The study of a nafion membrane (ISE membrane) for the inexpensive potentiometric detection of potassium ion in a flow injection system is reported. The nafion membrane doped with potassium ion showed that excellent selectivity could be achieved for various cations, except for Rb+, and some interference for Cs+. A linear potential response of 10-2 - 10~® M for the potassium ion was obtained with a detection limit of about 0.5 x 10-6 M. This approach can be employed for constructing a potentiometric detector for virtually any cation (soluble at basic pH values) by simply changing the electrode pretreatment (or doping solution)
