In crude penicillium juice, as exuded (pooped) straight from the mold, penicillin exists as the 100% free pure acid, swimming about in a lot of murky water.
An old rural uneducated woman living alone (the sort we used to burn as witches) can make this stuff.
By no coincidence, most of us would describe this penicillin (inaccurately) as highly impure.
But when we take a penicillin pill , it consists of lots of buffers and fillers and a little of that free penicillin acid now chemically bound to a metal in the form of a salt.
It takes a village of highly educated men (and about a billion dollars in equipment) to raise this child.
Again by no coincidence, most of us would (inaccurately) call this pill 100% pure penicillin.
The terms of pure and impure are really nothing but scientific rhetoric , as devoid of any fixed precise meaning as every term is in scientific rhetoric...