I reached 100 skill in alchemy, I have 1 perk to increase and I tried to activate "purity", the highest perk in Alchemy, I click on it but it's not available and I don't know why. I completed the left branch in this tree Alchemist-Physician-Poisoner-ConcentratedPoison-Snakeblood, why doesn't it let me pick the last one?

Bug

"Although the in-game perk tree indicates that the prerequisite perk for Purity is Snakeblood, in fact the only required perk is Experimenter"
http://www.uesp.net/wiki/Skyrim:Alchemy

Thanks, I'll have to wait and invest in Experimenter then, although in this stage of my game it's kind of useless to me. I hope 1/3 Experimenter will be enough.

Uhm, I have a question that I thought the thread was about, what does it do?

Purity? Whenever you make a potion, it's either a potion or a poison. I believe the first effect determines which it will be.
With the purity perk, it removes helpful effects from poisons and harmful effect from potions.

I.e. : You make a potion that is +health but has -stamina on it. With the Purity perk, the -stamina part will be taken off.

I.e. : You make a poison that is -health but has + stamina on it. With the Purity perk, the +stamina part will be taken off.