Flat cells

It is especially useflil for liquid samples in flat cells, which may extend through tlie entire height of the cavity. In the cylindrical cavity a TEq mode is frequently used because of its fairly high g-factor and the very strong along the sample axis. 

Fig. 2. Cutaway view of a Leclancbir flat cell used ia multicell batteries (1).

In most cylindrical carbon—zinc cells, the zinc anode also serves as the container for the cell. The zinc can is made by drawing or extmsion. Mercury [7439-97-6J has traditionally been incorporated in the cell to improve the corrosion resistance of the anode, but the industry is in the process of removing this material because of environmental concerns. Corrosion prevention is especially important in cylindrical cells because of the tendency toward pitting of the zinc can which leads to perforation and electrolyte leakage. Other cell types, such as flat cells, do not suffer as much from this problem. 

Endothelium Layer of flat cells lining blood vessels. 

Air space thickness 25 mm minimum Air space in cavity-wall construction Air space between tiles and roofing felt on pitched roof Air space behind tiles on tile-hung wall Loft space between flat celling and pitched roof lined with felt... 

Loft space between flat celling and pitched roof of aluminium cladding, or low-emisslvlty upper surface on celling... 

Loft space between flat celling and unsealed fiber cement or black metal cladding to pitched roof Air space between fiber cement or black metal cladding with unsealed joints and low-emlsslvlty surface facing air space... 

The electricity-producing system of electric fishes is built as follows. A large number of flat cells (about 0.1 mm thick) are stacked like the flat unit cells connected in series in a battery. Each cell has two membranes facing each other. The membrane potentials of the two membranes compensate for each other. In a state of rest, no electrostatic potential difference can be noticed between the two sides of any cell or, consequently, between the ends of the stack. The ends of nerve cells come up to one of the membranes of each cell. When a nervous impulse is applied from outside, this membrane is excited, its membrane potential changes, and its permeability for ions also changes. Thus, the electrical symmetry of the cell is perturbed and a potential difference of about 0.1 V develops between the two sides. Since nervous impulses are applied simultaneously to one of the membranes in each cell, these small potential differences add up, and an appreciable voltage arises between the ends of the stack. 

In an alkaline solution, the cuticle—the outermost layer of a strand of hair—swells up, softens, and becomes rougher. The cuticle is made up of translucent, flattened cells that line the hair shaft like shingles on a roof. The cuticle gets rougher when the cells do not lay flat. When the raised cuticle cells of one piece of hair get stuck on the raised cuticle cells of another piece of hair, the hair tangles. The raised cells also reflect light differently than smooth, flat cells, making the hair appear dull. 

An alternative solution to the dielectric water problem that applies in particular to standard TE102 rectangular resonators, is to use a flat cell a rectangular sample holder that is oversized in the vertical direction, and with one small cut-through dimension, say 1 mm, and the other dimension circa 10 mm, that is, approaching the bore of the cavity access holes (Figure 3.11). When in place in the resonator, this cell is rotated about its vertical axis to a position such that the 10 mm dimension points to a direction of minimal /i-field. 

FIGURE 3.11 Aqueous-solution cells. Two different flat cells are shown one is mounted with special clamps in a rectangular X-band resonator (the body has been opened for better view). Flat cells must be turned in the resonator to a position of minimal fq-field. 

The electrochemical cells employed in epr measurements are generally fabricated out of silica since glass contains paramagnetic impurities that can interfere with the signal. The need to minimise the interaction of the sample with the electric component of the radiation in the cavity, and the geometry and size of the cavity itself, have generally led to electrochemists employing flat cells that are c. <0.5mm thick. 

For qualitative investigations there is considerable latitude in experimental procedure. There are few limitations on solvent for reactions studied in the liquid phase, although standard considerations of susceptibility to radical attack must obviously be taken into account. With polar solvents it may be desirable to replace the normal silica sample tube with a flat cell, although spin-adduct concentrations are usually sufficiently great for this to be circumvented by the use of capillary tubes. 

Electrokinetic Measurements. Electrophoretic mobilities were measured with a flat-cell apparatus manufactured by Rank Brothers, Cambridge, England. In addition, several mobility values were checked for accuracy with a Zeta Meter, New York. Mobilities were determined with a small volume of the suspension (approximately 25 cc) that had been prepared for the adsorption experiments. The pH of the solution was measured prior to determining the electrophoretic mobilities, which involved measuring the velocities of five to ten particles in each direction. An average value of the mobilities was recorded. Samples containing the flocculated particles were dipped into an ultrasonic bath for approximately one second prior to making the pH and mobility measurements. 

Epidermal tissue of plants consists of flat cells, usually containing no chloroplasts, with a thick outer wall covered by a heavy waxy cuticle about 2 pm thick. Only a few specialized cells are found in the epidermis. Among them are the paired guard cells that surround the small openings known as stomata on the undersurfaces of leaves and control transpiration of water. Specialized cells in the root epidermis form root hairs, long extensions ( 1 mm) of diameter 5-17 pm. Each hair is a single cell with the nucleus located near the tip. 

Nonetheless, many significant EPR studies have been accomplished with flat cells. Most commercial versions are fabricated of quartz, since Pyrex is lossy and contains paramagnetic impurities that can be detected at the high modula-... 

The optimum orientation of the flat electrochemical cell is in the center of the rectangular cavity, so that the sample is in the region of the maximum magnetic field. In this configuration, the face of the flat cell is parallel to the end plate of the cavity. Usually the cell position along the length of the cavity... 

Figure 29.15 Commercial flat cell for internal generation of radical ions. [Courtesy of Varian Associates, Palo Alto, CA.]...

Figure 29.17 Simulation results showing the fraction of the total current at each of 25 segments of the working electrode at different times during the constant-current electrolysis of 2.5 mAf anthraquinone and in 0.1 M tetrabutylammonium iodide-dimethyl-formamide solution. Total current 100 / A electrode dimensions 0.5 cm x 3 cm flat cell dimensions 0.5 cm x 0.9 cm x 3 cm.

Because of the difficulties described earlier, electroanalytical studies are usually performed separately from radical generation studies. But a flat cell has been designed [26] (Fig. 29.19) to permit simultaneous monitoring of the electrochemical and EPR response of a free-radical system (SEEPR). The auxiliary electrode extends along the edges of the working electrode, which diminishes the problems of iR drops and provides better uniformity of current density than is possible with conventional electrode placement. This cell is used primarily for short-term (on the order of seconds) electrochemical experiments, such as... 

Figure 29.19 Flat cell used to minimize iR losses.

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