Pressure jump complexes

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. 

Milburn and Vosburgh 16) confirmed the formation of Fe2(OH)2 in a spectrophotometric study, and their equilibrium constants log = —2.89, log / 22 =—2.58 in 3MNaC104 at 25° are in reasonable agreement with those of Hedstrom. The dimer has an absorption band at 335 ra/i, whereas both Fe + and FeOH2+ apparently have a band at 240 vcifi 17). Kinetic evidence for dimer formation emerged from a pressure jump study by Wendt in 1962 18). As expected for a polynuclear complex, the rate constant for dissociation... 

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). 

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. 

The various elementary steps involved in the surface photoredox reaction, leading to dissolution of hematite in the presence of oxalate, are outlined in Figure 12.10. The two-dimensional stmcture of the surface of an iron(III) hydroxide given in this figure is highly schematic. The charges indicated correspond to relative charges. An important step is the formation of a hypothetical bidentate, mononuclear surface complex. With pressure jump relaxation technique, it has... 

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 objective of this chapter is to discuss the theory of chemical relaxation and its application to the study of soil chemical reaction rates. Transient relaxation techniques including temperature-jump (t-jump), pressure-jump (p-jump), concentration-jump (c-jump) and electric-field pulse will be discussed both as to their theoretical basis and experimental design and application. Application of these techniques to the study of several soil chemical phenomena will be discussed including anion and cation adsorp-tion/desorption reactions, ion-exchange processes, hydrolysis of soil minerals, and complexation reactions. 

The kinetics of formation and dissociation of Ni(SCN)2(aq) have been studied in water and in several organic solvents, using the pressure-jump and shock wave relaxation technique at 20°C. The concentration of Ni(SCN)2(aq) ranged between 0.001 and 0.1 M. In water, only the formation of the monothiocyanato complex was observed. No background electrolyte was used, and the activity coefficients were calculated by an extended Debye-Hiickel expression. Although, this activity model is not compatible with the SIT, the ionic strengths were low. Therefore, the reported result was corrected to 25°C and the resulting value was accepted with an increased uncertainty (log, = (1.79 0.10)). 

Elucidation of the kinetics and mechanisms of mineral-fluid interactions requires high-resolution X-ray scattering measurements on rapid time scales. Time series analyses are desired for addressing the evolution of structure and composition at the interface, on time scales as small as milliseconds or less. The high brilliance of the third-generation synchrotron sources affords new opportunities for such time-resolved studies, because we can observe in real time the processes of adsorption/desorption and complex formation at mineral-fluid interfaces. For example, experiments using a pressure-jump relaxation techniques yield rates of adsorption and desorption of protons and hydroxide at the surface of metal oxides in the range of milliseconds to seconds (reviewed by Casey and... 

Kinetics of Selenium Adsorption. Zhang and Sparks 4G) examined selenate and selenite adsorption and desorption on goethite using pressure jump relaxation techniques. Selenate produced a single relaxation, that was interpreted as outer-sphere complexation with surface protonation based on fitting to the triple layer model. The forward rate constant was 10 L mol s Selenite adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of inner-sphere complexes of both HSeOj and SeOj, based on optimized fits using the triple layer model. The model optimized fit for the pK, of the surface species was approximately 8.7. Forward rate constants for the first step were on the order of 10 L -mor -s for HSeOj and 10 L -mor -s for SeOj. Forward rate constants for the formation of the inner-sphere complexes were 100 and 13 s respectively for HSeOj and SeOj. Agreement between the equilibrium constant obtained from batch and kinetic studies was taken as confirmation of the proposed reactions. 

Kinetics of Molybdenum Adsoiption. Zhang and Sparks 41) examined molybdate adsorption on goethite using pressure jump relaxation experiments. Molybdate adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of an inner-sphere complex of Mo04, based on optimized fits using the triple layer model. Forward rate constants were on the order of 4x10 L mol s and 40 s for the first and second reaction steps. 

Dickert et al [Di 71, Di 72, Di 74] employed pressure jump and other relaxation methods to study the kinetics of the formation and dissociation reactions of various nickel complexes. They established that the substitution reactions of some of the nickel(II) complexes (taking place according to the following equation) varied linearly with the donicity of the solvent ... 

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