Adsorption isotherm metal solution interface

Most earlier papers dealt with the mercury electrode because of its unique and convenient features, such as surface cleanness, smoothness, isotropic surface properties, and wide range of ideal polarizability. These properties are gener y uncharacteristic of solid metal electrodes, so the results of the sohd met electrolyte interface studies are not as explicit as they are for mercury and are often more controversial. This has been shown by Bockris and Jeng, who studied adsorption of 19 different organic compounds on polycrystaUine platinum electrodes in 0.0 IM HCl solution using a radiotracer method, eUipsometry, and Fourier Transform Infrared Spectroscopy. The authors have determined and discussed adsorption isotherms and the kinetics of adsorption of the studied compounds. Their results were later critically reviewed by Wieckowski. ... 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

For dilute additive solutions, consumption can easily become limited by diffusion of the adsorbate to the interface. A particularly tractable situation occurs at the dilute limit of the Langmuir isotherm where C(- is proportional to the surface coverage, and in the limit of diffusion controlled adsorption CsoHd is directly proportional to the additive flux and thus the bulk electrolyte concentration. Such transport limited incorporation was reported in some radiotracer studies of thiourea incorporation in nickel and copper plating in the 1950-1960s [1-4, 17, 130, 131, 141-146], Consistently, a common observation was that the additive concentration in the solid was proportional to the additive concentration at the interface and inversely proportional to metal deposition rate, i, [1-4, 130, 131, 141, 142] such that ... 

It is assumed that the concentration of ions present in a liquid is extremely low and that the ions present are formed exclusively in the autodissociation process. All ions and neutrals other than those originating from liquid solvent are removed by purification as impurities. Under such circumstances, the system can be considered as an ideally dilute solution i.e., the solvent mole fractions is 1. Hence, the system, the electrode and the solvent, is assumed to obey Henry s law. The surface concentration of specifically adsorbed anions can be estimated on this basis from the Henry isotherm. Assuming that the specific adsorption equilibrium constant for OH ions in pure water can range from 0.1 to 100 dm /mol, one can obtain the surface concentration of adsorbed anions in the range lO -lO mol/cm i.e., the ratio of the adsorbed anions to the metal atoms of the electrode surface is 10 -10 . Having this in mind and remembering that an amount of possible solvated cations in the bulk of solution is very low, it can hardly be believed that the Helmholtz compact layer is formed in pure liquid. Thus, the electrode-liquid interface seems to... 

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