With XTol being somewhat unreliable these days, (My whole stock has been recalled) I am using D76 a bit more.
I'm curious about D76H which omits the HQ for slightly more metol. If this is a simpler formula that delivers the same results, with less instability, why did Kodak not adopt it as their standard version?
There must be some good reason to stick with the original formula...
Well when these formulae were developed, EK Research was developing cutting edge technology. Puns intended
The answer is Superadditivity.
https://en.wikipedia.org/wiki/Superadditive_developer
Mechanism (source Wikipedia )
There were several historical theories about the mechanism of superadditive development, until G. I. P. Levenson elucidated the regeneration mechanism. It is useful to review modern treatment of the
development mechanism described as an electrode process.
Of the two developing agents that are superadditive, the following is usually the case:
- Agent 1 has lower reduction potential but much higher adsorption to the silver halide crystals.
- Agent 2 has higher reduction potential but much poorer adsorption to the silver halide crystals.
Agent 1 is the developer that reduces the silver halide crystals. Oxidation product of Agent 1 is reduced back to the original form by Agent 2. In other words, the ultimate source of the electrons used to develop image comes from Agent 2. For this reason, in modern convention, Agent 1 is usually called
electron transferring agent or ETA. Agent 2 is usually called the main developing agent.
As obvious from the above mechanism, developing agent with very stable semiquinone radical form (the first oxidized form of the ETA) tends to make more superadditive combination. Also obvious from the above mechanism is that Agent 1 has to have an intermediate reduction potential
in the developer solution between the Fermi energy level of silver (the developing image silver) and the reduction potential of Agent 2 in the developer solution.
Examples of Agent 1 include:
Phenidone,
Dimezone S,
Metol,
p-aminophenol,
glycin,
Eikonogen. These agents have nitrogen atoms that give strong affinity to silver halide (argentophilic) as well as hydrophobic end, effectively acting as a
surfactant between the developer solution and silver halide crystals.
Examples of Agent 2 include:
hydroquinone and
ascorbic acid. These agents have stronger reduction potentials than those of Agent 1 group, but they have very poor adsorption on silver halide crystals, due to lack of argentophilic end.
However, there are many developer combinations that exhibit superadditive development.