Fabrication of standard ISE electrodes The membrane solution was poured into a plastic ring placed on a glass plate and dried overnight. The next day membrane discs were cut to appropriate size using a biopsy punch and mounted in electrode bodies (type IS 561, Philips, Willi Möller AG, Zurich, Switzerland). 0.1 KCl solution was used as an internal filling. The electrodes were conditioning in 0.01 M KCl.