Wednesday, April 11, 2012

Designing a developer. Part one.

When you are going to design a developer, you have some work to do.

First you have to define your goals.
  • Is it going to be a fine grain developer?
  • Is it going to have high accutance?
  • Is it going to give normal, high or low contrast?
  • It it going to be one-shot, or reuseable?
  • Is it going to have long shelf life?
  • It is going to be mixed as a concentrate?

All this questions must be answered before you start the design process.

Well, this time I am going to design a low contrast developer giving reasonable contrast and reasonable fine grain. It is supposed to be used for developing fine grain, high contrast document films.

I want the developer to be reuseable, and is will be mixed as a working solution. A shelf life of three months is enough for my use.

To meet this goals I have to make some choices.
First, what developing agent or agents to use.

Obvious choices are Metol or Phenidone.
Both makes a soft working and fine grain developer alone at low pH.

Both suffer from rapid oxidation without any chemical to protect them.
Adding Sodium Sulfite to a Metol developer will protect the Metol, but will also increase the acivity of the developer so the contrast may build to quick.

Phenidone will oxidize quickly even with some Sodium Sulfite present, but adding a minute amount og Hydroquinone will protect it quite well without increasing the contrast too much.

That leaves us with another problem. How do we keep the pH low enough that the Hydroquinone doesn't get activated as a developer?

Selecting the correct alkali will take care of that.
The buffering action of the alkali mustn't be to good, because this will reduce the acutance or sharpness.

So let's settle for Phenidone as the developing agent and a small amount of Sodium Sulfite and Hydroquinone  to protect it from going bad.

 The Hydroquinoine gets activated at a pH above 9 somewhere, so we want to keep the pH below that.

How much Phenidone do we need?
Let's settle for a reuseable developer where we want to be able to develop 10 films in one liter.
Developing one film requires about 0.016g Phenidone.
If we don't take the regeneration of Phenidone by the Hydroquinone into concideration, we will need 0.16g Phenidone in our liter of developer.
That's about 1/10 of the amount in the POTA developer.
This is the minimum amount. To have some headroom and avoid total exhaustion, let's double that amount to 0.3g Phenidone.
Since the Hydroquinone regenerates the used Phenidone, this may be enough for our task.
How much Hydroquinone do we need?
Since it only are used to regenerate the Phenidone, we don't really need much. 0.25g may be enough for the task.
We have to regenerate 0.016g Phenidone or 0.0000986 mol Phenidone per film. That isn't very much!
Let's see how much Hydoquinone that is if we accept that we need one molecule Hydroquinone to regenerate one Phenidone molecule.
We need the same amount of molecues, 0.0000986 mol Hydoquinone. That is 0.0108g Hydroquinone per film, or 0.11g Hydroquinone per liter developer.
So 0.25g HQ is more than enough.

To protect this from oxidation we need some Sodium Sulfite.
How much? 10g is enough for this task.

So far we have arrived at this formula.
Sodium Sulfite 10g
Hydroquinone  0.25g
Phenidone        0.3g

What do we need in addition to this?

Well, we need some kind of alkali as an accelrator and a restrainer to avoid fog.

This will be discussed in the next postings.

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