form dyes whose
color depends on the dye coupler molecule. In
other words, dye coupler provides one-half of a
dye molecule, andp-phenylenediamines the other
half; the developing reactions of the exposed crys-
tals combine the two halves. The developed metal-
lic silver is then removed, and the resulting image
consists of the color dyes. This is the major differ-
ence of chromogenic (color) development from
black-and-white development.
Practical chemical developer solutions usually
contain developing agent(s), an alkaline agent,
and a preservative. Many developers also contain
an antifogging agent. The alkaline agent in early
use was ammonia solution, but it was prone to
inconsistent results, and was replaced with more
stable agents such as hydroxides, carbonates, and
borates. Carbonates and borates form an effective
buffering system in the pH ranges commonly used
for developer solutions. Early chemical developers
lacked a preservative and had short useful lives.
Berkeley introduced sodium sulfite to the developer
solution in 1882, which prolonged the useful life of
the developer. Sulfite in various forms, including
sodium sulfite itself, is extensively used in modern
developer solutions. Developers of high developing
power often cause fogging that does not appear in
developers of lesser powers. This type of fog was
suppressed by bromide added to the developer
solution. The effect of bromide in developer was
studied from the very early days of sensitometric
investigation of silver-gelatin materials, and it
remains one of the most frequently used antifog-
ging agents today, along with newer agents such as
benzotriazoles and 1-phenyl-5-mercaptotetrazole.
With numerous developing agents discovered,
and many variables associated with them, a great
many developer formulae were concocted and pub-
lished for pictorial negatives and prints. It is con-
ceivable that each new developer was hoped to
improve the image quality, photographic speed,
tonal range of the developed image, graininess,
and perhaps other aspects of the image quality, as
seen in all kinds of advantages claimed for the for-
mulae. However, as photographic scientists and
engineers accumulated knowledge and experience,
it was realized that the entire range of developer
solutions could be formulated with a few standard
developing agents (Metol, Phenidone, and hydro-
quinone) and a few common agents. Today, pictor-
ial photographic films and papers are manufactured
to exacting photographic properties and technical
specifications, to be processed in one of the stan-
dard developers. Modifying some properties of mo-
dern material through adjusting the developer
formulation is possible only with often unaccepta-
ble costs in other aspects of the image quality.
As well, the technical requirement for developers
changed over time as photography innovated with
both photographic processes as well as equipment,
particularly in advances in the camera. In the early
years of dry plate photography taken with large-
format view cameras, the negatives were printed on
papers of generally low contrasts, by contact print-
ing, or with low magnification factors. Therefore,
high contrast negatives were preferred and graini-
ness was unimportant. However, when high-con-
trast print emulsions became common, especially in
combination with the 35 mm film format, a low
contrast developer with improved fine grain quality
became necessary. Eastman D–76 fine grain devel-DEVELOPING PROCESSES