What I do notice is that after about 15 or 20 rolls of film the volume of developer in the liter bottle is down by what seems to be as much 40%.
Yep - this is a physical effect (not chemical).
Basically, you put dry gelatin (coating on the film) into the developer, and the gelatin absorbs developer and swells. It "sucks up" a given volume of developer (on the order of 5000% by weight). Probably about 2-4 ml (?) per 36-exposure roll. When you pour the developer out to move to the next step, that volume of dev. remains in the swollen wet gelatin. It does not go back into the dev. bottle, thus your volume of bottled developer shrinks a bit with every roll.
If you started with the blix (a bad idea photographically ) it would be the blix that "shrinks" in volume with each new dry roll of film.
Ultimately, the 2-4 ml of fluid (now stabilizer, that has replaced the wash water in the gelatin, that replaced the blix in the gelatin, that replaced the developer the gelatin absorbed) evaporates from the film into the air when you dry the film.
If you pre-wet your film with water before starting development (I don't) you'll likely see less shrinkage in your bottled developer (it will be ever-so-slightly diluted, with pre-wet water from the swollen gelatin, but what you pour back into the developer bottle will be the same volume you poured into the film tank - developer minus 2 ml, plus 2 ml of pre-wet water).
Leuco cyan dye - colorless cyan in the negative "prints" (reversed to a positive) as too much cyan in reddish skin tones: http://www.fujifilme...ef?OpenDocument
Anyway, to your original question - blix is, of course, a mixture of bleach and fixer. Both of those react chemically with the silver in film - the bleach converts it to a soluble silver salt, the fixer then dissolves the soluble silver salt and removes it from the film.
Eventually, the reactions deplete the necessary original chemicals that came in the mix (and saturate the fixer part with silver), so the blix will cease to function adequately.
It is important to remember that the bleach (whether stand-alone or in a blix mix) plays a role in correct development of the film's cyan dyes - it "finishes" that dye creation to prevent the occurence of "leuco cyan" (colorless cyan) dye molecules. If the bleach goes bad, it can lead to poor color as well as failing to remove the silver. That is an oxidation reaction ), so unlike other photo chemicals, where oxidation is a bad thing, the more you can oxygenate your bleach (by agitating to the point of some "foaminess") the better it works.
Thus I treat my bleach as co-equal with my color developer - when it's time to replace one, it is time to replace both. I replace stand-alone fixer when the "clearing time" of color film starts to exceed 1 minute.
Edited by adan, 19 July 2017 - 05:11.