KINETICS OF ION EXCHANGE IN A CHELATING RESIN
Oct., 1961
182 1
KINETICS OF ION EXCHANGE I N A CHELATIXG RESIN BYRICHARD TURSEAND WM. RIEMAK I11 Ralph G. Wright Chemical Laboralory, Rutgers, The S u e University, New Brunswick, New Jersey Received January $0,1961
Failure to obtain satisfactory chromatographic separations of metals with a chelating cation-exchange resin, Dowex A-I, led to a study of t.he kinetics of ion exchange with this resin. A modification of the limited-bath technique of Kressman and Kitchener was used. The experimental results can be explained only by the assumption that the chemical exchange reaction (not diffusion) is the slow step.
An investiga.tion of chromatographic separation of metals with the chelating resin Dowex A-1 was undertaken in this Laboratory. The resin’ consists of the well-known crosslinked polystyrene matrix with --CH2N(CH2COOH)2 as the functional group. The behavior of this resin in elutions of non-chelating (cations is fairly similar to that of Dowex 50. For example, the elution of ammonium chloride with 0.5 M potassium chloride as eluent a t a. flow rate of 0.3 em. per min. gave a nearly Gaussian graph with an average of 18 plates per em. On thc other hand, the elution of :magiieisium chloride with 1 A ! sodium chloride a t a comparabie llow rate gave a badly tailing graph wit,h calculated plate numbers of 16 and 2.0 per em.. on the ascending and descending slopes, respectively. Cations that chelate with the iniinodiacetate group more strongly than magnesium ga’r-e evctn worse graphs. Separations of such metals by elution chromatography were entirely unsatisfactorg. Attempts to separate such metals by dispiacemcnt chromatography were equally unsuccessful. This led to the suspicion that the rates oi ion exchange of chelating metals with this resin were abnormally slo.n, and a study of the kinetics was undertaken , There is ample evidence in the lit’erature to indicate that the rate of ion exchange, where chelation is not involved, is governed by the diffusion of thc ions either within the resin particle (usually for solutions above 0.1 M ) or in the film of solution surroumding the resin particles (usually for solutions helow 0.001 M ) . At intermediate concentratioxis both processes exert an influence. Boyd, et a1.,2 investigated ion-exchange kinetics by the “shallow-bed” technique. They passed a rapid strea.m of electrolyte solution through a shallow bed of resin for a controlled length of time. Thus the concentration of the solution remained essentially constant during the process. liressman and K i t ~ h e n e r ,on ~ the other hand, used the “limited-bath” technique. They lowered a rotating, platinum-gauze cage containing a measured quantity of resin into a solution containing an exactly equivalent quantity of the exchanging cation. They stopped the reaction by simply removing and rinsing the cage. Since the two groups of investigators applied different conditions their equations are different; neverthe(1) “Dowex Chelai.ing Resin A-1,’’ The Daw Chemical Co., 1959, p. 2. (2) G. E. Boyd, A. Adamson and L. S. Meyers, Jr.. J . Am. Chsm. Soc.. 69, 2836 (1947).
W.
(3) T. R. E. Kreseman and J. A. Kitchener, Discussions Perday Soc., 7, 00 (1949).
less, their conclusions are in complete agreement. The kinetic equation for an exchange controlled by particle diffusion under the limited-bath condit i o n is ~~
Qt is the amount of exchange in time t; Qa is the amount of exchange at equilibrium; Qo is the amount of resin (and of solute) taken in the experiment (all quantities in milliequivalents) ; T is the radius of the resin beads in centimeters; and D is the coefficient of diffusion inside the resin bead. Thus a plot of Q t / Q m us. d t s h o u l d be a straight line for exchanges controlled by bead diffusion. The diffusion coefficient of t,he ions concerned can be calculated from the equation for the slope
An approximation iiitroduced in the derivation of equation 1 causes deviations from linearity when Q t / Q m exceeds about 0.6. The kinetic equation for an exchange controlled by a second-order chemical reaction in the limitedbath method4.5is In 2 = 212Qo(Qo
- Qm)t/Qm
(3)
where
Here the Q’S have the dimensions of equivalents per liter, and IC is the second-order rate constant. A plot of log 2 us. t should be linear for a chemically controlled exchange. The rate constant can be calculated from the slope of such a plot S
2kQa(Qo
-- &w)/2.30Qm
(4)
Another very useful method of identifying the slow step in an ion-exchange process is to study the kinetics of the process in two experiments performed under identical conditions except for a difference in the particle size of the resin. According to equation 1, the rate of a diffusion-controlled process is accelerated by a decrease in particle size. On the other hand, equation 3 indicates that the rate of a chemically controlled exchange is independent of the particle size of the resin. Stdl another method of identifying the slow step is to conduct two kinetic experiments under identical conditions except for a change in concentration. (4) A. A. Froat and R. G. Pearson, ”Kinetios and Mechanism, John Wiley and Sons, Ino., New York. N. Y., 1953, p. 174. (5) R. Turse, thesis, Rutgers, The State University. New Bruus wick, N. J., 1960.
RICHARD TCESEAND WM.RIEMANN I11
1822
0
Fig. l.-Partic
2
6 8 10 I2 14 SecondslJz. le-diffusion plot for Dowev 50-X8: reaction MgR? C a + + a t 25".
4
+
0 F l = O O Z t CM 0 F1=0016 CM
Vol. 65
the quantity (milliequivalents) of sodium in the resin. The exchange capacibies of other forms of the resins were calculated from those of the sodium form and the appropriate equivalent weights. Kinetic Experiments.-A modification of the limitedbath technique was used. One-hundred ml. of a standard solution of the electrolyte, about 0.1 M in all cases, was placed in a 250-ml. beaker and stirred atoa constant, rapid rate in a thermostat controlled to 0.01 . dfter thermal equilibrium was attained, a quantity of dry resin, exactly equivalent to the cation in solution, was added rapidly. After the desired time interval, an adequate portion of the solution was transferred rapidly with suction from the beaker through a sintered-glass filter into a flask. A suitable volume of the solution in the flask was subjected to analysis to determine the extent to which the ion-exchange reaction had occurred. The procedure was repeated at other time intervals with fresh portions of solution and resin. I n experiments a t 37.5 and 50" for long time intervals, excessive evaporation from the beaker was avoided by covering it with a metal foil provided with a hole for the stirrer shaft. The determination of ammonium ion was accomplished by titration with sodium hydroxide in the presence of formaldehyde.R Nickel, magnesium, cobalt, calcium, cadmium, manganese and copper were determined by titration with ethylenediaminetetraacetate.' Hydrogen ion was determined by titration with sodium hydroxide.
Results and Interpretation Dowex 50.-In the case of Dowex 50, plots of Q / Q m us. l/t are linear, indicating that these exchanges are controlled by diffusion within the resin beac'a. Figure I is typical of these plots. It is linear to about t'he same value of as found by Kressman and Kitchener. Table I presents the results of these exchanges. Fig. 2.-Partic:le.-diffusion plot for 11011-e~A-1 : reziction 2NaR + Ca,++a t 25". ,4 comparison of experiments 3 and 4 shows that the was more rapid (larger 8) with the Provided that the change in concentration is not finerreaction resin particles. Furthermore, the t,wo exso great as to exceed the limits within which dif- periments yielded the same value of L3, as is to be fusion in the resin is the slow step, the change expected in diff usion-controlled kinetics. -4comshould have no effect on a process controlled by parison of experiment's 5 and 6 confirms these resin diffusion. On t,he other hand, an exchange controlled by a second-order chemical reaction findings. Dowex A-1.-In t,he reaction between the should be markedly affected by a change in ron- sodium form of Dowex A-1 and ammonium ion, centration. good linear plots of Q t / Q m us. d t a l s o were obtained. Experimental Work Furthermore, identical values of D == 18 X Resins.-Dowex Chelating Resin A-1 wae supplied by were calculated from experiments at, 25' with The Dow Chemical Company as spherical beads, mostly of 40-50 mesh. It consists of a croselinked polystyrene rcsins of different average radii, 0.019 and 0.021 matrix with iminodiacetate groups. For purposes of com- (ern. These observations indicate that bead difparison, some kinetic experiments also were performed with iiisiou is the slow step in this exchange reaction. Dowex 50-X8, a sulfonated, crosslinked polystyrene. The ?'htl exchange of the sodium form of Dowex very fine particlea were removed from both resins by slurryA- 1 with calcium ion (numbers 1 and 2 in Table 11) ing the resin in. deionized water, allowing the main portion was studied. The plots of Q / Q m are shown in to settle and decanting the suspension of the fines. Portions of each resin in the sodium form were wet- Fig. 2. Although the approximate linearity of the screened to obtain fractions in each of three ranges of mesh graph supports the hypothesis t,hat diffusion inside size. Twenty randomly selected spheres from each of t~he six fractions then were examined with a micrometer rnicro- t,he resin is t,he slnw step, the fact. t,hat resiris of different particle sizes gave identical graphs is scope to find the mean diameter of each fraction. To convert Dciwex A-1 to any desired form, the usual very strong evidencc that neither bead diffusion nor procedure was as follows: The resin (unless in the sodium tilm diffusion is the slow step. The same dat,a form) wm convert.ed to the hydrogen form by treatment with hydrochloric acid in a column, then to the sodium form bl- :ire plotted as log 2 us. t in Fig. 3. Wot,h the linefilurrying with excess sodium hydroxide in a beaker, and ;Lrity and the coincidence of the graphs support finally to the desired form by treatment in a column with t,he hypothe:sis that the second-order chemical t)he appropriate cialt. Both column treatments w-ere continued unt,il the 'effluent had the same composition as the reaction is the slow step. Even more striking evidence for a chemically influent. A simple column treatment sufficed to convert controlled exchange reaction with Dowex A-I is Dowex 50 into any desired form. The exchange capacities of the sodium forms of the dry resins were det,ermined by drying the resin for 20 hours in tlaeuo a t 60",:putting a weighed portion into a column, passing a sufficient quantity of 0.1 M hydrochloric acid through t,he colurnn, and finally determining both hydrogen :inti chloridr icin:: in t,hr total c.ffliirnt. The diffrrmcr is
( 6 ) W. Rienian. J. D. N e w s and B. Naiman, "Quantitative Analysis," Third Edition, McGraw-Hill Book Co., N e w York. N. Y., 1951, p.
166. (7)F. J. Welcher, "The Analytical Uses of Ethylenediaminetetra-
iicc.tir Aci,l," D. Van Nostrsnd Co., N e w York, X. Y . , 1958.
KINETICS OF 10s EXCHANGE IN .J. CHELATING RESIN
o c t . , 1961
1823
TABLE I KINETIC EXPERIMENTS WITH DOWEX 50-X8 Mean
T,
Temp.,
u x io* d)m S "C. &a 25 0.0970 0.0448 0.135 45 1 NaR NH4+ 0.031 ,0750 .082 2.0 25 ,0926 2 2NaR Ca++ .031 25 ,0896 ,0548 ,073 6.8 3 MgR2 3-C a + + ,031" 25 ,0920 ,0564 ,083 6.8 4 MgRz Ca+* ,027" 50 ,0936 ,0570 ,106 15 5 MgR2 C a + .031" 6 MgFL Ca++ ,027" 50 ,09363 ,0570 ,120 14 The beads of magnesium resin were assumed to have the same radii as the beads of the sodium resin because the actual difference in thlt radii is less than the standard deviation of the measurements. NO.
Reagents
+ + + + +
CIII.
+
TABLE I1 SECOND-ORDER RATECONSTANTS YO.
I 2 3 4 7
ci
7 \j
!I 10 li 12 1:3 11
Reaction
+ (;a++ + Ca" Ca" + (:a++ + 2H+ + 2~ + 2H+ + 2H+
2KaR 2XaR 2NaR $2Nalt hlgIli M~IL? hlgR? MgR?
+
r , ?in.
0.022 ,019 ,022 ,022 ,022 .016 ,022 ,022 ,018
00
Qm
0 0924 0924 0936 ,0936 ,1130 IO51 ,0945 0934 0!Mi
0,0902 ,0902 0912 ,0912 0974 ,0921 .08ci2 0846 0544 ,0548 ,0550 ,0550
Tc,mp., O C ' .
25 25 37. ti
50 25 25 37. 5 50 25
sx
k
10:
t3.68
x
102
18.8 18,s 22.8 34.8 2 . 70 2.81 7.713 15.7 0 .40:;
:3 G 3
4.8;i 7 46 4.25 :3 (i:3 f i 10
13 :3 2 30 2 82 3 . 4G :3 .40
,022 3i.5 .0!13ti .4!JO .022 50 0936 .5!4:3 ,0936 , 5
