This post looks at the chemistry behind developing a film negative from the latent image stage to fixation.

In my first post in this series I described the Mott-Gurney model of explaining how a latent image is formed by the action of light on silver halide. The model is summarised in the following diagram and accompanying text:

A photon of light excites an electron of the bromine atom in the silver halide. The resulting photo-electron and bromine can move through the halide lattice crystal. The photo-electron gets trapped in holes caused by imperfections inn the lattice, making it negatively charged. This attracts interstitial silver ions and metaiilic silver is formed, which in turn can act as a hole. The process repeats until a structure of 4 silver atoms form a latent sensitive point. It is thought that a mimimum structure of 4 silver atoms is required in order that the next step ensues. – (from Kitts, E. Radiographics 1996, 16:1467).

In this post I want to move onto the next stage: developing an image.

Development of the silver image

Let’s start with looking at developing agents. A wide range of chemicals can be used to develop images, both organic and inorganic. The main function of the developing agent is to supply electrons which are required for the reduction of silver ions into metallic silver. When a grain with a latent image center is immersed in developer solution after exposure, these few silver atoms catalyze the reduction of the entire silver halide grain to approximately 1 billion atoms of metallic silver.

However, not any chemical that supplies electrons will do. The chemical has to tolerate a certain pH range, be soluble, operate within a certain temperature range, be stable and so on. The evolution of developers has been a see-saw of balancing between many objectives.

In general, black and white developing agents have a di- or poly- hydroxybenzene structure, that is, a benzene carbon ring with two or more OH groups.

Commonly used developers. Hydroquinone is found in many developers combined with other developers. p-Aminophenol is the basis of Rodinal. The structure of Metol is based on a aminophenol. p-Phenylenediamine forms the basis of Diamine and is used in colour development. Ascorbic acid is the developing agent in Kodak’s XTOL and can substitute for Hydroquinone. Phenidone can substitute for Metol, but resulting in different outcomes.

The most commonly used developing agents are Hydroquinone, Metol, Ascorbic acid and Phenidone. Less comonly used, although important for certain applications are Catechol, Pyrogallic acid, Para-Aminophenol and Amidol.

Hydroquinone adds density and contrast when paired with with other agents. It is relatively unstable, being easily oxidised in air. It is often paired with metol, (MQ developers) or phenidone (PQ developers) where it regenerates metol and phenidone respectively. Metol and phenidone are super-additive with Hydroquinone, meaning that they act synergistically together. A well-known MQ developer is Kodak’s D-76 (or Ilford’s ID11). PQ developers tend to give an increase in speed. Examples include HC-110, Acutol and Microphen. Metol-only developers are also used, such as D-23. and D-25. One might think that without Hydroquinone, D-23/D-25 would be low contrast developers but this view is unjustified.

For further information on the ‘pros and cons’ of each developer I recommend ‘The Film Developing Cookbook’ by Anchell and Troop; also ‘The Darkroom Cookbook’ by Anchell. Both are indispensible to the darkroom practitioner.

Common to these developers are benzene rings. There are usually one or two electron-rich groups of atoms, attached to the ring, which provide the necessary electron to initiate development. The diagram below gives the reaction between Silver bromide and Hydroquinone:

The benzene rings gives great solubility and stability of the developing agent, which allows for regeneration.

But this diagram, often reproduced in textbooks, is a very great simplification. And Hydroquinone-only developers are very specialist, often only used in current darkrooms for lith development in developers like Kodak D-85, Ansco 70 and NewLith.

The way in which the latent image (i.e. metallic silver sensitve points) catalyses image formation is much in dispute. One theory has it that the developing agent injects electrons into the halide crystals thereby effecting the deposition of more metallic silver in an autocatalytic process. Another theory has it that the negative charge on the surface of the halide cystal repels the electrons provided by the developing agent except where silver has already been deposited on the surface (the latent image). A third theory has it that the crystal is an electrical circuit which is put into play by the developer, depositing silver through electrolysis. What seems clear, though, is that MQ and PQ developers are synergistic because the metol/phenidone electron donors at the surface of the crystals are replenished by hydroquinone.

Developers require a soup of other chemicals to ensure that they act efficiently. Preservatives prevent premature oxidation of the developer as well as helping them to be regenerated, potassium bromides modify the speed of reaction to optimum (and thereby affect contrast) and alkalis provide the necessary environment for the reaction as a whole. An understanding of the effect of each component allows the darkroom practitioner to modify developers to a particular purpose.

Stabilisation (‘fixation’) of the silver image

Finally we come to stabilising the silver image. Once development has been stopped in an acid bath (or water bath) according to aesthetic taste, the remainder of the silver halides that have not been converted into metallic silver need to be removed. Failure to do this properly will mean that silver halides will continue to interact with light to darken the image over time.

Thiosulphate salts are used for this. Thiosulphate is cheap, soluble and relatively benign. The process is more complicated than might be assumed:

“Fixing” – Thisosulphate anions attach (but do not react) to silver ions from the remaining silver halide molecules; The silver thiosulphate cation picks up the residual bromide ions; The silver thiosulphate cations adsorb more thiosulphate and are thereby easily pulled into solution.

One can see that under-fixing would result in incomplete adsorption of silver ions and that over-fixing would give the opportunity of silver being re-halogenised (as thses are not chemical reactions but adsorptions). One can also see why proper wash times and temperatures are required to remove silver thiosulphates and intermediate complexes.

Final word

I am not a chemist. But one doesn’t need to be to get a rough idea of the chemical principles involved in developing a photograph.

Having a rough understanding of what happens in producing a photographic negative from unexposed film gives us a basis for understanding why controlled conditions are vital. It also enables us, as darkroom practitioners, to experiment with developer recipes and developer/film combinations. That way, we can better control results in pursuit of how we visualise images.

This and the last post are by way of technical input into an upcoming series of posts that will examine the technical stories behind some of the photographs from my darkroom.