Two-stock ECN-2 concentrate split ~70 rolls in - formulas and observations

[nograin_nogain]

Hey everyone,

I've been mixing my own ECN-2 developer at home for about a year now, and after ~70 rolls through the system I wanted to share the formulas and a few observations.

The base reference is Kodak's Processing KODAK Motion Picture Films, Module 7. The two-stock concentrate split was inspired by Koraks' simplified home formulation, with CD-3 held back as a dry component since it doesn't keep well in alkaline solution.

Formulas, normalized per liter:

• Stock A: 20g sodium sulfite, 13.88g potassium bromide, distilled water to 1L
• Stock B: 149.76g sodium carbonate monohydrate (Na₂CO₃·H₂O), 13.5g sodium bicarbonate, distilled water to 1L
• Working developer: 700mL distilled water + 100mL Stock A + 200mL Stock B + 4g CD-3, topped to 1L

In practice I mix 250mL one-shot batches for a 1-2 rolls as needed: 175mL water + 25mL A + 50mL B + 1g CD-3.

Process is 3:15 at 41.1°C (106°F) with constant rotary agitation in a Jobo, after a brief room-temperature alkaline prebath for remjet-backed stocks. Standard C-41 bleach and fixer for the support chemistry, no acid stop. Module 7 lists the working dev pH target at 10.25 ± 0.05 at 25°C, and a cheap pH meter has been enough to confirm I'm in range across batches.

Some things I've noticed over the year:

• While Stock A is the more oxidation-sensitive of the two (due to the sulfite), wine bags with the air evacuated have held it stable for several months without visible color shift or precipitation.
• Stock B has been uneventful in the same storage.
• CD-3 stays dry in a sealed amber jar until mix time. The fizz when it hits the alkaline base is consistent batch to batch.
• Results across batches have been close enough that I'd call the process reproducible at this level of measurement care

I also put together a video walking through the full mixing and process, for anyone interested:



A couple things I'd be curious to hear about from anyone with more experience here:

• How long are you getting out of a Stock A-style sulfite/bromide concentrate before you see meaningful degradation? My empirical sense is several months in evacuated polymer storage, but I haven't tested longer than that.
• For anyone who's tracked it closely: any noticeable activity differences within the 0.1-unit pH window Kodak's reference PDF calls out, or is that more of a formality once you're in range?

Happy to answer any questions about the setup or process.

 

[Rudeofus]

I have been wondering, why you put the sulfite and the bromide into a separate concentrate. Is there chance, that you saw the HAS in a separate concentrate and thought, that the same need for caution applies to sulfite? HAS is basically a weak photographic developer, which incidentally protects the CD-3/CD-4, and as most organic photographic developers its reduction potential rises with pH.

As far as I know, it's safe to keep it together with the alkali. It's interaction with Oxygen may be slowed down by factors different from its reduction potential. In fact the sulfite might even last longer together with the carbonate and the other stuff, since concentrated solutions dissolve much less Oxygen.

If your current sulfite solution has survived just a few months, then you'll get at least the same shelf life from a concentrated CD-3 stock solution with a few grains of metabisulfite. See the E-6 CD concentrate formulas for a reference, this one lasts at least 6 months in a glass bottle.

Regarding pH: if you look at Kodak's manual h2407 module 8, you will see quite a bit of process deviation with as little as +/- 0.05pH. Try to be accurate.

 

[koraks]

Great to hear that someone found my split ECN2 developer formula useful!

-3 stays dry in a sealed amber jar until mix time. The fizz when it hits the alkaline base is consistent batch to batch.

You can avoid the fizz by dissolving the CD3 in a small quantity of water. This avoids an issue that I've occasionally run into, where some of the CD3 lumps up into small hydrophobic beads and refuses to dissolve neatly.

I can't comment on your questions as I presently only mix the developer directly into working strength (not shooting ECN2 all that often). When I used this concentrate approach, the bottles were pretty much 'mix and forget' for me and I never noticed drift, but I also didn't do any measurements or anything. It'd be great if someone could bring some more clarity to this.

 

[nograin_nogain]

I have been wondering, why you put the sulfite and the bromide into a separate concentrate. Is there chance, that you saw the HAS in a separate concentrate and thought, that the same need for caution applies to sulfite? HAS is basically a weak photographic developer, which incidentally protects the CD-3/CD-4, and as most organic photographic developers its reduction potential rises with pH.

As far as I know, it's safe to keep it together with the alkali. It's interaction with Oxygen may be slowed down by factors different from its reduction potential. In fact the sulfite might even last longer together with the carbonate and the other stuff, since concentrated solutions dissolve much less Oxygen.

If your current sulfite solution has survived just a few months, then you'll get at least the same shelf life from a concentrated CD-3 stock solution with a few grains of metabisulfite. See the E-6 CD concentrate formulas for a reference, this one lasts at least 6 months in a glass bottle.

Regarding pH: if you look at Kodak's manual h2407 module 8, you will see quite a bit of process deviation with as little as +/- 0.05pH. Try to be accurate.

Thanks so much, this is the kind of substantive feedback that makes posting here worthwhile.

Honestly, the A/B split came from Koraks's earlier C41 formula rather than independent chemistry on my end, and your reply pushed me to look more carefully at whether that architecture actually applies to ECN-2. I think you're right that it largely doesn't.

Did some research and here's my current read on the relevant literature: sulfite oxidation in alkali looks to be mostly metal-catalyzed radical chemistry rather than pH-driven decomposition. Oxygen-containing sulfite solutions are reportedly stable up to pH 12 in the absence of trace metals and light, and the salting-out effect you mentioned should reduce dissolved O₂ in a combined sulfite-plus-alkali concentrate compared to my current Stock A in particular. The C41 "don't combine sulfite with HAS" rule also doesn't carry over since ECN-2 has no HAS in the formula. So the dry CD-3 separation seems to be doing the real architectural work, and the A/B split is something I inherited rather than something the chemistry requires.

The 6-month glass benchmark you gave for E-6 CD concentrate is helpful as a concrete anchor for what "good" longevity looks like. I won't be running a combined-concentrate test immediately since my current Stock A still has plenty of life left, but it's on the list for when I mix the next batch. If it tracks Stock A's stability, that's a meaningful workflow simplification.

 

[lamerko]

I followed the same approach - a two-part concentrate. I'm not a chemist, but I also figured that a high pH wouldn't be good for the sulfite. As for adding CD-3, I always do it after Part A, which is less alkaline. No bubbling is observed. Then I add Part B slowly with stirring. Like koraks, I also added sodium hydroxide to the concentrates, but in my case a much smaller amount was needed. In any case, it's not really possible to fine-tune it exactly.
One warning: I kept the two parts in tightly sealed PET bottles - it's convenient. But… after some time, Part B damaged the PET and the chemicals leaked out. There are no issues with Part A.
I can confirm (at least) one year of viability for the concentrates - probably they could last even longer.