I beg to disagree on this one.
Mesuring minute amounts of phenidone requires a really good scale.
If you do not trust the life of the solution, mix, use and discard.
I have used my phenidone reserve solutions both in absolute ethanol and isopropyl alchool for a couple of months without any problem, and many years ago I've used it solved in metabissulfite.
(all my developers, film and paper, are phenidone based - I was unable to find metol locally)