New modified polymeric electrodes selective to local anaesthetic compounds

New polymeric electrodes responding to the cationic forms of tetracaine (TC), lidocaine (LD), and procaine (PC) were constructed by incorporating their ion-pair complexes (the salts of TC, LD and PC with phosphotungstic acid) into ethylene-vinyl acetate (E/VAC) copolymer. Other ion pairing agents investigated were silicotungstate and tetraphenylborate. The phosphotungstic acid resulted in the best linear and Nernstian response. A 1:1 (v/v) mixture of dioctyl phthalate (DOP) and nitrobenzene (NB) was used as plasticizer. The electrodes exhibited linear response over the concentration ranges 10(-2)-5.6 x 10(-6), 10(-2)-2.5 x 10(-5) and 10(-2)-1.8 x 10(-5) M of TC, LD and PC, respectively. pH did not affect the electrode performances within the ranges 2.7-6.3, 2.6-6.7 and 2.8-7.5 for the three electrodes, respectively. Interferences are negligible for many organic base and alkali metal cations. Cations of similar structure interfere with LD and PC, but not appreciably with TC. Direct potentiometry was used to determine these compounds in pharmaceutical preparations with accurate results.