Dear Majid,

As you might know that each type of crown ether has a cavity which can fit ions with certain size. In order for the ion to be transported (escaped the trapping) through crown ether membrane, the size of the ion should be smaller (does not fit the cavity) than the cavity of the crown ether, thus, preventing any interaction between the ion and the oxygen atoms of the crown ether.

Please note, that crown ethers bind certain cations and they do not bind anions.

Hoping this will be helpful,

Rafik

when the ion absorbed by crown ether in the membrane, how is ion removed from it an being transferred?

Hi Majid,

The crown ether is not releasing the ions. The function of crown ether is just as a transportation for the ions or a passive carrier. For example, if you are using liquid contact ISE, which means the internal solution itself contained ion of interest at a constant activity. Once the membrane has  the ions, it will create gradients of activity across the membrane. By this gradient, using Gibbs Donnan equilibrium, the potential difference will be developed across the membrane.