Thin Disk Volume Element

As a basis, a differential element of volume, dVr = dxdydz, or time, dt, is identified. In cylindrical geometry, the element of volume may be a thin disk, Adz,. if changes occur only in an axial direction, or a ring 2-rrrdrdz, if changes occur both radially and axially. 

Because of the plug-flow assumption, it is natural to take a thin disk for the reactor volume element as shown in Figure 4.19. The concentration does not change over the volume element because there is complete mixing in the radial and angular directions and because the axial distance Az is small. The element has volume AV = A Az in which Ac is the tube cross-sectional area. Writing Equation 4.2 in this situation gives... 

Electrodeposition is the preferred method for preparing a thin, smooth, uniform, and stable source for counting, as discussed in Section 3.7. Empirically developed procedures are applied to the heavy elements in aqueous solution because they cannot be reduced to the metal the source is deposited at the cathode in what is believed to be hydroxy forms after partial reduction. Deposition is in acid solution, under various conditions that yield near-quantitative recovery from the solution. Test results specify the volume, reagent content, applied volts and amperes, and optimum time period (see Section 6.4.1). The cathode can be a platinum disk, but a polished stainless steel disk is satisfactory. 

---