You only need to open up a search engine and look for any film-developer combination, to find that there are as many preferences for specific combinations as there are people using them. Some prefer a pronounced grain and harsh contrast, while others want smooth gradation and tonality combined with minimal grain. Some may get away with a ‘thin’ negative, while others need considerable density for their further processing. But what is it that makes all these developers give different results, while the basic principles are the same? In this three part series on developers, I will cover the principles of development and developing agents and the respective differences (part I) and the other ingredients of developer solutions with their functions (part II). In the final installment, part III, we will dissect the recipe of what many consider the benchmark developer: Kodak D76 / Ilford ID-11 and compare it to the recipe for one of the many versions of the infamous Rodinal.

The preliminaries

When browsing the literature on the topic it becomes clear that the subject of developing is very vast and can by no means be covered in a three part series of blog posts. I tried to extract those topics that are relevant for today’s darkroom worker and the novice darkroom (al)chemist. We will briefly touch upon the chemical structure of developing agents, and explain what makes them different from one another. However, in depth discussions are left for further reading for the trained professional that knows his/her way in chemical synthesis. I think it is fair to assume that most of the readers of this blog will not be trying to develop new developing agents, and that this information can thus be safely omitted. (I know most content on this blog is for a niche audience, but not such a small niche.) Therefore, in this part we will focus on the reaction kinetics and the chemical processes underlying the developing process, and use generalized models to study the differences between several developing agents.

Before we get into development itself, we need to make sure everyone is up to speed with the basics of the analog silver-gelatin process. So, without further ado:

The film you load in the camera consists of a colored base material that is coated with a light sensitive ’emulsion’. The emulsion consists of small silver halide crystals, that are suspended in a water permeable gelatin. When energy is added to these crystals, for example because they absorb the incident light [1] or are excited by high energy x-rays [2], small clusters of metallic silver form. These clusters, however, are so small that they do not amount to any perceivable image. They can be considered to form the hidden registration of our scene, the ‘latent image’. By developing the image, we can enhance it and make it visible. If you are interested in how this latent image is formed, I recommend you review the relevant sections of refs. [1] and [2]. The process is schematically summarized in Figure 1. For the rest of this article, the formation of the latent image is of no further concern. You just need to remember that the light did not cause any perceivable image in the emulsion, and that without developing it, that image will never be apparent or of any practical use to us. In addition to this, we need to remember that the latent image is composed of small clusters of metallic silver in a silver halide crystal.

The principles of developing can be explained at several levels of complexity and depth. For this article I have chosen to focus on the most relevant parts of the chemistry and physics that are required to understand why developing agents give distinct results in the final image. I try to stay away from the stuff that requires a chemistry degree to understand, but forgive me if I veer too much in that direction at times. What follows next is largely based on Chapters 1, 3 and 4 of [3], Chapter 5 of [4], Chapter 4 ‘The progress of Development’ of [5] and Chapters 11, 13, 14 of [6]. Other references are added in the text where necessary.

Developing from molecular to microscopic scale

As said before, developing is required to amplify the invisible latent image and turn it into a visible image of sufficient density for further processing. This is achieved by chemically reducing the silver halides in the crystals that have been struck by light to convert them into metallic silver. In an ideal situation, the developing agent would be perfectly selective. In other words, the exposed and only the exposed molecules are reduced, while the grains that did not receive sufficient light remain untouched. Unfortunately, in reality also not illuminated crystals are reduced, but at a much slower pace than the lit ones. The silver specks of the latent image work as catalysts that amplify the formation of silver in one way or another. According to Mason [3], this amplification is in the order of 10 million to 1: for every silver atom present in the latent image 10 million new ones are formed in development. In this way, a visible image is formed which has from very low to very high values of density with a continuous gradation in between.

The basic reaction on molecular level

In developing the emulsion, a silver ion is converted into metallic silver through a redox (reduction-oxidation) reaction. In this type of reaction, the silver (which has a shortage of electrons) is reduced and receives an electron from the oxidizer.  In its simplest form, it can be written as

where R(red) represents the original developing agent in its reduced state, and R(ox)⁺ represents the developing agent its oxidized state, in which it has lost an electron. The developing agent releases a negatively charged electron, which is absorbed by the positively charged silver ion to form metallic silver. But wait, we mentioned silver halides before. Where did the halide part go? The light sensitive crystals in the emulsion are typically composed of silver bromide (AgBr) as its main constituent (but also AgI and AgCl can be found). If we look at the reduction process of this specific halide, the reaction can be written as

in which the bromide is normally released in the form of hydrobromic acid, HBr. The additional proton/hydrogen ion in this molecule can be released by the developing agent or come from other constituents of the developer solution.

Development on a microscopic scale

The chemical reaction described above happens on the molecular level. If we zoom out a bit, we can consider the development process at the level of the grains. At this scale, the process can be described by two co-existing theories. As we will see, these theories complement each other well, but are based on different underlying assumptions regarding the physical and chemical nature.

On the one hand, there is the “Triple Phase Theory”, that considers the reduction process as a heterogeneous catalysis process in which the silver specks of the latent image act as the solid state catalyst and where the catalysed process is more rapid than the uncatalysed reaction. On the other hand, we have the generalized “Electrode Theory”, that assumes that the silver specks act as electrical conductors that allow the exchange of electrons between the developer and the silver halide in the grain.

I can imagine that is quite something to digest, so let us break it down, starting with Triple Phase Theory (TPT). Microscopic studies of the developing grains show that developing starts at discrete sites within the grain (not the entire grain at once), until the complete grain is developed. These discrete sites are also known as the development centers, and there is little doubt that these developing centers are in fact the silver specks of the latent image that were created when the grain was illuminated. Because of this discrete nature, it comes natural to assume that these latent image specks may serve as catalysts that enable or speed up the local developing process significantly.

If we consider the latent image specks as a catalyst, then the catalyst (silver atoms) is in the solid phase, the reducing agent (the developing agent) is in the liquid phase and the silver ions can be either in the solid phase or in the solution phase. We call this heterogeneous catalysis. It is tempting then, to draw parallels with other heterogeneous catalysis processes in which 1) adsorption from the liquid phase onto the solid catalyst is highly probable, and 2) the catalysed reaction is more rapid than the uncatalysed reaction.

This notion is reasonable, as experiments with hydroquinone have shown that after an induction period, the developing rate can be written as

where is a reaction constant and [Ag] and [hydroquinone] are the concentrations of silver and hydroquinone, respectively. The fractional power of the silver concentration in this rate equation is characteristic of a reaction in the adsorbed state. In such process the rate is determined by the concentration in adsorption, rather than the concentration in the solution. This explains the fractional power. Further experiments have shown that the adsorption of the developing species to the silver halide directly around the latent image speck is important, rather than adsorption to the silver speck itself, because the silver specks themselves are too small to have sufficient activity.

In the generalized Electrode Theory (ET) by Mott, Gurney and Jaenicke (yes, those lads), the initiation of the developing of a developing center is considered to be due to the silver specks functioning as an electrical conductor between the bulk developer and the solid silver halide in the grain. This means that the developing agent does not have to be in direct contact with the silver halide for chemical reduction to take place. That is, electrons can reach the silver halide from the developing agent, by passing them via the silver specks as schematically shown in Figure 2. For this to happen, two processes occur at the same time. An anodic process occurs at the interface between the silver speck and the developing solution. This is where the electrons pass from the developing agent to the silver speck. And a cathodic process takes place at the interface between the silver speck and the solid silver halide. This is where a silver ion is added to the silver of the silver speck and where it accepts an electron from the silver. The result is a silver speck that grows at the silver/silver halide interface. Experiments give credibility to this view, most notably by showing that the silver halide could be reduced by a developing agent, even if the developing agent was only in contact with the silver via an installed conductor and no direct contact was possible.

Interestingly, many experimental results obtained through experiments at the grain level can be interpreted by using both theories separately, and both fail in cases where the other does not. The net result of both theories is the same, however, and both yield a rapid initiation of development of grains containing latent image specks, and the silver speck grows at the silver/silver halide interface. James has pointed out, that two stages can be identified in chemical development: a slow initial stage and a rapid continuation stage. Furthermore, factors that influence development, influence both stages differently. He therefore concluded that both phases are governed by different principles. Results obtained for the initial stage are in line with the Triple Phase Theory, while the continuation phase is in line with the Electrode Theory.

During development, we need a source of silver ions to be reduced. Depending on the source of these ions, we can distinguish two ‘types’ of development: physical development and chemical development.

Physical development

The name ‘physical development’ is in my view a misnomer, because it suggests that the silver ions are reduced by non chemical means. This is, however, not the case. One speaks of ‘physical development’, when the silver ions for reduction at the latent image are supplied from the developer solution, rather than from the emulsion. A case of pure physical development can be demonstrated when an exposed but undeveloped film is fixed and then developed. The fixer solution dissolves all silver ions from the emulsion and only leaves the silver specks of the latent image untouched. When the film is then placed in the developer solution, the silver ions in the solution are reduced and metallic silver is deposited near the silver specks of the latent image. The deposited silver has to be present in the developer. This is a slow process, however, and therefore not used as the primary developing method in conventional imaging.

Chemical development

In contrast to physical development, the silver ions for reduction are supplied only by the grain that contains the silver speck of the latent image. This reduction method is so quick acting in practice, that the metallic silver formed in the form of fine silver filaments, which no longer resemble the shape of the silver halide crystals we started with in the emulsion. These new masses of silver filaments are visible as the ‘grains’ we see in the final image. The size and shape of these filaments depends on the actual developing kinetics.

Developing agents at the molecular scale

Not every substance that can reduce silver halides to metallic silver is suitable as a developing agent. As mentioned before, it needs to be very selective and preferably only reduce the crystals that have been exposed to light. What makes for a useful developing agent from a molecular chemistry perspective is outside the scope of this article, but it is instructive to consider their composition and shape at least in some detail.

In an attempt to classify developing agents, Kendall and Pelz found the following empirical relation that is satisfied by most – but not all – known developing agents:

where are groups, is a carbon atom and is either a carbon or a nitrogen atom and is any integer equal to or greater than 0. The most common developing agents that are of commercial interest today or that were of commercial interest in the past are listed in Table 1. While the first 9 entries satisfy this empirical Kendall-Pelz rule, the last one, Phenidone, does not. Although people came up with different rules to classify developing agents, none of them seem to be capable of covering them all. We can therefore only consider general characteristics that occur often.

Table 1: Most common developing agents and their trade names. Information taken and condensed from [4].

Trivial name	Chemical name	Other trade names
Amidol	2,4-diaminophenol	Acrol, Dianol, Dolmi
Catechol	1,2-dihydroxybenzene	Kachin, Pyrocatechin
Chloroquinol	2-chloro-1,4-dihydroxy-benzene	Adurol, Chlorquinol
Glycin	N(p-hydroxyphenyl) aminoacetic acid	Athenon, Glycine, Iconyl, Kodurol, Monazol, Paraglycin
Hydroquinone	1,4-dihydroxybenzene	Hydrochinon, Hydrokinone, Hydroquinol, Quinol
Metol	p-methylaminophenol	Adilol, Agenol, Armol, Elon, Enol, Genol, Graphol, Manol, Monomet, Phenomet, Photol, Photo-Rex, Pictol, Planetol, Rhodol, Satrapol, Scalol, Serchol, Veritol, Verol, Viterol
p.a.p.	p-aminophenol	Citol, Diutall, Energol, Freedol, Kathol, Koledon, Para, Paraamidolphenol, Unal
p.p.d.	1,4-diaminobenzene	Diamine, Diamine P, Metacarbol, Paramine, Diamine H (HCl), P.D.H, p.p.d.
Pyro	1,2,3-trihydroxybenzene	Piral, Pyro, pyrogallic acid
Phenidone	1-phenyl-3-pyrazolidone	Graphidone

To put these developing agents in a bit of perspective it is interesting to see in which developer formulas they are used today. Catechol and Pyro, for example, are used in some staining developer formulae, and catechol is rumored be part of the Tetenal Emofin Blau formula. Hydroquinone is the developing agent in Table 1 that is most prominently used today, and is used in many commercial formulae in superadditivity with Metol (Metol-quinone, MQ) or in superadditivity with Phenidone (Phenidone-quinone, PQ). One of the most notable examples of MQ developers are Kodak D76 and Ilford ID11 (of which the formulations are nearly identical [5]), while Ilford Microphen and Diafine are two notable examples of PQ developers. However, Phenidone is not only found in super-additivity with hydroquinone, but can also be found in combination with catechin in, e.g., Sandy King’s Pyrocat-HD [7]. For Rodinal p-aminophenol is the developing agent of choice [7, 8].

Molecular structure

More differences become apparent when we consider the molecular structure of these developing agents. I have listed the structure of eight different developers in Table 2 below. The first six of these show a remarkable resemblance in their structure by having one benzene ring at its core with a few appendices on the perimeter. How can these ostensibly small differences make these molecules give different developing behaviour? This question is unfortunately too difficult to answer, but some differences become apparent.

Table 2: Chemical diagrams of developing agents.

Pyrogallol	Hydroquinone	Catechol	p–aminophenol
p-phenylene-diamine	Metol	Phenidone	Ascorbic acid (Vitamin C)

Lyalikov studied some of the developing agents listed in Table 2 via molecular orbital calculations. These calculations model the behaviour of the electrons in the molecule and give insight in how they move throughout the molecule. It was found that the activity of a developing agent was related to the density of the electron cloud at the active group that is attached to the benzene ring. In simpler terms, the more electrons are available in a certain place, the more likely it is that one of them can take part in the reduction reaction. This density of the electron cloud is influenced by the relative placement of the active groups in the molecule. Researchers working for the photographic industry try to find ways of synthesizing developing agents in which the right groups end up in the right places to control the developer activity.

To complement the work of Lyalikov, Tani found that some electrons are created more equal than others and concluded that developing agents of the 1,2-, 1,4-, 1,2,3- and 1,2,4-substituted derivatives are generally much more active than those of the 1-, 1,3-  and 1,3,5- substituted derivatives. An example of a 1,2-substituted derivative from Table 2 is catechol, while hydroquinone/p.p.a/p.p.d are 1,4-substituted derivates  and pyrogallol is a 1,2,3-substituted derivative.

One other cause for the differences we find between developing agents can be found in the solubility of these molecules in water. The solubility can be increased by additional hydroxyl groups (-OH, preferably directly connected to the benzene ring), and by the presence of carboxylic acid or sulphonic acid groups in the structure. Bigger molecules with a bigger aromatic core (bigger benzene group), however, have a decreased solubility. For example: catechin and hydroquinone have small molecules with two hydroxyl groups and have a high degree of solubility.

Superadditivity

The topic of superadditivity was already briefly mentioned earlier, but what is it and how does it work? Simply put, superadditivity means that the combination of two developing agents in one developer results in a developing rate that is higher than the sum of the rates attained with similar developers composed of either developing agents alone. Popular  and probably the most important superadditive mixtures are the Metol-Hydroquinone (MQ) and the Phenidone-Hydroquinone (PQ) pairs.

This superadditivity is in fact achieved via a four step process:

1. The primary developing agent is adsorbed to the grain. For MQ and PQ developers, this is the metol or the phenidone.
2. A silver ion is reduced by the adsorbed developing agent. This oxidizes the developing agent and makes it ineffective.
3. The adsorbed developing agent is regenerated by the secondary developing agent (typically hydroquinone). This is achieved by the transfer of an electron from the secondary developer (typically the hydroquinone) to the primary developing agent, making it active again. The secondary developing agent is oxidized in this process.
4. The oxidation products of the secondary developing agent is removed. This happens mainly through a reaction with sulphite.

As long as there is an excess of the secondary developing agent and the developer is regularly agitated to prevent local exhaustion, this is the only developing agent that will eventually exhaust. In this four step process, step 1 only determines the duration of the induction period. Steps 3 and 4 are very rapid under a large excess of sulphite and hydroquinone. The leaves step 2 as the rate controlling step. Because the interaction between the primary developing agent and the grain is depending on many factors both chemical and physical, this is where the differences between many developing agents arise.

The ratio between the primary and the secondary developing agent can be used to tune the overall developing rate. It depends on many factors, but it has been determined for both MQ and PQ developers by Axford and Kendall, and Shiba. Their results are listed in Table 3. The difference between the sources can be traced to different methods of measuring developer activity. In the last column, you find the commonly used ratio, which in modern developers are closer to the results of Shiba.

Table 3: Ideal mixture ratio for MQ and PQ developers.

Developer type	Axford and Kendall	Shiba	Commonly used
PQ	7 : 93 (molar) ; 1 : 9 (mass)	2.5 : 97.5 (molar) ; 1 : 26 (mass)	1 : 20 (mass)
MQ	22 : 78 (molar)	28 : 72 (molar)	between 1 : 2 and 1 : 4 (mass)

A quick browse through the formulas listed by Jacobson and Jacobson [6] and Anchell [7] shows that indeed MQ developers are often used in a ratio of 1:2 to 1:4. PQ developers come in a wider gamma, however, with FX37 at a ratio of 1:10 and Ilford ID-68 at a ratio of 1:38. The development rate between these two will therefore also greatly differ, especially at later stages in the development. 

How the developing agent affects the final image

As mentioned in the introduction, one combination of film and developer can give completely different results than the same film with another developer. However, the developer is more than just the developing agent, so how large is the influence of the developing agent on the final image?

Emulsion sensitivity

Let’s start with a tricky one. You may be familiar with the concept of ‘full speed’ developers, which are supposed to give you the full box speed of the film, while other developers typically give slower effective speeds. This is not a merit of the developing agent itself, but can be ascribed to the balance of several constituents of the developer. When the developer solution is tuned to give optimal results, a variation of at most 1.5 times was found in practice.

Fog

Earlier, I already briefly spoke about the selectivity of a developer. Ideally we only want to develop the exposed grains, and leave the not exposed grains untouched. The density obtained by developing the unexposed portion of the grains is called ‘fog’. With this term we do, however, not distinguish the causes of this density. It may be because of unwanted exposure of the grains by stray-light or x-rays, or by the non-selective action of the developer. To make matters worse, the local presence of developing centers from the latent image can also retard or promote local fog development, depending on the developer composition. As with many things, fog too, is a topic that is more complex than what I can describe here. It is the conglomerate of many potentially coupled effects. Therefore, let us focus on ‘developer fog’: the fog that is created purely by the action of the developing agent.

This type of fog can be formed when the developing agent directly attacks the silver ions that are in the solution or the unnucleated silver halide grains. This process in itself should form very little silver, or otherwise it would be a very poor developing agent. However, it can form tiny silver nuclei that act as highly catalytic centers, which result in local formation of larger silver specks.

When the developing agent oxidized by exposure to oxygen, products can be formed that promote fog formation (aerial fog). Research shows that oxidation by oxygen of hydroquinone and some of its derivatives, phenidone, and metol result in the formation of hydrogen peroxide . This product is, however, not formed in sufficiently large quantities to explain the measured fog densities, and there are indications that the intermediate formed free radical is the actual fogging agent.

The selectivity of developing agents (the ratio of the rates of image development and fog development) decreases when the temperature increases and can change in curious ways. For example, p.a.p. when used as sole developing agent has a very high selectivity, while that of hydroquinone in the same solution performs rather poorly. In a superadditive mixture however, the molar ratio (ratio of number of molecules of one to the other) of the primary developing agent to hydroquinone, should not exceed the optimum ratio, for fog density increases when this ratio increases. In practice, therefore, slightly lower ratios are used at the cost of speed to limit fog formation.

Formation of the grain

The grains that are deposited during manufacture of the film are not the grains we see on the developed film. In the former, the grains can have regular sizes and shapes, while in the latter, the silver is often in filamentary form. It is the scientific consensus that this happens by a complex combination of extrusion and mobile silver atoms that move along the forming filament. However, research has shown that the shape and size of this filamentary form is not so much dependent on the developing agent, but much more on the specific emulsion and the other constituents of the developer solution. Therefore, I will leave the story on how these filaments are formed to another time.

Conclusions and what’s next?

The developing agent is the key ingredient of any developer solution. Without it, no development would happen. By reducing the silver ions in the silver halide grains, silver is formed at the silver-halide / silver interface and the invisible latent image specks grow to sizes in which they become visible under visible light. As is true for many complex systems – and a developer certainly qualifies as one – even a key part is only one cog in the entire process, and the influence of the developing agent on the final image is (unfortunately) hard to isolate. We will have to consider the full picture, to really grasp the complex interactions between all the components and to tell why one developer works better for a given purpose than another.

If you like this content, stay put! The next part of this series will consider the other components that you will typically find in a developer, and I will (try to) explain how the components works together to form an image and keep the process under control.

References

[1] R.J.F. Bijster, “Reciprocity failure: the science”, Printer Attic, 2016.
[2] R.J.F. Bijster, “Travelling with film: the field test”, Printer Attic, 2016.
[3] L.F.A. Mason, Photographic Processing Chemistry, London, UK: Focal Press, 1975.
[4] D.H.O. John and G.T.J. Field, A Textbook of Photographic Chemistry, Londen, UK: Chapman and Hall Ltd, 1963.
[5] K.I. Jacobson an R.E. Jacobson, Developing: the negative technique, London, UK: Focal Press, 1976.
[6] T.H. James (eds), The Theory of the Photographic Process, New York, USA: Macmillan Publishing Co., 1977.
[7] S. Anchell, “The Darkroom Cookbook, 4th Edition”, New York, USA: Routledge, 2016.
[8] KennyE, “History of Rodinal”, The Digital Truth, Unknown. Accessed on 26 February 2017 via: http://www.digitaltruth.com/articles/historic-rodinal.php

Note: unless specified, the links to digital versions of these books may be to older versions than the ones I used and are just for further reading.

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Notice

 

On May 10, 2017, Malcolm Tremain accused me of plagiarizing parts of this article from “The Science of Photography” by Harry Baines and E.S. Bomback, in response to me advertising the article in the public facebook group ‘The Darkroom’.  I believe this accusation to be false and appalling. As a scientist that works with scientific literature on a daily basis – including as an author –  I do not take that accusation lightly. Although I write about photography in my spare time, I do so with the same integrity and to the same professional standards that I adhere to every day.

Although I am unfamiliar with the book that was proposed, I do not doubt that much of the information you read in this article can also be found there. In fact, all of the information presented in this article was found in the sources that I cited above. I subsequently organized and condensed this information in a form that is hopefully understandable to the lay man. I have not performed any original research in this field, nor do I pretend to have done so or do I claim that any of the notions above are my own. They are the result of many years of scientific research and debate by many skilled workers. The text above is nothing more than my interpretation and way of explaining that information.

Moreover, you will find that the information above can be found in written in similar ways in many of the sources that I have cited. The authors of these books cite the same papers, have been scientific contemporaries and have struggled with the same problems. However, they all represent and explain this information in different ways, but typically showing similarities. It is therefore not surprising that an explanation similar to mine can be found in other books.

I have acted with a clean conscience and without any intention or awareness of potential plagiarism. I will try to obtain a copy of “The Science of Photography” by Baines and Bomback and add additional citations in the article whenever necessary. Please forgive me that this will take time and effort. In the meantime, I will not retract the article, as I believe this not to be a case of plagiarism or misconduct.