Potentiometry cell voltage

The need for a rather lengthy discussion of the liquid junction potential was prompted by the fact that, in potentiometry, the information is obtained from the measurement of the cell voltage. 

Electrochemical transducers are commonly used in the sensor field. The main forms of electrochemistry used are potentiometry (zero-current cell voltage [potential difference measurements]), amperometry (current measurement at constant applied voltage at the working electrode), and ac conductivity of a cell. 

When thinking about the accuracy and precision of direct potentiometry, it must be kept in mind that 0.5 mV difference in cell voltage translates to almost 2% difference in sample concentration for univalent ions. For divalent ions, it is close to 4%, while for a trivalent ion, in which case S 20mV/decade, it is close to 6%. Therefore, for high accuracy, very well controlled measuring parameters such as cell temperature, junction potential, electrode selectivity, and so on, are needed. 

As mentioned previously, electroanalytical techniques that measure or monitor electrode potential utilize the galvanic cell concept and come under the general heading of potentiometry. Examples include pH electrodes, ion-selective electrodes, and potentiometric titrations, each of which will be described in this section. In these techniques, a pair of electrodes are immersed, the potential (voltage) of one of the electrodes is measured relative to the other, and the concentration of an analyte in the solution into which the electrodes are dipped is determined. One of the immersed electrodes is called the indicator electrode and the other is called the reference electrode. Often, these two electrodes are housed together in one probe. Such a probe is called a combination electrode. 

Equation 2.16 shows that potentiometry is a valuable method for the determination of equilibrium constants, ffowever, it should be borne in mind that the system should be in equilibrium. Some other conditions, which are described below, also need to be fulhlled for use of potentiometry in any application. The basic measurement system must include an indicator electrode that is capable of monitoring the activity of the species of interest, and a reference electrode that gives a constant, known half-cell potential to which the measured indicator electrode potential can be referred. The voltage resulting from the combination of these two electrodes must be measured in a manner that minimises the amount of current drawn by the measuring system. This condition includes that the impedance of the measuring device should be much higher than that of the electrode. 

Potentiometry is a method of electroanalytical measurement in which the equilibrium voltage of the cell consisting of an indicator electrode and a proper reference electrode is measured using a high-impedance voltmeter, i.e., effective at zero current. The potential of the indicator electrode is a function of particular species present in solutions and their concentration. By judicious choice of electrode material, the selectivity of the response to one of the species can be increased, and thus, interferences from other ions can be minimized. The method allows the determination of concentrations with detection limits of the order of 0.1 pmol per liter, although in some cases, as little as lOpmol differences in concentration can be measured. 

Direct potentiometric measurements are used to complete chemical analyses of species for which an indicator electrode is available. The technique is simple, requiring only a comparison of the voltage developed by the measuring cell in the test solution with its voltage when immersed in a standard solution of the analyte. If the electrode response is specific for the analyte and independent of the matrix, no preliminary steps are required. In addition, although discontinuous measurements are mainly carried out, direct potentiometry is readily adapted to continuous and automatic monitoring. 

Most ion-selective electrodes, however, cannot be used for the direct determination of functional groups in organic compounds unless they are converted into ionic species. Thus, direct potentiometry is performed rarely with commercial ion-selective electrodes. For example, the CN -selective electrode gives an almost Nernstian response in the determination of substituted phenylacetonitriles and benzonitriles. Thiols can be determined by direct measurement of the voltage of a cell with an Ag2S-based electrode. Such examples are, however, not typical for applications of ion-selective electrodes. 

In practice, electrochemistry not only provides a means of elemental and molecular analysis, but also can be used to acquire information about equilibria, kinetics, and reaction mechanisms from research using polarography, amperometry, conductometric analysis, and potentiometry. The analytical calculation is usually based on the determination of current or voltage or on the resistance developed in a cell under conditions such that these are dependent on the concentration of the species under study. Electrochemical measurements are easy to automate because they are electrical signals. The equipment is often far less expensive than spectroscopy instrumentation. Electrochemical techniques are also commonly used as detectors for LC, as discussed in Chapter 13. 

Potentiometry involves measurement of the potential, or voltage, of an electrochemical cell. Accurate determination of the potential developed by a cell requires a negligible current flow during measurement. A flow of current would mean that a faradaic reaction is taking place, which would change the potential from that existing when no current is flowing. 

Electrochemical methods are increasingly popular in the clinical laboratory, for measurement not only of electrolytes, blood gases, and pH but also of simple compounds such as glucose. Potentiometry is a method in which a voltage is developed across electrochemical cells as shown in Figure 27.3. This voltage is measured with little or no current flow. 

With an external DC power supply connected to the electrolytic cell, the applied voltage that gives no DC current flow in the external circuit corresponds to the equilibrium potential of the half-cell (or actually the cell). It is the same voltage as read by a voltmeter with very high input resistance and virtually no current flow (pH meter). In electrochemistry, potentiometry is to measure the potential of an electrode at zero current flow, which is when the cell is not externally polarized. To understand the equilibrium potential with zero external current, we must introduce the concept of electrode reaction... 

With no current through the electrolytic cell, it does not matter whether the electrodes are large or small the equilibrium potentials are the same. But with current flow, the current density and therefore the voltage drop and the polarization, will he much higher at the small electrode. An increased potential drop will occur in the constrictional current path near the small electrode, and in general the properties of the small electrode will dominate the results. The small electrode will be the electrode studied, often called the working electrode. It is a monopolar system, meaning that the effect is determined hy one electrode. The other electrode becomes the indifferent or neutral electrode. Note that this division is not true in potentiometry, electrode area is unimportant under no-current conditions. 

Other analytical techniques. Electroanalytical methods can also be used to differentiate between ionic species (based on valence state) of the same element by selective reduction or oxidization. In brief, the electroanalytical methods measure the effect of the presence of analyte ions on the potential or current in a cell containing electrodes. The three main types are potentiometry, where the voltage difference between two electrodes is determined, coulometry, which measures the current in the cell over time, and voltammetry, which shows the changes in the cell current when the electric potential is varied (current-voltage diagrams). In a recent review article, 43 different EA methods for measuring uranium were mentioned and that literature survey found 28 voltammetric, 25 potentiometric, 5 capillary electrophoresis, and 3 polarographic methods (Shrivastava et al. 2013). Some specific methods will be discussed in detail in the relevant chapters of this tome. 

Fig. 84. The circuit of potentiometry with constant current A a voltage measuring instrument B batteries C cell R resistor G galvanometer.

As noted above, voltage is developed between two electrodes in an electrochemical cell. The voltage depends upon the kinds and concentrations of dissolved chemicals in the solutions contacted by the electrodes. In some cases this voltage can be used to measure concentrations of some substances in solution. This gives rise to the branch of analytical chemistry known as potentiometry. Potentiometry uses ion-selective electrodes or measuring electrodes whose potentials relative to a reference electrode vary with the concentrations of particular ions in solution. The reference electrode that serves as the ultimate standard for potentiometry is the standard hydrogen electrode shown in Figure 8.13. In practice, other electrodes, such as the silver/silver chloride or calomel (mercury metal in contact with are... 

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