Jorge: PQ (Phenidone/Hydroquinone) in alcholol will work, but why not use propylene glycol or ethylene glycol as the preservative instead? I did (following Patrick Gainer' sugggestions) and it has lasted very well!

I just used some this morning that I mixed months ago - - it is the same color and activity now as when I first mixed it.

When Hydroquinone begins to oxidize, it turns brown, my stock PQ solution is still the slightly pinkish color it was when I mixed it.

To make the stock solution, I used just enough methanol to dissolve the PQ, then added ethylene glycol to make up the desired total solution volume.

Are you keeping the alkali as a separate stock solution?