Rotating electrode cells

Rotating electrode cell Wilson Process Systems Rotating cylindrical foil (usually stainless steel) or static cylindrical foil with rotating anode (larger cells) Discontinuous by manual scraping or flexing No V V ... 

The rotating electrode cell employs a working electrode which is rotated about its axis. Because the electrolyte is stirred in the vicinity of the electrode-electrolyte interface, convective mass transfer occurs, for which the following correlation can be employed ... 

Rotating electrode cells. Cells for industrial applications which utilise rotation of the cathode are either discs or cylinders. They can produce the metal deposit as a powder or a particulate which enables the cell to be operated in a continuous mode when the powder is displaced freely from the electrode surface. 

A typical 5 kA Eco cell has a cathode drum with a radius of 0.37 m, a height of 0.74 m and a cathode-membrane gap of about 1 cm. The cathode is rotated at 100-200 rev.min-1. In rotating-cylinder electrode cells, high fractional conversion can be obtained by employing an Eco cascade cell. 

There are a lot of further innovative cell constructions in the literature that may also be suitable for electroorganic syntheses, from laboratory up to industrial scale. Examples are rotating electrodes, application of ultrasound, and packed or fluidized particle bed three-dimensional electrodes for increasing the active electrode area and enhancing the mass transfer. A short overview is given, for example, in [1,2]. 

Ebel et al. have used a microliter vessel in the voltammetry and polarographic determination of small sample volumes of chlorpromazine [166]. The concentration of cells in glass or PTFE was described for use with a dropping-mercury electrode (sample volume 180 pL), a rotating disc electrode (sample volume 1 mL), or a stationary vitreous-carbon electrode (sample volume 80 pL). Chlorpromazine was determined using oxidative voltammetry at a 3 mm vitreous-carbon or a rotating electrode. 

Use of twin electrode cell [103] or rotating ring disc electrode [104] techniques permit the independent separation of the charge and amount... 

The cell for rotating electrodes, Fig. 7, is usually cylindrical and surrounded by a water jacket for thermostatting purposes, but as long as the cell walls are more than 1 cm or so from the rotating assembly, there are usually no cell edge effects. The auxiliary electrode is very often contained in a separate compartment behind a glass frit in order to avoid contamination problems. A Luggin capillary, where required, can be positioned in various ways unless it is more than 0.5 cm from the electrode, it must be placed under the centre of the disc in order to avoid a non-equipotential surface this can cause some problems with disturbance of the fluid flow. 

Fig. 7. Typical rotating ring—disc electrode cell. A, rotating ring—disc electrode B, reference electrode with Luggin capillary C, counter electrode D, teflon lid E, porous frit F, thermostatted water jacket.

Rotating electrodes characterize the BASF cell of Figure 19.19(b), which is used for making adiponitrile. The cell described in the literature has 100 pairs of electrodes 40 cm dia spaced 0.2 mm apart. The rapid flow rate eliminates the need for diaphragms by sweeping out the oxygen as it is formed. 

Voltammetry experiments are not often performed in flow cells for analytical purposes. One reason for this is the special problem of ohmic potential losses (iR drops) at an electrode in a confined stream. Another reason is the problem of precisely pumping solution at a carefully controlled velocity. In general, rotating electrodes are more easily controlled and do not involve serious plumbing problems. On the other hand, flow cells operated at a fixed potential (i.e., at one point along the steady-state voltammetric curve) are eminently useful for electrosynthesis, chromatographic detection, and automated analysis systems. These features will be described in later chapters. 

The cells shown in Figures 9.3 and 9.5 can be readily adapted for use in rotating-electrode experiments. Clamping the cell and top into position and drilling the cap to allow free rotation of the electrode shaft is required. Constant-speed rotating-electrode experiments usually have analytical objectives (e.g., amperometric titrations), and the cell volume may be any convenient size ranging upward from about 0.5 mL, as demonstrated in a study of sample volume requirements in rotating-electrode voltammetry [13]. 

Pine Instrument Company 101 Industrial Dr. Grove City, PA 16127 Instrumentation, electrodes, cells, accessories rotating electrode equipment... 

Comparable or larger errors are introduced by unwanted convective mass transport. Convection is caused by physical motion of the solution, sometimes purposefully introduced for techniques such as rotating electrode voltammetry. When a quiet solution is desired, however, convective errors may arise at longer experiment times (slow scan rates) from mechanical vibrations of the solution. Convection is a particular problem for cells inside inert-atmosphere boxes, on which fans and vacuum pumps may be operative. Convection raises the current... 

Rotating cylinder electrode cell High High Small... 

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