Electrodeposition of Actinide Elements at Tracer Concentrations RUSSELL
F.
MITCHELL
Reactor Monitoring Cenfer, Tracerlab, lnc., 2030 Wright Ave., Richmond 3, Calif.
b The electrodeposition of the actinides from an electrolyte of ammonium chloride-hydrochloric acid affords a rapid method for recovering trace concentrations of these elements in high yield. The plating procedure produces deposits which are adherent and uniformly distributed over the area of the plate. Normally, the deposition is carried out from an electrolyte volume of approximately 5 ml. However, volumes up to 15 ml. have been used without greatly affecting the yields. alpha-emitters are used as tracers, or when i t is desired to analyze alpha-emitting nuclides by pulse height analysis, it is important to produce samples having uniform invisible deposits. The best technique for meeting this requirement is electrodeposition. This laboratory has previously used a n electrolyte of ammonium sulfate-sulfuric acid for the deposition of the actinides on platinum at tracer concentrations. The ammonium sulfatesulfuric acid procedure was adopted for use here by L. F. Tischler in November 1953, following the suggestion of Wingerson (6). Good yields were obtained for all actinides except protactinium. Wentzler stated that the protactinium yields could be improved by increasing the ammonium sulfate concentrations (6). With this increase, 45 minutes of plating would normally give acceptable yields. Although good yields could be obtained from the ammonium sulfate-sulfuric acid baths, the plates often had a dark deposit of unknown composition and thickness. It was not known to what extent this deposit might affect counting results, so it became necessary
to adopt another plating procedure. Most of the procedures referred to in the literature have been adapted for plating micro to milligram amounts of actinides, and the methods usually require an hour or more for the deposition to be quantitative. Recently, Chopin has reported that americium and curium had been electrodeposited in good yield from a n electrolyte of ammonium chloride-hydrochloric acid ( I ) . Consequently, a study was initiated to see if this procedure could be extended to the plating of other actinide elements.
HEN
Table 1.
hctinide Th230 Pa”1 Pu239
ArnZ41
Cm244
326
850 575
640
Np237
All samples were electroplated using a Sargent-Slomin Electro-Analyzer with a rotating electrode. Electrodeposition cells, manufactured by Tracerlab, Inc., especially for use in electroplating tracer activities for counting, were used to contain the electrolyte. These cells consist of a metal frame, a t the bottom of which a platinum disk is placed to serve as the cathode. A Teflon gasket and a glass tower are secured on top of the disk. The platinum-iridium anode stirrers were fabricated by Western Gold and Platinum Works, Belmont, Calif. Platinum disks 5 mils thick and 2.2 cm. in diameter, precut, mirror finish, were obtained from General Plate Co., Attleboro, Mass. The Teflon gaskets were cut so the deposits would cover an area of approximately 3 sq. cm. Before use, the platinum disks were washed with water and ethyl alcohol, and then flamed to dull red over a Bunsen flame. The electroplated sampleswere counted with a Tracerlab Model CE-13 2 T Alpha scintillation counter or with a Tracerlab RLD-1 Frisch grid chamber in conjunction with a 50-channel analyzer. Materials. A11 chemicals used were
Electrodeposition of Actinides at Tracer Concentrations
DPM Tracer Added
2500
u233
EXPERIMENTAL
Apparatus.
758 143 129
ANALYTICAL CHEMISTRY
Per Cent Deposited 3 min. 57 35 48 70 58 35
30
5 min. 84
70 74
87
88 .
I
..
8 min.
98 80
88
93 .. 83 72
10 min. 99 85
90 91
100
92 83
15min. 100
95 98
94
100 100 98
reagent or c. P. grade. T h e radioactive materials in dilute hydrochloric acid solutions were obtained either from Oak Ridge National Laboratory or t h e University of California Radiation Laboratory. RESULTS A N D DISCUSSION
To determine the optimum conditions for plating the actinide elements, it was necessary to determine the effects of such parameters as current density, p H of electrolyte, salt concentration, volume of electrolyte, and the time of plating on the electrodeposition for these elements. To determine the minimum time required for maximum deposition of the actinide elements, aliquots of actinide alpha-tracers in the chloiide form were pipetted into plating cells containing 1 nil. of hydrochloric acid neutralized t o the methyl red end point with amnionium hydroxide. The solutions were then made acid with 2 drops of 2N hydrochloric acid in escess of the methyl red end point. They were then electrolyzed for 3, 5, 8, 10, and 15 minutes. At the end of the plating period, the solutions n ere made basic ’IT ith 1 ml. of ammonium hydroxide. The current p a s turned off and the anode removed from the solution. No noticeable effect was observed, whether the current n as turned off before or after removal of the anode. The electrolyte was removed from the cells and the platinum disks n ere rinsed with water. The disks 13-ere removed from the cell, washed with ethyl alcohol, and then flamed to a dull red before counting . The deposition yields of the actinides us. the time of deposition are shown in Table I. The amount of tracer deposited increases rapidly with time up to about 8 minutes, after which yield increases only slowly. For thorium and plutonium the deposition is essentially complete after 10 minutes. For uranium, neptunium, americium, and curium the yields approach 100% after 15 minutes. The deposition yields of protactinium are consistently lower than for the remaining actinides. The maximum yield appears to be approximately 95%. For the remaining experiments, plutonium-239 was selected as the tracer.
In practice, all the actinides behave in ncarly the same manner with respect to plating conditions. Therefore, it is felt that the optimum conditions determired for plutonium can be estended to other iictinides as well. FGr these experiments, a stock solution of ammonium chloride was prepared containing 0.194 mg. of chloride ion per ml. The pH of this solution was adjusted to 6.4 with ammonium hydroxide. Aliquots were added to the plating cells. The:tracer in 2N hydrochloric acid and additional acid. base, or water were then added to give the desired pH concentration conditions. At the end of the plating period the solutions were made basic and the disks washed and flamed as described above. In all the experiments, the anode was adjusted t o approximately 5 mni. above the cathode. T o investigate how plating yields vary with current density (current per square centimeter of cathode), plating solutions containing 0.115 gram of chloride ion per ml. (total volume 6.26 ml.) and 1880 dpm plutonium-239 tracer were made up and mere electrolyzed for 10 minutes. The results are shown in Figure 1. From these data it may be seen that maximum yields are obtained with current densities of 0.6 to l ampere per sq. em. Figure 2 shows how plating yields vary with chloride concentration at constant volume. For the constant volume study, alicjuots of the ammonium chloride solution were added to the cells. A constant volume of plutonium tracer (0.25 ml.) in 2 N hydrochloric acid and additional water to give a total electrolyte volume of 4.5 ml. mere then added. -411 solutions were electrolyzed for 10 minutes. I n this eyperiment, the chloride concentration varied from approximately 0.05 to 0.18 gram per ml. The deposition yields of the plutonium-239 varied from 74% to nearly 100%. Below a chloride concentration of 0.1 gram per ml. the deposition yields dropped below 90%. At the lower chloride concentrations (0.06 gram per ml.) the current through the cells also decreased, so that the lower yields a t these concentrations n ere due in part to the decreased current densities. To study the effect of volume on plating yields of plutonium, 0.25 ml. of plutonium-239 tracer in 2.V hydrochloric acid was added to varying volumes of the ammonium chloride solution (Figure 3). Below a volume of 6 nil. the yields remained greater than 90%. When the volume was greater than 6 ml., the deposition yields of plutonium dropped and were less than 70% at a volume of 8 ml. This might be explained by assuming that a t the larger volumes it takes longer for the
8ol 7
70
m
60
0 20
0 2
Ploting
"
3
C u r r e n t -Amps
g
Figure 1. Effect of current on deposition of plutonium
Sample 1 2
3
4 5 6 7 8 9 10 11 12 a
0.2
Cl/ml
Figure 2. Effect of chloride concentration on deposition yields of plutonium
plutonium ions to migrate into the region of the cathode. The rate of deposition is not a function of the total plutoniuin concentration, for it can be demonstrated that under the same conditions, 10 dpm of plutonium-236 tracer (8.9 X gram) can be deposited as efficiently as 10 dpm of plutonium-239 (7.3 X 10-11 gram). Similarly, for the remaining actinides, tracers a t concentrations of the order of gram or less can be deposited n i t h the same efficiency as tracer concentrations a t 1O-l1 gram or greater. Ilased on this, it was assumed that the decrease in deposition yields in Figure 3 n a s due to a volume effect and not to the concentration of plutonium. -1 filial experiment was made to determine horn variations in volume and ammonium chloride concentrations would affect the deposition of plutonium under normal operation conditions. An attempt was made to keep anodecathode separation and starting current constant, but no effort was made to keep the current constant throughout the plating period. Plutonium-239 tracer in concentrated hydrochloric acid \vas added to the
Table II.
.02 0 4 0 6 .08 0.1
Constant volume 4.5 mi.
6ol
a
"' 20
a"
0
1
2
3
4
5
6
7
8
9
Toto1 Volume
Figure 3. Effect of volume on deposition yields of plutonium CI- constant a t 0.19 g r a m p e r ml.
plating cells. The solutions were neutralized with ammonium hydroxide in slight excess of the methyl red end point. The volume of water added varied from 0 to 10 ml. and the total volume of the plating solution varied from 2.2 ml. to 15.7 nil. All samples
Plutonium-239 Solutions Electrolyzed for 15 Minutes a t 2 Amperes
Concd. HC1 Added,
HJO Added,
Total Volume,a
All.
311. 0 2 3
All. 2.2 4.2
1 1 1 1 1 2 2 " 7
2 3 3 3 3
5
10 2 3 5 10 2 3 5 10
5.2
7.0 12.0
5.9 6.8 9.1 14.0
7.9 8.8 10.7 15.7
Chloride Concn., G./MI. 0.20 0,105 0,085 0,063 0.037 0.149 0.129 0,097 0,063
Yield,b % 100 100 100 97 87 100 97 96 84
0.16; 0.150 0.123 0,084
90 90 90 84
13 After solution has been neutralized with ammonium hydroxide and reacidified with
2A%r hydrochloric acid. 7'24 dpm PuZ39added.
VOL 32, NO. 3, MARCH 1960
0
327
were plated for 15 minutes. The results of this experiment are given in Table 11. Here, although the volume and chloride concentration varied by a factor of nearly 7 , the amount of plutonium deposited varied only 16%. Even for the large electrolyte volume the plutonium yield is acceptable. The deposit formed by the plating procedure is assumed to be of the type described by Hufford, Scott, and others (9-4). During the deposition a hydrous oxide is formed in the basic region near the cathode. When the solution is made ammoniacal, the hydrous oxide film becomes fixed to the disk. Upon flaming, this film is converted to an oxide. CONCLUSIONS
Based on the above experiments the following conditions have been chosen for standard operations: volume of solution, 4 to 5 ml. a t p H about 1; chloride concentration, 0.1 to 0.2 gram per ml.; plating time, 15 minutes.
I n practice, the actinide t o be plated is evaporated to dryness several times a i t h hydrochloric acid. The residue is taken up in 1 ml. of hydrochloric acid and the solution transferred to the plating cell with two 1-ml. water washes. Methyl red is added, and then ammonium hydroxide, until the solution is basic. The solution is then acidified with 2-47 hydrochloric acid. This procedure gives a plating bath of the desired volume and concentration. The plating cell is set up on the electroanalyzer and the stirring anode is adjusted to approximately 5-mm. distance above the platinum disk. Voltage is set t o give a starting current of 2 amperes. Radioautographs show that, if the stirrer is set closer than 5 mm., the activity tends to concentrate toward the outer area of the disk. Because of the rapid evolution of hydrogen a t the cathode, this plating would not produce adherent films of actinides in the micro- or milligram region. With tracer quantities, however, very clean adherent deposits are produced. I n many cases it is difficult
to distinguish the plated platinuni from new platinum. This plating procedure offers R rapid method for depositing the actinide elements a t tracer concentrations in high yield. The conditions can be wried over a wide range of volume and chloride concentration without adversely aifecting the yields. LITERATURE CITED
(1) Chopin, -4., University of California
Radiation Laboratory, Berkeley, pri-
vate communicat,ions. 19.56.
( 2 ) H-eord, D. L., Scott; B . F., “Transuranic Elements,” NNES, IV-14B Part I, p. 1149, McGraw-Hill, New York,
i949. (3) KO, R., h’ucleonics 15, No. 1, 72 (1957). (4) Rulfs, C. L., De, A. K., Elving, P. J., J. Electrochem. SOC.104, 80 (1957). (5) Went,zler, H. L., private communications, 1956. (6) Wingerson, R. C., private communication to L. F. Tischler, 1953. RECEIVED for review November 10, 1958. Accepted November 12, 1959. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
Separation of Magnesium from Sodium and Potassium A Tracer Study ARNO H. A. HEYN, Department o f Chemistry, Boston University, Boston, Mass. HARMON L. FINSTON, Nuclear Engineering Department, Brookhaven National laboratory, Upton,
The production of magnesium-28 by pileproduced tritons simultaneously results in considerable sodium-24 contamination. The development of a routine chemical processing method for the production of pure magnesium-28 suggested a study of the separation of magnesium and sodium; the ready availability of both of the y-emitting isotopes, sodium-24 and potassium-42, led to further investigation to include potassium. The degree of contamination was determined for magnesium precipitates obtained by precipitating magnesium ammonium phosphate, magnesium oxalate from homogeneous solution, and magnesium-8-quinolinolate both by the conventional method and from homogeneous solution. The extent of sodium and potassium coprecipitated decreased in the order: phosphate method (single precipitaphosphate method (double tion) oxalate method precipitation) quinolinolate method quinolinolate method from homogeneous solution. In this last case the amount of sodium coprecipitated per 69 mg. of mag-
>
328
>
>
ANALYTICAL CHEMISTRY
>
nesium ranged from 0.02 to about 1 y for 0.4 to 4000 mg. of sodium added. Adsorption is believed to b e responsible for the contamination in most cases. The results presented show the quantitative accuracy and reproducibility of a new magnesium-8quinolinolate procedure for precipitation from homogeneous solution.
T
production of 21-hour magnesium-28 from magnesium-26 by a triton in-neutron out reaction (16) has renewed interest in the separation of sodium from magnesium. The magnesium used contains traces of sodium; also, sodium-24 can result from the following interactions: magnesium-24, neutron in-proton out; magnesium 25, triton in-deuteron out. Because the tritons are produced by a neutron irradiation of lithium-6 giving tritons and alpha particles, sodium-23 present gives the 15-hour sodium-24. Thus, the activity of sodium-24 produced will be significant because of long irradiation times and possible reactions involving the various magnesium isotopes. HE
I. I., N. Y.
The half lives of sodium-24 (15.0 hours) and magnesium-28 (21.3 hours) and the p- and y- energies (14, 15) are sufficiently similar to make differentiation difficult. While there are no feasible procedures for precipitating sodium quantitatively without also precipitating magnesium, several methods supposedly constitute a separation of magnesium from alkali metals. The classical method for determining magnesium is the precipitation of hydrated magnesium ammonium phosphate, but there has been no conclusive examination of the contamination of the precipitate by alkali metals. Previous workers (8,IO) have deduced the amount of alkali metal coprecipitated with magnesium from the errors in the gravimetric phosphate procedure. A possible compensation of errors makes the direct determination of the coprecipitation errors desirable. Other methods are the precipitation of magnesium-8-quinolinolate, and the precipitation of magnesium oxalate in 85% acetic acid. According to Moser
