Friday, April 13, 2012

Designing a developer part 2. Alkali and restrainer

This time we are going to look at the accelrator, the alkali in the devloper, and the restrainer.

The accelrator.
Looking at our design goals, it specifies that we want to make a low contrast developer for using document films for pictorial purposes.

To get to this goal, we need to make sure that only the Phenidone part of the developer does any devloping job. The hydroquinone must not under any circumstance be activated. Hydroquinone produces way to high contrast for our design goal.

That means that we have to stay below pH 9 to make sure that doesn't happen.
Phenidone produces low contrast when the pH is low, so let's place the pH even lower on the scale. Let's place it at about the same level as XTol. That is pH 8.2.

What commonly available alkali is suitable for that? Not carbonate for sure. Neither Kodalk. Both are way to high in pH.
Borax may be suitable, but a 10% borax solution is pH 9.2 to 9.3.
How can we get from that level and down to 8.2?

Well, Sodium Sulfite will help, so will Boric Acid.

We want this develper to have some buffer capacity, but not too much since that will reduce the acutance.

Some Sodium Sulfite is required to preserve the developer, but too much will reduce sharpness. Since this is a developer for extremely fine grain films, we don't need to use large amounts of Sodium Sulfite to get the solvent action for reducing grain.

Let's use 10g Sodium Sulfite.
How much Borax?
Well, since we don't want a large buffering capacity, lets use a small amount, about 2 to 4g.
Let's start with 2g.
So far we have settled for this formula:

10g Sodium Sulfite
0.25g Hydroquinone
0.35g Phenidone
2g Borax

In addition to this, we introduce a chelating agent to keep metal ions from water and chemicals  out of the picture.  Metal ions increases oxidation rate and shortens shelf life.
Fotoplex-2 is very effective for chelating iron and other metal particles without disturbing the development process. It does shift the pH, so we need to introduce it on an early stage so that we don't have to recalculate all of it at a later stage.

To calculate the pH for this mix we need to know some data for the ingredients that may inflict on the pH.

This data looks like this:
Hydroquinone pka=10.35 mol=110.11g/mol
Sodium Sulfite pkb=6.81 mol=126.04274 g/mol
Borax (decahydrate) pkb=4.74 mol=381.37g/mol
Boric Acid pka1=9.237 pka2=12.74 pka3=13.8 mol=61.83g/mol
Fotoplex-2 pKa1=1.35  pKa2=2.87 pKa3=7.03  pKa4=11.3 mol=206,28g/mol

Phenidone doesn't inflict on the pH, so we don't need to include it in the calculation.

The dissosiation constants, pka and pkb is used to calculate how each chemical inflicts on the pH.
The mol weight is a measure of how much of this chemical is required to make a solution with with 1 mol of the chemical. By using this, we can calculate with a known relative amount of molecules in the solution.
A 1 mol Borax solutiuon contains the same amount of molecules as a 1 mol Boric Acid solution.
The pH solver needs this information to work.

To find the concentration in mol for each ingredient, we have to divide the amount of the ingredient with the mol weight for the ingredient.
Borax 2g= 2g/381.37g=0.005244 mol

So let's set up the required parameters for the solver.

Hydroquinone pka=10.35 c=0.002271
Borax pkb=4.76 c=0.005244
Sulfite pkb=6.81 c=0.07934
Fotoplex-2  pKa1=1.35 pKa2=2.87 pKa3=7.03 c=0.004848

The solver can't use more than 3 pKa values, but since the pH is in the area 7 - 9, the last value isn't used for the Fotoplex-2.

Entering this data into the solver results in the ansver pH= 8.0700

This is a bit lower than we wanted.
Let's increase the amount of Borax a bit to bring it up to the level we want.
Let's use 4g.

The parameters with 4g Borax is like this:
Hydroquinone pka=10.35 c=0.002271
Borax pkb=4.76 c=0.010488
Sulfite pkb=6.81 c=0.07934
Fotoplex-2  pKa1=1.35 pKa2=2.87 pKa3=7.03 c=0.004848
This results in pH=8.3555

This is a bit more than wanted, but since we want some buffering capacity, let's introduce some Boric Acid to bring the pH a bit down and increase the buffering capacity.

Let's use 1g Boric Acid.

The parameters with 1g Borc acid added looks like this:
Hydroquinone pka=10.35 c=0.002271
Borax pkb=4.76 c=0.010488
Boric_Acid  pka1=9.237 pka2=12.74 pka3=13.8 c=0.01617
Sulfite pkb=6.81 c=0.07934
Fotoplex-2  pKa1=1.35 pKa2=2.87 pKa3=7.03 c=0.004848

Entered into the solver it looks like this:

The answer looks like this:

The pH is 8.253750. In reality, we don't use more than 2 decimals anyway, so pH is 8.25
This is what we wanted, so let's settle for the formula that looks like this:

10g Sodium Sulfite
0.25g Hydroquinone
0.35g Phenidone
4g Borax
1g Boric Acid.
1g Fotoplex-2 AKA Etidronic acid

We can leave out the Fotoplex-2 if we want, but the pH will be higher.
To counter this, you have to increase the amount of Boric Acid to 8-10g

I don't reccommend this as this increases the buffering capacity and by this reduces the sharpness of the developer. If you don't increase the amount of Boric Acid, the pH will be about 9.1 and you are in the vincinity of the pH where the Hydroquinone gets activated and the contrast will be too high.

The restrainer.
Almost every developer needs a restrainer to prevent buildup of fog.
Common restrainers are Potassium Bromide, KBr and Benzotriazole, BZT.
A less common restrainer is Potassium Iodide, KI.

BZT doesn't work very well at this low pH level, so we can rule this out.
KBr works well, but in a developer intended for re-use, the amount of Bromide in the developer will increase when the developer is used. This will increase the development time from film to film.
This is not an ideal situation.
Selecting KI as restrainer has a positive side-effect. It is less soluble in water, so when used in a developer, part of it will still be in the film emulsion when we pour out the developer. Because of this, the amount of KI will decrease for each film developed. At the same time, the amount of Bromide will increase. This counters the effect of loosing some KI for each film developed.
The net sum of the effect of the restrainers will be more or less the same for the first and the film developed. There will be some difference, but less than with just using KBr as a restrainer.

The amount of KI needed is about 1/100 of the amount of KBr to get the same effect.
Instead of using 1g KBr, let's use 0.01g KI.

The complete formula is like this:
750ml Water
10g Sodium Sulfite
0.25g Hydroquinone
0.35g Phenidone
4g Borax
1g Boric Acid.
1g Fotoplex-2 AKA Etidronic acid
0.01g Potassium Iodide.
Water to 1000ml.

Since this isn't tested yet, I can't be sure it will work, but in theory it will.

Next posting will contain practical information of mixing the developer and possibly first tests.


Data about chemicals may be found on the net. Just do a search. You will find both PKa, PKb and mol weight for chemicals.

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