Photohalides

The rose-red to purple-brown mixtures of silver subchloride and silver chloride are called by Carey Lea " photochlorides." They are formed by the interaction of colloidal silver and a suspension of silver chloride; by the action of organic reducers and of light on silver chloride; and by the electrolytic chlorination of sheet silver. The photochlorides are characterized by their great sensitiveness to light, blue light producing a blue coloration, and red light a red coloration. This action is reversible, the blue coloration being transformed into red by the action of red light, and so on. This phenomenon is inapplicable to the production of colour-photographs, for white light causes a darkening in colour.

There are several theories as to the constitution of the silver sub-halides in the latent image. The molecular theory regards the subhalides as definite chemical compounds. The adsorption theory regards them as adsorption-compounds of colloidal silver and subhalides. The molecular theory is advocated by Trivelli, who considers the colour-changes to indicate the existence of several silver subhalides, which yield solid solutions with each other and with the silver halides. He also regards the mechanism of " reduction " with ammonium persulphate as favouring the molecular theory.

Light reacts with silver halides, producing a series of subhalides containing a diminishing proportion of halogen, the colour-changes taking place for all the halides in the sequence green, bluish-green, blue, violet, red, orange, yellow. Guareschi has noted that the darkening of silver salts by light was observed before the time of Boyle (1663), and that investigations made by Schulze (1727), Beccari (1757), and Scheele (1777) were very important for the development of photography. In a more recent paper, Boruttau states that the colour-changes undergone by silver salts under the influence of light were first mentioned by Konrad Gessner in 1565 in his work De omni verum fossilium genere libri aliquot, where the darkening of native horn-silver is cited. Hydrogen peroxide has no action on silver bromide, but with the green, blue, or red photohalide oxygen is immediately evolved, with formation of silver bromide and Silver monoxide.

Luppo-Cramer prefers the adsorption-theory because the red, blue, or violet photohalides are formed from the hydrosols of the silver halides in presence of colloidal silver by precipitation with any electrolyte, and treatment of the resulting gel with nitric acid. He attributes the action to adsorption of colloidal silver by the gel of the normal silver halide. He states that identical silver halides are produced by the action of light on silver chloride and bromide, and regards the assumption of the existence of subhalides as unjustifiable. An investigation of the action of light on silver halides has been made by Hartung with the aid of the microbalance, and has furnished evidence in support of Luppo-Cramer's view. On exposure to light and air, silver bromide loses 2.4 per cent, of its total bromine, its colour changing from pale yellow to pale purple; but exposure to the action of bromine in absence of light restores the original colour, the initial weight being regained almost completely. In air, silver chloride loses 4.1 per cent, of its chlorine, and in vacuum 81.0 per cent., the original weight being restored by the action of chlorine in absence of light. In air, silver iodide loses 1.1 per cent, of iodine, and in vacuum the action is much more rapid, though relatively slower than with the chloride or bromide. The fact that Llippo-Cramer's photobromide plates, containing colloidal silver and silver bromide, are unaffected by hydrogen peroxide is advanced by Trivelli as an objection to the adsorption-theory.

Reinders has suggested that the photohalides are normal silver halides coloured by admixture of a small proportion of colloidal silver. The divergence in properties between the photohalides and the latent image is attributed by him to differences in the number, form, and distribution of these colloidal silver particles throughout the silver halide. He regards this theory as explaining, inter alia, the production of different colours by the action of light.

Lorenz and Hiege state that exposure of silver chloride or bromide to light causes the separation of colloidal, metallic silver, the particles increasing in size as the action continues. They regard this phenomenon as constituting important evidence in support of the view that the latent photographic image consists of colloidal silver in an ultra-microscopic form.

According to Sichling, the photohalides may consist of -

1. Amorphous silver and silver halide, either mixed together, or with the silver adsorbed by the halide.
2. A mixture of silver subhalide, Ag₂X, and silver halide.
3. A solid solution of silver subhalide, Ag₂X, and silver halide.
4. A solid solution of silver and silver halide. He considers the most probable theory is one assuming a condition of solid solution.