An Isotope Dilution Analysis Experiment with Phase Isolation by Electrodeposition R. A. Pacer1, W. D. Ehmann, and S. W. Yates2 University of Kentucky, Lexington, KY 40506 Isotope dilution analysis (IDA) is a sensitive analytical method t h a t stands as one of themaior applications of radiochemistry to chemical analysis. ~ h ' 1 ~ ~ ; e c h n i q uise generally characterized, in simple terms, by three steps: addition (or "spiking") of a known amount of radioactivity to the mixture t o be analyzed, phase formation and isolation, and determination of specific activity. One of the principal advantages of IDA is that the phase formationlisolation steo need not be carried out auantitatively, and, in many cas'es, quantitative isolation prbcedures may not be known. Various methods, such as recrystallization (1,2 ) , precipitation ( 3 , 4 ) ,and molecular sieve separation (51, have been chosen for the phase formationlisolation step. Moreover, if one sets out to determine volume rather than mass, the phase formation/isolation step may be eliminated (6). We would like to present a n IDA experiment based on the important, but seldom demonstrated, process of electrqdeposition for the phase formationlisolation step. The exneriment is suitable for the advanced undermaduate or graduate laboratory in analytical chemistry or idiochemistry and is especially desirahle hecause it allows the instructor to combine the many learning objectives associated with two techniaues-isotone dilution analvsis and electrode~osition-into a single experiment. Experimental Procedure (1) Two stainless steel disks (approximately 2.5 cm in diameter) should first be polished with steel wool, washed withdistilled water, then with acetone, and again with distilled water. Both disks are dried for 10 min under a heat lamp and weighed with an analytical balance. (2) To a small plastic beaker, the student should add about 20 drops of a standard "spike" solution containing a known amount of Co2+per milliliter, to which W o tracer3 has been introduced, and approximately 5 mL of 3 M (NH&S04 solution. This beaker is then laheled as the "spike". (3) To a second beaker, the student should add eraetly 0.500 mL of the standard "spike" solution, exactly 1.000 mL of the "unknown" cobalt-containing solution: and approximately 5 mL of 3 M (NHJnSOasolution. This beaker is the "unknown +spike". (4) Cobalt metal is then electroplated from the two solutions prepared in steps 2 and 3 onto the two clean, weighed disks. Approximately 30 min are required for each plating. The anode used in our plating cell was a 1-cm loop of Pt wire, suspended at least 1cm above the disk cathode. The stainless steel cathode was masked by a rubber gasket that sealed the cell so that
'
Permanent address: Indiana University-Purdue University at Fort Wayne. Fort Wayne. IN 46805. Author to whom correspondence should be addressed. Unlicensed amounts of %o are available from The Nucleus. Inc.. Oak Ridge. TN 37830. The specific activity of the standard spike solution used in our experiments was 1.0 X TO5 dprnlrng or 0.81 wCil mL. 'The unknown solutions were prepared by dilution of the same stock solution used in preparing the standard spike solution. In the trials described later, cobalt recovery averaged 78% (range of 59-84%).
the actual area plated was -1.5 cm in diameter. A plating cell similar to that used here has been illustrated by Chase and Rabinowitz (7). The solution was not stirred during plating, since the gas evolution provided adequate mixing. The cobalt deposit was grainy and black (but adherent) rather than shiny. Shiny deposits can be obtained using other procedures (a),but the electroplating yields are considerably lower. The plating conditions used were: i = 120 mA; E.,w = 3.8 V. ( 5 ) After plating, each disk is thoroughly washed (distilled water. acetone, distilled water) and dried, as before, and is reweighed with the analytical halan~e.~ (6) The electroplateddisks are then each counted with a gas flow (Q gas) Geiger-MBller counter with a 200 mglcmz A1 absorber hetween the disk and the counter. A backmound count should also be recorded. Ten-minute counting periods proved adequate in all cases. The absorber permits one to count only y radiation and thus allows one to avoid the problems of &particle backscattering and self-absorption in the deposit on the disk. With the geometry used, the counting efficiency was 4.15%. From five measurements of the activity of the standard "spike" solution made during the same day, avalue of 145 i 8 cpmlmg was obtained. All electroplated samples had count rates between 800 and 1200 cpm, and dead-time corrections were not required at these low counting rates. The background rate was 19 cpm. (7) From the masses of the two cobalt deposits, their activities, and the known cobalt concentration of the standard "spike" solution, the cobalt concentration of the "unknown" solution is then calculated and expressed as milligrams of Co per milliliter of solution.
Calculations It is important that student calculations of results be designed to reinforce the principles used in the experiment. IDA is based on the as&mp60n that, once thespike and unknown solutions are mixed, the specific activity of the mixture is not changed by further chemical manipulations (in this case, by electroplating Co metal). Therefore, the specific activity of the mixed spike and unknown solution before plating is equal to the specific activity of the Co metal electroplated, regardless of the amount of Co actually deposited. Hence, a 100% plating yield is not essential to obtain quantitative results. This fact becomes apparent to the student since the masses of cobalt are measured directly in this experiment and are clearly less than the amounts contained in the solutions. At this point, we depart from the customary discussion of IDA calculations presented in many radiochemistry texts. We feel that the following treatment illustrates the basic principle of isotope dilution analysis better than the memorization of more sophisticated working equations. We begin with the following statement of this principle: specific activity of mired spike and unknown solution
specific activity =
uf Co rnrlal
electroplated
and make the appropriate substitutions, Volume 66
Number 7
July 1989
603
The selectivity and detection efficiency of the y-rays from W o can he significantly improved by using scintillation or semiconducto~detectors, and the amount-of the radionuelide used can he reduced. However, the amount of GoCo employed is quite small, and we have opted for the much more common Geiger-Muller counting instrumentation. This experiment can easily becompleted within one threehour laboratory period, and the requirements for analytical instrumentation are minimal. Moreover..onlv " small amounts of W o , a readily available, long-lived y-ray-emitting radionuclide, are required. Radioactive waste disposal is thus not a serious problem (see, for example, ref 9 for a discussion of the disuosal of radioactive wastes), and "Co offers the advantage over shorter-lived tracers t h a t the prepared spike solutions can he used for several years. The pedagogical usefulness of this radiochemistry experiment, which combines IDA with electrodeposition, has been well documented through many years of student experience. ~
where a = countslminute 6"Co added from the 0.500 mL of thestandard spike solution, b = milligramsof Co in the0.500 mL of the standard spike solution, c = milligrams of Co in the 1.000 mL aliquot of the unknown solution, d = counts1 minute W o electroplated from the mixed spike and unknown solution, and e = milligrams of Co electroplated from the mixed mike and unknown solution. The quadtities a and b are calculated from the given Co concentration of the standard mike solution and the exuerimental specific activity of the co metal electroplated &om the pure "spike" solution. The values for d and e are ohtained by weighing and counting the Co metal electroplated from the mixed "unknown spike" solution, and c is then easily obtained from the above equation.
+
Results and Discussion This experiment has heen performed by students in our undereraduateleraduate radiochemistrv laboratorv with consis~entlygooh results. Student resuits rarely dizfer by more than 20% (relative oercent error) from the actual values. uncertaintiks in the'weighing pr&edures are generally dominant over those in counting the radioactivitv. In order to establish the reliability of the p r i e d u r e , we asked a single analyst to perform a series of determinations. Of the 17 ~easurekents;eported, 13yielded results having deviations which were within f10%of the actual value, and the average percent deviation was 7.3%.
604
Journal of Chemical Education
Literature Cited 1. Feldman, M.; Wheeler, J. W.J. Cham. Educ. 1967,44,464-485. 2. Ault,A.;KrsC, R. J. Chem.Educ. 1969,16,787-768. 3. Johnsfon, C. R.; Drake, G. W.; Wentworth, W.E. J. Chem. Edue. 1969,46,284-286. 4. Williams, K. R.: Lipford, L. C. J. ChwnEduc. 1985.62.894-896. 5. Pope, C. G. J. Chem. Educ. 1975,52,34M44. 6. Marshal1,R.A.G.J.Chem.Educ. 1976,53,32&321. 7. Chase, G. D.; Rabinowitz. J. L. Principlra of Rodiokofope Melhadology, 3rd ed.: Burgess: Minnespolis. 1967: p 365. 8. Bennett. H..Ed. The ChemicolFormulory: Chemical Publishing: Brwklyn, 1939:Vol. 4, pp 4W01. 9. Wang, C. H.; Willis, 0. L.: h e l a n d , W. 0.RodiofrocorMeLhodology in Lha Biological, EnMlonm~nlol,and Phyaicol Sciences; Prentie-Hall: Englewood Clifle, NJ,1975: pp 376379.
