You ask for the pure version.

"I want the full strength version, please" you tell the arcane merchant. "Are you sure? The effects of the full strength potion can be very intense, and will not go away on their own." the old man tells you, with a look of concern on his brow. "If you must though, I will trade you for those copper and zinc coins in your pocket, I do need the alloys for an artifact I've been commissioned to create, and you wouldn't believe how hard it can be to source."

After reading and signing a scroll that suddenly appears in your hands, you head off on your way, a new bottle in your hand and your pocket no longer jingling with the change that was in it.