Minimum amount of developer?

[Alastair Moore]

Hey all,

Planning on doing some experiments this weekend with my developer, Ultrafin, and to see what effects dilution has on contrast. I generally mix up 10ml of Ultrafin to 190ml of water and that's working fine for me. I planned to try 1/10 and 1/30 dilution but my concern is 6.666(666666 etc.) ml for 1/30 dilution of developer may not be enough. I'm developing a single sheet of 8x10 at a time. I could use a bit more water I suppose but my Paterson Orbital will only contain so much liquid before it spills over the top!

[Alastair Moore]

Oh and just a follow up to this question, the less the dilution, the less time I should have the film in the developer - so currently I have my film in 1/20th dilution for 12 minutes.

In my head I can't help but think if I reduce the dilution to 1/10th and reduce the time as a result then why does this not balance out and give a similar result?

[Maris]

A lot of complicated stuff happens when developer concentration changes.

A more dilute developer has a lower ph (depends of buffer capacity of the formulation) and is therefore less active.
The rate of diffusion of developer molecules into emulsion is faster at higher dilutions.
But fewer developer molecules available for mass transfer means the redox reaction between developer and exposed silver halide crystals is slower.
The rate of diffusion of development restrainers, usually liberated bromide ions, out of the emulsion is faster for more dilute mixtures hence less restrainer effect.
The effect of grain modifiers, usually silver solvents, is reduced at higher dilutions so some large (but fewer) grains grow more quickly: grainier negatives.
At high dilutions the relationship between developer molecules and silver halide crystals approaches stoichiometric and the developer can become exhausted before all available silver is reduced.
The preservative in a developer (usually sulphite) becomes less effective at high dilutions so more developer molecules get oxidised before they even get to the silver halide crystals.

The net effect is that more dilute developers work slower but the effect is non linear. Empirical testing (tedious) is the only sure way forward. In practice no one adjusts negative contrast by jogging developer dilution up or down.

[Walter Glover]

I might add, that many (most ?) chemistry manufacturers include in their data sheets some acknowledgement of the minimum amount of concentrate or stock solution necessary to process 80 square inches of film. This translates as the more heavily diluted the solution is the greater the volume of developer. This is of particular importance with things like Jobo machines where a minimal amount of chemistry is required and the exhaustion of the chemistry becomes a real concern.

Kodak T-Max RS recommends the following:

T-Max RS Concentrate Capacity (approx)
Concentrate per film 6.03125 ml