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Shelf-life
First developer is the most sensible to aging. Concentrate shelf-life can be increased by reducing contact to air.
Here are a couple of methods:
Introducing a heavy and non-reacting gas into the bottles.
This gas forms a layer over the concentrate and separates it from air.
You can find such gazes available commercially like Tetenal Protectan. But since I saw its ingredients, a mix of butane and propane, I replaced it by lighter gas, commonly available in Switzerland as used in heaters for our national meal, la Fondue ...
Open Kodak kit shelf-life is then around 10 months.
Concentrate is put into brown glass bottles, the kind you find in pharmacies, and kept in the dark.
I've chosen 200ml bottles as this quantity makes one liter of final solution which lets me develop 8 rolls 135-36 in a single batch.
Each bottle is filled with exactly 200ml of concentrate and some demineralized/distilled water is added to fill the bottle completely up to the cap.
Once needed, the bottle is diluted to the final liter.
I can't give you yet a precise shelf-life as I've used this method to E-6 kits only recently but as I use it since a long time with B/W developers with great success (Xtol test has gone beyond 4 years of shelf-life), I'm pretty confident it will give better results than the gas method.