Measuring electrodes, potentiometric cells

The electrochemical potential in a potentiometric cell is inevitably measured with respect to a standard electrode. Several types of electrodes are commonly used. The standard hydrogen electrode (SHE) is a hydrogen half-cell in which the cell reaction is as follows ... 

If a cell is operated potentiometrically, then only two electrodes are required. However, if closed-circuit operation is to be employed it is necessary to measure electrode overpotentials. To measure electrode overpotentials it is necessary to use a three-electrode DC system. The system consists of a working electrode which is exposed to reaction conditions, a counter electrode (which is exposed to a constant partial pressure of oxygen in the case of an oxygen-ion conducting electrolyte), and a reference electrode. The reference electrode remains unpolarized during the measurements and may be exposed to either reaction conditions or the constant conditions of the counter electrode. A power supply is used to polarise the working and counter electrodes while the current is recorded (see Figure 3). 

In constructing an electric cell for potentiometric titrations it is necessary, of course, to use a second electrode to complete the circuit, in addition to the measuring electrodes (commonly called indicator electrodes ) described above. Ideally the second electrode would be a hydrogen electrode which (as explained in the entry on electrode potential) is the standard reference electrode for which die potential, in equilibrium with its ions, is defined as zero. Since it is awkward to use, other electrodes of known potential, such as the calomel electrode or the glass electrode, are commonly used as reference electrodes. The arrangement of the apparatus is as shown in Fig. 1. 

In a potentiometric measurement with an ion-selective electrode, the cell can be represented as... 

Isopotential point — In potentiometric measurements with the use of a -> ion-selective electrode (ISE) cell, the isopotential point is the potential difference between the internal and external reference electrodes which is independent of temperature. The isopotential point is governed by a particular activity of the ion being determined. Both ISE and the outer reference electrode must be specified. When an isothermal cell is used with identical reference electrodes, the isopotential point is defined by the activity of the sensed ions that gives zero net - membrane potential, e.g., sensed activity is the same in the inner and outer (test) solution. Calibration lines for different cell temperatures have different slopes, but they intersect at a common activity point. Cells with temperature gradients are not recommended. 

These systems involve galvanic cells (p. 229) and are based on measurement of the potential (voltage) difference between two electrodes in solution when no net current flows between them no net electrochemical reaction occurs and measurements are made under equilibrium conditions. These systems include methods for measuring pH, ions, and gases such as CO2 and NH3. A typical potentiometric cell is shown in Fig. 34.2. It contains two electrodes ... 

For reliable high-temperature potentiometric studies, the measured open-circuit potential should be stable and reproducible within a few millivolts or less. Similar to the low-temperature electrochemical measurements, both the indicator and reference electrodes should be used in devising a high-temperature potentiometric cell. Both the electrochemical probes should be resistant to chemical degradation and pressure (mechanical) stress in high-temperature aqueous environment. The development of suitable indicator and reference electrodes that can reliably... 

To completely eliminate changes in reference electrode half-cell potentials, a three electrode potentiostat is often employed. In simple terms, the potentiostat appHes a voltage to the working electrode, which is measured versus a reference electrode via a zero current potentiometric type measurement, but the current flow is between the working electrode and a third electrode, called the counter electrode. Thus if reduction takes place at the working electrode, oxidation would occur at the counter electrode but no net reaction would take place at the surface of the reference electrode, since no current flows through this electrode. A potentiostat circuit is relatively simple to construct using modern operational amplifiers. 

Measurement of the cell potential of a potentiometric sensor should be carried out under zero-current or quasiequilibrium conditions. A high input impedance electrometer is commonly used. The advantage of potentiometric sensor is that the sensor output and the cell potential do not depend on the electrode surface area. This can make the manufacturing of practical sensors simpler. A major disadvantage of... 

The basic principle of the potentiometric ZrOz oxygen concentration cell is explained in Section 5.63.1, Fig. 5.6.2. The sensor consists of a Zr02 solid electrolyte body coated with two Pt electrodes, a measuring electrode exposed to the exhaust gas with an oxygen partial pressure of p 02, and a reference electrode exposed to a reference atmosphere with a constant oxygen partial pressure of p f2, typically ambient air. 

Rybkin and Seredenko reported the construction of empirical scales of oxoacidity (acidity rows) in molten KC1 at 800 °C and Csl at 650 °C [62, 63]. Estimation of the oxoacidic properties was performed for buffer solutions obtained by the addition of equimolar quantities of conjugated acid and base in the melt. E.m.f. (pO) measurements were performed in the potentiometric cell with the use of a membrane oxygen electrode Pt(02)lYSZ. 

The work of some of us (Cherginets, Banik and Rebrova) is devoted to studies of the reversibility of various oxygen electrodes such as Pt(02)lYSZ and NilNiO, in the KCl-LiCl eutectic melt at 400, 500, 600 and 700 °C. The potentiometric cell for the measurements at 400 and 500 °C consisted of the lead reference electrode (PblPbCl2, 0.05 mol kg-1) and one of the indicator oxygen electrodes [232] ... 

A simple but reliable experimental routine was used by Combes, Vedel and Tremillon to investigate the pyrohydrolysis of the KCl-NaCl equimolar mixture [240]. For this purpose, a mixture of HC1 and H20 obtained by passing inert gas through aqueous solutions of HC1 of certain concentrations was supplied into the inner space of the potentiometric cell with an indicator oxygen electrode (NilNiOlYSZ). The measurements of the equilibrium oxide-ion concentrations, performed by the potentiometric method, permitted calculation of the equilibrium constants of reaction (3.6), and determination of their thermal dependence ... 

In the foregoing treatment of electrodes and cells we assumed implicitly that the electrode or cell was in equilibrium with respect to certain chemical and electrical transformations. By definition such an electrode or cell is reversible. To correlate measured values of cell potentials with the ones calculated by the Nernst equation, the measured values must be equilibrium or reversible values the potentiometric measurement in which no current is drawn from the cell is ideally suited for the measurement of reversible potentials. 

In contrast with amperometric electrodes, potentiometric electrodes are passive in that they do not convert the analyte. The main use of potentiometry is to measure the concentration (strictly activity) of a target analyte and so it is usefiil to revisit the Nemst equation, relating the cell potential to activity. For this purpose, consider a simple redox reaction at the right-hand electrode of a cell, with SHE as the left-hand electrode. The process at the right-hand electrode is... 

Accurate measurement of pH is achieved by the use of electrical pH meters. Potentiometric meters consist of a measuring electrode (hydrogen gas, quinone, glass, or antimony) and a reference electrode or standard electrode (usually calomel) connected to a potentiometer circuit which measures the voltage generated by the electrical cell when the electrodes are immersed in the solution to be tested. These meters... 

Mesmer and Baes (P6) studied the dissociation equilibria potentiometrically over a wide temperature range. Using a hydrogen-electrode concentration cell measurements were made to SOO C. The equilibria were expressed as neutralization reactions ... 

Potentiometric Measurements. The potentiometric responses of the icm-sensor constructed above were carried out in 0.1 M Tris-HCl buffer, pH 7.0, and using a double junction calomel electrode with the cell assembly ... 

The open circuit potential difference (AE) of the above cell was found to be -30 mV. This potential difference is highly reproducible for continuous measurement of the cell for two months which shows the non-polarizable behavior of the modified sol-gel electrode to be exploited in the construction of solid-state potentiometric ion-sensOT. 

The potentiometric method is based on measurement of the cell emf in an electrochemical cell in which one of the electrodes is selective for hydrogen ions and the other electrode serves as a reference. An important consequence of this fact is that the change in potential on moving the electrodes from the buffer to the sample is the sum of all changes that occur in the contributions to the cell potential as shown in Fig. (7-1), where ... 

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