Potassium (K+) is the most abundant intracellular cation. Highest concentration of potassium is seen in:

There is evidence that active potassium secretion occurs in the distal colon. This secretion combined with potassium in bacteria and colonic mucous in stool may explain the relatively high concentration of potassium, 50-90 mmol/L, in stool. Ref: Essentials of General Surgery, 4e edited by Peter F. Lawrence, Richard M. Bell, Merril T. Dayton, 2006, Page 55.