Bulk methods

Although there are only three principal sources for the analytical signal—potential, current, and charge—a wide variety of experimental designs are possible too many, in fact, to cover adequately in an introductory textbook. The simplest division is between bulk methods, which measure properties of the whole solution, and interfacial methods, in which the signal is a function of phenomena occurring at the interface between an electrode and the solution in contact with the electrode. The measurement of a solution s conductivity, which is proportional to the total concentration of dissolved ions, is one example of a bulk electrochemical method. A determination of pH using a pH electrode is one example of an interfacial electrochemical method. Only interfacial electrochemical methods receive further consideration in this text. 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

In a bulk method of analysis there is little need for concern over the number of results necessary to achieve a representative... 

The molecular beam method has an advantage as compared to bulk method because translational energy can be controlled in the molecular beam method. However, in bulk method the reactant molecules can be formed in much wider range of vibrational and rotational states. 

A slight increase in oxidation state is detected by DR UV-vis spectroscopy after 1500 min. However, it was not detected when the experiment was repeated under nitrogen rather than in air. Oxygen is likely to dissolve in the solvent, oxidizing the VPO particles surface, which is not captured by the bulk method titration. 

The absolute methods (meaning no independent calibration is necessary) for determining H20t include manometry (Newman et al., 1986), Karl-Fischer titration (Behrens et al., 1996), and nuclear reaction analyses (NRA). NRA is a bulk method for H20t and requires a density estimate to convert data to weight percent. Secondary ion mass spectrometry (SIMS) can measure H20t, but it requires... 

After the experiment, the experimental charge is prepared for analysis of the diffusion component or species. The analytical methods include microbeam methods such as electron microprobe, ion microprobe, Rutherford backscatter-ing, and infrared microscope to measure the concentration profile, as well as bulk methods (such as mass spectrometry, infrared spectrometry, or weighing) to determine the total gain or loss of the diffusion component or species. Often, the analysis of the diffusion profile is the most difficult step in obtaining diffusivity. 

For minerals, if diffusion is anisotropic, the bulk method gives only an average diffusivity for an assumed effective shape, but cannot determine the diffusivity along different crystallographic directions. The profiling method is necessary to quantitatively resolve the anisotropy. 

Surface type and surface Surface structure, including bond length [and layer spacing] relaxations, relative to bulk Method References Comments... 

Hydrolysis procedures have been, developed for the removal of the acetal protecting groups without accompanying crosslinking of polydiene backbones. Dilute solution hydrolyses are preferred over bulk methods ( ). (Eq. 6-9). 

Photoelectron spectroscopy (ESCA) and thermal evolved gas analysis (EGA) have been applied to characterize sulfur- and nitrogen-containing species in atmospheric particulate matter. Particulate amines and amides previously identified only by ESCA have been detected by EGA, a bulk method, for the first time. EGA and ESCA results suggest the existence of a sulfate similar to ammonium sulfate but with some of the ammonium ions replaced by a charged organic nitrogen complex. 

The supports themselves are usually made by the bulk methods. Examples of impregnated catalysts indude the Pd/C hydrogenation catalysts, the Pt/Sn/Al203 dehydrogenation catalysts and the automotive three-way catalysts. There are also catalysts made by hybrid synthesis approaches, e.g., where the active precursors are mixed with a powdered support, and the mixture is then agglomerated. 

His measurements were made on "bulk" coal fragments extracted from the vicinity of pyrite particles. Electron-optical methods, though, can make such measurements in situ and have better spatial resolution than his bulk methods allowed. 

It is fair to say that over the last 8 years there has been a significant growth in microfluidic-based methods for synthesizing nanoscale materials. Significantly, such materials are of a quality that matches or exceeds materials produced using traditional bulk methods. Furthermore, the continued development of integrated multicomponent systems will create invaluable tools for improving both the properties and yields of this important class of materials. 

The transfer system is capital- and power-intensive compared with both the traditional and bulk methods. It also requires more space and labor than does the bulk method and less than the traditional method that employs hand riddling. Mechanical riddling reduces space and labor requirements. 

Due to its high surface area, surface chemistry and physics dominate the properties of fumed silica. The O—Si-O being 0.3 to 0.4 nm let estimate about only 20 silicon dioxide units spanning the diameter of a primary particle of amorphous silica. Fumed silica therefore has an extremely high surface to bulk ratio up to about 10 %. This is why even bulk methods of chemical analysis are suitable to follow chemical reactions on its surface elemental analysis, IR or NMR methods, etc. 

Irreversible redox reactions result in unrecoverable samples in bulk methods. 

Spectroelectrochemistry has become a valued technique coupling spectroscopy and electrochemistry. Spectroelectrochemistry is a bulk electrochemical technique and as such many of the cell requirements discussed above that pertain to BE apply for spectroelectrochemistry. Often concentrations for spectroelectrochemistry are much lower than most electrochemical techniques due to the spectroscopic absorbance requirements. The bulk solution must still be oxi-dized/reduced in spectroelectrochemistry. Large surface area working and auxiliary electrodes are employed as in the bulk methods described above. Cells designed with optically transparent electrodes like thin films of Sn02 or In203 or optically transparent mesh electrodes are employed, otherwise the electrode must be manually removed to record spectra. Optically transparent electrodes can be constructed such that the solution volume to electrode surface area ratio is very small making the BE occm rapidly. 

The technique is limited at high velocity, when the characteristic time of the data acquisition system is comparable to 1/v, leading to a decrease of the signal to noise ratio. It is also limited at low velocities, when the bleached pattern relaxes by diffusion faster than the appearance of the oscillations due to the flow. The easily available range is typically lO pm/s high velocity limit is reached, we switch to a bulk method which uses another bleaching pattern only one beam (width - 50 Xm) is shined in the sample, at an angle smaller... 

Figure 2. 2.a) Principle of the bulk method a line is first bleached through the sample (a), and then deformed by the flow. It is tilted without slip (b) or both tilted and translated with slip (c). The translation distance and the velocity profile are characterised through the deformation of the fluorescence intensity profile monitored when the distorted bleached line is translated back and passes in front of the laser beam (2.b). 

Another feature that has not been systematically covered concerns additional means of determining properties of adsorbates. Examples here are the classical spectroscopies, with their surface variants (secs. 1.7.10-12), reflection methods, including elllpsometry, reflectometry and evanescent wave studies, NMR. X-ray analysis, neutron diffraction and dielectric spectroscopy. The theory of the last mentioned phenomenon for bulk phases has been discussed in sec. I.4.5f if applied to adsorbates, the technique can give information on the various degrees of freedom that polar molecules may have, say, for water adsorbed on oxides. For thicker water layers containing ions, measurement of the surface conductivity may yield additional information see also sec. I.6.6d. The reason for not systematizing these techniques is that we do not consider them typically "surface methods, but rather surface variants of bulk methods. 

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