A New Method of Separating *lOPbfrom Ra-DEF for a Radioactive ~quilibrium~xperiment C. M. Wai and J. M. Lo University of Idaho. Moscow, ID 83843 The decay of 21"Ph t o 210Bi provides a good system to illustrate secular equilibrium between a long-lived parent and a short-lived daughter.
A general nrocedure of senaratine 210Pbfrom 210Bi for this experiment is by means 6f a pkcipitation technique ( I ) . Carrier solutions made of bismuth nitrate and lead nitrate are generally used for the precipitation of either radioactive isotope. Incomplete separation of 210Pb and 210Bi may result if pH of solution is not carefully controlled in the precipitation nrocedure. Another difficulty involved is to handle small quantities of precipitate a n d t o make i t into a uniform and thin-layer sample for counting. We have recently developed a new method of separating 210Pbfrom Z10Bi and 210Po by means of solvent extraction of their diethyldithiocarbamate (CzH&CSz- complexes. This method involves a simple extraction procedure which allows complete separation of 21"Pb from Ra-DEF within several minutes. Since the radioisotopes are separated in different solutions, they can he easily transferred and made into uniform thin samples by evaporation. The experimental procedure and the basis of this separation method are described in this article. Experimental -
T h r proposed extraction proredure requires a diethyldithioc.8rhnmnte (DDC) comldrx, RitDDCh, which ran br easily synthesized in the laboratory. The starting materials for synthesizing Bi(DDC13 are sodium diethyldithiocarbamate and a bismuth salt such as bismuth nitrate. The former can be obtained from Baker or other major chemical companies. The synthetic procedure involves the reaction of Bi3+ ion with DDC in aqueous solution followed by extraction of the metal complex with chloroform. After discarding the aqueous phase, ethanol was added to the solution and the mixture was heated to 70°C to drive off chloroform. T o avoid inhalation of chloroform vapor, t h e whole experiment should be carried out in a hood. After evnoorntion. BilDDCl. u,hich is insoluble in ethanol, is recovked by filtration. The metal comnlex can be stored in a bottle for a nroloneed neriod of t i m i Radioactive Ra-DEF solution use2 in t h i experiment can be obtained from Amersham or New England Nuclear. Separation of Z1OPbfrom Ra-DEF About 0.1 to 1pCi of Ra-DEF is sufficient for a typical radioactive equilibrium experiment. 210Pb is separated from a solution of Ra-DEF by extracting 210Bi and zloPo into a chloroform solution containing Bi(DDQ3. The extraction can he performed in a 5-ml polyethylene liquid-scintillation vial with a plastic cap. The Ra-DEF solution is introduced into 1 ml of a 2.4% HN03 solution placed in a plastic vial. An equal volume of CHC13 solution containing about 1.5 X 10-3 M Bi(DDC)3 is then added to the radioactive solution. The acid solution is used to prevent adsorption of radioactivities to the
container walls and to control the forms of the ionic species in solution. After shaking the mixture for about a minute, the system is allowed to stand still for the separation of two bhases. If a centrifuge is used, phase separation can be achieved within a minute. The aqueous phase which contains pure 2'0Pb'is ready for the secular equilibrium experiment. T h e time when two phases are separated is taken as the zero time for the experiment. T o prepare a pure 210Pbsource, an aliquot of the aqueous solution is taken from the vial and nlaced in a ~lanchet.Depending on the activities of the R;-DEF soluiion used, the aliouot size can varvfrom 20 ul to 200 ul. The radioactive solution is evaporated to dryness either a t room temperature or under a lamp. Results and Discussion Theoretical Basis of the Sewration Method The proposed method of separating 210Pb from 210Bi and 210Po is based on the difference in their complex formation constants with diethyldithiocarbamate. The extraction constants of some metal-DDC complexes are given in the table. The extraction constant of Pb(DDC)2 is much smaller than that of Bi(DDC)3 or Po(DDC)4. Therefore, Pb2+can be separated from Bi3+ and Po4+ in aqueous phase using a metalDDC complex which has an extraction constant between those of PbiDDCb and Bi(DDCL. In this eiperiment, we ;Lose to use Bi(DDC)3 to extract from the Ra-DEF solution. The concentration 210Biand 21T0 of Bi(DDC)s used in this experiment (about M ) is much greater than that of 210Bi Dresent in the aaueous ohase (about X lo-" M).Because df the large extrktionconstant for Bi(DDC13, virtually all 210Bipresent in the aqueous phase should be extracted into the organic phase by isotope exchange with nonradioactive Bi in Bi(DDC). The extraction constant for Po(DDC)a has been determined to be greater than that of Bi(DD03 (4). Therefore, in the extraction process, 210Po will replace Bi3+ to form Po(DDC)4 leaving essentially pure 210Pbbehind in the aqueous phase.
4
Growth of 210Bifrom 210Pb In general, the growth of a radioactive daughter product from a pure parent can be expressed by the following equation (5):
Extraction Constants * of Some Metal-Dlethyldithiocarbamate Complexes Metal ion
Complex
pb2+
WDDCh
Cu2+
Cu(DDC)p Bi(DDC)a Po(DDC)~
BiSt PO'+
Extraction constanta
9.48 13.4 13.8 14.1
'Exb-action mnstem = l l m log Kwcar.. where Krmn,ir [M+] [ D D C I and m is the charge of the metal ions.
Reference
.
(2) (2)
(3
(4
defined as [M(DDC),ld
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0.1
0
100
200
500
400
T h e (hr)
Figure 2. Plot of (A.
Tim* (hour) Figure 1. CrowVl of Bi from a pureZ'OPb sample 0 and thedecay ofa pure 2'0Bi sample A.
where A and A represent activity and decay constant and subscript 1 and 2 are for the parent and the daughter, respectively. If A2 is much greater than A1 as in the case of a long-lived parent and a short-lived daughter, eqn. (1) can be simplified as The beta activities from the decay of Z10Bi can be measured by a Geiger-Miiller counter. A thin aluminum absorber (about 10 mg/cm2) should be placed between the sample and the G-M tube to remove weak beta emitted from 210Phand to absorb the Po alpha particles. The growth of 210Bifrom a pure ?lOPb source obtained by this method is shown in Figure 1. The decay of a pure 210Bisource obtained from the same experiment is also shown in Figure 1. Since a t equilibrium A1 = Az, eqn. (2) can also he expressed as
258
Journal of Chemical Education
- AVA,
for the growlh of *'OBi.
If Z'OPb and 210Bi are in equilibrium in the Ra-DEF solution to start with, the activity of 210Bifound in the organic phase after Bi(DDC)a extraction should represent Aze%Another graphical illustration of the secular equilihrium is by plotting ln(Azeq - A 2 ) I A p ) versus t as shown in Figure 2. The slope of the line should be -Az. This new method of separating Ra-DEF provides asimple and fast procedure for studying the secular equilibrium between 21"Ph and 210Bi. It illustrates several important concepts such as solvent extraction, complex formation, and isotope exchange which are relevant to undergraduate laboratory experiments in radiochemistry and in analytical chemistry. The mathematical aspects of this experiment are important for understanding the relations between parent and daughter in a radioactive decay series. Literature Cited (1) C h s c , G . D., and Rshinowitz. J. L., "PrincipleeofRadioisotopeMethodology.'. 3rd ed.. Burgess Publishing Co,Minneapolis, 1967, p. 179. 12) Shen,L.H., Yeh,S. J.,snd Lo, J. M.,Anol. Chsm.,S2,1882 11980). 131 Yeh,S. J.,Lo. J. M.,andShen,L. H.,Anal.Chem.,S2,52811980J. ($ Wai.C. M.,and 1.0. J. M..Rndiorhem. Rodimnal. Lett., 50.293 11982). 151 Friedlander, G., Kennedy.J. W., M a c k E. S., and Miller, J. M., "Nuclear and Rsdiochemistry.i'3rd ed., John Wiley & Sons. New York, 1981.
