Improved detection limits and sensitivities of potentiometric titrations

Zero-current ion fluxes through polymer membranes of ion-selective electrod es (ISEs) may lead to biased endpoints of potentiometric titrations. The bi as is eliminated, and the sensitivity of the end-point detection is improve d through reducing transmembrane ion fluxes by an appropriate choice of the inner solution. Surprisingly, ISE membranes that have a significant primar y ion flux toward the inner solution show much larger sensitivities than ex pected by titration theory; however, depending on the experimental conditio ns, their application may bias the endpoint. With the optimal systems, endp oint detection is now possible with total sample concentrations below 10(-6 ) M, as demonstrated by the titration of EDTA with a Pb2+ solution.