Pressure jump relaxation method

The Heterogeneous Case. Hachiya et al. (1984) and Hayes and Leckie (1986) used the pressure-jump relaxation method to study the adsorption kinetics of metal ions to oxide minerals. Their results support in essence the same adsorption mechanism as that given for homogeneous complex formation. 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Pressure-jump relaxation methods (Takahashi and Alberty, 1962 Eigen and DeMaeyer, 1963 Hoffman et al., 1966 Knoche, 1974 Gruenewald and Knoche, 1979 Yasunaga and Ikeda, 1986) and theory (Takahashi and Alberty, 1969 Bernasconi, 1976) have been reviewed extensively, and the reader is referred to these references for in-depth discussions. The p-jump methods are based on the fact that chemical equilibria are dependent on... 

Ikeda, T., Sasaki, M., Astumian, R. D., and Yasunaga, T. (1981). Kinetics of the hydrolysis of zeolite 4A surface by the pressure-jump relaxation method. Bull. Chem. Soc. Jpn. 54, 1885-1886. 

Pressure-jump relaxation methods and theory have been reviewed by Sparks (1989). In-depth treatments can also be found in a number of reviews 11 nkahashi and Alberty, 1969 Bernasconi, 1976 Yasunaga and Ikeda, 1986 Uitienewald and Knoche, 1979 Knoche, 1974). 

Ikeda, T., M. Sasaki, K. Hachiya, R.D. Astumian, T. Yasunaga, and Z.A. Schelly. 1982a. Adsorption-desorption kinetics of acetic acid on silica-alumina particles in aqueous suspension, using the pressure-jump relaxation method. J. Phys. Chem. 86 3861-3866. 

The second method for the study of relaxation in spin-state equilibria makes use of the rapid change of pressure. Single-step pressure-jump relaxation requires an observation time of about lO s which is too slow. However, the... 

Another method to determine time-dependent properties is pressure jump relaxation. In a simple equilibrium between two states A and X,... 

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. 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

Another consideration in choosing a kinetic method is the objective of one s experiments. For example, if chemical kinetics rate constants are to be measured, most batch and flow techniques would be unsatisfactory since they primarily measure transport- and diffusion-controlled processes, and apparent rate laws and rate coefficients are determined. Instead, one should employ a fast kinetic method such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4). 

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. 

The temperature-jump relaxation method and other relaxation methods avoid mixing and therefore the limitation due to rate of mixing. Instead, the relaxation technique starts with a system at equilibrium and disturbs it by a sudden alteration of temperature or pressure. The discharge of a capacitor provides a short-duration pulse of electric current which gives a sudden increase in temperature. Detections of changes at some distance removed from the electrodes will not be complicated by the chemical... 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

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). 

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Figure 1. Typical relaxation curve in the aqueous y-A1203 -Cu(N03)2 suspension observed by using the pressure-jump method. [P] = 30 g/dm3, and I = 7.5 x10-3 at 25 °C sweep 20 ms/div.

Table III suggests some of the proton transfer kinetic studies one is likely to hear most about in the near future. The very first entry, colloidal suspensions, is one that Professor Langford mentioned earlier in these proceedings. In the relaxation field, one of the comparatively new developments has been the measurement of kinetics of ion transfer to and from colloidal suspensions. Yasunaga at Hiroshima University is a pioneer in this type of study (20, 21, 22). His students take materials such as iron oxides that form colloidal suspensions that do not precipitate rapidly and measure the kinetics of proton transfer to the colloidal particles using relaxation techniques such as the pressure-jump method.

The appearance or disappearance of the U.V. absorption of the carbonyl group can in principle be used for kinetic measurements. Bell and Jensen (1961) applied this method to 1,3-dichloroacetone the reaction is too fast in pure water, but proceeded at a convenient rate in 5% water-I-dioxan mixtures, in which there is about 50% hydration at equilibrium. Catalysis by many acids and bases was observed. Much faster reactions can be studied by relaxation methods, and the pressure-jump technique has been applied to the reaction Me0(OH)2.CO2H MeC0.C02H-hH20 by Strehlow (1962). 

Relaxation methods can be classified as either transient or stationary (Bernasconi, 1986). The former include pressure and temperature jump (p-jump and t-jump, respectively), and electric field pulse. With these methods, the equilibrium is perturbed and the relaxation time is monitored using some physical measurement such as conductivity. Examples of stationary relaxation methods are ultrasonic and certain electric field methods. Here, the reaction system is perturbed using a sound wave, which creates temperature and pressure changes or an oscillating electric field. Chemical relaxation can then be determined by analyzing absorbed energy (acous-... 

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.]...

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