Membranes electrode

When ions are transferred between two phases, say a solution and a membrane, this gives rise to a Galvani potential difference (see also Chap. 1.2.5, where the ion transfer between two immiscible electrolyte solutions is considered). This happens also when both phases are ion conductors and electrons are not involved. The term membrane denotes a thin plate separating two liquid phases. Membranes are divided into three groups  

Large meshed membranes. Rapid mixing of different electrolyte solutions is delayed and a diffusion potential arises due to the different diffusion coefficients of different ions (see also Chap. IIL3, Eq. IIL3.33). 

Close meshed membranes. Only ions or molecules up to a certain size can pass the membrane. They work similarly to those which are thick with respect to adjacent mixed phases but they are able to host certain ions or molecules and, at least in principle, may transport them from the side of higher electrochemical potential to the side of lower electrochemical potential. Such membranes are named semipermeable. Here, an electric potential difference between the two solutions exists, which is called Donnan potential. 

Thick membranes. Two Galvani potential differences can occur at the two interfaces, and diffusion potentials may build up in the membrane. Among the thick membranes, those of glass are most important (the glass membrane is geometrically thin however, in the sense of the nomenclature given here it is thick). 

Semipermeable Membrane Without Inner Diffusion Potential 

The following brief classification of membrane electrodes can be used [42] inert membranes (cellulose, some sorts of porous glass) ion exchange membranes. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Membrane Potentials Ion-selective electrodes, such as the glass pH electrode, function by using a membrane that reacts selectively with a single ion. figure 11.10 shows a generic diagram for a potentiometric electrochemical cell equipped with an ion-selective electrode. The shorthand notation for this cell is 

Interaction of the analyte with the membrane results in a membrane potential if there is a difference in the analyte s concentration on opposite sides of the membrane. One side of the membrane is in contact with an internal solution containing a fixed concentration of analyte, while the other side of the membrane is in contact with the sample. Current is carried through the membrane by the movement of either the analyte or an ion already present in the membrane s matrix. The membrane potential is given by a Nernst-like equation 

A potential developing across a conductive membrane whose opposite sides are in contact with solutions of different composition. 

An electrode in which the membrane potential is a function of the concentration of a particular ion in solution. 

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Pt/Ru Catalyst Polymer Pt Catalyst Porous Gas Layer Electrolyte Layer Diffusion Membrane Electrode... 

The concept of the reversed fuel cell, as shown schematically, consists of two parts. One is the already discussed direct oxidation fuel cell. The other consists of an electrochemical cell consisting of a membrane electrode assembly where the anode comprises Pt/C (or related) catalysts and the cathode, various metal catalysts on carbon. The membrane used is the new proton-conducting PEM-type membrane we developed, which minimizes crossover. 

Electrochemical cell for potentiometry with an ion-selective membrane electrode. 

Replacing Na20 and CaO with Li20 and BaO extends the useful pH range of glass membrane electrodes to pH levels greater than 12. 

A second complication in measuring pH results from uncertainties in the relationship between potential and activity. For a glass membrane electrode, the cell potential, Ex, for a solution of unknown pH is given as... 

The relative measurement error in concentration, therefore, is determined by the magnitude of the error in measuring the cell s potential and by the charge of the analyte. Representative values are shown in Table 11.7 for ions with charges of+1 and +2, at a temperature of 25 °C. Accuracies of 1-5% for monovalent ions and 2-10% for divalent ions are typical. Although equation 11.22 was developed for membrane electrodes, it also applies to metallic electrodes of the first and second kind when z is replaced by n. 

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

Potentiometric electrodes are divided into two classes metallic electrodes and membrane electrodes. The smaller of these classes are the metallic electrodes. Electrodes of the first kind respond to the concentration of their cation in solution thus the potential of an Ag wire is determined by the concentration of Ag+ in solution. When another species is present in solution and in equilibrium with the metal ion, then the electrode s potential will respond to the concentration of that ion. Eor example, an Ag wire in contact with a solution of Ck will respond to the concentration of Ck since the relative concentrations of Ag+ and Ck are fixed by the solubility product for AgCl. Such electrodes are called electrodes of the second kind. 

The potential of a membrane electrode is determined by a difference in the composition of the solution on either side of the membrane. Electrodes using a glass membrane respond to ions... 

Radio, N. Komijenovic, J. Potentiometric Determination of an Overall formation Gonstant Using an Ion-Selective Membrane Electrode, /. Chem. Educ. 1993, 70, 509-511. 

Watanabe and co-workers described a new membrane electrode for the determination of cocaine, which is a weak base alkaloid with a piC of 8.64d The response of the electrode for a fixed concentration of cocaine was found to be independent of pH in the range of 1-8, but decreased sharply above a pH of 8. Offer an explanation for the source of this pH dependency. 

Mifflin and associates described a membrane electrode for the quantitative analysis of penicillin in which the enzyme penicillinase is immobilized in a polyacrylamide gel that is coated on a glass pH electrode. The following data were collected for a series of penicillin standards. 

Meyerhoff, M. E. Fu, B. Bakker, E. et al. Polyion-Sensitive Membrane Electrodes for Biomedical Analysis, Anal. Chem. 1996, 68, 168A-175A. 

Hypochlorous acid can be distinguished from other chlorine species by amperometry using a membrane electrode (135). Spectrophotometry can also be used to measure HOCl via its absorbance maximum at 235 nm. Gaseous mixtures of CI2, CI2O, HOCl can be analyzed by mass spectrometry. 

The paper presents the experimental and theoretical data regarding the realization and characterization of three liquid-membrane electrodes, which have not been mentioned in the specialized literature so far. The active substances whose solutions in nitrobenzene have constituted the membranes on a graphite rod, are simple complex combinations of the Cu(II) and Ni(II) ions with Schiff base N-[2-thienylmethylidene]-2-aminothiophenol (TNATPh). 

LaFs crystals developed by J. W. Ross and M. S. Frant as the first non-glass membrane electrode... 

Insertion of Eqs. (18) in (15) results in an Equation for a dissolving solid state membrane electrode in the absence of complexing agents. In case aA = aB = 0 (e.g., pure H20) the stationary potential can be expressed as ... 

Amalgam electrodes, and liquid and glass membrane electrodes... 

As has been shown 82 85 88), the behavior of amalgam electrodes under conditions of cementation is very similar to that of liquid and glass membrane electrodes under stationary state conditions. Here, Eq. (2) should be written in the following way ... 

Lakshminarayanaiah, N. Membrane Electrodes, New York, Academic Press 1976... 

The glass membrane of the electrodes discussed above may be replaced by other materials such as a single crystal or a disc pressed from finely divided crystalline material it may be advantageous to incorporate the crystalline material into an inert carrier such as a suitable polymer thus producing a heterogeneous-membrane electrode. 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

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