Chemical reactions and the partitioning of potassium between metallic iron and potassium-silicate were studied at the core--mantle boundary (CMB) condition, 2900 km deep in the Earth, in a laser-heated diamond-anvil cell (LHDAC). Analytical transmission electron microscope (ATEM) analysis of the recovered samples from 134 GPa and 3500 K, prepared with Focused Ion Beam (FIB) techniques, revealed significant dissolution of potassium (0.8 wt.%) into molten iron, indicating that the partition coefficient of potassium between iron and silicate is 0.15. Our results show that the Earth's core can contain 35 ppm total potassium, i.e., 4.1 ¿ 10-3 ppm of 40K, which could serve as a heat source corresponding to 0.23 TW in the Earth's core.