Observations on the rate of ion exchange in Dowex 50W resins are reported. These measurements were made under conditions where solid diffusion was the governing phenomenon for six pairs of ions sodium-zinc, sodium-silver, silver-aluminum, zinc-copper, zinc-aluminum, and aluminum-cerium (trivalent). The effects of temperature and of resin cross-linkage were studied with the first system. Exchange in both directions was studied for each pair except the last.Interpretation with a Fick's law model was accomplished, and diffusion coefficients were The kinetic behavior of ion exchange resins is of much theoretical and practical interest. The ultimate aim in the design of a ftred-bed ion exchange installation i s the prediction of break-through curves for a particular set of conditions. Formerly, this was accomplished by the observation of breakthrough curves on a laboratory scale and the interpretation of these curves in terms of a suitable mathematical model to yield the fundamental parameters such as fluid-phase transfer coefficients and solidphase dgusivity. This method has always suffered from the fact that the simpmed assumptions, made so that the appropriate equations could be solved, were in varying degrees unrealistic. The advent of high-speed computers has made possible a reversal in this approach. One can now advantageously measure the pertinent parameters first and let the computer combine these with a minimum of assumptions into a predicted breakthrough curve.One of the fundamental parameters is the dsusivity in the solid phase. The accurate measurement of this property becomes highly desirable, and such measurement was the aim of this work. The experimental method involves EXPERIMENTAL Size SeparationSince diameter of the resin particles is a quantity of great importance in governing the rate of exchange, it was necessary to make a fine cut in particle size. This was accomplished with an elutriation technique in which the resin particles were fluidized by a rising water column. The particles selected were those which flowed over the top of a 2.4-cm. I.D. column but did not flow over the top of a 2.6-em. I.D. column in series with the first. Measurements of the wet, swollen resin diameters were made with a calibrated filar micrometer mounted on a microscope at 50X magnification. The standard deviations in the diameter of a 50-bead sample varied from 4.5 to 8.5% with most sets having deviations of less than 6%. Ian Exchange ApparatusThe ion exchange apparatus is shown in Figure 1. The resin beads were held in a basket, H , 9.6-cm. I.D. x 19.2-cm. high, made of monofilament saran cloth. The weave was roughly equivalent to a 200-mesh screen. The basket was supported on a lucite frame, A, by means of the support rods, J. The frame provided strength and rigidity for the necessary auxiliaries. A baffle, D, made up of 5-mm. O.D. glass rods 15.2-cm. long which connected upper and lower ring plates, extended from the frame into the basket. A feed injector, I(, made of 6-mm. I.D. glass tubing could be introduced thr...