Electrochemical Processes Neutrons

Fig. 2. 22. The radial distribution functions andflicici(r) fora4.35 /Wsolutionof NiClg in D2O (neutron results). (Reprinted from J. E. Enderby, Techniques for the Characterization of Electrodes and Electrochemical Processes, in R. Varma and J. R. Selman, eds The Electrochemical Society Series,...

Understanding of the modification from bulk liquid water behavior when water is introduced into pores of porous media or confined in the vicinity of metallic surfaces is important in technological problems such as oil recovery from natural reservoirs, mining, heterogeneous catalysis, corrosion inhibition, and numerous other electrochemical processes. Water in porous materials such as Vycor glass, silica gel, and zeolites has been actively under investigation because of their relevance in catalytic and separation processes. In particular, the structure of water near layer-like clay minerals [11,12], condensed on hydroxylated oxide surface [13], confined in various types of porous silica [14-22] or in carbon powder [23] has been studied by neutron and/or x-ray diffraction. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of /ton-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review. 

Analytical techniques used for clinical trace metal analysis include photometry, atomic absorption spectrophotometry (AAS), inductively coupled plasma optical emission (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). Other techniques, such as neutron activation analysis (NAA) and x-ray fluorescence (XRF), and electrochemical methods, such as anodic stripping voltammetry (ASV), are used less commonly For example. NAA requires a nuclear irradiation facility and is not readily available and ASV requires completely mineralized solutions for analysis, which is a time-consuming process. 

Fundamentals. Neutrons can interact with matter in several ways. Depending on the neutron-nucleus interaction, they can be scattered coherently or incoherently and both processes can occur elastically or inelastically. For structural studies in electrochemical systems, diffraction, i.e. elastic coherent scattering, is of particular interest. Fundamentals of these modes of interaction, including spectroscopic aspects relevant for mobility studies, have been reviewed [989]. 

Radiochemical methods, such as tracer methods [1-3], Mossbauer spectroscopy [4], neutron activation [5], thin layer activation (TLA) [5], ultrathin layer activation (UTLA) [5], and positron lifetime spectroscopy [6], are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows adsorption and electrosorption occurring on the surface of electrodes the role of electrosorption in electrocatalysis deposition and dissolution of metals corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes... 

---