Growth and Decay An Experiment Demonstrating Radioactivity Relationships and Chelate Solvent Extraction Separations D. M. Downey, D. D. Farnsworth, and P. G. Lee West Virginia University. Morgantown, WV 26506 Many important techniques discussed in lecture material frequently are not covered by laboratory experiments due to limitations in available lahoratory time. Thus, experiments which demonstrate more than one concept are often more beneficial to the student than those which cover only one topic. We have attempted to include in the undergraduate radiochemistry laboratory course at West Virginia University experiments which teach both important radiochemistry concepts and analytical chemistry techniques that are not covered in our formal analytical laboratory courses. These experiments include the determination of solubility product and solvent extraction partition coefficients, quantitative paper and thin-layer chromatography, metal separations by ion exchange and solvent extraction, X-ray fluorescence and activation analysis. One of the experiments, the separation of lead and bismuth by chelate solvent extraction, is of interest because of the simplicity which the use of radiotracers allows in its demonstration. Chelate solvent extraction metal separations are usually discussed in the introductory analytical course hut are seldom demonstrated by experiment. Yet the separation of lead and bismuth may be conveniently followed with 212Ph and 2'2Bi which are readily obtained from natural thorium. Furthermore, the parent-daughter relationship and the relatively short half-lives of the tracers make possible the teaching of the laws of radioactivity in the same experiment.
Theory Metal ions mav be extracted verv effectivelv from aaueous solutiuns 115.the formatinn uf rhrlates with wiak acidswhirh arr solublr in orranic solvents. The eeneral chelate extraction process may be represented by
-
M"+
+ n(HR), s (MR,), + nH+
(1)
for a metal ion, Mn+, and chelate, HR, with the subscript "0" denoting the organic phase and no subscript denoting the aqueous phase. A nonthermodynamic equilibrium constant mav be used to describe the Drocess. That is
aqueous hydrogen ion concentration and the equilibrium chelate concentration in the organic layer, i.e.,
Where K', is the extraction constant for the metal ion of larger charge value. I t is useful t o express equ. (2) in logarithmic form:
+ n log [H+] - log [M"+] - n log [HA] (7)
log K.. = log [MR,]. Rearrangement gives pH
1 n
= -log
[MR.],
1
- -log n
1
[M"+] - log [HA], --log K., n
(8)
Equation (8) may be used to calculate the threshold pH value which is the lowest value for complete extraction of the metal. It may be seen that the threshold pH is dependent on the concentrations of extracted metal chelate, residual (unextracted) metal ion, and unreacted chelating reagent in the oreanic laver. Thus. a threshold DH is calculated for a partichar metal extraction based onihe anticipated ronce$rations: then a DH value hirher than the threshold DHisselected and "sed in the extraction to allow for variableconditions. The separation of Bi3+ from Pb2+ with dithiine (diphenylthiocarbazone) is a classic example of a chelate solvent extraction separation ( I ). The published log K., values for these metals are 9.98 and 0.44, respectively (2) and these values allow the calculation of threshold pH values by eqn. (8) for the separation conditions presented in the experiment below. If a final aqueous metal concentration of 1X 10WM is the criterion for complete extraction (-100% extracted) from a solution of initial metal concentration 2 X 10-5 with an initial organic phase of initial chelate concentration 8 X 10-5M, then the threshold pH values are 2.46 for Bi3+ and 5.83 for Pb2+. Other values for the extraction of Bi3+ and Pb2+ have been calculated from eqn. (8) and are plotted in terms of the percentage of metal extracted versus pH in Figure 1.These curves
Quantitative separation of metal M' from metal M" is defined as
for the organic phase, and
for the aqueous phase. The separation factor, a,is defined as
and must be greater than 104 for quantitative separation. I t may be seen from eqn. (2) that a is simply K'JK"., for two equally charged metal ions. For the separation of metal ions which differ by one unit in charge value, a is dependent on the
Figxe 1. Effect of pH on the emactim of BP+ and PbZ+ wim difiizane in carbon tetrachloride (initialconcemrations: CM= 2 X to-' M; CD = 8 X I O v M). Volume 61 Number 3 March 1984
259
are developed on the assumption that the dithizone concentration is negligible in the aqueous phase and there are no secondary complexing reagents present. Experimentally, it is found that optimum extraction for Bi3+ occurs at pH 3.2-3.4 and for Pb2+ a t pH 8.6-9.2 (3).At pH 3.2, the a value for the Bi3+Pb2+senmation calculated from eon. (6) is well in excess of lo4and qGantitative separation shohd be obtained. Several natural radioactive series were discovered early in the history of nuclear science. One of these series has its origin in 232Th,which is 100%of natural thorium. The transformation of the radioactive 232Th to stable 208Ph proceeds through a variety of intermediate radioactive daughters by means of six alpha and four beta decays. This group of radionuclides is called the "4n" series since all the mass numbers are evenlv divisible by four. The overall decay scheme of the thorium (4;) series is shown below.
counts above background. However, the 212Bi will "grow" with time in the parent 212Ph according to the equation
where the A values are the decay constants (0.693/t1/2) and N represents the number of atoms at a specific time after preparation of the radiochemically pure 212Pb.Solution of eqn. (9) for the number of z12Biatoms a t any decay time gives hpb (10) N0pb ( e ~ ~ f - h ~-b exp(-bit)) t) h ~-i hpb when there is no initial 212Bi present and N o p b is the initial number of 212Pb atoms. Figure 2 illustrates the growth and decay of the radioactive materials. I t may be seen that a maximum activity of the daughter is attained after 2.7 h and that radioactive equilibrium is attained after 3.8 h. After 3.8 h the 212Bidecays '"Th a t the same rate as i t is produced and the activity of the mixture decreases with the cbaracteristic half-life of the parent z12Ph. Na=-
Experimental
u'pb
m a n
mePo
0.16 s
Materials
- 01
'%ORn
54 s
n
"
4
3.64 d
1.9 Y
L 1 1 2 ~ i 60.5 mi"
,
It may he seen that 220Rn is an intermediate product of the decay series. The radon is released as a gas from the solid parent material and provides a ready source of the short-lived daughter products that follow its decay. T o collect these dauehter &ducts a olatinum wire electrode at hieh " neeative potential may be suspended over a solid thorium compound and charred due to the loss of . . - ~the - ~ 220Rn. which is ~ositivelv electrons in the decay of its parent, migrates to the electrode where it deposits its daughters as it decays. Such an apparatus for collecting the short-lived daughters may be built with common laboratorv eauinment and is known as a "thorium cow" because i t may 6e"milked" at 2-3-day intervals (4). Other orocedures for collectinn the short-lived daunhters have also been suggested ( 5 , 6 ) . Washine the wire electrode with nitric acid nrovides a tracer i = 60.5 solution consisting of 212Pb(t1/2 = 10.6 h ) , i l z ~ (t1/2 min) and 208T1 (t,,., = 3.1 min) in radioactive eauilihrium. 'I'hese tracers are very convenient for use in the monitoring of I he Ri Pb se~arationdiscussed above. Furthermore they exhibit proper$es which make them useful for studying the laws of radioactivity. The 212Bi may he separated from the 212Pb by the dithizone extraction procedure. The separated 212Bimay then be monitored with a Geiger-Miiller detector to determine its half-life. More 21rBiwill begin to form in the separated z12Pbas it decays and this activity may be monitored to demonstrate the growth of daughter products in a radioactive parent. The 212Pbemits up to 0.4 MeV P- in its decay whereas 212Bi emits up to 2.2 MeV 0- particles. If an aluminum absorber of 80-100 mg/cm2 thickness is placed between the separated ?I2Phand the G-M tube, the detector will initially record no
- . ~~~~
260
-
-
Journal of Chemical Education
A thorium cow should he prepared in the manner suggested hy Morimoto and Kahn (4). Enough Th(0H)d should be present to give 500 countsls by G-M counting on the wire to provide sufficient activity for good counting statistics. The apparatus may be set up in a fume hood and left as a permanent source of the short-lived dauehters. Two ~ e i ~ e r - ~ i i lcounters, ler each consisting of a G-M tube in a shielded lead castle and scaler/timer with aluminum absorbers (80 mgl cm2), and a NaIiTI) Scintillation Detector with signal led into a ~ & i c h a n n e lAnalyzer (MCA) should be available for P- particle counting and y spectra collection. A standard pH meter with glass electrode and reference electrode is used for pH adjustment. Chemicals. Electrode calibration buffers (pH 4.0,7.0, and 10.0); 2 M nitric acid; 1/10 nitric acid; concentrated aaueous ammonia; 1/10 aqueous ammonia; ammonia/ammbnium chloride buffer; Ph2+ carrier (0.1 mg/mL); Biz+ carrier (0.1 mg/mL); dithizone (20.0 m g L ) in carbon tetrachloride. Other materials. Test tube, Bunsen burner, separatory funnel, calibrated pipets, volumetric pipets, 10-mL volumetric
+~ - 2"l-h
~
-
.
P l!L-
F~~~~~ 2. ~
Time, hrs.
~variations ~ wfih time. i (A) ~ decayi of mBi: ~ ,B) of 21zpb; (C) growthand decay of 2'2Bi in initially pure 212~b:(D)totalactivityotthemixiure (B) (c).
+
flasks, counting planchets, heat lamp, and disposable gloves. Procedure
Warning: T h e student should b e cautioned of t h e haza r d s of radioactive materials a n d volatile solvents. All work except counting should be carried out in fume hoods and students should wear safety glasses, lab aprons, and disposable gloves a t all times. Film badges, pocket dosimeters, and/or survey meters should he used, and appropriate containers for radioactive waste disposal must be provided. The student should be aware of the particular hazard of accidentally ingesting alpha-emitting radioisotopes and therefore pipetting by mouth must n o t ~ b edone. Radon gas is released upon opening the thorium cow, and it must be kept in a properly vented fume hood to prevent inhalation. Transfer the wire electrode from the thorium cow to the small test tube. add 2 mL of 2 M nitric acid. 0.5 mL of each carrier solution, and heat to dissolve the radioactive daughter products. Carefully transfer the cooled solution to a small beaker, immerse the calibrated pH electrode and add several drops of concentrated ammonia to neutralize the acid. Carefully adjust the pH to 3.0 using 1/10 ammonia and/or 1/10 nitric acid. Transfer the solution to a 10-mL volumetric flask and dilute tovolume with distilled water. Pipet exactly 1.0 mL of this solution into a counting planchet (planchet "A") (Note 2) and evaporate to dryness with an infrared lamp. Planchet "A" serves a reference as i t contains both 212Bi and 212Pb. Count the planchet in the G-M castle with the aluminum absorber in place and determine the original 'counts per minute (cpm) due to 2% (Note 3). Meanwhile, quantitatively transfer the remainder of the aqueous solution to a separatory funnel with as small a volume of distilled water as possible (Note 1) and extract twice with 4-mL portions of the dithizone solution (Note 4). Quantitatively transfer the extract to a second 10-mL volumetric flask and dilute to volume with CCL. Piuet 1 mL of this solution into a nlanchet (Planchet " ~ " jan2 evaporate to dryness. Transfer planchet " A to the NaI(T1) detector and record the cpm for the 0.24 MeV peak of 212Pb. Meanwhile count planchet "B" in the G-M castle under the same conditions as planchet "A" and record the 212Bi cpm. When the beta counts of 212Biare recorded, transfer planchet "B" to the NaI(T1) and record the gamma spectrum. Adjust the pH of the residual aqueous layer in the separatory funnel to 8.8-9.2 with a few drops of ammonia/ ammonium chloride buffer. Extract twice with 4-ml portions of the dithizone solution. Quantitativelv transfer the extract to a third 10-mL volumetric flask and dilute to volume with
CClc Pipet 1 mL of this solution into a planchet (Planchet "C") and evaporate to dryness. Allow a 10-min decay of planchet "C" to reduce the activity of any coextracted 208T1 (tllz = 3.1 min) and count in the GM castle and NaI(T1) detector as for planchets "A and "B". The half-life of 212Bi may be determined graphically from the data recorded by counting planchet "B" in successive 10-min intervals for up to 2 h. The growth of 212Biin 212Pbmay be observed by counting planchet "C" for successive 10-min intervals in the second G-M castle with an aluminum absorber in place. Results
Planchet " B should contain 212Bi free from 212Pb contamination. Comparison of the beta activity cpm as determined by the G-M counting of planchets "A" and "B" will indicate the com~letenessof extraction and mav be used to indicate the studeks' laboratory technique. The students may he asked to comment on the "error" in the beta counts of planchet "A" due to the gamma activity of 212Pb. The radiochemical purity of the extracted 212Bi will be evident from the lack of a 0.24 MeV 212Phpeak in its gamma spectrum. The com~letenessof the 212Phextraction will be indicated bv the comparison of the 0.24 MeV peak counts for planchets""~" and "C".Any residual 212Bithat is coextracted with the 212Pb will be evident in the G-M count for planchet "C". The halflife of 212Bi may he determined from a plot of log activity (cpm) versus decay time. The growth of 212Bi in Z12Pb may be demonstrated on the same graph by plotting the log beta activity of planchet "C" versus time. Notes 1)
The dilution should not significantly change the solution pH.
2) The extraction of Bi3+ proceeds slowly. Shake the separatory
funnel for 4 6 min per extraction. 3) The aluminum absorbers orevent the weak beta oarticles of %12Ph 4,
h r n reaching thed~twt(,r. The student should calculate the npt full energy ppak counts a i explained by Bauer et al. (7).
Literature Cited (1) Sandell, E. B.,"ColorimotncD~hinationofTraes ofMetala," 2ndpd.. Interscience, New York, 1950, pp. 3 W 0 2 . (2) Ruzicks, J., and Stary, J., '"Substoichiornetryin Radimhemiaal Analysis," Pe~gmm, New York. 1968. p. 12.
(3) Iwantscheff,G.,"DaaDithiiiund~ieineAnruendunginderMiiiiiidSp~~~~~~1~e.~~ Verlag Chemie. Weinheirn. 1958. (4) Morirnata,E. M.,and Kahn, M . J.,J.CHBM.EDUC.,36.296(1959). (5) Kusle, E., and Sksrestad, M.. J. CHEM EDUC.,51,756 (1974). (6) Brals, E..snd Schonfeld,T..J.Cwe~. Eouc.,54.577 (1977). (7) Bauer, H. H., Christian, G. D., and O'Reillcy. J. E., "lnstrumentel Aoalysia,"Allvn and Bacon, Baston, 1975. pp. 576-577.
Volume 61 Number 3 March 1964
261
