Electrical potential drop method

Few experimental studies have been reported on the behavior of short cracks. However, in one study. Prater et al. [101] used an electrical potential drop method to monitor the growth oif surface cracks in carbon steel in oxygenated (8 ppm O2) high-temperature (288 °C) water. The cracks were semielliptical in shape and the crack... 

The electrical potential drop method uses the electrical resistance of the specimen to measure the crack length. A constant electrical current is applied between two points of the specimen far away from the crack and the potential drop in the vicinity of the crack is measured. Comparison with a calibration curve allows calculation of the crack length. Obviously, the specimen has to be electrically isolated from the testing machine and the displacement transducer. 

The aperture impedance principle of blood cell counting and sizing, also called the Coulter principle (5), exploits the high electrical resistivity of blood cell membranes. Red blood cells, white blood cells, and blood platelets can all be counted. In the aperture impedance method, blood cells are first diluted and suspended ia an electrolytic medium, then drawn through a narrow orifice (aperture) separating two electrodes (Fig. 1). In the simplest form of the method, a d-c current flows between the electrodes, which are held at different electrical potentials. The resistive cells reduce the current as the cells pass through the aperture, and the current drop is sensed as a change in the aperture resistance. 

What is next Several examples were given of modem experimental electrochemical techniques used to characterize electrode-electrolyte interactions. However, we did not mention theoretical methods used for the same purpose. Computer simulations of the dynamic processes occurring in the double layer are found abundantly in the literature of electrochemistry. Examples of topics explored in this area are investigation of lateral adsorbate-adsorbate interactions by the formulation of lattice-gas models and their solution by analytical and numerical techniques (Monte Carlo simulations) [Fig. 6.107(a)] determination of potential-energy curves for metal-ion and lateral-lateral interaction by quantum-chemical studies [Fig. 6.107(b)] and calculation of the electrostatic field and potential drop across an electric double layer by molecular dynamic simulations [Fig. 6.107(c)]. 

The details of the influence that electrostatic surface forces on the stability of foam films is discussed in Section 3.3. As already mentioned, the electrostatic disjoining pressure is determined (at constant electrolyte concentration) by the potential of the diffuse electric layer at the solution/air interface. This potential can be evaluated by the method of the equilibrium foam film (Section 3.3.2) which allows to study the nature of the charge, respectively, the potential. Most reliable results are derived from the dependence foam film thickness on pH of the surfactant solution at constant ionic strength. The effect of the solution pH is clearly pronounced the potential of the diffuse electric layer drops to zero at certain critical pH value. We have named it pH isoelectric (pH ). As already mentioned pH is an intrinsic parameter for each surfactant and is related to its electrochemical behaviour at the solution/air interface. Furthermore, it is possible to find conditions under which the electrostatic interactions in foam films could be eliminated when the ionic strength is not very high. 

Therefore, each realisable reaction is comparable to a kind of scale which allows the comparison of chemical potentials or their sums, respectively. But the measurement is often impossible due to any inhibitions, i.e., the scale is jammed. If there is a decline in potential from the left to the right side, that only means that the process can proceed in this direction in principle however, it does not mean that the process will actually run. Therefore, a potential drop is a necessary but not sufficient condition for the reaction considered. The problem of inhibitions can be overcome if appropriate catalysts are available or indirect methods including chemical (using the mass action law), calorimetric, electrochemical and others can be used. Because we are interested in a first knowledge of the chemical potential, we assume for the moment that all these difficulties have been overcome and consider the values as given, just as we would consult a table when we are interested in the mass density or the electric conductivity of a substance. ... 

Chapter 11 focused attention on methods of analysing conductance data where the effects of non-ideality have been ignored, i.e. it has been assumed that there are no ionic interactions. The movement of ions in solution is then a result of motion induced by an applied potential gradient, i.e. an external field superimposed on random Brownian motion. The applied electric field will cause the positive ions to move in the direction of the field and anions to move in the opposite direction. The direction of the field is from the positive pole to the negative pole of the electrical system, and the field is set up by virtue of the potential drop between the two poles. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

This interface separates hydrophilic and hydrophobic ions , such as simple alkah-halides and organic ions, respectively. When two such salts are dissolved in this system, one composed of hydrophilic and the other one of hydrophobic ions, they form back-to-back EDLs and the interface can be polarized [289, 290] standard electrochemical methods can be used for the characterization of this interface. The resulting electric field across the interface affects a set of phenomena that occur at the interface. A controllable variation of the potential drop across this interface can shed extra light on these phenomena. 

The capillary electrometer and dropping electrode both gave for the potential difference in the normal calomel electrode +0 560 volt (mercury positive). J. Billitzer tried another method. Particles of colloidal metals are charged and move in an electric potential gradient. By adding electrolytes the... 

In the resistivity method, an electric current is introduced into the ground by means of two current electrodes and the potential difference between two potential electrodes is measured. It is preferable to measure the potential drop or apparent resistance directly in ohms rather than observe both current and voltage. The ohms value is converted to apparent resistivity by use of a factor that depends on the particular electrode configuration in use (see below). 

Thus far very little has been said about how intact ionized molecules, e.g. those formed by soft ionization techniques like the API methods (Sections 5.3.3-5.3.6), can be induced to dissociate for subsequent MS/MS analysis. (In fact any ion produced in an ion source can be miz selected and subjected to MS/MS analysis). Soft ionization does not produce metastable ions (see above) in any abundance if at aU. Historically the most common method of ion activation has heen coUisional activation (CA), wherehy ions are accelerated through a defined potential drop to transform their electrical potential energy into kinetic (translational) energy, and are then caused to collide with gas molecules that are dehherately introduced into the ion trajectory the history of this approach has heen described in an excellent overview (Cooks 1995). This involves conversion of part of the ions kinetic energy into internal energy that in turn leads to fragmentation. It is still by far the most commonly used method. 

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