Phosphate Rocks Chapter 12: Potash

Potash is the name given to a range of natural salts of potassium²⁷. It was Humphry Davy^(x), a brilliant Cornish chemist and poet, who first isolated potassium metal. Davy is also remembered as the inventor of a safety lamp for miners and for naming laughing gas, although he quipped that his greatest ever discovery was Michael Faraday^(xiii), the young man he hired to help after an accident in the lab damaged Davy’s eyesight²⁸. 

In 1807, Davy passed an electric current through molten potash and, after many false starts, punctuated by sparks, burns and explosions, finally isolated the element he was seeking. 

Potassium is a silvery-white metal, soft enough to cut with a dinner knife. You may remember a school chemistry experiment demonstrating the increasing reactivity of Group 1 metals²⁹. Your teacher donned a white coat, goggles and gloves and, with all the theatre a fine performer can muster, ordered the class to retreat to a safe distance. Unscrewing the lid from a little glass vial containing a silvery lump in a clear, protective oil, teasing out a little pearl with tweezers, dropping a nubbin of metal onto a dish of water. If the metal bead was lithium or sodium, it sizzled and raced across the surface in mad zigzags, a metal hovercraft on a raft of hydrogen. But if it spontaneously burst into flames, the hydrogen igniting and burning with tongues of lilac fire as it popped and jumped and danced across the water, then the metal was potassium. 

Potassium is highly reactive, which is why you never find elemental potassium in nature. Humphry Davy had to use electrochemistry to coax it out of hiding when, all the time, it was right there in plain sight. 

There are over five-hundred trillion³⁰ tonnes of potassium in the sea alone. Potassium reacts with oxygen to form flaky white potassium peroxide, and it reacts with water to form potassium hydroxide, a strong alkali which in turn will react with almost anything acidic to form stable potassium salts. Most potassium salts are soluble, so when it rains, the potassium compounds in the rocks and the soil are washed into the sea.