Concentration conductivity probe methods

Trioctylmethylammonium chloride [7] has been widely used as a phase transfer catalyst. This compound is slightly soluble in water and forms aggregates at very low concentrations (Okahata et al., 1977). Figure 3 shows surface tension data, which indicate aggregation occurring at 10-4-10-5 M. The dye probe method and conductance measurements suggest that the... 

LFP-Probe Method. In cases where the radicals of interest do not contain a useful chromophore, the LFP technique can be modified by incorporation of a probe radical reaction that gives a product with a chromophore. The probe reaction can be unimolecular or bimolecular, a constant concentration of probe reagent is employed in the latter case. Formation of the detectable species occurs with an observed first-order or pseudo-first-order rate constant equal to k0. In the presence of another reagent X that reacts with the original radical, the rate constant for formation of detectable species is kohs = k0 + kx [X], and the bimolecular rate constant is determined (as before) by conducting the reaction at varying concentrations of X. Note that the LFP-probe technique is a direct method even though the reactant or product of interest is not monitored. 

Improvement of the techniques for monitoring local instantaneous concentrations down to the viscous dissipation microscale (e.g. spatial and time resolution of conductivity probes), development of new techniques (e.g. optical, radioactive tracer methods). 

Conductivity Probe for Local Solids Concentration Measurements. On the basis of the preceding discussion, local solids concentration can be determined by measuring the mixture conductivity, then using a calibration curve, for example, equation 20. However, using this method to measure solids concentration or dispersed-phase concentration is not an easy task. In the following sections, the development a new conductivity probe will be summarized (24). Also, various problems encountered with conductivity methods will be discussed. 

Figure 31 shows the resistivity measured by the conductivity probe and the local concentration measured by the isokinetic sampling method. Measurements were made for slurries of polystyrene particles of 1.4-mm mean diameter at the pipe center and at radial positions of R = 0.8. Good agreement with calibration results is seen in these tests. Because the particles used in these experiments were large, no samples could be withdrawn from the center of the pipe at concentrations higher than 35%. Also, no voltage measurements could be taken closer to the pipe wall because particles tended to be trapped between the probe and the wall. 

The thermoelectric properties were measured at 300 K for the FGM and its component layers separated from the FGM. The electrical conductivity ((T) and Hall coefficient (Rjj) were measured by the 6-probe method for the FGM and by the van der Pauw configuration for the components cut from the FGM using Pt-wire electrodes. The carrier concentration (n) and Hall mobility (/ h) calculated using the equation n=lleR (e electric charge) and / H= h respectively. The thermoelectric power (a) at 300 K was estimated from the linear relationship between thermoelectromotive force (EMF) and temperature difference within 5 K. 

The electrical conductivity(<, carrier concentration(n) and Hall mobility(/i ) were measured at 300 Kfor the p and n-type sintered PbTe. The characterization of the p-n jimction was conducted at 300 K by measuring thermoelectromotive force within temperature difference of 5 K, and voltage (electric potential) distribution using 4-probe method and current(])-voltage(V) relationship in forward and reverse bias. 

Conductivity methods have been successfully used to measure solid concentration in multiphase systems, where the conductivity of continuous phase is greater than zero, for example, sand-water slurries and oil-in-water emulsions (65). These methods have been used to measure bulk and local solids concentration in the slurry pipelines. Nasr-El-Din et al. (27) developed a four-electrode conductivity probe for measuring local solids concentration in slurry pipelines (Figure 20). The probe is... 

Various techniques are available to measure velocity and solids concentration profiles in slurry pipeline. Sample withdrawal using an Li-shaped probe can give a representative sample at isokinetic conditions. Other sample devices will produce significant errors that must be corrected. Conductivity probes can be used to measure local velocity and concentration profiles simultaneously. However, the carrier fluid should be conductive. NMR imaging methods do not disturb the flow with a probe however, they are limited to pipes of small diameter. 

Polyaniline-ZnO nanocomposites were synthesized by in-situ oxidative polymerization of aniline monomer in the presence of different amounts of ZnO nanostructures. ZnO nanostructures were prepared in the absence and presence of surfactant. The effect of ZnO nanostructure concentration on the conducting behaviour of nanocomposites was evaluated by a two-probe method. The results showed that the conductivity of nanocomposites was increased with an increased concentration of ZnO nanostmctures as compared with neat polyaniline. Optimum conductivity was observed with incorporation of 60% ZnO nanostructures into the polyaniline matrix [248].Polymethylmethacrylate-ZnO nanocomposites were fabricated by solution radical copolymerization of methyl methacrylate and oleic acid-modified ZnO nanoparticles using 2,2 -azobisisobutylonitile as initiator in toluene. The UV-vis analysis showed that resulting nanocomposites exhibited high absorption in the ultraviolet region and low absorption in the visible region [249]. 

Figure 7.13 Temperature dependence of conductivity of free-standing biofilm formed by G. sulfurreducens strain CL-1 and pili filaments of strain KN400 measured with a four-probe method. Error bars represent SD. Data is a representative of several replicates (n = 4 for biofilm n = 3 biological replicates for pili). The difference in conductivity between pili and biofilm at 300 K is due to lower pili concentration required on smaller electrodes for four-probe measurements. Control experiments containing media buffer or with biofilm, but not bridging the gap, showed very low conductivity (<10 pS cm" at 300 K) that did not change with temperature. Inset Arrhenius fit for exponentially decreasing conductivity. Figure adapted from Ref. [4] with permission.

A variety of other techniques have been used to investigate ion transport in conducting polymers. The concentrations of ions in the polymer or the solution phase have been monitored by a variety of in situ and ex situ techniques,8 such as radiotracer studies,188 X-ray photoelectron spectroscopy (XPS),189 potentiometry,154 and Rutherford backscatter-ing.190 The probe-beam deflection method, in which changes in the density of the solution close to the polymer surface are monitored, provides valuable data on transient ion transport.191 Rotating-disk voltammetry, using an electroactive probe ion, provides very direct and reliable data, but its utility is very limited.156,19 193 Scanning electrochemical microscopy has also been used.194... 

The extent of gas dispersion can usually be computed from experimentally measured gas residence time distribution. The dual probe detection method followed by least square regression of data in the time domain is effective in eliminating error introduced from the usual pulse technique which could not produce an ideal Delta function input (Wu, 1988). By this method, tracer is injected at a point in the fast bed, and tracer concentration is monitored downstream of the injection point by two sampling probes spaced a given distance apart, which are connected to two individual thermal conductivity cells. The response signal produced by the first probe is taken as the input to the second probe. The difference between the concentration-versus-time curves is used to describe gas mixing. 

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