Radiotracer adsorption

Pioneering work was done in this field by Schwabe [34, 35]. Substantial and very informative radiotracer adsorption studies carried out by him and his coworkers using Pt, Ni, and Fe sheets and evaporated films, using labeled anions in order to interpret the role of anions in the corrosion and passivation processes [34, 35]. 

Radiotracer adsorption studies (- tracer methods) furnish information on this latter phenomenon by the use of labeled anions at low concentration in the presence of a great excess of supporting electrolyte containing the metal ion forming the -> adatoms [ii]. 

Various methods have been developed for in situ radiotracer adsorption studies depending on the requirements of the problems to be studied. In the case of the in situ studies the central problem is how to separate the signal (radiation) to be measured from the background radiation, and how to attain the optimal ratio of these quantities. 

Smit. W. and Holten, C.L.M., Zeta-potential and radiotracer adsorption measurements on EFG (z-AhO, single crystals in NaBr solutions, J. Colloid Interf. Sci., 78,... 

Figure 1 illustrates the apparatus used for these measurements. Detailed description of a -radiotracer adsorption method is given in the paragraph dealing with mucin and collagen adsorption at interfaces. 

The introduction of new methods resulted in a change in the philosophy on how to separate specific and nonspecific adsorption. According to the approach presented in the previous section, the determination of specific adsorption required the deduction of the adsorption in the diffuse double layer, calculated on the basis of model assumption, from the total adsorption determined by well-defined and reliable thermodynamic methods. In the case of some new methods, the specific adsorption of a given species is studied in the presence of a great excess of a supporting electrolyte. If this excess is several orders of magnitude higher than the concentration of the species studied, there could be no doubt that in the case of a measurable adsorption of these species (more than 10 to 10 mol cm ) the value obtained should be ascribed to specific adsorption. This consequence can be clearly demonstrated in the case of radiotracer adsorption studies. 

Radiotracer adsorption studies furnish rapid and reliable information on these phenomena, using labeled anions in low concentration in the presence of... 

The elaboration of a technique for the radiotracer adsorption measurements is equal to the search of the optimal conditions for the minimalization of the role of the radiation coming from the solution background. The usual classification of the various methods follows this pattern, that is, how this requirement is fulfilled. According to this principle, the methods can be divided in two main groups the radiation of solution background is governed and minimized by self-absorption of the radiation (thin-foil method) and the background radiation intensity is minimized by mechanical means (thin-gap method). The most important and widely used representative of the first approach is the so-called foil method. 

Horanyi G and Rizmayer E M 1984 Radiotracer study of anion adsorption at silver electrodes in acidic medium J. Electroanal. Chem. 176 339-48... 

A correlation between the amount of adsorbed ions and the electrode potential, in particular E. , has been identified apparently for the first time by Frumkin and Obrutschewa [26Fru]. A minimum of ionic adsorption was found at E, this is equivalent to the absence of specific adsorption at Ep c- The measurement of the amount of adsorbed ions was performed by measuring the ionic concentration in the solution as a function of the electrode potential or by measuring the surface concentration of adsorbed ions by e.g. radiotracer techniques (see also 4.2). (Data obtained with this method are labelled lA). 

Figure 9.5 Anion adsorption on Pt in several electiolytes bisulfates and TFMSA as determined by radiotracers (RT), and OH and H as reflected in A/r, the ampUtiide of the XANES peak. The effect of 6 M TFMSA suppressing OH formation can be seen. (Reproduced with permission from Teliska et al. [2007].)...

Corrigan DS, Krauskopf EK, RiceLM, Wieckowski A, Weaver MJ. 1988. Adsorption of acetic acid at platinum and gold electrodes— A combined infrared spectroscopic and radiotracer study. J Phys Chem 92 1596-1601. 

Such analytes require carefully chosen extraction conditions in terms of pH, solvent composition and technique. Also, these analytes tend to become lost by adsorption on (glass) surfaces or undergo conjugation so that a chemical or enzymatic deconjugation step may be required. Often only the use of radiotracers... 

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. ... 

Adsorption of TU on active and passive iron electrodes was studied by Bockris et al. using radiotracer and Fourier Transform Infrared Spectroscopy techniques. The high values of AG° (the standard states 0° = 0.582 and c° = 1 mol T ) equal to -17.20 kJ/mol and -18.98 kJ/mol have been obtained for the active and passive iron electrodes, respectively. They are markedly higher than those obtained for other metals (Table 1). 

The influence of the metal on electrosorption was also considered in the adsorption of aromatic compounds. Extensive studies of adsorption of different aliphatic and aromatic compounds on a polycrystalline Pt electrode in O.OIMHCI solution were done by Bockris etal. Using different techniques (radiotracer, FTIR, and ellipsometry), they were found that aromatic molecules are adsorbed parallely to the electrode surface and that the potential dependence of adsorption is symmetrical around the pzc. It was concluded that the dependence of adsorption on the potential is mostly determined by the interaction of water with the Pt surface. 

The adsorption of sulfate was confirmed by radiotracer technique as shown in Fig. 14, which clearly shows that the sulfate adsorption feature depends on crystal faces, as reviewed in Ref. 17. 

A recent paper by Lairdinvestigated the efficacy of HPAM flocculation of kaolinite, illite and quartz by carrying out visible absorption experiments. He concluded that HPAM more effectively flocculates kaolinite than quartz or illite. This was also the conclusion of previous work by Allen et al. who studied the adsorption of HPAM onto kaolinite, quartz and feldspar at various HPAM concentrations and solution pH by X-ray photoelectron spectroscopy (XPS). Much of the previous work on polyacrylamide adsorption onto aluminosilicates monitored the adsorbed amount by viscometry, carbon analysis and radiotracer techniques. These methods rely on following adsorption by subtraction from that detected in solution. 

In the last few decades, the study of adsorption phenomena at nickel electrodes by a radiotracer method has been the subject of several studies (see [34-40] and literature cited therein). M ost of these studies were carried out at smooth electrodes however, in some cases, nickel powder and Raney-nickel catalysts were used. 

A radiotracer method was elaborated for the study of the adsorption phenomena on nickel electrodes formed by electrodeposition of nickel ( nicke-lized nickel electrode). Adsorption studies were carried out with labeled HCl, Fi2S04, H3PO4, thiourea, phenylacetic, and acetic acids in 0.1 mol dm HCIO4 supporting electrolyte. The potential dependence of the adsorption of these species was determined in the potential range —200 to +200 mV vs. relative hydrogen electrode (RHE). It was found... 

It was demonstrated that the radiotracer method, using labeled anions, is an adequate tool to follow anion adsorption in the course of voltammetric measurements and to gain simultaneous information on hydrogen and anion adsorption [163]. Coupling voltammetric and radiometric measurements in the study of platinized platinum electrodes gave insight in the anion-hydrogen atom coadsorption process. 

The specific adsorption of H2P04 ions that accompany the UPD of Zn + ions on Pt (platinized electrodes obtained by Pt electrodeposition on the gold-plated plastic foil) electrodes was studied by using a radiotracer method with P phosphate species in the presence of a great excess of C104 ions [196]. It was found that the specific adsorption of species induced... 

The same authors [197] have studied the specific adsorption of other anions, which accompany the Zn UPD on Pt electrodes. Again, radiotracer technique with S-labeled sulfate and Cl-labeled chloride ions has been used. It was found that sulfate ions adsorb on the top of Zn adatoms, and the adsorption strength of Cl ions on the Zn adlayer is significantly lower than that of HS04 (S04 ) ions. 

Zelenay etal. have applied CV and radiotracer technique to study adsorption of benzoic acid from 0.1 M HCIO4 solution on a pc-Au electrode prepared by electroplating [286]. Chemisorption occurred within the entire potential range... 

Adsorption of chlorobenzene on pc-Au electrode has been studied by Czerwinsld and Sobkowski [316] as early as in 1980, using a radiotracer technique. The rate-determining step of this adsorption was diffusion of the reactant. Chlorobenzene was adsorbed in multilayers at sufficiently high bulk concentration. The adsorbed molecules were probably oriented vertically with respect to the Au surface and bounded to it via a Cl atom. 

Fig. 6.94. Comparison of adsorption properties of different electrode surfaces. Bisulfate adsorption as a function of electrode potential on different platinum planes (110), (111), (100) and on polycrystalline platinum. Data obtained by the radiotracer technique. (Re-printed from Y.-E. Sung, A. Thomas, M. Gamboa-Aldeco, K. Franaszczuk and A. Wieckowski, J. Electroanal. Chem. 378 131, copyright 1994, Figs. 14 and 15, with permission from Elsevier Science.)...

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