J. F. Mucci, Dorothy E. Spiegel and Robert 1. Stearns Vassor College Poughkeepsie, N e w York
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I I
I
Ion Exchange-Radiochemical Experiment
This ion exchange experiment serves the dual purpose of introducing the undergraduate to the concept of the total exchange capacity of a column while giving him an opportunity to use radioisotopes in a quantitative analysis. A small column (10 cm long and 1.2 crn i.d.) contaiuing the styrene type, snlfouic acid cation exchange resin Dowex 50-X2 (50-100 mesh) is put in the H+ form by use of a 1 M HC1 solution. The column is then washed clean of excess H + and Cl- with distilled water. The influents are introduced into the column by siphon action and the rate of flow is regulated with a Hoffman clamp (see Fig. 1). The column is not allowed to go dry during the entire experiment. A 0.060 M aqueous solution of CsC1, spiked with a trace amount of Cs'3' (half-life 30 years) is allowed to run through the column a t the rate of 1 m1/3 min. Fractions of the effluent are collected in previously weighed and dried test tubes by use of a sample collector (Model 1205, Automatic Fraction Collector, Research Specialties Co., Richmond, California). The volume of each fraction is determined by weighing and assuming that the density of the effluentis 1.00 g/ml. A 6-ml aliquot of each sample is then pipetted into t,he receiving cup of a dip counter (Radiation Counter
348
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Journal of Chemical Education
Laboratories, Inc., Nodel 10301). The geometry of the counting system is kept constant by housiug the receiving cup and couuter in a machiued metal holder. .IBaird-Atomic Model 123 G.M. Scaler is used for the
+-HOFFMAN
R
-0OWEX
~i~~~~ 1. ion
CL&W
U T"B>LG ~
50-XZ
~
~
~
I Curve
05
!
04
:
Figure
2.
"b'; Cr'
Cvrvo..w.Co'+
L*.
Exshonge Eichong.
Plot of c/co verwr volvme far C ~ + a n dCo+'exchongeon Dowex
50-X2 in H+-form.
counting measurements and a 2-in. thickness of lead is used to shield the counter. If c is the number of counts per second from each sample of effluent and co is the number of counts per second from the influent, a plot of c/c0 versus the effluent volume (mid-point of each sample) can be made as shown in Figure 2-A; c and co are, of course, directly proportional to t,he molar concentrations of the effluent and influent, respectively. The area above the curve in the c/co versus volume plot determines the total exchange capacity of the ion exchange column (number of meq per column). Although there are more efficient n~ethods'.~ for determining the total exchange capacity, we have used this one because it fulfills in a simple way the dual purpose of the experiment as stated above.s The column is again put in the H+ form by allowing 1 :lI HCI to flow through the column displacing the Csf. The excess H + and C1- are again washed off with dist,illedwater. At this point a 0.060 Ill aqueous CoC12 solution, spiked with a trace of Co" (half-life 5.3 years), is
' "Dowex : : IOK Exchange," Dow Chemical Co., Midland, hlirhigan, 1058, p. 301. ~ K U N I NROBERT, , 'Ton Exchange Resins." John Wiley & Sons.. Ine... New York. 2nded.. 1958.u.340f. .. 3 General references for this type of experimental measurement are to be found in such texts as ~ ~ E I T E SL., , AND THOMAS, H. C., ".idvanred Analytical Chemistry," MtGraw-Hill Book Co., Inc., New York, 1958; and CHASE, GRAFTON D., "Principles of Radioisotope Methodology," Burgers Publishing Co., Minnespolis, Minn., 1959.
allowed to flow through the column and samples of the effluent are collected as in the case of Cs+. A plot of c!co versus effluent volume for the Co+%exchange is shown in Figure 2-B. The trace amounts of radioactive cesium and cobalt. were determined by adjusting the counting rates co to give about 100 counts per second. Small counting rate corrections for background and resolving time lvere made when appropriate. I t should be noted that in both runs the colum~l initially contained water as well as the resin. The volume of the water remaining in the column is known as the interstitial or "hold up'' volume. This "hold up" volume has been determined for Dowex 50-X2 and is 30.1% of the bed ~ o l u m e . The ~ area above the curve in t,he c/co versus effluent volume plot which is contributed by this interstitial volume must of course be subtracted from the total area in determining the total exchange capacity. The todal exchange capacity of the column is a measure of the number of available exchange sites in the resin. This means that, for instance, an ion wit,h a +2 charge can satisfy two exchange sites while it takes two ions of charge +1 to satisfy the same number of sites. In the runs just described, the molar concentrations of the two influent solutions were the same and therefore the area above the c/co versus volume plot for Cs+ exchange should be just twice that for the Co++ exchange since the total exchange capacity of the column is constant. The ratio of the areas shown in Figure 2 was found to be 1.97. The ratio of t.he areas above the curves for solutions of the same molarity will only give us the relative charge carried by the exchanging ions when only one ion in known concentration is in solution. For example, some ions such as A P , VOz+, Th+(, ZnO+?, and Fe+3 are only stable in appreciable H f concentration and therefore the area above the curve mould not be a measure of the charge carried by the ion under investigation. The authors wish to express their appreciation for financial aid received through an Undergraduate Research Training Program Grant awarded by the Kational Science Foundation. r,Xksaio, G. D.. TCRSE, R., Chhn. Acta, 21, :J83(1959).
AND
RIEMIS,WM.,111, Anal.
Volume 38, Number 7, July 196 1
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