Potentiometric Solid-Contact Ion-Selective Electrode for Determination of Thiocyanate in Human Saliva.

A new solid-contact potentiometric ion-selective electrode for the determination of SCN- (SCN-ISE) has been described. Synthesized phosphonium derivative of calix[4]arene was used as a charged ionophore. The research included selection of the ion-selective membrane composition, determination of the ISEs metrological parameters and SCN-ISE application for thiocyanate determination in human saliva. Preparation of the ISEs included selection of a plasticizer for the ion-selective membrane composition and type of the electrode material. The study was carried out using ISE with liquid internal electrolyte (LE-ISE) and solid-contact electrodes made of glassy carbon (GC-ISE) and gold rods (Au-ISE). The best parameters were found for GC sensors for which the ion-selective membrane contained chloroparaffin as a plasticizer (S = 59.9 mV/dec, LOD = 1.6 ´ 10-6 M). The study of potentiometric selectivity coefficients has shown that the thiocyanate-selective sensor could be applied in biomedical research for determination of SCN- concentration in human saliva. The accuracy of the SCN- determination was verified by testing 59 samples of volunteers' saliva by potentiometric sensors and UV-Vis spectrophotometry as a reference technique. Moreover, SCN- concentrations in the smokers' and non-smokers' saliva were compared. In order to investigate the influence of various factors (sex, health status, taken medications) on the thiocyanate level in the saliva, more extensive studies on a group of 100 volunteers were carried out. Additionally, for a group of 18 volunteers, individual profiles of SCN- concentration in saliva measured on a daily basis for over a month were collected.

Lead(II)-selective ionophores for ion-selective electrodes: a review.

In potentiometry, high selectivity of the ion-selective membranes must be assured in order to reliably measure various analytes in clinical, industrial and environmental samples. Due to the toxic nature of lead(II) it is important to monitor Pb(2+) distribution in natural waters. This may be achieved by implementation of ion-selective electrodes (ISEs) with high selectivity towards lead(II) and low detection limit. A great number of Pb(2+)-ionophores were synthesized and studied. In this work lead(II)-selective ionophores, starting from late 90s, are gathered and discussed. This work gives a comprehensive description and discussion on the novel and available lead(II)-selective ionophores for ISEs.

Solid-contact ion-selective electrodes with highly selective thioamide derivatives of p-tert-butylcalix[4]arene for the determination of lead(II) in environmental samples.

Thioamide derivatives of p-tert-butylcalix[4]arene were used as ionophores in the development of solid-contact ion-selective electrodes based on conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) which was synthesized by electrodeposition on the glassy carbon electrodes. The typical ion-selective membranes with optionally two different plasticizers [bis(2-ethylhexyl)sebacate (DOS) and 2-nitrophenyl octyl ether (NPOE)] were investigated. The potentiometric selectivity coefficients were determined by separate solution method (SSM) for Pb(2+) over Cu(2+), Cd(2+), Ca(2+), Na(+), and K(+). High selectivity toward Pb(2+) was obtained. By applying two conditioning protocols, a low detection limit log(a(DL)) ≈ -9 was achieved. The fabricated ion-selective electrodes were used to determine Pb(2+) concentration in environmental samples. The obtained results were compared to analysis done by inductively coupled plasma mass spectrometry (ICPMS).

Selectivity of lithium electrodes: correlation with ion-lonophore complex stability constants and with interfacial exchange current densities.

Lithium-selective electrodes with solvent polymeric membranes based on two different dicyclohexylamide neutral ionophores are studied systematically. The selectivity of lithium response is studied by means of the ordinary potentiometric experiments. Stability constants of lithium, sodium, and potassium ions with the neutral ionophores are measured by means of the segmented sandwich membrane method. Charge transfer through the membrane bulk and across the membrane/solution interface is studied by means of electrochemical impedance spectroscopy. Well-resolved Faradaic impedance semicircles are obtained, allowing calculation of exchange current densities for lithium, sodium, and potassium. It is clearly demonstrated that the potentiometric selectivity coefficients correlate well with thermodynamic equilibrium parameters. The correlation with exchange current densities also exists, although it is low, and seems rather qualitative than quantitative. The results are treated in favor of equilibrium at the membrane/solution interface. It is also concluded (tentatively) that the kinetic description is equivalent to the equilibrium one, giving evidence that ion-ionophore complexes form directly at the interface.