Yep - chemist knives and keys are futile. I would throw out every key you take over from the recent act. It only confuses me, as I can't keep track which keys I now have to find or not. Adn the same goes for quest items I have left over due to act change => out. I hate a cluttered inventory.
My waste disposal for this is either the 1st room in act change or the BF - mostly BF. the first thing I do then is: sell all old stuff (set/unique items) - drop stuff I don't need anymore and start to explore the new act.
