Conductance pressure-jump

Dynamic processes of complex formation of metal ions with poly-4-vinylpyridine (PVP) (Eqs. (4) and (5)) have been studied by means of the conductance stopped flow (CSF) and conductance pressure-jump (CPJ) technique 30). 

Other perturbations have been demonstrated. The pressure,, jump, similar to the T-jump in principle, is attractive for organic reactions where Joule heating may be impractical both because of the solvent being used and because concentrations might have to be measured by conductivity. Large (10 —10 kPa) pressures are needed to perturb equiUbrium constants. One approach involves pressurizing a Hquid solution until a membrane mptures and drops the pressure to ambient. Electric field perturbations affect some reactions and have also been used (2), but infrequentiy. 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

A pressure perturbation results in the shifting of the equilibrium the return of the system to the original equilibrium state (i.e., the relaxation) is related to the rates of all elementary reaction steps. The relaxation time constant associated with the relaxation can be used to evaluate the mechanism of the reaction. During the shift in equilibrium (due to pressure-jump and relaxation) the composition of the solution changes and this change can be monitored, for example by conductivity. A description of the pressure-jump apparatus with conductivity detection and the method of data evaluation is given by Hayes and Leckie (1986). 

Kinetic experiments were conducted using a pressure-jump apparatus with conductivity detection. Details of the apparatus and its operation can be found in Appendix A. Sample equilibration time can have an effect on the kinetic results (e.g., slow processes (on the order of hours-days) occurring concurrently but not monitored in the time frame of the p-jump technique (milllseconds-seconds)) hence, it is important to run kinetic experiments on samples with similar equilibration history. All samples were equilibrated between 3 and 4 hours for the p-jump kinetic studies. The temperature of the p-jump apparatus, which includes sample and reference solution cells, was maintained at 25.0°C 0.1°C. 

Pressure-Jump Apparatus with Conductivity Detection... 

T. Okubo and A. Enokida, J. Chem. Soc. Faraday Trans. 1, 1639, (1983) flow and pressure-jump with conductance. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Conductivity and Optical Detection Using p-Jump Relaxation 75 Evaluation of p-Jump Measurements 76 Commercially Available p-Jump Units 78 Application of Pressure-Jump Relaxation Techniques to Soil Constituents 81 Stopped-Flow Techniques 91 Introduction 91... 

In 1959 Strehlow and Becker developed a pressure-jump apparatus that enclosed a conductivity cell containing the reaction solution, and a reference cell under xylene in an autoclave. The reaction and reference solutions were pressurized to about 6.1 MPa with compressed air. By the blow of a steel needle, a thin metal disk used to close the autoclave was punctured and the pressure was released within about 60 s. 

Figure 4.3. Schematic diagram and sectional views of the autoclave of the pressure-jump apparatus of Knoche and Wiese (1974) 1, conductivity cells 2, potentiometer 3, 40-kHz generator for Wheatstone bridge 4, tunable capacitors 5, piezoelectric capacitor 6, thermistor 7, 10-turn helipot for tuning bridge 8, experimental chamber 9, pressure pump 10, rupture diaphragm 11, vacuum pump 12, pressure inlet 13, heat exchanger 14, bayonet socket. [From Knoche and Wiese (1974), with permission.]...

Knoche and Wiese (1974) made a number of alterations to the autoclave (Fig. 4.3) originally proposed by Strehlow and Becker (1959). The energy released at the pressure jump is partly needed to break the rupture disk but can cause the autoclave to oscillate. This disturbs the determination of the cell resistances. To minimize the energy, the experimental chamber (8) volume was reduced and the pressure pump (9) was built as an integrated part of the autoclave to reduce all supply lines to a minimum. With this autoclave, water was used as the pressure transducing liquid instead of than kerosene (Strehlow and Becker, 1959), to reduce the compressibility. The conductivity cells were also mounted on a small incline so that no air... 

Conductivity Detection. Pressure-jump measurements can be detected using either optical or conductivity detection. However, conductivity detection is usually preferred since the equilibrium displacement following p-jump is usually small (Bernasconi, 1976). Conductometric detection has been exclusively used by researchers investigating the rapid kinetics of reactions on soil constituents (to be discussed later) because of the high sensitivity, obtained using conductivity and because suspensions are studied. Optical detection would not be desirable for suspensions. 

Figure 4.4. Typical oscillograms of pressure-jump experiments. Relative change in conductivity for pressure-jumps of 13.1 MPa in solutions of 0.05 M InCI, pH = 3.25. (a) At 383 K showing only pressure decay, (b) At 300.5 K, r = 50 15 /us. (c) At 273.7 K. r = 215 10 jj.s. (d) Solution of 0.10 M a-ketoglutaric acid, pH 1.69. at 274 K, t = 25.8 s. [From Knoche and Wiese (1974), with permission. ...

Figure 4.12. Typical relaxation curves in aqueous y-ATOj-PbfNO, suspension observed by the pressure-jump method with (a) electric conductivity and (b) turbidity detection. Concentration of A1203, Cp, is 15 g dm 3 at 293 K sweep, 2 ms/division wavelength in (b), 525 nm, [From Hachiya et al., 1979), with permission.]...

Now the pressure-jump technique will be described with a system consisting of lanthanide oxalate complexes [17]. The technique used a sudden change in pressure to perturb the equilibrium and a conductivity bridge to detect and follow the changes in the system. The course of re-equilibration is recorded by the use of an oscilloscope and a camera. 

When adsorption from solution Is monitored by the depletion method. It Is very difficult to measure changes in bulk concentration over time Intervals down to milliseconds. Perhaps this Is the reason that such systematic studies are not abundant in the literature. Fast measurements require stopped-flow, pressure-jump or temperature-jump techniques. The method used to determine concentrations must also be fast suitable methods include certain spectroscopies and, for charged substances, conductivity. When adsorption on Fresnel surfaces Is studied, say by reflectometry, concentration measurements in the solution are not needed. 

A number of soil chemical phenomena are characterized by rapid reaction rates that occur on millisecond and microsecond time scales. Batch and flow techniques cannot be used to measure such reaction rates. Moreover, kinetic studies that are conducted using these methods yield apparent rate coefficients and apparent rate laws since mass transfer and transport processes usually predominate. Relaxation methods enable one to measure reaction rates on millisecond and microsecond time scales and 10 determine mechanistic rate laws. In this chapter, theoretical aspects of chemical relaxation are presented. Transient relaxation methods such as temperature-jump, pressure-jump, concentration-jump, and electric field pulse techniques will be discussed and their application to the study of cation and anion adsorption/desorption phenomena, ion-exchange processes, and hydrolysis and complexation reactions will he covered. 

The p-jump method has several advantages over the t-jump technique. Pressure-jump measurements can be repeated at faster intervals than those with t-jump. With the latter, the solution temperature must return to its ini-lial value before another measurement can be conducted. This may take 5 min. With p-jump relaxation, one can repeat experiments every 0.5 min. One can also measure longer relaxation times with p-jump than with t-jump relax-mion. As noted earlier, one of the components of a t-jump experiment is It heat source such as Joule heating. Such high electric fields and currents can destroy solutions that contain biochemical compounds. Such problems lIo not exist with the p-jump relaxation method. 

A counter-diffusion experiment is usually conducted as follows. First, the activated adsorbent wafer is loaded at a given experimental temperature with a certain amount of component 1 (e.g., benzene) according to a certain partial pressure jump of this component in the gas stream (vide supra). 

The kinetics of intercalation and deintercalation of alkali metal ions were investigated in pressure-jump experiments while monitoring the electrical conductivity of the samples (32). These studies indicate biphasic kinetics whose magnitudes are in milliseconds the rates of the fast and slow components increased with increased concentrations of the metal ions. The forward and reverse rates depend on the interlayer distances, and the fast and slow components have been attributed to the ingress of ions into the galleries and interlayer diffusion, respectively. Similar biphasic kinetics on millisecond-second time scales were also observed in pressure-jump experiments for the deprotonation-reprotonation of a-ZrP (33). In the latter case, the slow and fast components have been attributed to deprotonation from the surface and from the interlayer regions of the solid, respectively. 

Details of the pressure jump equipment with detection of electrical conductivity have been discussed in previous reviews... 

Apparatus. (a)Pressure-jump apparatus The Pressure-jump apparatus used is a modification of that described by Knoche and Wiese. A pressure jump of 100 atm was realized within 80 ysec. An advantage of this apparatus is that chemical relaxation induced by pressure jump can be observed simultaneously by both changes in electric conductivity and optical absorbance. (b)Electric field pulse apparatus The electric field pulse apparatus used has already been reported . The electric field... 

The kinetic measurements were carried out in the aqueous suspension of y-Al203 containing PbCNOg) at 20°C. A typical relaxation curve obtained by means of the pressure-jump technique with conductivity detection is shown in Figure 1(a) in which the electric conductivity increases with the pressure. As seen from Figure 1(a), at the beginning of the relaxation a very fast change in conductivity was also found. To resolve this, measurements were performed by the electric field pulse technique. 

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