LIPOPHILIC AND IMMOBILIZED ANIONIC ADDITIVES IN SOLVENT POLYMERIC MEMBRANES OF CATION-SELECTIVE CHEMICAL SENSORS

Lipophilic borate salts are frequently used as anionic additives in potentiometric and optical cation-selective sensors based on solvent polymeric membranes. The lifetime of such membranes may be limited owing to chemical decomposition and leaching of the components. Borate salts, in particular, are decomposed in the presence of acids in the membrane. Adequately substituted borate salts and sulphonic acids, such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate and dinonylnaphthalene-sulphonic acid (DNNS), are shown to be sufficiently stable as membrane additives. Furthermore, lipophilic mobile or immobilized sulphonic acids [DNNS or poly(2-acrylamido-2-methyl-1-propanesulphonic acid-co-styrene), respectively] were also tested as anionic additives. Their influence on the selectivity behaviour of the sensor is attributed to their strong association with positively charged species in the membrane phase. It may be kept small by choosing ionophores that form stable complexes with the analyte.