Which crown ether forms a complex with potassium ions?

The crown ether that forms a complex with potassium ions (K⁺) is 18-crown-6.12

This molecule consists of an 18-membered ring containing 6 ether oxygen atoms. The potassium ion fits into the center of this ring, where it is solvated by the oxygen atoms to form a 1:1 complex. Because the selectivity of a crown ether depends on the size of its ring relative to the cation, 18-crown-6 is specifically suited for the size of the potassium ion.12