Galvanic potentiometric measuring

Potentiometric measurements are usually performed under zero-current conditions in a galvanic cell of the type ... 

Determination of the substance concentration with a potentiometric ISSs and C02-GSS is realized through the measurement of the electromotive force (EMF) of the galvanic cell (potentiometric measurement). In the case of the 02-GSS, however, the current is measured (amperometric measurement). 

To allow the practical potentiometric measurement of the EMF of a galvanic cell it is ensured that the measurement takes place in the open... 

Fig. 2.1. Schematic illustration of the galvanic electrochemical cell used to conduct potentiometric measurements. The disposable indicator electrode shown is fashioned from plastic pipette tips and PVC tubing. Note that the top compartment (shown dashed) is only necessary in cases where the inner filling solution does not contain well-defined concentrations of Cl" or Ag+.

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

It is the aim of the potentiometric measurements to determine the distribution of oxygen activity in the catalyst along the fixed bed in parallel to the concentration profile in the gas phase. In order to avoid the use of one galvanic cell at each sampling port the side streams collected passed sequentially over the cell shown in Fig.2. 

Therefore the setup where the dissolution reaction is followed by potentiometric measurements of the pertinent H -concentration within the galvanic cell (glass electrode solution salt bridge reference electrode) is an optimal system for solubility measurements. This method is called potentiometric method of solubility measiuements ( Potentio in Table 1.1). An example of such measurements could be tiiat of solubility of metal oxides at temperatures up to 300°C performed in ORNL using simultaneously sampling method and the hydrogen-electrode concentration cell (HECC). The details of such measurements and experimental setup can be found in Chapter 3 in this Book. 

The potentiometric measurements most often are carried out at no or negligible current flowing through the cell, i = 0. In this case, the measuring cell is actually a galvanic cell, composed of two half-cells. The potential difference is called the electromotive force (emf). 

Two types of methods are used to measure activity coefficients. Potentiometric methods that measure the mean activity coefficient of the dissolved electrolyte directly will be described in Section 3.3.3. However, in galvanic cells with liquid junctions the electrodes respond to individual ion activities (Section 3.2). This is particularly true for pH measurement (Sections 3.3.2 and 6.3). In these cases, extrathermodynamical procedures defining individual ion activities must be employed. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

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. 

Potentiometric methods measure a potential in a galvanic cell arrangement. Amperometric and voltammetric methods measure a current in an electrolytic cell. 

This type of detection has achieved much development in the last few years due to its simplicity. A specific revision on conductimetric (and potentiometric) detection in conventional and microchip capillary electrophoresis can be found in Ref. [57]. It is considered a universal detection method, because the conductivity of the sample plug is compared with that of the solution and no electroactivity of the analytes is required. Two electrodes are either kept in galvanic contact with the electrolyte (contact conductivity) or are external and coupled capaci-tively to the electrolyte (contactless mode). An alternating current potential is applied across the electrodes and the current due to the conductivity of the bulk solution is measured. As the signal depends on the difference in conductivity between solution and analyte zones, the choice of the electrolyte is crucial. It is necessary that it presents different conductivity without affecting sensitivity. 

The ion sensitive field-effect transistor (ISFET) is a special member of the family of potentiometric chemical sensors [6,7. Like the other members of this family, it transduces information from the chemical into the electrical domain. Unlike the common potentiometric sensors, however, the principle of operation of the ISFET cannot be listed on the usual table of operation principles of potentiometric sensors. These principles, e.g., the determination of the redox potential at an inert electrode, or of the electrode potential of an electrode immersed in a solution of its own ions (electrode of the first kind), all have in common that a galvanic contact exists between the electrode and the solution, allowing a faradaic current to flow, even when this is only a very small measuring current. 

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 ... 

The first term in this equation, Ejnj, contains the information that we are looking for—the concentration of the analyte. To make a potentiometric determination of an analyte, then, we must measure a cell potential, correct this potential for the reference and junction potentials, and compute the analyte concentration from the indicator electrode potential. Strictly, the potential of a galvanic cell is related to the activity of the analyte. Only through proper calibration of the electrode system with solutions of known concentration can we determine the concentration of the analyte. 

The potentials of other electrodes are expressed in reference to the normal hydrogen electrode. To evaluate the potential for any other single electrode, it is necessary to couple it with a standard or normal hydrogen electrode and the e.m.f of the galvanic cell is measured potentiometrically. Since the e.mf of the cell is known and is equal to the algebraic sum of the two electrode potentials of which the potential of the other electrode is obtained. If... 

With regard to in vivo gas-sensing devices, the majority of the work reported to date has involved oxygen-sensitive devices which operate as an electrolytic, not galvanic, type of electrochemical cell (i.e., current measured, not potential). Since such oxygen-sensing catheters are not based on ISEs, they will not be considered in this review. There has been, however, some limited work concerning the development of potentiometric sensors, particularly for in vivo COg measurements. One approach has been to devise... 

The properties of electrochemical cells allow us to. use them in a variety of ways for determining the concentrations of individual ions on physical-chemical properties. We will not deal with all types of electrochemical measurements here rather we will select examples to illustrate the use of electrochemical techniques in water chemistry. Our examples will include the measurement of activity (concentration) by potentiometric methods using galvanic cells and specific ion electrodes, and the measurement of activity (concentration) by electrolytic cells using techniques such as polarography and amperometric titration. 

The inner filling solution for the sensors is usually 0.01 M NaCl, which is also used to condition the potentiometric sensors. Electrochemical potential is measured with the following galvanic cell Ag/AgCl/bridge electrolyte/sam-ple solution/ion-selective membrane/inner filling solution/ AgQ/Ag. A high impedance pH-mV meter is used to measure the electrochemical potential. Selectivity coefficients are evaluated by the matched potential method (also known as method of mixed solutions), or via the separate solution method. 

A Nernst equation may be written for every galvanic cell at equilibrium. Therefore the equation is the basis of all thermodynamic applications of potentiometry (i.e. measurement of open-circuit cell potentials by means of a potentiometer or other zero-current device). Cells may be constructed and appropriate Nernst equations written to find, for example, the dissociation constant of water, and many electrolyte activity coefficients and stability and solubilit,v constants. Potentiometric titration curves are also interpieted by means of the appropriate Nernst equation. 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

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