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Theory of Development

silver, bromide, charge, negative, ions, positive and solution

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DEVELOPMENT, THEORY OF The old theory of development was that the reducing agent or developer reduced the exposed silver bromide or latent image to metallic silver, and that the bromine combined with the alkali to form an alkaline bromide, and this is usually expressed by the following equation : AgBr + DNa = Ag + Na Br + D in which D merely stands for the developing agent. This was satisfactory as far as it went, but it really explains very little. The later theories, which involve a consideration of the ionic theory, assume that when a salt is dis solved in water it is split up into so-called ions, which are considered to be atoms of the elements carrying an electric charge. Metallic or basic ions are usually termed kations, and the acid ions are termed anions, the former carrying a positive and the latter a negative charge. Chemical reactions are now considered to take place between ions, and only when the substance goes into solution, and thus becomes dissociated or ionised. According to this we might represent the formation of silver bromide by the following equation : + – + – + – K + Br + Ag + = Ag Br + K + which roughly shows the dissociation of potassium bromide into the potassion K carrying a + or positive charge, and bromion carrying a – or negative charge, silver nitrate being sociated into positive Ag + and negative nitrion – then the final result would be :— + – + – + – + – KBr + = AgBr + and as a positive and negative charge meet in AgBr this becomes unionised, and is precipi tated as an insoluble precipitate.

If now we apply this to development, and we assume the formation of an alkaline phenolate, as in the case of pyrogallol with only sufficient caustic soda to form this, we might represent the action as follows :— That is to say, the pyrogallol loses a negative charge which neutralises the charge on the silver ion. No account is here taken of any change in the pyro, though such must take place, but the oxidation products are not well known.

In the case of hydroquinone, however, where we know that the product formed from it in development is quinone, we can simply write the equation as follows :— and the ionised hydroquinone has merely lost two negative charges which neutralise the positive charges of the silver in the ionised silver bromide, and the two oxygen ions combine to form quinone.

Obviously there are certain physical pheno mena which one must take into consideration, and an emulsion consists of a number of particles of silver bromide embedded in a jelly. The modern theory of a jelly is that it consists of a number of minute cells with passages in between ramifying in all directions, and the cells and passages contain a weak solution of gelatine, whilst the cell walls are formed of a very strong solution. In each cell we may imagine a grain or particle of silver bromide, and for the developer to reach this it is obvious that it must first traverse the passages, and then diffuse through the cell walls. The first is termed macro-diffusion, which takes place at a rapid rate, and the later action, which is comparatively slow, is known as micro-diffusion. It has already been pointed out that a chemical reaction can only occur when the silver bromide goes into solution and is ionised into + Ag and — Br. The instant the developer reaches the dissolved silver it is reduced to the metallic state and deposited, provided there be some nucleus or germ on which it can deposit. This nucleus is the latent image (which see). Were there no nucleus, then the silver would accumulate in solution till super saturation occurred, and then the chemical action would cease. As soon as the dissolved silver is deposited, fresh silver takes its place, and so the process proceeds till the whole of the silver available is reduced.

In the above rough sketch of what is supposed to take place, we must not lose sight of the important fact that, as pointed out in the note on the latent image (which see), Scheffer has proved that the action of light is to cause the protrusion of filaments or threads from the sensitive salt grain, and therefore these would rapture the cell walls, and so render the access of the developing agent much easier.

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