The dependence of the bulk resistance of membranes of ionophore-based ion-selective electrodes (ISEs) on the composition of mixed electrolyte solutions, within the range of the Nernstian potentiometric response, is studied by chronopotentiometric and impedance measurements. In parallel to the resistance, water uptake by the membranes is also studied gravimetrically. The similarity of the respective curves is registered and explained in terms of heterogeneity of the membranes due to the presence of dispersed aqueous phase (water droplets). It is concluded that the electrochemical equilibrium is established between aqueous solution and the continuous organic phase, while the resistance refers to the membrane as whole, and water droplets hamper the charge transfer across the membranes. In this way, it is explained why the membrane bulk resistance is not constant within the range of the Nernstian potentiometric response of ISEs.
The possibility of voltammetric ion sensing is demonstrated, for the first time, for ion-selective electrodes (ISEs) containing an internal aqueous solution. ISEs selective to calcium, lithium and potassium ions are used as model systems. The internal solution of the ISEs contains a chloride salt of the respective cation and a ferrocenemethanol or ferrocyanide/ferricyanide redox couple. A platinum wire is used as the internal reference electrode. It is shown, theoretically and experimentally, that the dependence of oxidation and reduction peak potentials on the sample composition obeys the Nernst law, while the peak currents virtually do not depend on the sample composition. Thus, the electrode behavior is similar to that reported by Bakker’s group for solid contact ISEs with ultra-thin membranes (200–300 nm). It is shown that the use of classical ISEs with relatively thick membranes (100–300 µm) and internal aqueous solution allows for the sensor lifetime of about one month. It is also shown that use of a suitable background electrolyte allows for improvement of the detection limits in voltammetric measurements with ISEs.
Unsubstitutedand 1-methyl-substituted 1,3,4,8btetrahydrodiazirino[3,1-a]isoquinolines (diaziridines 1 and 2) demonstrate different reactivities toward aryl isocyanates and aryl isothiocyanates. Diaziridine 1 reacts with aryl isocyanates as a nucleophile to afford novel 1,5,6,10btetrahydro[1,2,4]triazolo[5,1-a]isoquinolines. The addition of a Lewis or Brønsted acid switches the reaction pathway, producing seven-membered 2,3-benzodiazepines. In reactions of diaziridine 1 with aryl isothiocyanates, unusual zwitterionic compounds -2-thioxo-5,6-dihydro-2H-[1,2,4]triazolo[5,1a]isoquinolin-1-ium-3-ides -and products of the addition of two molecules of diaziridine 1 to one molecule of aryl isothiocyanate are obtained. However, in reactions with aryl iso(thio)cyanates under heating or acid catalysis, diaziridine 2 is first converted into an azomethine imine, the 1,3-dipolar cycloaddition of which to the dipolarophile produces substituted triazolo[5,1-a]isoquinolin-2(3H)-(thi)ones. Proposed is a new method for azomethine imine generation from diaziridine 2 enabled by Brønsted acid catalysis. Antitumour activity of select compounds obtained was assayed to help guide future drug discovery efforts.
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