Chloride/resistivity measurements

The resistive measurement principle is based on a humidity-dependent electrical resistance. The early probes used lithium chloride as the hygroscopic resistive material. Such probes are still available under the name Dunmore sensors. The measurement range of such devices is quite narrow, and the resistance versus humidity relationship is extremely nonlinear. 

In order to examine the effect of solvents, films of a solvent-free epoxypolyamine were cast, mounted in cells and their resistances measured in dilute and concentrated potassium chloride solution . All the films had / properties with resistances in the range 10 -I0 flcm. ... 

The treatment of LB films of copper behenate (10-50 layers) with H2S gas resulted in formation of the semiconductor CU2S [177]. In this case, the LB films of behenic acid alone were formed and then exposed to solutions of copper chloride. Conversion of the carboxyl groups to carboxylate groups upon copper complexation was confirmed by infrared spectroscopy. Resistivity measurements versus temperature confirmed the formation of semiconducting CU2S in one case, and showed a linear increase in log(R) versus IT K). All of the samples became insulators on exposure to air maintaining the conductivity required storage under vacuum. The formation of CuiS sheets in some of the sample was concluded from optical microscopy and resistivity data. 

In most of these systems there is clear evidence for the formation of the reduced ion For example, in NdX2 salts this is on the basis of magnetic studies (3J), and with the praseodymium chloride and bromide phases, from qualitative resistivity measurements and their structural relationships to the neodymium chlorides according to x-ray data. Cryo-scopic data for all the systems listed are also consistent with the formation of a as opposed to solute in dilute solution in MX3 (3, 7). 

This analysis technology was applied to screening of chemosensitive properties of polyanilines and copolymers of anilines and aniline derivatives. Combinatorial libraries were prepared by electrically addressed polymerization.42 43 Gaseous hydrogen chloride was selected as an analyte. The measured signal was electrical resistance detected simultaneously by 2 and 4-point techniques (s24 technique44,52), and the ratio of these values provides information on the contribution of the polymer/contact resistance into the total resistance measured by 2-point configuration. 

Figure 3.17 Organic fouling of an anion exchange membrane. Electrodialysis was carried out using 0.05 N sodium chloride solution containing sodium dodecylbenzene sulfonate (100ppm) at a current density of 3.5 mA cm 2 at 25.0 °C. (Membrane strongly basic anion exchange electrical resistance measured in 0.05 N sodium chloride solution 3.5 Q cm2).

Chloride content. By embedding the combined chloride/resistivity sensor elements mentioned above (Figure 17-2), the activity of the free chloride ions in the pore solution of concrete can be monitored over time at different depths. The potential of the embedded chloride sensors is measured versus a Mn02 reference electrode and converted by Nernst s law to chloride concentration. In several field applications, hundreds of chloride sensors worked well over several years [15]. A more detailed description of the chloride sensor, its calibration and long-term stability is given in references [20,22]. 

W. Morris, A. Vico, M. Vazquez, Chloride induced corrosion of reinforcing steel evaluated by concrete resistivity measurements, Electrochim. Acta 49 (2009) 4447-4453. 

The resistance of the concrete against chloride penetration was mainly tested by use of the Rapid Chloride Migration (RCM) method [6] and electrical resistivity measurements based on the two-electrode method [7], The testing was carried out on 50 mm cut slices from 0100 x 200 mm concrete cylinders, and the concrete samples were mainly cured in water with a temperature of 20 2 C and tested at regular intervals of up to 90 days. In addition, some samples from two of the concrete mixtures (PO O and Pl. O) were wrapped in plastic sheeting after demoulding and stored in air with a temperature of 20 2°C until time of testing. 

Resistivity measurement showed large increases after treatment, to over 200 kfl cm in the field and 5 kO cm on untreated lab specimens to 30 kO cm after treatment on specimens vacuum saturated with distilled water. Petrographic microscopy showed the treatment to block pores, especially close to the steel reducing transport of water, oxygen and chloride ions. This is possibly due to redistribution of calcium hydroxide. There was also an improvement in freeze/thaw resistance (Buenfeld and Broomfield, 1994). 

A crystal or pressed pellets of an insoluble salt can act as ion sensitive elements operating in much the same way as the salt dispersed in an inert matrix. The crystal is precision ground into a disc shape and fixed onto an electrode body. The manufacturing process is closety controlled to avoid the crystal developing an internal crack or leak. The crystal should also not have high resistance. Examples can be cited of the lanthanum fluoride electrode (measure fluoride) and silver chloride electrode (measures chloride). 

As discussed previously, some soil environments and ground waters can be corrosive to buried metal structures. Resistivity measurements, which can be made both in the field and in the laboratory, indicate how corrosion currents will flow through soils or ground waters. High concentrations of chlorides and sulfates contribute to a reduction in resistivity and an increase in the corrosion activity of a material. In the presence of oxygen, chloride ions can be extremely corrosive to steel. Similarly, high levels of sulfates can cause a reduction in soil or groundwater resistivity and corrosion of steel and concrete. 

Figure 14.1 Electrical resistance measurement of a sodium chloride solution (NaCI).

One procedure makes use of a box on whose silk screen bottom powdered desiccant has been placed, usually lithium chloride. The box is positioned 1-2 mm above the surface, and the rate of gain in weight is measured for the film-free and the film-covered surface. The rate of water uptake is reported as u = m/fA, or in g/sec cm. This is taken to be proportional to - Cd)/R, where Ch, and Cd are the concentrations of water vapor in equilibrium with water and with the desiccant, respectively, and R is the diffusional resistance across the gap between the surface and the screen. Qualitatively, R can be regarded as actually being the sum of a series of resistances corresponding to the various diffusion gradients present ... 

Antimony Oxide. The effect of antimony trioxide on the oxygen index of flexible poly(vinyl chloride) containing from 20 to 50 parts of plasticizer is shown in Figure 2. The flame resistance as measured by the oxygen index increases with the addition of antimony oxide until the oxygen index appears to reach a maximum at about 8 parts of Sb202. Further addition of antimony oxide does not have any increased beneficial effect. 

In determining the chemical resistance, color changes of pigmented binder surfaces are measured after their exposure to various chemicals, such as water—sulfur dioxide or water—sodium chloride systems. These systems imitate the environment to which the colored articles could become exposed. 

Remedial measures include reduction or elimination of chlorides or replacement of the 304 stainless steel with a metal that is resistant to chloride stress-corrosion cracking. 

The specific electrical resistance of concrete can be measured by the method described in Section 3.5. Its value depends on the water/cement value, the type of cement (blast furnace, portland cement), the cement content, additives (flue ash), additional materials (polymers), the moisture content, salt content (chloride), the temperature and the age of the concrete. Comparisons are only meaningful for the... 

In 3% sodium chloride solution at 60°C the austenitic irons again show superior characteristics to the ferritic. The breakdown potentials determined in this environment, which provide a relative measure of the resistance to attack in neutral chloride solutions, are generally more noble for the austenitic irons than for the ferritic (Table 3.47). This indicates that the austenitic irons should show better corrosion resistance in such environments. 

Kittelberger and Elm measured the rate of diffusion of sodium chloride through a number of paint films. Calculations based on their results showed clearly that the rate of diffusion of ions was very much smaller than the rate of diffusion of either water or oxygen. Furthermore, they found that there was a linear relationship between the rate of diffusion and the reciprocal of the resistance of the film. This relationship suggests that the sodium chloride diffused through the membrane as ions and not as ion pairs, since the diffusion through the film of un-ionised material would not affect the resistance, because if a current is to flow, either ions of similar charge... 

The films were then soaked in water and removed from the plates. Portions were mounted in glass cells which were filled with potassium chloride solution two Ag/AgCl electrodes were inserted into the limbs of the cells and the unit was placed in a thermostat. The resistance of the films was determined, from time to time, by connecting the cells in series with a known resistance and applying a potential of 1 V to the combination the potential drop across the standard resistance was measured by means of a valve potentiometer. 

In view of this, the properties of / films were examined after they had been subjected to increasing amounts of ion exchange . In order to do this, detached films were exposed at 65°C for 7 h to a universal buffer adjusted to a suitable pH and the resistance of the film measured at 25°C in 3 n and O OOlN potassium chloride. The results obtained with a pentaerythritol alkyd are shown in Fig. 14.4 from which it can be seen that as the pH of the conditioning solution increased, the resistance of the film fell, until at a pH of about 7.5 it suddenly dropped. The resistance of the film then followed that of the solution in which it was immersed, i.e. it became a D-type film. Similar results were obtained with films of a tung oil phenolic varnish, although in this case the change-over point occurred at a higher pH, i.e. about 9. 

Measurement of conductivity of a fibre. If a fibre is impregnated with an electrolyte, such as lithium chloride, its electrical resistance will be governed by its moisture content, which in turn depends on the humidity of the atmosphere in which it is situated, Iri a lithium chloride cell, a skein of very fine fibres is wound on a plastic frame carrying the electrodes and the current flowing at a constant applied voltage gives a direct measure of the relative humidity. 

Monolithic columns, formed from the co-polymerization of divinylbenzene and vinylbenzyl chloride or styrene, were observed to be resistant to bubble formation.11 Application of pressure in electrochromatography, discussed below, also reduces bubble formation. A massively parallel detector capable of scanning up to 1000 capillaries using planar confocal fluorescence has been used for DNA sequencing.1213 Recovery of fluorescence following pho-tobleaching has been used to measure DNA mobility in agarose gel.14... 

Olafsson [472] described a similar procedure, in which the sample (450 ml) is acidified with nitric acid, aqueous stannous chloride is added, and the mercury is entrained by a stream of argon into a silica tube wound externally with resistance wire and containing pieces of gold foil, on which the mercury is retained. The tube and its contents are then heated electrically to about 320 °C and the vaporised mercury is swept by argon into a 10 cm silica absorption cell in an atomic absorption spectrophotometer equipped with a recorder. The absorption (measured at 253.7 nm) is directly proportional to the amount of mercury in the range 0 - 24 ng per sample. 

Hardy effect.248-249 The internal return part of the ionization equilibrium is particularly hard to detect since it is almost completely independent of the concentration of anything in the bulk of the solution outside of the solvent cage. The extent of internal return will depend on the reactivity of the cage walls and their resistance to the escape of either ion. Unless internal return has been eliminated by the use of an extremely reactive cage wall, the measured rate is not that of the ionization but the lesser rate of ion pair dissociation. In the case of the acetolysis of a, a-dimethylallyl chloride (XXXIX), internal return is detectable by virtue of the fact that the chloride ion can return to either of two allylic carbon atoms.248... 

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