A COMPARATIVE-STUDY OF 2 DIFFERENT APPROACHES FOR ACTIVE OPTICAL SENSING OF POTASSIUM WITH A CHROMOIONOPHORE

A potassium optical-sensing layer based on reflectance change measurements of the Takagi reagent (4'-picryl-3'-nitrobenzo-18-crown-6) immobilized on a plasticized PVC membrane has been developed. The analytical characteristics of this sensing-layer design have been studied in detail and are critically compared with those observed by immobilization (adsorption) of the same reagent on non-ionic resin beads packed at the end of an optical fiber (optrode). The immobilization of the indicator on the resin surface and its use in the optrode form have proved to be clearly superior to PVC immobilization and its use in connection with a typical flow-injection analysis (FIA) system. A probable recognition-process mechanism for both sensing phases of potassium is proposed. In the case of the sensing layer using a PVC membrane a very limited operational lifetime is observed due to leaching of the reagent and water absorption in the membrane. Detection limits are in the range 10-15 ppm K+ and the precision observed is around +/- 2% for both devices. The optrode type of sensor has been further developed for the determination of potassium in urine, milk and sea water.