Improvement of the Upper Detection Limit of Ionophore-Based H-Selective Electrodes: Explanation and Elimination of Apparently Super-Nernstian Responses.

The response range of an ion-selective electrode (ISE) has been described by counterion interference at the lower and Donnan failure at the upper detection limit. This approach fails when the potentiometric response at the upper detection limit exhibits an apparently super-Nernstian response, as has been reported repeatedly for H-selective electrodes. While also observed when samples contain other anions, super-Nernstian responses at low pH are a problem in particular for samples that contain phthalate, a common component of commercial pH calibration solutions.

A rare positively charged nicotinic acid di-sulfide: 2,2'-di-thio-dinicotinic acid hydro-chloride monohydrate.

The title compound {systematic name: 3-carb-oxy-2-[2-(3-carb-oxy-pyridin-2-yl)disulfan-1-yl)]pyridin-1-ium chloride monohydrate}, CHNOS·Cl·HO, crystallizes in the triclinic space group . A pair of 2-mercaptonicotinic acid moieties is connected by a 2,2'-di-sulfide bond with a dihedral angle of 78.79 (3)°.