Passivity definitions

References to diminished responsiveness to environmental stimuli, diminished intellectual initiative, indifference and a slight general malaise, and a certain passivity definitively describe the deactivating, brain-disabling effects of lithium (chapter 1). The language used is identical to that used to describe lobotomy effects. 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

According to the definition, a passive technique is one for which no appHed signal is required to measure a response that is analytically usehil. Only the potential (the equiHbrium potential) corresponding to zero current is measured. Because no current flows, the auxiHary electrode is no longer needed. The two-electrode system, where the working electrode may or not be an ion-selective electrode, suffices. 

After the inherent hazards are reduced, layers of protection are frequently used to protect the receptors of the hazard—the public, the environment, workers, other processes, or the process itself (Figure 1.1). In the strictest sense, one could argue that the definition of inherently safer applies only to elimination or reduction of the hazard. In the broad sense, the strength of a layer of protection can be improved by features that are permanent and inseparable from that layer. Thus, layers of protection can be classified into three categories, listed in decreasing order of reliability passive, active, and procedural. A passive layer of protection can be described as inherently safer than an active... 

The potentiostat is particularly useful in determining the behaviour of metals that show active-passive transition. Knowledge of the nature of passivity and the probable mechanisms involved has accumulated more rapidly since the introduction of the potentiostatic technique. Perhaps of more importance for the subject at hand are the practical implications of this method. We now have a tool which allows an operational definition of passivity and a means of determining the tendency of metals to become passive and resist corrosion under various conditions. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

Consider first the case of a simple combination-dissociation reaction, which for definiteness we shall take to be the passivation of an acceptor A... 

Procedural controls, process controls, 98-99 Process controls, 96-100 active controls, 98 inherently safer approach, 97 mitigation techniques, 99 passive controls, 97-98 procedural controls, 98-99 safe operating limits, 99-100 Process definition, documentation, 102-104 Process design, documentation, 105 Process hazard analysis (PHA) risk assessment, 92-93 screening methods, 63 Process risk management decisions, documentation, 105-106... 

Because ARMS is a form of passive surveillance that is based on spontaneous reports, it has a number of limitations. Most importantly, it cannot establish a definitive, causal relationship between the ingestion of an incriminated substance and the occurrence of symptoms. Since people are exposed daily to a myriad of food ingredients, it is inherently difficult to attribute an adverse event to a specific food substance. Moreover, symptoms reported are often vague or general in nature. Other confounders, which were discussed by Bradstock et al. (1986), include ... 

In an effort to optimize the solvent-containing passive sampler design, Zabik (1988) and Huckins (1988) evaluated the organic contaminant permeability and solvent compatibility of several candidate nonporous polymeric membranes (Huckins et al., 2002a). The membranes included LDPE, polypropylene (PP), polyvinyl chloride, polyacetate, and silicone, specifically medical grade silicone (silastic). Solvents used were hexane, ethyl acetate, dichloromethane, isooctane, etc. With the exception of silastic, membranes were <120- um thick. Because silicone has the greatest free volume of all the nonporous polymers, thicker membranes were used. Although there are a number of definitions of polymer free volume based on various mathematical treatments of the diffusion process, free volume can be viewed as the free space within the polymer matrix available for solute diffusion. 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

As a compromise between the above two approaches, the third approach adopts nonactive (inert) materials as working electrodes with neat electrolyte solutions and is the most widely used voltammetry technique for the characterization of electrolytes for batteries, capacitors, and fuel cells. Its advantage is the absence of the reversible redox processes and passivations that occur with active electrode materials, and therefore, a well-defined onset or threshold current can usually be determined. However, there is still a certain arbitrariness involved in this approach in the definition of onset of decomposition, and disparities often occur for a given electrolyte system when reported by different authors Therefore, caution should be taken when electrochemical stability data from different sources are compared. 

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