Anion exchange studies. XXVIII. Comparison of diffusion in anion exchange resins and quaternary amine solutions

Abstract

Diffusion coefficients of ions in crosslinked organic ion exchangers are generally one or more orders of magnitude smaller than in dilute aqueous solutions. Since it has become useful to consider crosslinked ion exchange resins as concentrated electrolytes, one wonders to what extent the properties of concentrated electrolyte solutions may account for these low diffusion rates in resins.We have studied t,he diffusion of t,racer bromide ( B F , ti/, = 35.9h) in solutions of benzyltrimethyl-ammonium chloride, an amine salt arbitrarily selected because of its availability and high solubility, as a "model electrolyte" for resins of the quaternary amine polystyrene--divinylbenzene type. The results are compared with the diffnsion of tracer bromide in Ilowex-1 resin crosslinked with 8% DVB. Although benzyltrimethyammonium chloride (abbreviated BCl) is not strictly the repeating unit of the resin network, the data presented below show that its solutions represent a reasonably good model for diffusion in the resin.Diffusion coefficients in the resin, D R~(~) , were determined by a shallow bed elution technique.' Elution was carried out a t sufficiently high supporting electrolyte concentrations and flow rates so that diffusion through the resin particle may be assumed to tie the rate controlling step. Diffusion coefficients in the aqueous phase, D B~? were determined by a. modification of the frit method.2 Small porous porcelain slabs (ca. 0.5 X 8 X 12 mm.) were equilibrated with solutions containing BrB2 and rapidly eluted with the same solution not containing tracer. From the decrease in counting rate of t.he slabs with time, Dflr was computed2 after calibration of the slab with an electrolyte (NaBr) of known diffusion rate.The diffusion coefficients found in the resin as a function of aqueous LiCl concentration (~L~c I , where 771 represents moles per 1000 grams of water) are shown in Table I. With increasing LiCl concentrat,ion decreases rapidly from 8.2 X lo-' cm.l/ser. a t ~L~C I = 1 to 1.6 X 10-8 cm.2/sec. at n i~i c i = 16. The value in pure water, Dir = 2.07 X cm.l/rc., calmlated from the limiting equivalent conductanre of bromide ion ( A i r = 78.14 X lo6 ohm-' ~m .~/ e q u i v a l e n t )~ and the Nernst limiting equation, is considerably larger, as expected.The decrease of D B~(~) with increasing m L i c i in the aqueous phase is paralleled by invasion of the. resin by LiC1, i.e., by an increase in the total chloride concentration of the resin. At low m r x i invasion is negligible; it increases with increasing LiCl concentration, and a t high i n~i c i the concentration of LiCl in the resin may exceed that of the exchange sites. For example, the 8% I>VB resin equilibrated with a 16 molal LiCl solution contains, per 1000 g. of water in the resin, 16.8 moles of LiCl, 9.0 moles of exchange sites, or a total of 25.8 moles of C1-.Diffusion coeffirients, DB,, of trarcr tiromide in the aqueous phase were measured in various BCl-LiCl mixtures having compositions similar to those of the resin immersed in LiC1 solutio...

Accelerated Analyte Uptake on Single Beads in Microliter-Scale Batch Separations Using Acoustic Streaming: Plutonium Uptake by Anion Exchange for Analysis by Mass Spectrometry

“…Alternatively, the diffusion layer thickness can be calculated from k observed and values for the other parameters. Although eq is strictly valid for pure isotopic exchange between a solution and an ion-exchange resin, it can be applied to the exchange of trace components . In our experiments we are exchanging divalent Pu(IV)(NO 3 ) 6 2− complexes for NO 3 − anions, the composition is not at equilibrium, and the concentration of the analyte in solution declines as the system moves toward equilibrium.…”