Joseph F. Mucci, Robert 1. Steorns and Harriet F. Fleishman Vassar College Poughkeepsie, N e w York
I
I
The Cedm-CobclCDo~ex50-X2 System in H + - F O ~
This experiment vividly displays the nature of the ion exchange process involved when an aqueous solution of cesium chloride' and cobalt chloride' is allowed to run through an ion exchange column which contains Dowex 50-X2 in the H + form. In addition, a scintillation counter is used to advantage as an analytical tool in the quantitative analysis of the influent and effluent concentrations. A small column (10 cm long and 1.2 cm id) containing the styrene type, sulfonic acid cation exchange resin Dowex 50-X2 (50-100 mesh) is put in the H + form by use of a 1 M HCl solution. The column is then washed clean of excess H + and C1- with distilled water. The influent is introduced into the column by siphon action and the rate of flow is regulated with a Hoffman clamp.2 The column is not allowed to go dry during the entire experiment. An aqueous solution of 0.010 M CsCl and 0.010 M CoC12' (spiked with enough lnCs and W o to give about 4,000 counts/min each of Co and Cs) is allowed to run through the column a t a rate of about 1 m1/2 min. Fractions of the effluent are collected in previously dried and weighed 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 effluent is 1.00 g/ml. A 2-ml aliquot of various effluent samples is then pipetted into a test tube which has been carefully washed, dried, and used throughout the entire experiment. The samples are counted in a well-type scintillation detector, Baird Atomic 810B, using a model 250 Baird Atomic single channel pulse height analyzer. The geometry and total volume of each sample (4 ml; i.e., 2 ml aliquot 2 ml H?O) are kept constant. The effluent samples are analyzed for the concentration of Cs+ and C O + ~ .Details of the calculations follow. I n this ion exchange system, many effluent samples contained Cs+ but no Cof2. It was simple to analyze these samples since all one need do is adjust the "window" of the analyzer to include only the photo peak of Cs+ (see Fig. 1) and compare the counting rate with a standard sample. However, in effluentsamples which contain both Cs+ and CoC2other considerations need to be made. Radioactive InCs produces a 0.66 Mev gamma while the O°Co yields two gammas (1.33
and 1.12 Mev). Nearly any amount of cobalt can have a considerable effect on the cesium counting rate since many pulses resulting from Compton electrons produced in the crystal by the cobalt gamma rays will fall within the pulse height range used for the cesium. The presence of a small quantity of cesium does not interfere with the cobalt counting rate as strongly as cobalt effects cesium since the pulses producing the cobalt photo peaks are about twice as large as those from cesium. However. both must be taken into consideration in our calcul&ions.
v
-
1
GOrnrnO ROY Energy
Figure 1.
+
'The ions of interest are C O ( H ~ O )and ~ + ~Csf (aq);we will refer to them as Co+2 and Csf. Mrrcc~,3. F., SPIEREL, D. E., AND STEARNS, R. L.,J. CHEM. EDUC., 38, 348 (1961).
+ +
Cobalt Standard: 2 ml of 0.020 M CoCL stock solution 2 ml H*O = 4 ml of 0.010 M CoCb. Cesium Standa~d:2 ml of 0.020 M CsCl stock solution 2 ml HrO = 4 ml of 0.010 M C8CI. Injevent Solution: Prepared by mixing equal volumes of 0.020 M CoCI. stock solution and 0.020 M CsCL stocksolution. This results, of course, in an influent concentration of 0.010 M CoCIs and 0.010 M CsC1. Counting of Efluent Samples: 2 ml aliquot pipetted into special 2 ml HzO. test tube (same tube used throughout experiment) This means that counted sample h a the same geometry and volume necessary to reproduce results and compare with standards.
+
Counting Rate of Cs+ in Effluent Sample Counting Rate of Cs' Standard
X
(If only Cs+ is present in the effluent, then X determines the Volume 47, Number 3, March 7964
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163
(i.e., conslant tolal influent and total effluent concentration at t h i ~ stage of exchange process)
concentration of Cs+ in sample) Counting Rate of Co +% in Effluent Sample = Counting Rate of Cot' Standard X/Y = Experimental [Csfj/[Cofq in efluent sample. In samples containing both Cg+ and Cof: it is necessary to experimentally obtain a correction m ~ v eto overcome the interference in counting the isotopes in the presence of each other. Samples containing &own ratios of [Cs +I /[Co+'] are counted. The following information is recorded:
Over the range B to C (approximately) in Figure 3, (A/Ao)c.+ Y 2 (within experimental error) and (A/Ao)o,+, = 0; however, total is still 2. Therefore using the above expressions one can obtain both (A/A&.* and (A/Ao)c.+a in efluent samples containing the two isotopes. Discussion
Counting Rate of Co+Z in Known Ratio Sample Counting Rate of Cat> in Standard
=
W
Z/W = Experimental [ C s + l / [ C ~ + ~inl Known Ratio Sample. ( Z / W )(Correction factor)
=
Actual Ratio of [Cs+l/[Co+~l.
Z/W and its corresponding correction factor are shown in Figure 2. To determine Actual [Cs+l/[Cac2I one simply uses Figure 2 and multiplies the experimental [Cst']/[Co+'] by the correction factor. I n this column experiment,!X/Y is the experimental [Cs +]/[Go +=I.
It was only necessary to use correction factors from Figure 2 for eight samples in Figure 3 for which significant amounts of both isotopes were present. About 60,000 counts were taken for each measurement giving about 0.5% counting statistics based on the v%/n where n is the total number of counts. The background wm measured and subtracted. Since slight changes in the gain of the counting system are inevitable, the system was adjusted a t the start of each day's counting so that the cesium peak came a t the same window setting. I n addition to this, all cesium and cobalt counts were normalized to standard cobalt and cesium samples to compensate for slight gain changes. As the influent (i.e., 0.010 M CoCl? and 0.010 M CsC1) goes through the column, both cobalt and cesium displace the H+, and therefore the concentration of Co+Z and Csf in the effluent is zero (Fig. 3). Furthermore, since the order of selectivity of exchange in this ~ system is C O + ~ >Cs+l> H + (i.e., Ka order is C O +> Cs+ > H+), we note in Figure 3 that C O +does ~ not show up in effluent until the exchange capacity of the column for Co+? is closely approached and surpassed. The greater selectivity of Co+? forces the A / A o value of Cs+ to go above 1.0. As the column approaches exchange site saturation with Co+? both A/Ao for Cs+ and A/Ao for Co+*approach and finally reach a value of 1.0 as shown.
Let Actual [Cs+]/[Co+'] in effluent = R.or [Cst] = R ~[C0+2]. Let A. represent the concentration of CoC2 and of Csf in the influent and A represent the concentration of cobalt and of cesium in the effluent. A plot of (A/Ao)c.+ and (A/Ao)c,+r versus efluent volume (mid-point of sample) is shown in Figure 3. One can obtain (A/Ao)c.+ and (A/Ao)oq+l as follows: In effluent eamples with Cs+ a l n e (in this experiment we had none with Co+*alone) (A/Aa)c.+ = X In effluent samples containing bath Cs+ and Co+a we know from above, that (A/Ao)o.+ = (AIAO)C**~R) At exchange site saturation with CoC' (beginning at point D in Figure 3) (A/Ao)c.+ = (A/Ao)o.+a = 1 or (A/Ao)c.+
+ (A/Adco* = 2
Jut prior to exchange site saturation with C~+~:(RangeC to D (approximately) for Cs' and E to D for C O +in ~ Figure 3) (A/A.)c.+
> 1 and (A/Ao)c,+. < 1
hut (A/Ao)c.+
164
/
+ (A/Ao)cO+l= 2
Journal of Chemical Education
Figure 3 then gives us a rather detailed picture of the ion exchange process under investigation. I n addition we have shown how Cs+ and Co+? can be counted in the presence of each other by use of a scintillation counter. This same type of experiment could be carried out using ions of the same charge and even in that case the exchange will have a preference for one over the other (or others). It should be noted that the column initially (before introducing the influent) contained water as well as the
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resin. The volume of the water remaininn in the column is known as the illterstitial volume. This volume has heen determined for Dowex 50-X2 and is 30.4y0of the bed v01ume.~ The area above the curve in the A / A o versus volume plot contributed by the interstitial volume is pointed out in Figure 3. The authors wish to express their appreciation for financial aid received through an undergraduate nesearch Training Program grant awarded by the National Science Foundation. ' M m a ~ o ,G. D., TURSE,R., Chern. Ada, 21, 383 (1959).
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Volume 41, Number 3, March 1964
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Biblioara~hv EvANs, It. D., ',The i\tomic sueleus,23 .\IcGmw.Hill New Ywk, 195.5. "Dowex: I O N Exchange," Ilow Chemical Co.. Midland. Michigan, 1958, p. 30f. KUNIN,ROBERT,"Ion Exchange Resins," 2nd ed., John Wiley and Sons, Inr., New York, 1958, p. 340f. MEITES,L., AND THOMAS, H. C., "Advanced Analytical Chemistrv," McGraw-Hill Book Co., Inc., New York, 1958. CAASE,GRAFTON D., "Principles of Radioisotope Methodology," Burgess Publishing. Co., Minneapolis, Minn., 1959. "Scintillntion Spectrometry," a handbook prepared by BairdAtonk, Ine., 33 University Rd., Cambridge, Mass., 1960.
