Potassium is a chemical element, number 19 on the periodic table.
When the crew of the USS Exeter was reduced to crystalline minerals with all water drained from their bodies, 35% of their remains were potassium. (TOS: "The Omega Glory")
Potassium is a component of the chemical compound yominum sulfide (K4Ym3(SO73Es2). (Star Trek IV: The Voyage Home)
During a scan of Aldean DNA, Doctor Beverly Crusher was able to confirm that potassium overexposure was not responsible for the Aldeans sterility. (TNG: "When The Bough Breaks")