Electrode glass membrane

At the surface of a glass membrane (Fig.3.2) that is in contact with an aqueous solution, a layer, called the Haber-Haugaard layer, is established with a thickness of about 100 nm. In this layer, a fraction of the positive ions, typical for the glass structure, are exchanged by hydrogen ions (H+). Owing 

At the other side of the membrane, a similar hydrogen-ion exchange is obtained (not shown in Fig.3.2). At both interfaces, a potential is obtained (E2 and Eu respectively), and the potential difference over the entire membrane is defined as AE=E2-E1 (Equation3.1). This potential difference, which is related to the hydrogen-ion acitivity in solution (see also Equation 3.3), can be used for analytical purposes to measure the hydrogen-ion activity in solution and thus to determine the pH. The relation between pH and hydrogen-ion solution is given by  

3 Potential variation over a glass membrane in contact with two solutions of different composition, one being of constant and known hydrogen-ion activity. 

In practice, the trick to obtain the pH is to use a membrane that is on one side exposed to a solution of known and constant hydrogen-ion activity (e.g. a buffer solution) and on the other side is exposed to the solution of unknown pH (Fig.3.3). On the first side of the membrane, an exchange of H+ occurs, but because of the constant composition of the buffer solution 1, this will result in a constant potential, Ex. On the other side of the membrane, a potential, E2, is established which is determined by the activity of hydrogen ions in the solution of unknown pH. Therefore, AE is dependent only on E2, and thus the pH of this solution can be determined using a glass membrane. 

Of course, before such a glass membrane can be used, the measured potential, AE must be linked to the pH. This can be done through calibration by measuring the potential differences when the membrane surface is immersed in buffer solutions of known pH. In that case, E2 is linked to the pH of the buffer and a calibration curve. Finally, a solution of unknown pH can be analysed by measuring AE and calculating the pH using the calibration curve. 

It is well known that glass electrodes require soaking before use to allow certain components of the glass and the solution to exchange and produce an ion exchanging layer on the membrane surface. The condition of the electrode after this operation is completed is summarized in Fig. 6.3. Water 

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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... 

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

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 ... 

Explain the main mechanistic differences between a glass membrane electrode and an ion-sensitive field effect transistor (ISFET). 

The half-cell of glass-membrane electrode may be expressed as ... 

The crystalline membrane electrodes have a very close similarity to those of glass-membrane electrodes (see Section 16.3.1.2.2.1 ) except that glass has been replaced with crystalline membrane. In fact, these electrodes offer a means to devise responsive to anions by making use of a membrane containing specific anionic sites. 

The following is a schematic of an electrochemical cell consisting of an AgCl-coated Ag wire reference electrode and a pH-sensing glass membrane electrode. The bars represent phase boundaries. 

They are classified by membrane material into glass membrane electrodes, crystalline (or solid-state) membrane electrodes, and liquid membrane electrodes. Liquid membrane electrodes are further classified into liquid ion-exchange membrane electrodes and neutral carrier-based liquid membrane electrodes. Some examples are shown in Fig. 5.36 and Table 5.3. If the membrane is sensitive to ion i of charge Z and the activities of i in the sample and internal solutions are equal to (i) and a2(i), respectively, the membrane potential, m, which is developed across the membrane, is... 

New polymer membrane-based ISEs for nitrate and carbonate exhibit detection limits and selectivities that may be applicable for ocean measurements. In addition, a number of these ISEs can be used as internal transducers for the design of useful potentiometric gas sensors. For example, dissolved C02 can be detected potentiometrically by using either a glass membrane electrode or a polymer-based carbonate ISE, in conjunction with an appropriate reference electrode, behind an outer gas permeable membrane. Novel differential pC02 sensors based on two polymer membrane-type pH sensors have also been developed recently. 

One of the most important and extensively used indicator electrode systems is the glass-membrane electrode that is used to monitor hydronium ion activity. Although developed in 1909, it did not become popular until reliable electrometer amplifiers were developed in the 1930s. When the outside surface of the glass membrane is exposed to an ionic solution, a response for the hydronium ion activity meets with the Nicholsky equation, which is similar to the Nernst expression. In view of the importance and widespread use of the hydronium or pH electrode, this system is discussed in a separate chapter. 

This chapter examines various probes for pH measurement such as ion-selective and glass-membrane electrodes as well as simultaneous cellulose removal and bleaching of textiles with enzymes. 

Glass-membrane electrodes, such as pH electrodes. In this type of electrode, a glass body acts as a membrane and shows affinity for different... 

Alternative shapes of the measuring zone of glass-membrane electrodes. 

Thus, online measurements of composition are usually limited to some overall property. A typical example is pH, defined as the absolute value of the logarithm of the molar concentration (or, more exactly, activity) of hydrogen ion pH can be measured by exploiting the electric potential established between two proper electrodes immersed in the sample fluid, usually a glass membrane electrode and a reference electrode [15], Notwithstanding the temperature dependence and the alkaline error (at high pH, a marked sensitivity to the effect of Na+ and of other monovalent... 

Usually, rather than using a hydrogen gas electrode, a glass membrane electrode is used for the measurement. As discussed in Sec. 8, the potential across such a membrane can be proportional to the difference in pH s of the solutions on each side of the membrane. One design for a membrane-type pH electrode, which incorporates a Ag/AgCl reference electrode in a tube concentric to the membrane electrode, is shown in Fig. 6. The electrode is immersed in the solution whose pH is to be measured, with the solution level above the porous plug. 

The development of membranes that exhibit both sensitivity and selectivity for species of interest is paramount in the application of ISEs. Between these two properties, selectivity is by far the more difficult one to achieve. Three basic types of membranes have been developed, because of their selectivity, for ISEs. These are liquid and polymer, solid state, and glass membranes. Electrodes are commercially available for numerous inorganic ions such as Na , Ag , Cu , ... 

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