Radiotracer adsorption technique

In the case of indirect radiotracer methods, instead of labeling the species to be studied another adequately chosen labeled species (the indicator species) is added to the system and the adsorption of this component is followed bythe usual radiotracer measuring technique. The adsorption of the indicator species should be related to that of the species studied. The nature of this link could vary in different systems. 

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. 

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). 

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... 

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. 

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.)...

Fig. 6.117. Adsorption isotherms for various organic species on platinum in 0.01 N HCI solution, obtained with the radiotracer technique valeric acid (a), butanol ( ), benzoic acid (a), naphtoic acid ( ), naphtol (O). All the data are taken at the maximum adsorption potential. (Reprinted from J. Jeng, dissertation, Texas A M University, 1991.)...

The radiotracer technique offers a unique possibility of demonstrating the occurrence of -+ specific adsorption... 

In spite of several uncertainties, this method was found to be useful for the study of the adsorption of potential fuels on platinum electrodes. A comparative study of the adsorption of benzene on platinum by the radiotracer and the electrochemical technique showed reasonably good agreement, as seen in Fig. 9J. 

The most significant sources of error in trace element analysis are contamination or losses by adsorption or volatilization. The key property of radiotracers with respect to the investigation of the accuracy of analytical techniques is the emission of easily detectable radiation in any stage of an analytical procedure with extraordinarily high sensitivity. 

One of the most useful experimental methods to be applied to protein adsorption in recent years is the radiotracer technique (Mura-matsu, 1973). Proteins labeled with, 31I and 125I (Brash et al., 1974) and [14C]acetyl derivatives of proteins (Phillips et al., 1975) have been used as tracers. As well as measuring adsorption directly, this method has the great advantage that it can detect exchange between interface and bulk even when the total amount adsorbed does not vary. A technique that has been used to obtain independent measurements of the amount of protein adsorbed by measuring film thickness is ellipsometry (Trurnit, 1953). 

The nature and structure of surface intermediates in hydrocarbon adsorption has been investigated using galvanostatic (constant current) and potential sweep techniques (7, 10, 172-174 or radiotracer methods (175. Niedrach s (172, 173 galvanostatic results with C1-C4 alkanes and with ethylene indicate the existence of common, partially oxidized surface species, despite differences in the initially adsorbed hydrocarbons. Methane adsorption is very slow, but higher saturated hydrocarbons adsorb faster and at similar rates. Potentiostatic adsorption followed by an anodic potential sweep gives two peaks [Fig. 14 (174 corresponding to different adsorbed species. The intermediate responsible for the peak at low potentials (0.7-... 

Horanyi, G. and Joo, R, Application of the radiotracer technique for the study of the specific anion adsorption on AI2O, in acidic medium, J. Colloid Interf. Sci., 231, 373, 2000. 

Among the ex situ methods that can be employed in surface analysis, low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) can give the crystal structure and the nature of the surface ad-layers after the electrochemical and adsorption experiments as explained in this chapter [31,32]. Among the in situ non-electrochemical techniques, the radiotracer method [33] gives information about the adsorbed quantities however, infrared spectroscopy in FTIR mode [34] allows the identity of the bonding of the adsorbed molecules, and finally ellipsometry [35] makes possible the study of extremely thin films. Recently, some optical methods such as reflectance, x-ray diffraction, and second harmonic generation (SHG) [36] have been added to this list. 

There are many other experimental method for studying the dynamics of adsorption at liquid interfaces. First of all, many other techniques exist to measure dynamic surface and interfacial tensions. Only a subjective selection of some more experimental developments are given in the following section. Moreover, other than surface and interfacial techniques are discussed in this chapter too, such as radiotracer, ellipsometer, electric potential, and spectroscopic methods. 

Beside the very frequently used methods of dynamic surface and interfacial tension measurements, adsorption kinetics processes at liquid interfaces can also be studied by other methods, such as dynamic surface potentials, ellipsometry and other light scattering and reflection methods, X-ray techniques, neutron scattering, radiotracer techniques. These methods yield more or less relative information on the change of adsorption with time at different time resolutions. 

One of the older techniques for measuring directly the adsorbed amoimt of surfactant molecules or polymers at liquid interfaces is the radiotracer technique. Its idea is the measurement of the radiation emitted by radio-labelled molecules, adsorbed at an interface (Sally et al. 1950, Flengas Rideal 1959). Because of the background radiation the method yields relative data only. Using equilibrium adsorption isotherms, the dynamics of adsorption can also be followed by the radiotracer method. Experiments were performed with various surfactant systems (Matuura et al. 1958, 1959, 1961, 1962, Tajima 1970, Konya et al. 1973, Muramatsu et al. 1973, Okumura et al. 1974) and adsorbed polymers (Frommer Miller 1966, Adams et al. 1971, Graham Phillips 1979b). Due to the development of more efficient methods the use of this technique has been reduced. 

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