Constant Potential Coulometric Measurements with Ca2+-Selective Electrode: Analysis Using Calibration Plot vs. Analysis Using the Charge Curve Fitting

The possibility of analysis using charge curve fitting in constant potential coulometric mode instead of using a calibration plot is explored, for the first time. The results are compared with the analysis based on the use of a calibration plot. A Ca2+ ion-selective electrode, with and without an electronic capacitor in series, is used as a model system in pure solutions of CaCl2. Both techniques delivered good results (error within 2%) when the final and the initial concentration values differed by not more than three times. Larger differences result in 10–25% error. The presence of an electronic capacitor in the measurement circuit and in series with the electrode, allows for significantly faster response

Voltammetric Ion Sensing with Ionophore-Based Ion-Selective Electrodes Containing Internal Aqueous Solution, Improving Lifetime of Sensors

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

Electrochemical Properties of Nitrate-Selective Electrodes: The Dependence of Resistance on the Solution Concentration

The electrochemical properties of ion-exchanger-based solvent polymeric ion-selective electrodes (ISEs)—bulk and interfacial resistance, capacitance, and polarization under a galvanostatic current step—are studied, with a nitrate ISE based on tetradecylammonium nitrate (TDANO3) as a model system. The study is performed by chronopotentiometric and impedance measurements, and focuses on the dependence of the aforementioned properties on the concentration of NO3− anions in solution. The impacts from the bulk and the interfacial charge transfer to the overall membrane resistance are revealed. It is shown that the bulk resistance of the membranes decreases over an increase of NO3− concentration within the range of a Nernstian potentiometric response of the ISE. This fact, also reported earlier for K+- and Ca2+-selective ISEs, is not in line with current views of the mechanism of the ISE response, or of the role of ion exchange in particular. The origin of this effect is unclear. Estimates are made for the concentration of ionized species (NO3− and TDA+) and, respectively, for the TDANO3 association constant, as well as for the species diffusion coefficients in the membrane

Transfer of Sodium Ion across Interface between Na⁺-Selective Electrode Membrane and Aqueous Electrolyte Solution: Can We Use Nernst Equation If Current Flows through Electrode?

Electrochemical impedance and chronopotentiometric measurements with Na+-selective solvent polymeric (PVC) membranes containing a neutral ionophore and a cation exchanger revealed low-frequency resistance, which is ascribed to Na+ ion transfer across the interface between the membrane and aqueous solution. The attribution is based on the observed regular dependence of this resistance on the concentration of Na+ in solutions. The respective values of the exchange current densities were found to be significantly larger than the currents flowing through ion-selective electrodes (ISEs) during an analysis in non-zero-current mode. This fact suggests that the interfacial electrochemical equilibrium is not violated by the current flow and implies that the Nernst equation can be applied to interpret the data obtained in non-zero-current mode, e.g., constant potential coulometry

Ion-Selective Optical Sensors: A New Look at Well-Established Techniques of Signal Acquisition

It is demonstrated for the first time that the registration of the optode signal under non-equilibrium conditions reduces analysis time and shifts the sensor working range. The fabrication of optode-like color standards for digital color analysis (DCA) is described, and a multi-parameter color scale for calibration-free sensor arrays is proposed. Advantages and limitations of monochrome and color camera for DCA with colorimetric optodes are discussed

Chemiresistors based on ultrathin gold nanowires for sensing halides, pyridine and dopamine

Two distinctively different approaches to the deposition of the ultrathin gold nanowires (NWs) on chips for the development of the chemiresistor sensors are studied. The first approach is the deposition of the pre-synthesized NWs on a chip using a microfluidic technique, and the second is the direct synthesis of the NWs on chips. It is shown that the 2nd approach ensures a better contact between the NWs and electrodes, lower resistance of the NW sensor device, and a longer life time of the devices. The current-voltage dependencies obtained for the chips with the NWs in contact with air, and aqueous solutions of NaF, NaCl, NaBr, NaI, pyridine, and dopamine indicate significant changes of the NWs resistance caused by sorption of halides, pyridine and dopamine on the NWs surface. This change of the NWs resistance can be used for quantification of Cl− over a concentration range of 10−5–10−3 M and dopamine over a concentration range of 10−8–10−5 M. Tentatively, it is concluded that the data obtained can be explained in terms of the Langmuir adsorption isotherm