FAQ overflow

QUESTION

The most common drying agent I encountered when using a vacuum dessicator was phosphorus pentoxide in the form of Sicapent. But I've recently encountered people using KOH as a cheaper alternative (in a practical course for students, where such costs are amplified due to the high number of students).

What are the differences between KOH and phosphorus pentoxide as a general purpose drying agents to remove water? Is there a difference in effectiveness? Are there classes of substances where you can't use one of those drying agents?

Phosphorous pentoxide (really P4O10) is an anhydride of phosphoric acid. Phosphorous pentoxide reacts with water to form phosphoric acid: $$\ce{P4O10 + 6H2O -> 4H3PO4}$$ Phosphoric acid is also hygroscopic and absorbs water to form a solution.