Potentiometric behaviour of lanthanum polyalkoxylate complexes in PVC membrane electrodes: effect of plasticizers and applications

Tetraphenylborate salts of lanthanum complexes of nonylphenoxypolyethyleneoxyethanol (Antarox CO-880) and polypropylene glycol-425 (PPG-425) have been prepared and examined for their applicability as sensing species for lanthanum ions by incorporating them in a poly (vinyl chloride) membrane system in the presence of 2-nitrophenyl phenyl ether (NPPE), dioctylphenyl phosphonate or a mixture of both as plasticizers. Almost all of the systems studied showed an excellent response for lanthanum ions in pure solutions with a near Nernstian response of 18-20 mV per decade between 10^-5 and 10^-1 M lanthanum and a static response time of less than 1 min. Among the systems studied, lanthanum(Antarox CO-880)TPB/100% NPPE (TPB=tetraphenylborate) was the best in terms of tolerance to interfering ions. K⁺, Ba²⁺, Pb²⁺ and Al³⁺ were serious interferents, but most other interferents investigated were tolerable at [La³⁺] > 10^-4 M. Plasticizers were found to be decisive in determining the sensor selectivity towards the different metal ions. A lanthanum(Antarox CO-880)TPB/100% NPPE PVC membrane electrode was utilized for the potentiometric titration of fluoride, in sodium fluoride, against lanthanum nitrate solutions. It proved to be successful for fluoride concentrations of ≥ 10^-3 M.