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

Lipophilic borate salts are frequently used as anionic additives in po tentiometric and optical cation-selective sensors based on solvent pol ymeric 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 membra ne. Adequately substituted borate salts and sulphonic acids, such as s odium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, sodium 3,5-bis(2 -methoxyhexafluoro-2-propyl)phenyl]borate and dinonylnaphthalene-sulph onic acid (DNNS), are shown to be sufficiently stable as membrane addi tives. 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 str ong association with positively charged species in the membrane phase. It may be kept small by choosing ionophores that form stable complexe s with the analyte.