Sir Humphry Davy discovered potassium in 1807. He exposed very dry molten potash to an electric current, and potassium collected on the end of the cathode. Potassium was the first metal to be isolated in this manner.
The name comes from the English "potash" and the Arabic word for alkali, "qali," which also meant plant ashes. The symbol "K" comes from the Latinate word "kalium." Potassium and sodium were indistinguishable for many centuries because they tend to form compounds with the same elements. Once Sir Humphry Davy had accomplished the separation of potassium, he was able to disassociate sodium later the same year.