Using Two-Part Developers

[Jay DeFehr]

Using Two-Part Developers:

Using a two-part developer like Hypercat II is more complicated than using a single solution developer like 510-Pyro. A two-part concentrate is actually mixed in three parts; A solution + B solution + water, and that can complicate mixing when deviating from the standard mix. Keeping the basics in mind can help to avoid confusion.

The first thing to keep in mind is the role each part of the developer plays in the working solution. The part A contains the developing agent(s) and other supporting compounds, and the ratio of Part A to final solution volume determines the developer concentration. Let's forget about the part B for now, and consider the ratio of part A to final solution volume. To keep things simple, let's use 1 liter as the final solution volume in all the examples.

Hypercat II

Standard dilution- 1:100 (part A : final solution volume)

Another way to think of the ratio 1:100 is as the fraction 1/100. Part A = 1/100 the final solution volume, so 1000/100 = 10

Part A 10ml

If we want a more dilute solution, we simply adjust the ratio to 1:200. 1000/200 = 5

Part A 5ml

or, even more dilute, 1:300. 1000/300 = 3.3

Part A 3.3ml

Okay, that's simple enough, but what about part B?

Part B is the alkali, and it's role is to adjust the pH of the working solution. Generally speaking, higher pH produces a more active developer, but there are other effects, as well. As pH increases, so does the appearance of grain, chemical fog, and to some extent, film speed. Excessive pH can also produce harsh gradation. Too low pH can result in a loss of film speed, long development times, sub-optimal staining, or below the activity threshold, no development at all, so the goal is to identify the optimum pH for the formula and for the processing conditions.

Since few home darkrooms are equipped with pH meters, the standard practice is to indicate the alkali concentration in the developer formula. For Hypercat II, sodium carbonate concentration should be in the 5-10g/ Liter of working solution, depending on the processing conditions. For most uses including printing on silver paper or scanning, 5g/Liter is standard, but for alternative printing processes requiring very high contrast negatives, 10g/Liter is recommended.

To calculate the volume of part B:

20% sodium carbonate contains 0.2g/ml, or 1g/5ml sodium carbonate. We want 5g, so we'd use 25ml.


Whatever dilution of part A we use, we want to use 25ml part B per Liter of working solution. The simplest and best way to make up a working solution is as follows:

Add 750ml of tempered water to mixing container
Add 25ml part B, stir
Add desired amount of part A
Top up to 1 Liter, stir

Step 3 determines developer dilution. For a 1:100 dilution, we'd use 10ml part A, for 1:200 we'd use 5ml, and for 1:300 we'd use 3.3ml.

The above mixes could be written as:

1:2.5:100 (multiply all values by 10 for 1 Liter)
1:5:200 (multiply all values by 5 for 1 Liter)
1:7.5:300 (multiply all values by 3.3 for 1 Liter)

All of the above contain 5g/ Liter of sodium carbonate and will be approximately the same pH.

I hope the above clarifies more than it confuses.

[Max Hao]

Thanks Jay. Just to clarify, so the amount of carbonate in part B is only to determine the PH of the working solution. Therefore no matter how much of reducers is in the working solution, we just use the same amount of carbonate relation to the volume to get the same PH. Does this apply to all two-part developers?

[Jay DeFehr]

Hi Max,

Yes, that's right, and it applies to all developers of similar formulation, Like PMK Pyro, WD2D, Pyrocat, etc., in which the A part is very dilute in the working solution, and the B solution is a simple alkali. In these developers the concentration of the Part A is generally too low to much affect the pH of the working solution, which is determined almost entirely by the concentration of the B solution. You can think of the part A as the solute (the part to be dissolved) and the part B + water as the solvent, and in this case, the dilution can be written as two parts, ie 1:100 = one part A : 100 parts of a 0.5% solution of sodium carbonate, or 1:200 = 1 part A : 200 parts of a 0.5% solution of sodium carbonate, etc.

You had it right, so I hope I haven't confused you.

[Max Hao]

Hi Max,

Yes, that's right, and it applies to all developers of similar formulation, Like PMK Pyro, WD2D, Pyrocat, etc., in which the A part is very dilute in the working solution, and the B solution is a simple alkali. In these developers the concentration of the Part A is generally too low to much affect the pH of the working solution, which is determined almost entirely by the concentration of the B solution. You can think of the part A as the solute (the part to be dissolved) and the part B + water as the solvent, and in this case, the dilution can be written as two parts, ie 1:100 = one part A : 100 parts of a 0.5% solution of sodium carbonate, or 1:200 = 1 part A : 200 parts of a 0.5% solution of sodium carbonate, etc.

You had it right, so I hope I haven't confused you.

Thanks. I think I got it. The confusion came in when I was reading The Darkroom Cookbook (page 60, 3rd ed.), which says dilution ratio of Part A controls contrast, and the carbonate in Part B only affects film speed and developing time. So if we increase the concentration of carbondate in the working solution, keeping Part A the same, it won't affect contrast, and only need shorter development time. But if we still develop for the same time as before, should we get a negative with more contrast? Sorry if I'm making this complicated. I do want to find out, as I just started to use pyro developers.

[Jay DeFehr]

Hi Max,

I don't have a copy of TDC handy, but I suspect the author was referring to two-bath developers, in which the film is processed in each bath successively, and not two-part developers mixed before processing to make a single working solution. My remarks refer only to the latter, two-part developers, not to two-bath developers. I hope that makes the differences clear.

Incidentally, I'm currently testing a new staining, two-part developer made up in water for those who have trouble sourcing glycol, or who prefer solutions made up in water. There are some advantages to aqueous solutions my new formula exploits, and it offers some improvements over existing, similar formulas. I hope to have it verified within the next two weeks, and ready to publish.

[Max Hao]

Hi Max,

I don't have a copy of TDC handy, but I suspect the author was referring to two-bath developers, in which the film is processed in each bath successively, and not two-part developers mixed before processing to make a single working solution. My remarks refer only to the latter, two-part developers, not to two-bath developers. I hope that makes the differences clear.

Incidentally, I'm currently testing a new staining, two-part developer made up in water for those who have trouble sourcing glycol, or who prefer solutions made up in water. There are some advantages to aqueous solutions my new formula exploits, and it offers some improvements over existing, similar formulas. I hope to have it verified within the next two weeks, and ready to publish.

Thanks Jay. I read the paragraph from TDC again and figured out actually it confirms what you are saying here: the amount of carbonate doesn't influence contract much, it only influence film speed and development time.
I'll be very interested in the two-part developer in water. What would you recommend in terms of water and glycol if I could get hold of glycol with no problem? But water definitely costs less :)

[Jay DeFehr]

Hi Max,

I think glycol is the safest bet for keeping properties, but it does add considerably to the cost of the developer, and especially so if the glycol must be shipped. My water-based developer is designed with shipping in mind, for best economy. The idea is to source locally as many ingredients as possible, to avoid shipping costs that can exceed the value of the chemical being shipped, and to maximize the keeping properties of the part A stock solution. This developer part A should keep better than any other staining developer made up in water, but it will take a while to verify that.:)