Polarographic Determinati0 n of Technetium and Ruthenium Radionuclides in Fission Products DANIEL L. LOVE and ALLEN E. GREENDALE Analytical and Standards Branch, Chemical Technology Division, laboratory, San Francisco 24, Calif.
b A rapid method using polarographic techniques has been developed for the determination of technetium-99m and ruthenium-1 03,106 radionuclides in fission product mixtures. A known fraction of the technetium and ruthenium radionuclides is selectively reduced a t the dropping mercury electrode to an oxidation state soluble in mercury, and the resulting amalgam is removed from the fission-product solution by falling into carbon tetrachloride. An assay of the separated radionuclides is made by gamma-counting the amalgam. The precision obtained for technetium-99m is within about 170, the accuracy agreed with the radiochemical procedure (for molybdenum99) within 2%, and the decontamination factor from other fission products is about lo6. A single separation requires about 3 minutes; if enough activity is present, only a few seconds. The rapidity of the entire procedure (including calculation of the number of fissions) i s illustrated by the fact that 50 analyses can b e completed in 1 day by two people.
T
using polarographic methods in the analysis of radioactive isotopes (4, 5 ) consist of using the mercury from a dropping mercury electrode to remove the radioactive isotope as an amalgam and assaying this amalgam by a radioactivity measurement. An application of this technique is reported here, an accurate and rapid procedure for the determination of technetium-99m and ruthenium103,106 in fission product mixtures. The availability of accurate and rapid methods for determination of technetium-99m and ruthenium-103,106 is dpsirable because assay of technetium99m serves as a determination of its parent molybdenum-99, which is commonly used as a comparison base in nuclide fission yield measurements and also serves as a monitor of the number of fissions contributing to the radioactivity of radiological fallout samples ; 39.8-day ruthenium-103-rhodium-103 and 1.OO-year ruthenium-106-rhodium106 contribute a large percentage of the gammas emitted in long-lived fission ECHNIQUES
780
ANALYTICAL CHEMISTRY
U. S.
Naval Radiological Defense
product mixtures from reactor irradiations or from radiological fallout. Radiochemical determinations using polarographic methods are simply a combination of techniques used in radiochemistry and polarography. The primary requirement is that the nuclide be polarographically reducible (or oxidizable) to a n oxidation state that is soluble in mercury. Such procedures are rapid because only a one-step decontamination is made from other radionuclides present and a nonradioactive isotopic carrier is not used. Polarographic oxidation and reduction reactions a t almost infinite dilution are the same as those experienced in 10-5 to 10-3M solutions with conventional polarography; thus separations described in the polarographic literature are, in general, applicable to the techniques described here. Such applicability appeared to exist in the case of technetium and ruthenium, although the polarographic information concerning these elements is limited. A polarographic study of 105->-ear technetium-99 has been made in a 2 X sodium hydroxide supporting electrolyte ( 8 ) . The diffusion current of technetium reached a constant value a t about -0.9 volt us. the saturate6 where techcalomel electrode (S.C.E.) netium was separated from molybdenum and rhenium. No other polarographic m-ork has been reported on technetium. although electroplating technetium from acidic solutions has been used for radiochemical separations (6). I n the polarographic determination of ruthenium in noncomplexing media, the oxidation potential of ruthenium is positive enough to be reduced to the metallic state by contact with mercury. Ruthenium dioxide and ruthenium sesquioxide might also be plated out on mercury if in certain supporting electrolytes their potentials are between those of mercury and the cation of the supporting electrolyte. Rillis (11). using a dropping mercury electrode, found no reduction waves for ruthenium (IV) or (111) in any of the supporting electrolytes he tried, including some that were basic. Silverman and Levy (IO), using platinum electrodes and a supporting electrolyte of sodium per-
OMPARTMENT A STOPCOCK I
POLAROGRAPHIC
CELL
Figure 1.
Polarographic cell
chlorate, obtained half-wive potentials of 1.00 for the ruthenium(VI1)-(VIII) 0.59 for the (VI)-(VII) couple and couple. Similar results were found with & R U ( C N ) ~in 0.2M potassium chloride (3). In perchloric acid solutions of ruthenium(1V) an additional wave was observed for the (11)-(111) couple a t -0.10 volt us. N.H.E. in 5-11 acid and -0.14 volt in 0.1M acid ( 7 ) . h number of preliminary experiments, made to establish applicability of the polarographic method to assaying technetium and ruthenium, had indicated that the polarographic method would ultimately permit better accuracy and precision than conventional radiochemical methods. These conventional radiochemical determinations (1) of molybdenum-99 are based on the anion exchange separation of hIo+6 followed by a-benzoin oxime precipitation and require about 1 day for 12 analyses. The precision is within +l%, and the accuracy is probably within k 5 % . I n conventional radiochemistry, radioruthenium is separated from other fission products by distillation of the volatile tetroxide and, after sufficient
+
+
decontamination, is finally precipitated as the metal. Duplicate analyses can be performed in approximately 3 hours. The precision and accuracy are usually larger than those of molybdenum-99technetium-99m. Hence, on the basis of available polarographic data and preliminary experiments, application of the subject method to technetium and ruthenium radionuclide separation from fission products appeared feasible. Both nuclides have a reduced state soluble in mercury (therefore removable from solution); extent of reduction was found proportional to concentration of radionuclide; other fission product radionuclides did not influence reduction; and sufficient radionuclide was removed to permit radiometric assay in a reasonable time. EXPERIMENTAL
Apparatus. A Leeds &- Northrup Electro-Chemograph, Type E, was used to obtain the polarographic data and also t o apply a known potential to the electrodes immersed in the radioactive solutions to be analyzed. T h e polarographic cell with Teflon stopcocks shown in Figure 1 is made ready for an analysis in the following manner: Compartment A is washed with 4N nitric acid, 1N sodium hydroxide, water, and alcohol, and then is air-dried. With stopcock 2 closed and stopcock 1 open, enough carbon tetrachloride is added to compartment B so that when the aqueous radioactive solution is added to compartment A with a transfer pipet the carbon tetrachloride-aqueous interface is about '/z inch above stopcock 1. The dropping mercury electrode and the saturated calomel electrode are then inserted into the radioactive solution and the cell is thermostated t o 25' i 0.05' C. The tapered dropping mercury electrode has a diameter a t its tip of about 2 mm. and should have a drop time of 4 seconds with a mercury head of about 40 cm. This capillary electrode is cleaned by rinsing with 4 S nitric acid and water after each change of radioactive samples. The saturated calomel electrode (Figure 1) is placed in contact with the radioactive solution through a potassium chloride-saturated agar gel bridge. Less than O . O O l ~ oof the radioactivity in a solution is held by the salt bridge after mashing. Reagents. The radioisotopes of molybdenum-99 and ruthenium-106 were obtained from the Oak Ridge National Laboratory. The molybdenum-99 had a specific activity of > 10 me. per gram of molybdenum and the ruthenium-106 was carrier-free. The uranium-235 fission products were obtained from irradiations at the Materials Testing Reactor. Total gamma activity of these radionuclides was measured with a high-pressure gas ionization chamber or a sodium iodide (thallium) crystal vel1 counter, and energy distribution with an Argonne 256-channel analyzer (9). The total
efficiencies (counts or milliamperes per disintegration) of all the isotopes used or separated are known on all three instruments. Procedure. T h e fission product sample containing a n equilibrium mixture of molybdenum-99 and technetium-99m is evaporated just t o dryness in a siliconed flask or test tube. This evaporation is not necessary if the sample volume is small compared t o the supporting electrolyte volume or is known. Exactly 1 ml. of a supporting electrolyte of 1M sodium citrate and 0.1M sodium hydroxide is added. A transfer pipet is used t o place the solution in the cell and - 1.55 volts (21s. S.C.E.) are applied to the electrodes. The height of the mercury reservoir is fixed to give a drop time of 4.0 seconds. A stopwatch is started when a mercury drop breaks away from the capillary with stopcock 1 open and stopcock 2 closed (Figure 1). Stopcock 1 is then closed and stopcock 2 is opened and closed (to remove the mercury drops that accumulated before the start of the collection) and stopcock 1 is opened. ilfter 3 minutes, the stopwatch is turned off, stopcock 1 is closed, stopcock 2 is opened, and the 3-minute collection of mercury drops and a small amount of carbon tetrachloride are removed to a siliconed test tube. The carbon tetrachloride is sucked away using plastic tubing 1 mm. in inner diameter, connected to a vacuum line. Two gentle water washes of about 5 ml. each are made of the mercury. The mercury is then dissolved in exactly 1 ml. of concentrated nitric acid, so that the percentage of 140-k.e.v. gammas absorbed by the mercury is constant and the radioactivity in the resulting solution is assayed in the sodium iodide (thallium) crystal well counter. Sufficient counts of the sample are taken to give good counting statistics. Evaluation of Counting Data. Table
I s h o w a n example of the corrections applied to the counting rate to determine the amount of technetium-99m in the fission product sample. The data taken a t the time of the experiment are listed in columns 1, 2 , 4, and 9. The counting rates recorded in column 4 are from both technetium-99m and ruthenium-103,106. Several days after these counts were made, samples were recounted to obtain the counts due to the long-lived ruthenium-103,106; subtraction yields the total counts of technetium-99m. This correction is not necessary if the count rate of the ruthenium-103,106 is only a small or known per cent of the column 4 total count rate. Three other corrections must be applied to the counts of technetium99m. The first is for the decay of technetium-99m from the time of the start of the separation to the time of the start of the count. For longer separations or longer counts, it is necessary to take the time from the mid-point of the separation to the mid-point of
the count. Another correction arises from the impossibility of collecting the mercury drops for exactly 180 seconds. The final correction that is applied is necessary because the technetium-99m is continually being removed from the solution a t the rate of about 0.07% per minute as soon as the potential 1s applied to the electrodes. This is a rather complex correction, as not only is a constant percentage of the technetium99m being removed but more technetium-99m is growing in from the decaying molybdenum-99 in the solution in order to re-establish the molybdenum99-technetium-99ni equilibrium ; moreover, the molybdenum-99 itself is decaying at a significant rate during the time of analysis. The number of atoms of technetium-99m, -Y&,in the solution at the time the voltage is applied to the electrodes is
because the technetium-99ni is in equilibrium with the molybdenum-99. I n this expression 0.875 is the fraction of molybdenum-99 disintegrations that decay to technetium-99m and T indicates half life. The number of atoms in the solution of technetium-99m, STo, a t any time, t , after the start of the electrodeposition is :
e-IK
r'
hTo)t]
where K is the experimentally determined constant rate of removal of technetium-99m plating out on the mercury drops. The expression for N T ~ has been used to derive column 11 of Table I. This column is the ratio of the number of atoms of technetium99m in the solution a t t = 0 to the number of atoms of technetium-99m a t some time after the start of the electrodeposition. A table including all three corrections can conveniently be prepared for 1-minute intervals, but because K is different for different electrodes and experimental conditions, it is not included here. The number of fissions in the sample may be computed from the experimentally obtained count rate of technetium99m by calculating the count rate of technetium-99m a t the time of irradiation in the entire milliliter of sample, and converting to the disintegration rate of molybdenum-99 by a previously determined calibration factor and thence to the number of fissions from a knowledge of the fission yield of molybdenum99. This same type of analysis may be made on the count rate of the rutheniumVOL. 32, NO. 7,JUNE 1960
* 781
Count Rate of Tcs9m on a 3-Minute Collection of Mercury Drops from Corrections Applied to Observed Count Rate C.P.M. Tc$$m C.P.M. Corr. for C.P.M. Tc99m At of ColTc'dorn - Corr. for lecting Hg C.P.M. Corr. Tcg$m C.P.M. TcSDm A f for for X0Tc99m Time of Start D~~~~ RulO3~106 Decay Collecting (COl. 8 Factor C.P.M. C.P.M.b C.P.M.. (Col. 4 - (Col. 7/ HgDrops, x 180 NoTcgQmN T Of Of separation" counta for At Observed R u Q ~ , U ~Ru1o3v106 Col. 6) Col. 3) Seconds Col. 9) N TcDgm (Col. 10 X Col. 11) 180.0 60,847 1.0075 61,303 1314 1320 0.98856 274 540 60,151 60,847 60,691 61,423 60,966 60,966 1322 0.98478 60 ,038 60,578 1.0126 62,195 1320 1326 0.98856 277 546 60,516 179.4 61,421 61,216 61,062 62,136 1328 0.98478 60,227 61,363 61,158 60 ,773 1.0168 61,305 271 534 59,337 179.2 60,292 59,871 60,024 1325 1331 0.98856 60,649 59,647 58,478 59 ,382 59,012 1333 0.98478 1.0251 60,886 262 517 58,487 179.3 59,395 59 004 59,164 1335 1341 0.98856 60,922 59,430 58,298 59,199 58 815 1343 0.98478 1.0292 61,531 59,553 253 499 58,985 179.3 59,785 1340 1345 0.99046 59,484 61,960 60,202 59,968 59,055 1348 0.98478 59,554 1.0335 61,011 264 521 57,857 58,639 178.8 59,033 0.98667 58,378 1345 1352 61,110 57,731 78,735 59,129 1354 0.98290 58,252 Av. 61,369 =t505 0 Teeted March 12, 1959. b Tested April 20, 1959. Table 1.
Table II. Precisions for Technetium and Ruthenium Polarographic and Radiochemical Analyses Polarographic A. Bb Radiochemical Std. dev., Std. dev., Std. dev., Soh. Compn. Analyses % Analyses % Analyses yo M~gg-T~ggm 78 f0.81 13 f1.20 Tcggm in fission products 195 0.97 24 1.4 132 f1.8 RulOe 35 1.7 RulO3 in fission products 75 2.4 4 Error involved in successively sampling one solution (as in Table I). b Different sample for each determination.
103,106 separated from mixed fission products. RESULTS AND DISCUSSION
Precision and Accuracy of Technetium-99m Procedure. T h e method used t o assay the amount of technetium-99m polarographically removed from fission products is based on t h e amount of technetium-99m deposited in 3 minutes from a known equilibrium mixture of pure molybdenum99-technetium-99m in 1 ml. of solution. This can be expressed a s the ratio of the disintegration rate of amalgamated technetium-99m to the disintegration rate of technetium-99m in the equilibrium mixture of molybdenum-99-technetium-g9m, or
chlo
where C is counting rate, 782
0.962
+ To B
ET^
is the effi-
ANALYTICAL CHEMISTRY
ciency (counts per disintegration) of the counter, A is the disintegration rate, and 0.962 is the ratio of the disintegration rates of technetium-99m to molybdenum-99 in a n equilibrium mixture. Evaluation of the expression for a number of individual determinations yielded a ratio value of 0.223 X The determinations were carried out under the following conditions: temperature, 25" C.; mercury drop time, 4.0 seconds; supporting electrolyte, exactly 1 ml. of 1M sodium citrate and 0.1M sodium hydroxide; voltage, - 1.550 volts us. S.C.E.; and collection time, 180.0 seconds. This value of 0.223 X lod2 is the same for removing technetium-99m from mixed fission products, as shown by the agreement in the polarographic and radiochemical determination of molybdenum-99 in mixed fission products. The precision of the determination of technetium-99m by the polarographic technique is summarized in Table 11. It appears from later experiments that usual concentrations of gelatin improve
this precision. The test of accuracy of the polarographic procedure for technetium-99m in fission products was t o apply the value of 0.223% of the technetium-99m removed by a 3-minute collection of mercury drops from solutions of molybdenum-99 to the per cent of technetium-99m removed by a 3-minute collection of mercury drops from fission products, and to compare this value of technetium-99m (and hence number of fissions) with the value obtained by conventional radiochemical analysis for molybdenum-99. Table I11 shows the good agreement between the two methods. Time Required for Analysis. The time required for a single polarographic technetium-99m separation is about 3 minutes (neglecting aliquoting and counting). The rapidity of the entire procedure may be estimated by the fact t h a t the number of fissions in 12 fission product samples analyzed in quadruplicate was determined by t x o people in 1 day. This time will be increased, however, if the amount of molybdenum-99 in the sample is small and long counts are required to get good counting statistics, or if it is necessary to wait for the decay of technetium99m and to recount and subtract the ruthenium-103,106 background. Amount of Radioactivity Required. The aliquot size is, of course, dependent on the number of fissions in the sample and the age of the sample. Figure 2 shows the disintegrations per minute expected from various activities of uranium-235 slow-neutron fission samples a t various times after fission. T h e radiochemist usually knows approximately the number of fissions in a sample, either by a knowl-
24,000
106
I
l
i
-02
-04
-06
I
ro
z -
103
rn
P
LL
b m E
I-"
102
+02
10
0
Figure 2. Amount of technetium-99m and ruthenium103 expected in 3-minute collection from various activities of fission product samples
trolyte that is used. Several supporting electrolytes were tested to effect a clean separation of technetium and ruthenium from fission product mixtures. The ruthenium radionuclides were separated in almost all cases, but the technetium in very few. Tellurium-132 vias separated over short voltage ranges in most supporting electrolytes and also small amounts of radioiodine appeared to absorb on the mercury a t low negative voltages. I n a supporting electrolyte of 1N sodium citrate and 0.lN sodium hydroxide very good separations of technetium-99m and ruthenium-103,106 from fission products are obtained. Figure 3 shows the activity of radionuclides that are removed by electrodeposition of mercury from sodium hydroxide-sodium citrate a t various applied voltages from 3-day-old fission products. I n the region of - 1.55 volts
-14
-16
-18
-20
Sample NO.
Radiochemical 8.7 x 1.23 X 4.11 X 1.83 x
1 2 3
5
b
vs. S.C.E. a good separation of technetium-99m and ruthenium-103 is made from the other radionuclides. The data for Figure 3 were obtained from the gamma spectra taken of the amalgam collected a t each potential. A better estimate of the small amounts of impurities expected in the separated technetium-99m and ruthenium-103 was determined by assays of solutions containing individual pure radionuclides electrodeposited under the same conditions as the technetium99m and ruthenium-103 analysis. Approximately 100 pc. per ml. of each radionuclide was analyzed. Column 3 of Table I V lists the disintegration rate of each radionuclide retained by the 3-minute collection of mercury drops. Each of these values represents the average of a series of collections made a t -1.55 volts us. S.C.E. in a supporting electrolyte of 1M sodium citrate and 0.1M sodium hydroxide. Greater than a lo4 decontamination was obtained for
Polarographic and Radiochemical Determinations of Number of Fissions in Samples of Mixed Fusion Products
Table 111.
~
-12
Figure 3. Radionuclides polarographically deposited from 3-day-old fission products
TIME AFTER FISSION (DAYS)
edge of the history of the sample or by a gross activity measurement of the sample. Approximately IO1' fissions are taken for a radiochemical determination of molybdenum-99. Figure 2 shows that only slightly hotter samples are required for the polarographic determination because it is not necessary to wait for the technetium-99m to grow in as it is when molybdenum-99 is determined radiochemically, and the efficiency of well counting is much greater than the end-on gamma counting used when chemical yields must also be determined. However, it is always possible to improve the counting statistics by counting for longer times, collecting the mercury drops for longer than 3 minutes, or reducing the volume of the solution. Interfering Radionuclides. Uranium-235 fission products 3 days old contain about 15 radionuclides, each of which contributes between 1 and 20% of the total disintegration rate present, and many others t h a t contribute less than 1%. Thus one primary requirement in the polarographic separation of technetium99m and ruthenium-103,106 from fission products is to reduce the concentration of all other radionuclides present by a t least a factor of lo3. The degree of contamination of separated technetium-99m and ruthenium103,106 by other fission products will depend mainly on the supporting elec-
- 0 8 -10 VOLTS
Polarographic. x 1013 A 0.09
8.91
51 1.28 x 10'9 Zk 0 . 0 2 12 4 . 2 1 X 10" Zk 0.05 5 4 1.84 x 1013 0.02 86 Aliquots of about 10" t o 101*fissions taken for anal sis Number of analyses represents 4 or 5 assays of each d q u o t taken from sample. ~~
1013 lo1* 10" 1013
Polarographic Analyses Madeb
~
VOL. 32, NO. 7, JUNE 1960
783
13 12 I 1 E
0,
e
27 25
J5
45 -EVPERA-URE
55
I05
65
"C
Figure 4. Variation of technetium-99m deposited with temperature 60~10~:
the radioelements that are most abundant in early time fission products, with the exception of tellurium-132-iodine132, which had a decontamination of lo3. Complete polarographic spectra were made with macro amounts of these radionuclides t o determine if a better decontamination (smaller removal) a t nearby voltages was possible. The above information indicates that high yield fission product radionuclide impurities do not interfere with the technetium-99m separation. Several half-life measurements, made with polarographic separations of technetium -99ni from fission products and from molybdenum-99, all agreed within =t2% of the accepted value of 6.04 hours. The decontamination from technetium99m of all the isotopes listed in Table IV, with the possible exception of tellurium-132, s e e m adequate. I n opposition to the data of Table IV, tellurium-132 has never been observed by gamma spectrometric analysis in ruthenium-103,106 polarographic separations from fission products after the decay of the technetium-99m (about 3 days). Fission product samples of separated ruthenium-103,106 counting 105 c.p.m. showed no photopeak of
Table IV.
_I
Zrg6-Nbg6(C.F.) S F (C.F. ) Te132-II32
I
,
2
3
tellurium-132 (or any other radionuclide), This indicates a tellurium132 decontamination of much greater than lo3. The relatively large amount of tellurium-132 shown in Table IV mag be due to the large quantity of isotopic tellurium carrier which was present here but not in the fission product mixture. Electrode Reactions for Reduction
of Technetium-99m. The shape of the technetium-99m activity-voltage plot shown in Figure 3 deserves consideration. Exactly the same activity-voltage curve is obtained for technetium-99m from pure solutions of molybdenum-99. The normal type of concentration-voltage curve (as in regular polarograms) is not observed. because the oxidation states of technetium that are not amalgamated with the mercury are not separated from the radioactive solution and assayed. It seems likely that the only two forms in which the technetium mill amalgamate with mercury are the +4 and the zero
D.P.hf./Ml. Added.
D.P.M./3-Minute Collection on Mercury.
Radionuclide Collected on Mercury
3.76 X lo8 7 72 X lo8
41 113
1.1 x 10-7 1 . 5 x 10-7 4 . 8 x IO-@ 1 . 3 x 10-7 7 . 9 x 10-8 1 . 9 x 10-6 2.23 X lo-" 1.27 x lo-"
1 -
87 108 _. K ,~_
2.86 X lo8 2 . 5 2 X lo8 3.75 x 106 108
9
38 20 7
Decontamination of Radionuclide from T c 9 9 m 2 . 0 x 104 1 . 5 x 104 4 . 6 x 104 1 . 7 x 104 2 . 8 x 104 1 . 2 x 103
Tcg8m(C.F.) 2 . 2 3 X lo5 Rulo6(C.F.) 108 1.27 x lo6 Of parent only. * Carrier-free. c Average values determined from pure solutions of Mog9-Tcg9m and Ru106-Rh106.Hypothet,ical values given in columns 2 and 3. 0
~
784
ANALYTICAL CHEMISTRY
4 5 6 DROP TIME (SFCJ
'
e
9
Figure 5. Variation of technetium-99m and ruthenium-1 06 activities with drop time of dropping mercury electrode
Fission Product Impurities in Polarographically Separated Tcg9" Fraction of
Radionuclide Ce144-Pr144 Csl3' (C.F.)* Rn 133
1 1
oxidation states. Technetium is initially in the +7 oxidation state and it seems reasonable from the known oxidation potentials of technetium (2') to say that the wave a t -0.25 volt is due to the reduction of Tc+7 to Tct4 as TcOz. The TcOz is further reduced before completion of its wave to TcT3and a sharp decrease in activity on the mercury drops after -0.3 volt is observed. At negative potentials greater than - 1.0 volt there is a gradual increase of technetium-99ni activity until a fully developed wave is obtained a t - 1.4 volts; this could indicate reduction of Tc+3 to the metal. .It -2.0 volts a sharp increase in activity is observed because of a rapid increase of drop time (see belon-) and the slight evolution of hydrogen gas. There are probably other unobserved oxidation steps as the voltage is increased, because 1-electron steps are found from the slope of the wave a t -1.4 volts as me11 as for the earlier wave. I n fact, a slope corresponding to a 1-electron reduction is found for the decreasing portion of the curve at -0.35 volt. A normal polarogram along with an activity-voltage plot of Tc998O4- in the supporting electrolyte used here would help to clarify the oxidation states involved. The reduction of technetium to technetium metal a t - 1.4 volts is similar to the polarographic reduction of l\In+Zto manganese metal in basic complex ion solutions. This is expected because the chemistry of technetium in its higher oxidation states is unmistakably most like rhenium; its lo\yer oxidation states resemble manganese more than rhenium. The above proposals are only tentative and an exact explanation awaits more experimental work.
One-electron steps are involved in each of the observed reductions according to the following analysis, which is simply an extension of the type of derivation that has often been applied to the number of electrons involved in normal polarographic reductions. I n considering the reduction a t - 1.4 volts the follon-ing reversible and rapid reaction may be assumed to take place at the dropping mercury electrode.
+ ne + Hg = Tc(Hg)
Tc'n
Tc(Hg) is the very dilute amalgam formed a t the surface of the mercury drop. The potential for this reaction may be writter. [Tc:: Tc Tnj:
RT
Ed,,. = E." - - In
nF
U H[~
when [Tc]; is the concentration of the amalgam formed on the surface of the mercury drops and [Tc+n]: is the concentration of Tc-% ions in the layer of solution a t the surface of the mercury drops. There is no need to introduce activity coefficients, as the solutions are essentially a t infinite dilution-the concentration of Tc+n ion in a typical fission product solution is of the order of 1 0 - 1 4 ~ . It may be derived that the activity, 9, of technetium ions a t the surface of the mercury drop, a t any potential is [Tc+"]P =
-A
A,,,
KS
where K , is a constant. The concentration of technetium in the amalgam a t any potential is directly proportional to the activity, A , [Tc]," =
A K.
Substitution of concentrations of technetium a t the surface interface in the solution and the mercury a t any potential into the original equation for the potential of the dropping electrode, Ed is RT RT K Ed,,. = E." In uHg - - In *nF nF KO
+
The last term becomes zero \Then the wave is half-formed or a t -4 = Amax,'2, so that E,,,
=
E."
RT K + RT - In aHg - - In nF nF K,
and the equation for Ed,*.becomes
A plot of log A/(A,,, - A ) 2s. Ed e for the wave a t -1.4 volts gives a straight line u hose slope is 0.055, which indicates a 1-electron reduction (theoretical = 0.059). This analysis assumes,
COLLECTION T I M E ( S E C )
Figure 6 . Effect of amount of mercury on absorption of technetium-99m y-rays Drop time 4.0 seconds
of course. that the electrode reactions are reversible and the diffusion process is the rate-controlling step. One-electron steps are also measured for the first technetium-99m wave a t -0.2 volt and for its disappearance (which would be equivalent to the next reduction wave in a normal polarographic sequence of waves). If 1-electron reductions are occurring at each step, unobserved reductions of technetium from Tc+7 to Tc+5 and from T c +to ~ Tc+' must be taking place. Variables Involved in Technetium99m Procedure. The degrees to which several parameters govern quantitative separations of technetium-99m were investigated. Variations in the technetium-99m activity deposited rrith change of temperature, drop time, collection time, and ionic strength were measured. The effect of temperature changes from 25" to 65" C. on the amount of technetium-99m deposited is shown in Figure 4. The temperature was determined by a thermometer placed next to the dropping mercury electrode. The temperature of only the citrate solution was varied; the temperature of the calomel electrode remained constant a t 25" C. -4value of approximately 0.6% change in activity deposition per degree was obtained for the temperature coefficient. This value is loiv compared to the temperature coefficients of about 1.5 to 2% per degree normally obtained in polarography. The slight sigmoid shape of the curve in Figure 4 suggests that perhaps competing ratecontrolling reactions are taking place at the different temperatures. One of these effects might be due to the increased rate of reaction that, causes the decreases in technetium-99m activity after -1.5 volts. This reaction might be a t an increased rate a t higher temperatures in the s l o ~ reduction of technetium metal to Tc-'. The temperature should be controlled to within * l o C. for a precision to i l % .
Figure 5 shows the variation of technetium-99m deposited with drop time. I n normal polarographic analyses the optimum range of drop time is 3 to 6 seconds. In this work a drop time of 4.0 seconds was chosen. The amounts of technetium-99m deposited are dependent on drop time and there is almost a 100% increase in activity deposited per unit time for a drop time of 2 seconds compared with that deposited a t 9 seconds (Figure 5 ) . The amount of increase in technetium-99m with decreasing drop time is about that expected and is similar to the situation found in normal polarography. The errors involved in the quantity of technetium-99m deposited for drop times close to 4 seconds is about 1% for every 0.1 second. The drop time can easily be controlled to 0.1 second but should be checked periodically during a series of runs. The variation in observed technetium99m activity with the amount of mercury collected at a constant drop time is shown in Figure 6; variations in the corrected data have been normalized to a 180-second collection time. The observed decrease in activity is due almost entirely to the absorption of the 11-eak technetium-99m y-rays in the increased amounts of mercury collected. The ratio of the heights of the 140-k.e.v. photopeak of technetium-99m to the x-ray peak of mercury was lower when larger amounts of mercury were present. Figure 6 shows that only 67* of the technetium-99m counts are lost by absorption of mercury with a 180second collection, n-hile 54% are lost with a 1000-second collection. However, less than 0.5% of the technetium99m counts are lost with each 10second increase in collection time in the neighborhood of 180 seconds. Because with a 4-second drop time, the collection time can be controlled to \\ithin 180 =k 2 seconds, no significant error is involved in variations of a few seconds. If collection times much different than 180 seconds are involved. corrections can be made from Figure 6. Effect on the removal of technetium99m by increasing the ionic strength of the supporting electrolyte with sodium chloride has been studied because of the interest in applying this polarographic technique to analysis of sea water samples of fallout. With amounts of sodium chloride that are found in sea water and with sea nater itself containing mixed fission products, a slight negative shift in the technetium-99m wave is observed which results in a slight increase to 0.246 in the percentage of technetium-99m removed for a 3-minute collection. This value of the per cent of technetium-99m removed would be different for different concentrations of salt or sea water and would necessitate recalibration. VOL. 32, NO. 7, JUNE 1960
785
Polarographic Determination of Ruthenium-103,106 in Fission Products. Most of the emphasis of this work has been on the development of a fast procedure for technetium-99m because of its common use as a monitor for a number of fissions. However, ruthenium activities are separated from fission products under t h e same experimental conditions as the technetium-99m, so that another analysis may be obtained by only a n extra count of the separated ruthenium activities after the technetium-99m has decayed out. The deposition of ruthenium-103,106 from fission products is shown in the activity-voltage plot in Figure 3. More ruthenium is deposited at lo\Ter than at higher potentials. The same type of counting corrections is applied to the ruthenium-103,106 counting values as to the technetium-99m counts: corrections for collection time and for the continuous depletion of ruthenium activities from the solution after the potential of - 1.55 volts have been applied. This last correction is required by the 0.127% ruthenium removal during each 180-second collection. This value of 0.127% is the mean value of many determinations with pure solutions of ruthenium-106-rhodium-106. I n these measurements, the ruthenium-106 solution was counted in the well counter before deposition and the dissolved ruthenium-106 amalgam was counted after deposition. Because there are no short-lived parent products involved for ruthenium-106 and the half life of ruthenium-106 is about 1 year, the calculations are simple. The same is true for ruthenium-103, which is the main ruthenium constituent of earlytime fission products. To count only ruthenium-103 from early-time fissionproduct separations of ruthenium activities, the test tube containing the dissolving mercury and ruthenium activities is placed within a paraffin envelope in a 100-ml. Lusteroid tube. About 1 cm. of paraffin separates the ruthenium activities from the well crystal and most of the hard betas from the small fraction of ruthenium-106 present are adsorbed and do not contribute t o the gamma count. The rhodium-103m must be allowed to grow in before the amalgam is counted. Table I1 illustrates the precision obtained in polarographic determinations of ruthenium-106 in ruthenium-106 solutions and ruthenium-103 in fission product mixtures. The accuracy of the
786
ANALYTICAL CHEMISTRY
polarographic determination for ruthenium-103 in 15 fission product samples (based on the radiochemical determination of ruthenium-103) was t o =t3.3%. The ruthenium-103 separated polarographically and radiochemically was counted under exactly the same conditions and a t the same time to eliminate errors due to absolute determinations of ruthenium-103 disintegration rates. The precision and accuracy obtained for the ruthenium-103,106 analysis in fission products were not as good as for the technetium-99m separation because the count rate of the deposited ruthenium-103,106 was usually low. Moreover. the ruthenium in the amalgam appeared to be more soluble in water and carbon tetrachloride and adhered to the walls of the siliconed test tubes more readily than did the technetium-99m. When a mercury drop containing deposited ruthenium activities was shaken for 1 minute with water or rvith carbon tetrachloride, about 1% of the ruthenium activity dissolved in the water solution, about 2y0 appeared in the carbon tetrachloride, and about 1% n.as deposited on the lvalls of the siliconed test tube. Many variations in the mashing procedure were attempted in order to get more reproducible results for the ruthenium activities. These consisted of washing the mercury drop n i t h a continuous slow-moving jet of water immediately after falling through the carbon tetrachloride; adding aerosols to the radioactive solution or the carbon tetrachloride in the cell to prevent small quantities of the radioactive solution from adhering to the mercury drops; collecting and washing the mercury drops in polyethylene test tubes; and using various organic and inorganic solutions for 11ashing the mercury drops. The procedure described in the technetium-99m analysis was the best. The same decontamination factors from fission product radionuclides apply to the ruthenium-103,106 separated a t -1.55 volts as to the technetium-99m. No contaminations have been observed either by gamma spectrometric analysis or half-life measurements of the ruthenium-103,106 separated from either new or old fission products. Figure 2 describes the disintegration rate of ruthenium-103 that will be deposited in a 3-minute collection from fission products of various ages. Only about one third of the ruthenium-103 disintegrations are actually counted, SO it is probably necessary to have a n
unfractionated fission product sample of at least 10l2 fissions in order to get good counting statistics. Studies were made of the variation of ruthenium activity removed with variation in drop time, time of collection, temperature, and ionic strength. The variation with drop time is shown in Figure 5 , where the slope of activity us. drop time at 4 seconds is slightly steeper than that for technetium-99m. This means that the drop time again must be carefully controlled to *0.1 second. The time-of-collection studies show that there is less than 1% adsorption of ruthenium-106 gamma radiation for a 180-second collection. This value is about 4% for ruthenium-103 because of the weak 40-k.e.v. gamma of rhodium103m. The temperature coefficient is a straight line over the temperature range of 25” t o 65” C. and is equal to 0.89% change in activity deposition per degree. Finally, the effect of sodium chloride in a concentration equal to that in sea water on the amount of ruthenium-106 separated is not noticeable. Hence, this polarographic procedure may be applied to the determination of radioruthenium in sea water. LITERATURE CITED
(1) Barnes, J. W., Lang, E. J., “Collected
Radioc!;mical Procedures, Molybdenum, 2nd ed., Los Alamos Scientific Laboratory, La-1721 (Aug. 18, 1958). (2) Cartledge, G. H., Smith, W. T., Jr., J . Phys. Chem. 59,1111 (1955). (3) Ford, D. D., Davidson, A. W., J. Am. Chem. SOC.73, 1469 (1951). (4) Love, D. L., Anal. Chim. Acta 18, 72 (1958). (5) Love, D. L., “New Techni ues in Radiochemical Determinations %rough
Polarographic Methods,” U. S. Naval Radiological Defense Lab., Tech. Rept. USNRDL-TR-225 (March 31, 1958). (6) Motta, E. E Larson,. Q. V., Byrd, G. E., “Pluto&xn Project Records,’, Tech. Rept., Clinton Laboratories, Mon C-99 (April 1947). (7) Niedrach, L. W., Tevebaugh, A. D.,
J . Am. Chem. SOC.73,2835 (1951). (8) Rogers, L. B., Ibid., 71,1507 (1949). (9) Schumann, R. W., McMahon, J. P., Rev. Sci. Instr. 27, 675 (1956). (10) Silverman, M. D., Levy, H. A., J . Am. Chem. SOC.76,3319 (1954). (11) Willis, J. B., Ibid., 67,547 (1945).
RECEIVEDfor review January 11, 1960. Accepted March 30,.1960. Third Copference on Analytical Chemistry in Nuclear Reactor Technology, Gatlinburg, Tepn., October 28, 1959. Part I1 of a series of articles on “Radiochemical Analyses through Polarographic Methods.” Part I has been published (4).
