Electrical field jump method

The electric field-jump method is applicable to reactions of ions and dipoles. Application of a powerful electric field to a solution will favor the production of ions from a neutral species, and it will orient dipoles with the direction of the applied field. The method has been used to study metal ion complex formation, the binding of ions to macromolecules, and acid-base reactions. 

The electric-field-jump method with electric conductivity detecting system can be applied to reactions occuring on the order of milliseconds to microseconds. The rise time of the applied electric field is much faster than 0.1 ys. The strength of the electric field is 20 kV/cm. The details of the electric-field-jump apparatus can be found elsewhere (9). 

Commonly, a 10 volt/cm field will produce a 1% change in conductance of weak electrolytes. The measurement of very short relaxation times ( 50 ns) is possible by the electric-field jump method but the technique is generally complicated and mainly restricted to ionic equilibria. "... 

Pressure jump and electric field jump methods have also been used, as have methods depending upon periodic changes in some property. For example, absorption of ultrasonic sound causes a periodic change in the pressure of the system. 

Ehlers-Danlos syndrome 438 Eicosanoid 565 Eigen, Manfred 84 Elastase 66,609,610,611s cryoenzymology 616 P-cylinder in 78 Elastic fibers 436 Elastin 15,72,436 Electrical double layer 400 Electric field jump methods 468 Electrochemical gradient 410 Electrochemical transference 311 Electrode(s)... 

An important turning point in reaction kinetics was the development of experimental techniques for studying fast reactions in solution. The first of these was based on flow techniques and extended the time range over which chemical changes could be observed from a few seconds down to a few milliseconds. This was followed by the development of a variety of relaxation techniques, including the temperature jump, pressure jump, and electrical field jump methods. In this way, the time for experimental observation was extended below the nanosecond range. Thus, relaxation techniques can be used to study processes whose half lives fall between the range available to classical experiments and that characteristic of spectroscopic techniques. 

The electrical field-jump method is applied to reactions involving ions and dipoles. The field induces a shift in the solution equilibrium in the direction of producing more ions, or in the orientation of dipoles. Thus, when a high electrical field is applied to a solution containing a weak eleetrolyte, the extent of dissociation increases. The method has been used to study metal ion complex formation reactions and acid-base reactions. 

The several experimental methods allow a wide range of relaxation times to be studied. T-Jump is capable of measurements over the time range 1 to 10 s P-jump, 10 to 5 X 10" s electric field jump, 10 to 10 s and ultrasonic absorption, 10 to 10 " s. The detection method in the jump techniques depends upon the systems being studied, with spectrophotometry, fluorimetry, and conductimetry being widely used. 

Conventional, flow, temperature-jump, ultrasonic absorption, electric-field jump and nmr line broadening have all been used to measure the rates. UV-vis spectrophotometry and conductivity are the monitoring methods of choice. A variety of solvents have been used. The focus has been often on the dissociation since the dissociation rate constant appears in general to be the main controller of the overall stability. 

Method. When rapid techniques are involved the following abbreviations apply F, flow TJ, temperature jump PJ, pressure jump E, electrochemical NMR, nuclear magnetic resonance ESR, electron spin resonance SA, sound absorption EF, electric field. Classical methods for investigating kinetics are not specified unless low temperatures (LT) have been used. 

Previous investigations of helix-coil transition kinetics, which used a variety of fast relaxation methods (electric field jump, ultrasonic absorption, dielectric relaxation and temperature jump), encountered many difficulties (12). The systems studied were long homopolymers (>200 residues) that often had hydrolyzable side chains. Controversial results have been reported, depending on the experimental technique employed, because unwanted side chain reactions or molecular reorientation were often difficult to distinguish from the helix-coil conformational change. However, as observed here, a maximum in the relaxation times was detected for these experiments ranging from 15 ps to 20 ns and was attributed to the helix-coil transition. 

Measurements of the relaxation times by relaxation methods (involving a temperature jump [T-jump], pressure jump, electric field jump, or a periodic disturbance of an external parameter, as in ultrasonic techniques) are commonly used to follow the kinetics of very fast reactions. 

The kinetics of hydrolysis of Fe(III) perchlorate in dilute solution have been investigated by means of an electric field jump relaxation method [155]. At 25°C and at a low ionic strength (<3 x 10 the hydrolysis step... 

AiST Methods. After a rapid alteration of K by changing an external condition, the solution composition readjusts at a finite rate in an attempt to reattain equilibrium this process of adjustment is called relaxation. Several experimental techniques for achieving the necessarily rapid alterations of K have been developed. These include a pressure-jump (sound-absorption) method, an electric-field (dissociation) method, and a temperature-jump method. A temperature jump is brought about by passing an electrical current through the solution in a special cuvette, producing an abrupt, nearly instantaneous, rise in the temperature of the solution. A reaction then takes place as the concentrations adjust to the new temperature. Regardless of the type of perturbation used, the treatment of the data is essentially the same. 

Structural interconversions of dichloro-l,l,7,7-tetraethyldiethylenetriamine-nickel(n) (NiLClg) in acetonitrile have been studied by relaxation methods in which the equilibria were suddenly perturbed by an electric-field jump or by means of a pulse of radiation from a Q-switched neodymium laser. The results are interpreted in terms of the mechanism... 

Such high rate constants are relatively easy to determine by temperature-jump, electric-field jump, ultrasonic absorption, dielectric absorption, flash, fluorescence-quenching, and e.s.r. methods. The stopped-flow method is not commonly used to determine rate constants above about 10 M s ... 

Among the relaxation techniques (1) such as pressure-jump, electrical field-jump and ultrasonic absorption the temperature jump method is most widely used because almost every chemical equilibrium shows a variation with temperature. 

The two basic requirements of relaxation experiments are The sufficiently fast disturbance of the equilibrium and the specific observation of the transient helix-coil transition. Stationary relaxation methods, such as sound absorption and dielectric relation, do not fulfill the second requirement. The application of electric field-jump and laser temperature-jump perturbations in combination with optical rotation detection not only satisfies both requirements, but as a bonus it provides also for the use of a large variety of solvents and ionic strengths. 

Apart from the temperature-jump technique, other relaxation methods that have been used are those of ultrasonic absorption" " and electric-field pulse. Another technique that has been used for some of the more slowly included guest molecules is that of stopped-flow. ... 

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). 

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-... 

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