Ion exchange accompanied by a neutralization reaction has been analyzed for the general cases where the bulk solution contains a neutral salt and an acid or a base, e.g., [R · H] + (NaCl + NaOH). Both intraparticle and liquid film diffusion are considered. The Nernst-Planck equation was applied for the fluxes of the ionic species in the liquid film and the resin phase with a mobile reaction plane in the liquid film. When the solution contains acid or base more than 50%, the exchange rate is approximated by the case for pure acid or base solution: the reaction plane is located at the solid/liquid interface. When the solution contains acid or base less than 50% (this situation is generally encountered in the case where a neutral salt flows through a mixed bed of OH-form and H-form resins), the mobile reaction plane should be considered. When the ratio of intraparticle diffusion resistance to liquid diffusion resistance defined in the text is larger than 2, the exchange rate can be approximated by intraparticle diffusion control. The larger the ratio, the faster the reaction plane moves to the resin particle surface. Experimental uptake curves for the ion exchange systems [R · H] + (NaCl + NaOH) and [R · OH] + (NaCl + HCl) confirm the above conclusions.
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