I think for liquids, purifying will have to involve some method that does not involve instability.
I'm guessing the reason we can't put buckets of water or whatever in the current alchemical items is because the instability could be removed by simply mixing two buckets after the cooking is done and 'resetting' the stability for another go. That's something that can't be done with solid items and they probably couldn't figure out how to keep that from being done with water and so just excluded water from alchemy, so far.