Electrical methods

Changes in electrical conductivity have occasionally been used to study the setting chemistry of AB cements. Conductivity has been particularly used in the study of dental cements, notably the dental silicate (Wilson Kent, 1968), the zinc polycarboxylate (Cook, 1982), the glass-ionomer cement (Cook, 1982) and the ZOE cement (Crisp, Ambersley Wilson, 1980). 

In a typical study of conductivity. Cook (1982) used a cell consisting of two platinum disc electrodes, 12 mm in diameter and 1-5 mm apart. The setting AB cement was examined in this cell which had been calibrated using a standard solution of0 02 M potassium chloride. Plots were recorded of spedfic conductance against time for each of the setting cements. For zinc polycarboxylate there was found to be a rapid drop in spedfic conductance about 10 minutes after the start of mixing. This behaviour was consistent with the replacement of relatively mobile protons by significantly less mobile zinc ions in the polycarboxylate chain. Con- 

Electrical methods involve the detection and analysis of electronic pulses generated by droplets in a measurement volume or on a wire. The electronic signals are then converted into digital data and calibrated to produce information on droplet size distribution. A detailed review of electrical methods for droplet size measurements has been made by Jones.[657] 

The hot wire technique appears to be the most useful one among the various electrical methods. However, the hot wire anemometer can operate only at relatively low droplet velocities. 

For the most part in the study and practice of lubrication, the bounding surfaces are metallic and the film of oil or other lubricating fluid acts as an electrical insulator. The system can then be treated either as an electrical resistance or as a capacitance. 

The surface charging of materials that show a certain degree of electric conductivity can be measured directly by electrical methods. Such measurements are not used to determine the PZC. The potential of a hematite electrode prepared as a coating on Pt [243] was Nemstian (59 mV/pH unit) at 20°C in 0.005 M KCl. Also, a monocrystalline hematite electrode [644] had a nearly Nemstian potential in the acidic range in 0.0005 M NaNO, but the slope was lower in the basic range and 

Qualitatively similar results were obtained in [652], but only one salt was used. Different crystallographic faces of corundum produced different intersection points in the pH range 4-5. Possible sources of discrepancies between the pH, obtained by the methods discussed in this section on the one hand and by standard methods on the other are discussed in Section 2.3.2. 

---

Combination electrical methods Tomashov and Mikhailovsky describe a method developed in the Soviet Union. This test is essentially a combination of resistivity measurement and polarisation rates on iron electrodes in soil in situ. The usefulness and value of this procedure has not as yet been determined by practical application by corrosion engineers. The development of this combination test does, however, represent an attempt to integrate some of the complex factors controlling corrosion rates in soil. Much more research on these factors and methods of measurement should in the future enable the corrosion engineer to evaluate soil properties with respect to application of corrosion-alleviating operations. 

An electrical resistance methods which directly measures loss of metal from a probe installed in the corrosive system under study is described in Section 19.3. It is reported that corrosion equivalent to a thickness loss of as little as 2-5 X 10 cm can be detected . This technique is most useful as a means of monitoring steps taken to reduce corrosion, e.g. by inhibitors, or to detect changes in the corrosivity of process streams. Electrical methods of determining corrosion rates are considered subsequently. 

D 2776 1979 Test methods for corrosivity of water in the absence of heat transfer (electrical methods)... 

Electrical methods of analysis (apart from electrogravimetry referred to above) involve the measurement of current, voltage or resistance in relation to the concentration of a certain species in solution. Techniques which can be included under this general heading are (i) voltammetry (measurement of current at a micro-electrode at a specified voltage) (ii) coulometry (measurement of current and time needed to complete an electrochemical reaction or to generate sufficient material to react completely with a specified reagent) (iii) potentiometry (measurement of the potential of an electrode in equilibrium with an ion to be determined) (iv) conductimetry (measurement of the electrical conductivity of a solution). 

It follows that any electrical method of increasing E, as for example improved amplification, will make the technique more sensitive. 

In a search for an accurate method of measuring moisture in foods, one cannot overlook the essential requirements of convenience, speed, and precision. Many currently used methods meet these requirements without necessarily yielding accurate results under the conditions used. Probably most important are the electrical methods (IS, 24, 26, 36), the air- and vacuum-oven methods (/, 2, 6, 18, 25, 28, 36), distillation with organic solvents (1, 3, 7, 12, 13, 26, 35), and the Karl Fischer reagent method (9, 11, 26, 31, 32). Without discussing the relative merits of these methods, it can be assumed that accurate results could be obtained with each method by calibration against some accurate reference method. 

Gibbs energy for the adsorption of water and the interfacial parameter, 187 Gokstein and the piezo electric method for the determination of the potential of zero charge, 42 Gold... 

Perkins and Andersen, 31 photo emission method (Barker), 41 potentiostatic scrape method, 38 piezo electric method (Gokstein), 42 rapid emergent method (Packham), 38... 

Of the electrical methods, only two representative methods will be considered here. Other electrical methods have been discussed previously, e.g., in Refs. 11 and 96. Capillary methods have been used often and discussed for their sources of errors and are therefore not particularly considered here. 

An alternative electrical method that has been used in the study of glass-ionomer cements has been the measurement of dielectric properties. Tay Braden (1981, 1984) measured the resistance and capacitance of setting cements at various times from mixing. From the results obtained, relative permittivity and resistivity were calculated. In general, as these cements set, their resistivity was found to fall rapidly, then to rise again. Both these results and the results of relative permittivity measurements were consistent with the cements comprising highly ionic and polar structures. 

Stability may be inherent or induced. In the latter case, the original system is in a condition of metastable or neutral eouilibrium. External influences which induce instability in a dispersion on standing are changes in temperature, volume, concentration, chemical composition, and sediment volume. Applied external influences consist of shear, introduction of a third component, and compaction of the sediment. Interfacial energy between solid and liquid must be minimized, if a dispersion is to be truly stable. Two complementary stabilizing techniques are ionic and steric protection of the dispersed phase. The most fruitful approach to the prediction of physical stability is by electrical methods. Sediment volumes bear a close relation to repulsion of particles for each other. 

Because stabilization in the Verwey and Hamaker picture is electrical, the use of electrical methods for predicting the stability of a dispersion appears to be mandatory. Other than the work of Voet described above, little has been done in this direction. Beyond doubt, the important developments in the area of dispersion stability that will come forth will be based either on electrokinetics (for high dielectric media, especially hydrocolloids) or electrostatics. 

Roy B. (1991) The Outranking Approach and the Foundations of ELECTRE Methods. Theory Decision, 31, 49-73. 

All existing techniques of EEP detection may be divided into several groups spectral, calorimetric, chemical titration, electrical methods, and also a method of sensor detection. 

High sensitivity is featured by the electrical methods used to detect EEPs. These are based on measuring small currents that occur in the course of selective ionization of EEPs, or currents of secondary emission of electrons or ions knocked by EEPs out of the surface of solid targets (if such emission is taking place). 

A general disadvantage that limits the field of employing the electrical techniques of detecting EEPs is in the complexity of the hardware involved. As a rule, all electrical methods call for use of superhigh vacuum, the technique of excited particle bundles, the electron beam optics, and need high noise inununity. 

---