PH measurement glass electrode for

The glass electrode for pH measurements has long been a standard laboratory sensor and the subject of several monographs [74, 118, 349]. Consequently, only the basic theory will be given here, though electrodes sensitive to the alkah metal ions and silver will be considered in somewhat greater detail [95]. 

Electrodes that are sensitive to the concentration of a particular ion are called ion-selective electrodes, of which the glass electrode for pH measurement is just one example. Glass electrodes can be made sensitive to ions such as Na +, K+, or NH4 + by changing the composition of the glass. Other ions can be detected if an appropriate crystalline solid replaces the glass membrane. For example, a crystal of lanthanum(III) fluoride (LaF3) can be used in an electrode to measure [F-]. Solid silver sulfide (Ag2S) can be used to measure [Ag+] and [S2-]. Some of the ions that can be detected by ion-selective electrodes are listed in Table 11.2. 

Solution of unknown pH Figur 3-3 Glass electrode for pH measurements. 

It is important to note at the outset of this discussion that membrane electrodes are fundamentally different from metal electrodes both in design and in principle. We shall use the glass electrode for pH measurements to illustrate these differences. 

The most important ion selective electrode using a solid membrane is the glass electrode for pH measurement. Despite their outstanding selectivity for H+ ions glass electrodes are used only seldom in enzyme electrodes because their sensitivity is affected by the buffer capacity of the measuring solution. 

Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

Figure 15.7 (a) A schematic glass electrode for pH measurement, (b) A complete pH measurement cell, with the glass indicator electrode and an external saturated calomel reference electrode, (c) A commercial combination pH electrode, with built-in internal Ag/AgCl reference electrode. 

The glass electrode for pH measurement is sufficiently stable to allow calibration at reasonably large intervals of time, most probably of the order of once a week and hence it is generally the case that pH electrodes are calibrated manually. 

Glass electrode for pH measurement A bulb or other form of hydrogen ion-sensitive glass is attached to the end of a glass tube made from high-resistance glass. Commercially it is usually obtained outfitted with internal filling solution and an internal reference electrode. For special applications, e.g., blood pH measurements, other forms, such as capillaries, are more convenient. 

Since 1906 when Cremer ( ) first observed the potentiometric relationship of a glass membrane with a pH solution and the first systematic study of the glass electrode for pH measurement was by Haber and Klemenciewicz 2) in 1909, many investigators have attempted to explain the glass membrane potential. Table I lists major investigators and their mechanisms for the past 80 years. 

We begin our discussion of how pion membrane electrodes are constructed and how they function by examining in some detail the glass electrode for pH measurements. The glass pH electrode predates all other membrane electrodes by several decades, and it is the most widely used electrode in the world. ... 

The unknown constant E must be determined by calibration with known buffers, as described in Topic C4. Modem glass electrodes for pH measurement often incorporate a reference electrode. 

The use of electrodes, particularly the glass electrode for pH measurements and the wide range of other ion selective electrodes (ISE) described in Topic C3, enables titrations to be studied throughout the addition of titrant, so that small changes may be detected. It also allows automation of the titration. 

Application of glass electrodes for pH-measurements in non-aqueous media caused some controversies in the past " and in more recent literature. All the above remarks, related to accuracy and repeatability of results in non-aqueous and mixed-solvent media testify well on account on its applicability in such media. Some troubles and the non-compatibility of the results obtained with the expected ones may arise when the organic solvent (e.g. acetonitrile) is aggressive towards the electrode" or a solute tested. 

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