Metal/electrolyte contact

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

The preparation of the metal surface to receive the protective coating is of prime importance since a coating which is not bonded to the metal surface can allow electrolytes to contact the metal, with resultant corrosion. If water films develop between the metal and the electrically non-conductive coating, cathodic protection becomes ineffective. 

Two solid electrolytes in contact with a common ion show behaviour analogous to two metals in contact. 

Carbon, Lithium tetrachloroaluminate, Sulfinyl chloride Kilroy, W. P. et al., J. Electrochem. Soc., 1981, 128, 934-935 In electric battery systems, lithium is inert to the electrolyte components in absence of carbon, but in presence of over 10% of carbon (pre-mixed by grinding with the metal), contact with the electrolyte mixture leads to ignition or explosion. 

Hoopes An electrolytic process for refining aluminum metal. The electrolyte is a mixture of fluoride salts. The cell is constructed of graphite. The electrolyte in contact with the side-walls of the cell is frozen, thus preventing short-circuiting of electricity through the walls. Developed by W. Hoopes and others at Aluminum Company of America in the 1920s. 

Double-sided electrolytic contacts are favorable for this method of diffusion length measurement because they are transparent and the required SCRs are easily induced by application of a reverse bias. Therefore homogeneously doped wafers need no additional preparation, such as evaporation of metal contacts or diffusion doping, to produce a p-n junction. Furthermore, a record low value of surface recombination velocity has been measured for silicon surfaces in contact with an HF electrolyte at OCP [Yal], Note that this OCP value cannot be further decreased by a forward bias at the frontside, because any potential other than OCP has been found to increase the surface recombination velocity, as shown in Fig. 3.2. Note that contaminations in the HF electrolyte, such as Cu, may significantly increase the surface recombination velocity. This effect has been used to detect trace levels (20 ppt) of Cu in HF [Re5j. 

With metal-metal contact, it is also possible to obtain an electrolytic type contact potential if adsorbed water films are present. Deaglio (as quoted by Harper, H3), in investigating the contact potential between silver and nickel balls, observed that when he carefully controlled the gap between the surfaces the sign of charge reversed at a gap width of the order of 100 A. This... 

Having discussed the way in which blocking interfaces behave we must now consider how blocking metallic contacts can be made on a given material, e.g. a ceramic electrolyte. Frequently a relatively inert metal such as Pt or Au is evaporated onto a ceramic material which has been polished... 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

We now come to internal metal contacts in ISEs without an internal solution. As discussed above, systems without internal electrolytes are used very often, with both solid and liquid membranes. Obviously, the condition of thermodynamic equilibrium requires that common electrically-charged particles (ions or electrons) be present in electrically-charged phases that are in contact (see chapter 2). ISEs with a silver halide membrane to which a silver contact is attached are relatively simple. In the system... 

Electrons, generated near the semiconductor-electrolyte interface are unable to stay in this region because of the electric field there which drives them into the bulk of the TiOz crystal, out through the metallic contact, the external circuit (where the photo-current may be measured) and into the catalytically active metal. At the interface of this metal with the electrolyte solution, reaction occurs ... 

The effect of fluoride ions on the electrochemical behaviour of a metal zirconium electrode was studied by Pihlar and Cencic in order to develop a sensor for the determination of zirconium ion. Because elemental zirconium is always covered by an oxide layer, the anodic characteristics of a Zr/Zr02 electrode are closely related to the composition of the electrolyte in contact with it. These authors found the fluoride concentration and anodic current density to be proportional in hydrochloric and perchloric acid solutions only. In other electrolytes, the fluoride ion-induced dissolution of elemental zirconium led to an increase in the ZrOj film thickness and hindered mass transport of fluoride through the oxide layer as a result. The... 

Fig. 7.2 Sandwich (or stacked) configurations placed in an electrolyte solution (a) p-n type, (b) semiconductor-metal type photochemical diodes. Both p-type and n-type semiconductors are provided with ohmic contacts. In p-n type light is incident from both directions and ohmic contacts are connected through metal contacts.

It is the electrode potential

electrochemical experiments it represents a potential difference between two identical metallic contacts of an electrochemical circuit. Such a circuit, whose one element is a semiconductor electrode, is shown schematically in Fig. 2. Besides the semiconductor electrode, it includes a reference electrode whose potential is taken, conventionally, as zero in reckoning the electrode potential (for details, see the book by Glasstone, 1946). The potential q> includes potential drops across the interfaces, i.e., the Galvani potentials at contacts—metal-semiconductor interface, semiconductor-electrolyte interface, etc., and also, if current flows in the circuit, ohmic potential drops in metal, semiconductor, electrolyte, and so on. (These ohmic drops are negligibly small under experimental conditions considered below.)... 

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

Since metals become unstable (undergo the events named above) when they come into contact with the moist atmosphere, it is reasonable to conclude that this instability of metals results from charge-transfer reactions at their interfaces. This is why the rate of corrosive destruction of a metal s surface is greatly reduced by removal of moisture from the atmosphere. Keeping a metal in a vacuum is equivalent to removal of the electrolyte in contact with the metal and therefore to the prevention of charge-transfer reactions. Thus, the spontaneous instability (or corrosion) of metals results from the charge-transfer reactions at the electrified interface between the metal and the moist, CO 2 or NaCl-containing air (Wolaston, de la Rive). 

The position of magnesium in the electrochemical or galvanic series indicates that it has a potential of reacting chemically with a great number of other metals when there is electrical connection. The conductive path could be caused by direct metal-to-metal contact, an aqueous solution in which there is an electrolyte (e.g., chloride ions in solution), or by other ways. The key in any assembly involving magnesium is to design and assemble the parts in such a manner that the conductive path is eliminated. 

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