Ammonium bichromate ( is in smaller crystals and distinctly redder in It is unaffected by exposure to the air, and is very soluble in water, more than 20 per cent at F., and approximately ioo per cent in boiling water. About three times their volume of alcohol or acetone can be added to aqueous solutions at a concentration of 10 per cent at most without precipitation occurring.
A simple solution of bichromate in water is quite stable. A solution to which organic sub stances have been added will gradually turn brown if exposed to the air, and can then produce spontaneous insolubility of gelatine.
The addition of strong mineral acids—sul phuric, nitric, etc.—to solutions of bichromate liberates a proportionate quantity of chromic acid. The addition of hydrochloric acid produces a chlorochromate. Acetic acid has no action on bichromates.
The addition of ammonia or a caustic alkali to a solution of bichromate changes the colour gradually from orange to lemon-yellow, by con verting the bichromate into a neutral chromate. Inversely, neutral chromates in solution are restored to the condition of bichromates by the addition of acids, even the weakest, e.g. acetic acid and carbonic acid.
Ammonia is frequently added to solutions of the bichromates used for sensitizing gelatine for the pigment process up to the point of forming the neutral chromate. The ammonium chromate thus formed does not induce the insolubilization of the gelatine which results always in bichrom ated gelatine. On exposure to light the am monium chromate is first decomposed into ammonia and ammonium bichromate, the sensitiveness being then scarcely inferior to that of gelatine sensitized with bichromate (E. Kopp, 1864). The mixture of chromate and gelatine, when the latter has not been mixed with the pigment, is much lighter in colour than bichromated gelatine. The image of brown oxide, being as dark as with bichromated gelatine, is therefore much more strongly visible, a fact which has led many workers to assume that the addition of ammonia to bichromate solutions produces greater sensitiveness.
638. Physiological Action of Bichromates. The chromates and bichromates are poisonous compounds, but the colour of their solutions is sufficiently distinctive to prevent their being mistaken for any drinkable liquid.
Contact of any abrasion of the skin with a cold solution of bichromate, of the strength used for sensitizing gelatine in the pigment processes, can produce a painful sore, and possibly a serious ulceration. Any scratch or abrasion
ought to be protected by an application of collodion, or by wearing an indiarubber finger stall. In the event of an accidental contact with the solution, copious rinsing should be followed. by washing with commercial peroxide of hydro gen, which decomposes the bichromate.
Frequent immersion of the hands in warm_ solutions of bichromate, even very dilute solu tions such as the baths used in dissolving the gelatine which has remained soluble after ex posure to light, may be the cause of painful skin eruptions in those who are predisposed to the effects of bichromate. These eruptions are not only troublesome, but generally take a long time to cure. Those who have once suffered from them are far more susceptible afterwards to the poisonous action of the bichromates, and are very liable to a recurrence of the trouble.
In the commercial working of the carbon process, the employment of large tanks of warm water for developing the prints should be avoided, as the bichromate dissolved from a series of prints accumulates. By the use of baths of small size, the risk of skin troubles is almost entirely avoided, as the water is con stantly being renewed by the addition of fresh hot water. This renewal may also be effected by a jet of hot water the temperature of which can be regulated as desired. There are various types of heater available, heated by gas or electricity. An amateur, having only a small number of prints to develop, will throw away the water as soon as it becomes discoloured, and renew it as required from a kettle of hot water. The fingers should never be dried after having been immersed in a solution of bichromate without first being rinsed in plain water. After working with a solution of bichromate, the hands and forearms should be scrupulously cleansed with soap and a nail-brush.
The first symptoms of bichromate poisoning are generally shown by irritation between the fingers and on the back of the hands. This is followed by the formation of watery pimples, which increase in number and size and become purulent. The skin then becomes dry, cracks, and peels off in scales. The use of carbolic soap, with a lotion of carbolized glycerine and oint ment containing mercuric nitrate, generally produces a rapid improvement. The general use of a carbolic soap may, in addition, be regarded as an efficient safeguard.
639. Transfer of the Film. The first experi ments in obtaining photographic prints by means of a film of gelatine, sensitized with