IIYDROQUINONI3 (Formula ; molecular weight, Ito ; synonyms, quinol, hydro kinone, hydrochinone, pyroguinol, pyrogentisic constant product of the action of reducing agents on quinone. The most convenient method of preparing it is by passing sulphur dioxide (sulphurous acid gas) through a warm saturated solution of quinone. It is also prepared com mercially by oxidizing aniline sulphate with potassium dichromate. Hydroquinone is isomeric with pyrocatechin,* but is distinguished from it by the action of oxidizing agents, such as ferric chloride, which converts hydroquinone into fine green metallic prisms of green hydroquinone or quinhydrone, which may also be obtained by mixing aqueous solutions of quinone and hydroquinone.
Hydroquinone is dimorphous, crystallizing in monoclinic plates and from solutions in hex agonal prisms. It has a slightly sweet taste, and is soluble in water, ether, alcohol, and glycerine. An aqueous solution slowly turns brown on exposure to the air, losing its reducing power, and if it be alkaline it will brown more rapidly.
It has recently been discovered to be an excellent developing agent for dry plates, bromide paper, transparencies, etc , bringing out the detail to perfection. Further, it allows of a great latitude in the exposure, as it is slow in action, and consequently easily controlled.
Commercial hydroquinone is usually in the form of nearly white or pale yellowish needle like crystals. These must be kept protected from light and damp, otherwise they will darken.
The best method of preserving it is in solution with alcohol and glycerine, and used either with ammonia or the fixed alkalies, potash, and soda, as accelerators. The following are two such solutions:t Hydroquinone 8 grams Glycerine x drachm Methylated spirit 7 drachms.
Keep in stoppered bottle, excluded from light.
Hydroquinone 8 grains Citric acid..... 8 grains Distilled water 8 drachms The use, however, of a preservative such as potassium metabisulphite, or solution sulphite, is always an advantage, as it enables a large number of plates to be treated with the same devel oper without fear of producing stains or discoloration.
A good formula for a hydroquinone developer for dry plates is the following :— Hydroquinone z grain Sodium carbonate 2 grains Sodium sulphite 2 grains Potassium bromide z ounce One of the chief features of this developer is that a restrainer is rarely necessary, as it does not possess the vigor of pyrogallic acid in attacking the very lightly exposed portions of the image. It does not, however, suit every manufacture of plates. With some it hardly seems to have any developing action, while with others it brings up a clear, sharp, and brilliant negative.
It is also a very suitable developer for bromide paper, or developing contact prints or enlargements. For this purpose the formula reads :—