Crystalline solid-state electrodes

TABLE 5-1 Characteristics of Solid-State Crystalline Electrodes ... 

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. 

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... 

Calcium in beer can be determined by filtering the sample, adjusting the pH to 6, and employing a known addition of Ca. The copper-ion content of tap water can be determined with a solid-state crystalline copper electrode using a multiple standard-addition procedure [20]. Tap water is mixed 1 1 with a complexing antioxidant buffer (sodium acetate, acetic acid, sodium fluoride, and formaldehyde) to buffer the pH at 4.8, to complex the Cu uniformly with acetate, and to complex the Fe " interferant with fluoride. Copper in tap water can be determined down to about 9 ppm with a standard deviation of about + 8%. The recovery of Cu + added to natural waters, an indication of the accuracy of the method, averaged 103% for samples in the range of 3 to 60 ppm Cu. 

Unlike ion-selective electrodes using glass membranes, crystalline solid-state ion-selective electrodes do not need to be conditioned before use and may be stored dry. The surface of the electrode is subject to poisoning, as described earlier for a Ck ISE in contact with an excessive concentration of Br. When this happens, the electrode can be returned to its original condition by sanding and polishing the crystalline membrane. 

Figure 4.11 A solid-state electrode showing a first-order response. An electrode designed to measure the activity of silver ions uses a crystalline membrane of silver sulphide. An equilibrium between the mobile silver ions of the membrane and the silver ions in the solutions results in the development of a potential difference across the membrane.

In solid-state electrodes the membrane is a solid disc of a relatively insoluble, crystalline material which shows a high specificity for a particular ion. The membrane permits movement of ions within the lattice structure of the crystal and those ions which disrupt the lattice structure the least are the most mobile. These usually have the smallest charge and diameter. Hence, only those ions that are very similar to the internal mobile ions can gain access to the membrane from the outside, a feature that gives crystal membranes their high specificity. When the electrode is immersed in the sample solution, an equilibrium is established between the mobile ions in the crystal and similar ions in the solution and the resulting potential created across the membrane can be measured in the usual manner. 

The desire to realise technological goals has spurred the discovery of many new solid electrolytes and intercalation compounds based on crystalline and amorphous inorganic solids. In addition an entirely new class of ionic conductors has been discovered by P. V. Wright (1973) and M. B. Armand, J. M. Chabagno and M. Duclot (1978). These polymer electrolytes can be fabricated as soft films of only a few microns, and their flexibility permits interfaces with solid electrodes to be formed which remain intact when the cells are charged and discharged. This makes possible the development of all-solid-state electrochemical devices. 

The electrical conductivity of a material is a macroscopic solid-state property since even in high molecular-weight polymers there is not just one conjugated chain which spans the distance between two electrodes. Then it is not valid to describe the conductivity by the electronic structure of a single chain only, because intra- and interchain charge transport are important. As with crystalline materials, some basic features of the microscopic charge-transport mechanism can be inferred from conductivity measurements [83]. The specific conductivity a can be measured as the resistance R of a piece of material with length d and cross section F within a closed electrical circuit,... 

Membranes prepared from cast pellets of silver halides have been used successfully in electrodes for the selective determination of chloride, bromide, and iodide ions. In addition, an electrode based on a polycrystalline Ag2S membrane is offered by one manufacturer for the determination of sulfide ion. In both types of membranes, silver ions are sufficiently mobile to conduct electricity through the solid medium. Mixtures of PbS, CdS, and CuS with Ag S provide membranes that are selective for Pb-, Cd-+, and Cu-", respectively. Silver ion must be present in these membranes to conduct electricity because divalent ions are immobile in crystals. The potential that develops across crystalline solid-state electrodes is de.scribed by a relationship similar to Equation 21-10. 

Potentiometric detection of anions is feasible when an electrode is available that responds quickly, reversibly and reproducibly to the concentration (or more precisely to the activity) of sample ions. It is often possible to detect a given ion or class of ions with excellent selectivity. For example, solid-state or crystalline ion selective electrodes have been used in IC to detect halide anions. The fluoride ion-selcclivc electrode is particularly selective [20,21]. A copper wire electrode has been used to detect anions such as iodate, bromide and oxalate [22]. 

The most important advantage of photoelectrochemical cells with semiconductor electrodes, as compared to, for example, solid-state semiconductor solar cells, is a relatively low sensitivity of their characteristics to the crystalline perfection of the semiconductor and the degree of its purification. Polycrystalline semiconductor electrodes in electrochemical solar cells exhibit both high absolute and high relative (as compared to single-crystal electrodes) conversion efficiency. This opens, at least in principle, the way of... 

The results obtained from the investigation of the crystallochemical properties of a family of mixed Li-vanadates of general formula LiCoyNi(i.y)V04 are reported. This kind of material is believed to be a valid alternative to the traditional positive electrode materials in the Li-ion cell, an electrochemical power source of outstanding importance in the portable electronics field. The powders, prepared via two different methods (a wet chemistry route and a solid state method), have been characterised by XRPD analysis, NMR and diffuse reflectance Vis-NIR spectroscopy. The findings allowed to correlate the electrochemical performance of the vanadates, which is higher for the samples prepared via wet chemistry, to the crystallinity of the powders and to the transition metal cations distribution. 

Beginning in 2000, numerous modifications were made to the fluoride instrument design to greatly enhance the sensitivity, stability, and accuracy of the fluoride determination. Several electrodes were evaluated to determine the most sensitive and durable fluoride electrode available. The ELIT 8221 , a solid state sensor with a mono-crystalline membrane, was chosen. 

Table 2.5. Typical Properties of Commercial Crystalline Solid-State Electrodes...

The lower limit of detection for liquid ion-exchange electrodes is determined primarily by the solubility of the ion exchanger in aqueous media. As with crystalline solid-state electrodes, Nernstian response is obtained until the activity of the solution is within a factor of about 100 of the solubility of the membrane salt. Then the response deviates and levels off at a constant potential reflecting this solubility. 

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