HYDROQUINONE, QUINOL, or DI HYDROXY-BENZENE (CH4(OH).), the most used of all photographic developing agents, being practically indispensable in the making of photographic prints on developing paper. Its characteristic property is the great density it gives to the developed image, to the point of harshness of contrast, and for this reason it is almost always combined with a ((soft)) acting developer, such as metol, edinol, eikonogen, ro dinal, paramidophenol, etc. Hydroquinone fails to work if cold, when it has a tendency to deposit as crystals from the solution. The proper temperature is 65' F. in summer and 70° in winter. At higher temperatures it is lia ble to produce fog in the shadows and exces sive intensity of the high lights.
Hydroquinone is prepared commercially from aniline. A dilution of sulphuric acid is made by dissolving 8 parts of the acid in 30 parts of water: this, when cold, is added to 1 part of aniline, and after cooling, 36 parts of potassium bichromate, or sodium bichro mate, is added so slowly as to avoid much rise in the temperature of the mixture. To the thick brownish liquid resulting is passed a stream of sulphurous acid gas until the mix ture smells strongly of it. It is then filtered
to remove the precipitate, and the filtrate is shaken with ether, which dissolves out the hy droquinone. The ether layer is drawn off and distilled, the ether being recovered, and the hy droquinone remaining in the forms of a yel lowish (almost white) crystalline powder. It is purified by preparing a saturated solution with water and boiling it with animal charcoal and a little sulphuric acid, filtering and crystal lizing.
If chlorine gas be passed through a sidu tion of hydroquinone in benzol, monochlorhydro quinone is obtained, and this also is used largely as a photographic developer under the commercial name of Previous to the war in Europe the entire American supply of hydroquinone had come from the laboratories of Germany. The chem ical workshops of America were called upon to meet the demand, and are producing a qual ity of the reagent which is unsurpassed. The market, however, has been flooded by shame lessly adulterated samples of all photographic developers, and rigid tests are necessary for products where there is no guaranty by a manu facturer of known integrity.