High-current techniques

Each of these two procedures can be varied by proceeding from a low to a high current density (or potential) or from a high to a low current density (or potential) the former is referred to as forward polarisation and the latter as reverse polarisation. Furthermore, there are a number of variations of the potentiostatic technique, and in the potentiokinetic method the pwtential of the electrode is made to vary continuously at a predetermined rate, the current being monitored on a recorder in the pulse method the electrode is given a pulse of potential and the current transient is determined by means of an oscilloscope. 

Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion-recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples. 

The most important current technique for the thermal destmction of waste is incineration, where the energy required for destmction is provided by oxidation of the waste, sometimes supplemented with a fossil fuel. The major question about all thermal destmction techniques is whether products from the process—either traces of umeacted parent compound or compounds synthesized from the parent compound at high temperature— will pose a health hazard. 

TOF-SIMS can be applied to identify a variety of molecular fragments, originating from various molecular surface contaminations. It also can be used to determine metal trace concentrations at the surface. The use of an additional high current sputter ion source allows the fast erosion of the sample. By continuously probing the surface composition at the actual crater bottom by the analytical primary ion beam, multi element depth profiles in well defined surface areas can be determined. TOF-SIMS has become an indispensable analytical technique in modem microelectronics, in particular for elemental and molecular surface mapping and for multielement shallow depth profiling. 

A prolific author, Professor van Eldik has been responsible for some 580 papers in refereed journals, and four books as editor or co-editor. His current research intrests are the application of high pressure techniques in mechanistic studies metal-catalyzed autoxidation processes and bioinorganic studies. As such he is eminently qualified to edit the prestigious Advances in Inorganic Chemistry. We are confident that he is a worthy successor to Professor Geoff Sykes and that he will maintain the high standards for which the series is known. 

Generally, irrespective of the technique for which they are used, electrochemical cells are constructed in a way which minimizes the resistance of the solution. The problem is particularly accentuated for those techniques which require high current flows (large-scale electrolysis and fast voltammetric techniques). When current flows in an electrochemical cell there is always an error in the potential due to the non-compensated solution resistance. The error is equal to / Rnc (see Chapter 1, Section 3). This implies that if, for example, a given potential is applied in order to initiate a cathodic process, the effective potential of the working electrode will be less negative compared to the nominally set value by a amount equal to i Rnc. Consequently, for high current values, even when Rnc is very small, the control of the potential can be critical. 

Polymethylpentenes have very weak loss factors, about 2 X 10 , and do not heat up under high-frequency current. They can be used for microwave ovenware but cannot be welded by the high-frequency technique. 

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