The batch I made yesterday is already showing signs of weakness. Two things come to mind. The water I used undoubtedly contained oxygen in solution. Second, oxidized Metol is not regenerated by sulfite, but made inactive in the form of a sulfonate. Oxidized Metol is, however, regenerated by ascorbic acid. Oxidized Metol is a restrainer of development. If it is removed, the remaining active Metol concentration is reduced. If it is not removed, the developer activity is chemically reduced.
Originally Posted by Photo Engineer
I added 18 grams ascorbic acid to the remaining 900 ml of "Metonal". The base was, of course, precipitated. I added KOH until the flocculent precipitate dissolved. Time will tell if the life of the concentrate is extended. Its activity seems to have been restored.
The basis of my actions is on p.367 of "The Theory of the Photographic Process", third edition.