Electrodeposition of Actinides for Alpha Spectrometric Determination N. A. Talvitie Western Environmental Research Laboratory, Environmental Protection Agency, P.O. Box 15027, Las Vegas, Nev. 89114
A procedure for the electrodeposition of Pu, Th, U, and Am as hydrous oxides is given. Deposition recoveries from 1M ammonium sulfate at pH 2 ranged from 98.8 to 99.8% in 120 minutes of electrolysis at 1.2 A. A 40-minute electrolysis period was adequate for the alpha spectrometric determination of plutonium using za6Puas an internal tracer standard. In the absence of an internal standard, the electrodeposition efficiency can be determined by repeating the electrodeposition. The complexing property of the sulfate iqn prevents premature hydrolysis and precipitation of actinides and iron. Oxalic acid can be used for additional suppression of iron interference but it inhibits the deposition of actinides. Cross contamination in low-level analyses of environmental and biological samples is avoided by use of low-cost disposable polyethylene cells and electropolished stainless steel planchets. HIGHRESOLUTION alpha spectrometric determination of actinides requires a uniform and nearly weightless sample mount such as can be obtained by electrodeposition. Experiences with commonly used electrodeposition methods have been reported ( I ) . The methods are based on cathodic deposition of the hydrated oxides either from an alkaline medium (2) or from mildly acidic media (3-5) with or without the addition of oxalic acid and containing varying amounts of ammonium or alkali salts. In view of the variety of conditions reported, a study was made t o select a more closely defined set of conditions for the electrodeposition of actinides in general and plutonium in particular. The criteria were the rapid deposition of trace amounts at predictably high yields on stainless steel planchets mounted in disposable cells and coordination with the usual methods of chemical separation from environmental and biological samples. Deposition from a n organic medium (6, 7) was not considered in the study because of its lack of selectivity. EXPERIMENTAL
Apparatus. Disposable electrodeposition cells (Figure 1) were constructed from 20-cm3, linear-polyethylene, liquid scintillation vials. A 6/~-inchhole was cut in the bottom for introduction of the anode. The foil-lined caps were replaced by 22-mm Polyseal caps having a GCMI 400 thread design. The tubular portion of the polyethylene liner was removed and the conical portion retained as a cover for the assembled cell. A Q/&nch hole having a beveled inside edge was bored through the center of the cap. A a/4-inch diameter washer having a l/s-inch hole was cut from l/aZ-inch neoprene and placed in the cap. The shank of a hollow brass rivet (Dot Speedy Rivets, No. BS4830, Carr Fastener Co., Cambridge, ( I ) A. Holmes, Ed., “The Determination of Radionuclides in Materials of Biological Origin,” AERE-R5474, Harwell, England,
1967. (2) H. W. Miller and R. J. Brouns, ANAL.CHEM., 24, 536 (1952). (3) R. F. Mitchell, ihid., 32, 326 (1960). (4) P. E. Kauffman, Northeastern Radiological Health Laboratory, Winchester, Mass., mimeographed report, 1966. ( 5 ) F. L. Moore and G. W. Smith, Nucleonics, 13, (1955). (6) VJ. Parker, H. Bildstein, and N. Getoff, Nature, 200,457 (1963). (7) T. H. Handley and J. H. Cooper, ANAL.CHEM.41, 381 (1969). 280
-44,44
PLANCHET
Figure 1. Disposable electro-deposition cell and support
c 4 4 4 # $ CAP
ASSEMBLY
Mass.) was passed through the washer and cap t o serve as a n electrical contact for the planchet cathode. The cathodes were 3/4-in~h, 15-mil, type 304 stainless steel planchets either prepolished to a mirror finish with abrasives or polished electrolytically after mounting in the cell. The exposed cathode area was 2.3 cm2. To assemble the cell, the planchet was centered on the threaded end of the cell and held in place by vacuum applied through one of the holes of a two-hole rubber stopper, butted against the other end, The cap assembly was screwed on and leakage checked by adding water t o the cell and observing the rise of air bubbles when vacuum was reapplied. Flexing the cell by alternately applying and releasing the vacuum improved the seal of leaky cells. The combined resilience of planchet and washer maintained the liquid seal and electrical contact during electrolysis. The anode was a l/z-inch diameter, 1/32-inchplatinum-iridium disk having six l/s-inch perforations and attached a t the center to a 4-inch length of ‘/16-inch platinum-iridium rod. Electrolyses were conducted without stirring using a 10volt, 5-ampere Sargent-Slomin Electrolytic Analyzer. The cell support and cathode socket (Figure 1) consisted of a ‘isinch shaft lock (James Millen, Type A062) attached to a machined Lucite base or to a large bottle cap with a noninsulating banana jack (H. H. Smith, Type 109). Abrasive polishing of planchets was accomplished with a n aqueous slurry of 5-km alpha alumina o n a canvas lap rotating a t 550 rpm (Standard Polisher, Buehler 41-1500). The planchet, with the convex side facing out, was hand-held against the smaller end of a No. 3 rubber stopper by means of a 3-inch tab of a/4-inch self-adhesive masking tape. After degreasing with detergent and acetone, the planchets were immersed for 10 minutes in hot 4 M nitric acid-2x sodium dichromate and stored under water until needed. For electropolishing (8), the mill-finished planchet was degreased, cleaned in hot 4 M nitric acid, rinsed with water, ~
_
_
_
_
_
~
(8) C. L. Faust, in “Metals Handbook,” 8th ed, T. Lyman, Ed., American Society for Metals, Metals Park, Ohio, 1964, Vol. 2, pp 484-488.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
and then mounted in the electrodeposition cell with the burr side facing out. The cell was half-filled with a n electrolyte composed of 200 c m 3 of water, 450 c m 3 of 85% phosphoric acid, and 350 c m 3 of 1 8 M sulfuric acid. The platinum electrode was placed a t the midpoint of the electrolyte depth. Electrolysis was conducted a t a mean reversed current of 1.2 A for 6 minutes after which the cell was thoroughly rinsed with water and left filled with water until needed. Procedure. Evaporate a n ion-exchange eluate or the strip solution from a solvent-extraction separation t o dryness in a 50-cm3 beaker. Wet-ash, if necessary, with nitric and perchloric acids. Dissolve the residue in 2 cm8 of concentrated nitric acid, add 0.5 cm3 of concentrated sulfuric acid, and evaporate the solution until the nitric acid has volatilized and fumes of sulfuric acid appear. Dilute the sulfuric acid with 3 c m 3of water and add 4 drops of 0.02% sodium salt of thymol blue. Neutralize to the p H 2 end point by blowing gaseous ammonia over the surface while swirling the solution. The gaseous ammonia is obtained from a polyethylene wash bottle having the inner portion of the delivery tube removed and containing concentrated ammonium hydroxide. Pour the neutralized solution into the cell and rinse the beaker several times with a total of 6 c m 3 of 1 :99 sulfuric acid. Neutralize again to the p H 2.0-2.3 salmon-pink t o straw-colored end point. Use drops of 1 :9 sulfuric acid to acidify if the end point is overstepped. Introduce the anode through the polyethylene conical cover and position it about 2 mm above the shoulder of the rell. Electrolyze a t 1.1 t 9 1.2 amperes for 60 minutes if an internal tracer standard is used and for 120 minutes in the absence of tracer. One minute prior to the end of the electrolysis period, add 10 c m 3 of 1 :9 ammonium hydroxide. Terminate the electrolysis by raising the anode, pour out the electrolyte, and flush the cell three times with 1% ammonium nitrate-1 :99 ammonium hydroxide. Disassemble the cell and quickly rinse the planchet with a stream of ethyl alcohol adjusted to pH 8 with ammonium hydroxide. Touch the edge of the planchet with filter paper t o absorb the film of alcohol and heat the planchet a t 200 t o 250 "C o n a hot plate. Higher temperatures oxidize the planchet surface and result in loss of resolution. To check recovery, pour the electrolyte and cell rinses into the 50-cm3 beaker. After removing the planchet, cap the cell with a Polyseal cap and rinse successively with 5-cm3 volumes of concentrated nitric acid, concentrated hydrochloric acid, and water. Add these rinses t o fhe beaker and evaporate a t steam-bath temperature until only sulfuric acid remains. Repeat the electrodeposition for a 120-minute period. F o r highest accuracy, use silica-free distilled water and reagents. RESULTS AND DISCUSSION
Hexavalent plutonium can be deposited from alkaline media but quantitative recovery is dependent upon the absence of elements which precipitate in alkaline solution. Actinides in general can be deposited as hydrous oxides from dilute (0.01N) nitric, hydrochloric, perchloric, and sulfuric acids or from mildly acidic electrolytes containing salts of these acids. Deposition from the free acids is impractical for routine analysis because the optimal concentration of acid is too low for the quantitative dissolution and transfer of ashed residues. Electrolytes preadjusted in p H are also poor solvents for dry residues. The residues are preferably dissolved in a n adequate amount of strong acid and the excess is neutralized to the appropriate p H with ammonia. Several ammonium salts were tested for suitability as electrolytes. An ammonium nitrate solution became alkaline too rapidly because of reduction of nitrate ion at the cathode; e.g., a 1 M solution initially adjusted t o p H 2.2 attained a p H of 7.5 within 2 minutes of electrolysis at 1.2 amperes. Am-
monium chloride, on electrolysis, had a corrosive effect on stainless steel which led to thick and nonadherent deposits. The highest recoveries from lo-cm3volumes of 1Mammonium perchlorate, adjusted t o p H 2 and electrolyzed for 20-,40-,80-, and 120-minute periods, were: 82 % for z3aPu,98 % for 23W, 69% for 230Th,and 82% for 241Am. The low recoveries were attributed primarily t o a change in p H which decreased t o a value of 1.6 during 60 minutes of electrolysis at 1.2 amperes. The ammonium sulfate solution used in the proposed procedure also tends t o become more acidic during electrolysis because of oxidation of ammonium ions at the anode, but the greater buffer capacity prevents significant p H change. Prior neutralization of sulfuric acid solutions was necessary t o prevent corrosion of the stainless steel planchets by the strong acid but as much as 10% of plutonium remained adsorbed t o the beaker unless the subsequent rinses were definitely acidic. Gaseous ammonia was used for the neutralizations t o avoid introduction of silica which separates as flocs during electrolysis and tends t o adsorb actinides. The polyethylene cells were presumed to have less adsorptive affinity for actinides than glass cells but had a disadvantage in that spray collected as droplets above the liquid level. After 10 minutes of electrolysis, the droplets began to coalesce and return t o the liquid. Holdup was less when the polyethylene was rendered hydrophilic by immersing the cells in a chromic acid-sulfuric acid cleaning solution for 3 t o 4 hours. A 1hour immersion was adequate to decontaminate and condition the cells for reuse. Traces of chromium were removed by soaking for 1 hour in 4 M nitric acid. The polyethylene retained its hydrophilic property if kept continuously wet and if the assembled cells were filled with water until needed. The highest deposition rates for plutonium and thorium occurred in the p H interval from 2 t o 3. Thymol blue in 1 M ammonium sulfate is pink below p H 1.9, yellow above 2.6, and salmon-pink to straw-colored in the preferred pH range of 2.0 t o 2.3. When deposition was attempted from 5 t o 6 cm3 of electrolyte, depositions began a t a high rate but ceased entirely within 30 minutes. The cause was traced t o a n increase in p H due t o holdup of acidic anode spray on the cell wall above the electrolyte level. The effect was eliminated by increasing the volume of electrolyte t o 10 cm3 which provided a greater depth of liquid above the anode. To avoid trapping gas bubbles in the restricted neck and t o allow free circulation of electrolyte, the anode was positioned 2 mm above the shoulder of the cell. This placed the anode 13 mm above the cathode and a n equal distance below the liquid surface. The effect of temperature on deposition rate was investigated by maintaining the cell at approximately constant temperature in a controlled water bath. The following counts per minute of plutonium alpha activity were obtained after 15-minute periods of electrolysis at constant current: 740 a t 0 "C, 730 at 25 "C,1000 at 50 "C, and 1300 at 75 "C. Under ambient conditions, the electrolyte temperature increased during the first 15 minutes of electrolysis and stabilized at about 60 "C at 1.0 A, 80 "C a t 1.2 A, and 95 "C at 1.5 A. A current of 1.0 t o 1.2 A was chosen t o avoid the excessive agitation of the electrolyte at the cathode surface as the boiling point is approached. The optimal concentration of ammonium sulfate, as determined by deposition of plutonium, was in the range of 0.5 t o 1.OM. Below 0.5M, the reduction in current t o prevent boiling led t o a lower deposition rate. Above I.OM, the equilibrium between deposition and dissolution increasingly favored dissolution. A 0.5-cm3 volume of concentrated
ANALYTICAL CHEMISTRY, VOL. 44,
NO. 2, FEBRUARY 1972
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101
0
Rotating Unpolished Anode Stationary Polished Anode
1c
P
-
g ts
a h’ 1
*
Rotating Polished Anode Rotating Platinized Anode
60
30
0.1
ELECTROLYSIS TIME, MIN.
40 80 ELECTROLYSIS TIME, MIN
Figure 2. Electrodecomposition of oxalate in 8 cm3 of 1M ammonium sulfate-0.1M oxalic acid adjusted to pH 2 and electrolyzed at 1.2 A
sulfuric acid provides for a n allowable loss of 50% during fuming. When the organic content is negligible, the nitric and sulfuric acids can be added to solutions prior t o evaporation t o minimize adsorption of the nuclide t o glass and t o traces of gelatinous silica. If the evaporation is conducted a t steam-bath temperature without fuming, the volume of sulfuric acid can be reduced to 0.25 cm3 to improve deposition yield. Iron and thorium tend t o precipitate as insoluble sulfates in fuming sulfuric acid. The thorium precipitate dissolves instantaneously when a drop of 30% hydrogen peroxide is added to the diluted acid. Warming the diluted acid or the addition of oxalic acid was effective in dissolving an iron precipitate. Iron contributes to the thickness of the deposit and inhibits the deposition of actinides. The amount normally present in 1 dma of sea water or urine (less than 0.1 mg) did not adversely affect recovery or spectrometric resolution. The precipitation of traces of iron at pH 2 is prevented by the complexing action of the sulfate ion. Oxalic acid can be added to the electrolyte to prevent the precipitation and to inhibit the electrodeposition of 0.05- to 0.5-mg amounts of iron but, as shown below, the presence of oxalate ion lowers the rate of plutonium deposition. Medium 1M (NH&SOI 1 M (NH4)2S04-0.01 M H2C204 1 M (NHa)zSO4-0. 1M H2CzO4
Deposition of Pu in 15 min,
z
90 80 55
The oxalate concentration decreases during electrolysis because of oxidation a t the anode causing a corresponding increase in the p H of the electrolyte. A 1M ammonium sulfate-0.1M oxalic acid electrolyte initially adjusted to p H 2 increased to p H 3 during electrolysis. When the initial oxalate concentration was O.OlM, the p H increased 0.1 unit. Electrolysis experiments were conducted to determine the rate of oxalate decomposition. The results, obtained by permanganate titration at the end of the electrolysis period, 282
l
120 130
Figure 3. Electrodeposition of actinides from 10 cm3 of 1 M ammonium sulfate adjusted to pH 2 and electrolyzed at 1.2 A
are given in Figure 2. The disappearance curves are similar in shape to the curves reported for actinides in Figure 3. As shown in Figure 2, the rate of oxalate oxidation can be decreased by increasing the concentration polarization of the anode. A brightly-polished anode can be expected to lower also the anodic oxidation rate of ammonium ion and thus diminish the tendency of the electrolyte to become more acidic. Of possible merit would be the selection of conditions to give a gradually increasing p H concurrent with the oxidative removal of oxalate ion as has been suggested for ammonium nitrate electrolytes (9). In tests of this technique, a 0.5M ammonium sulfate electrolyte initially adjusted t o p H 4 using methyl orange as an indicator and then acidified by adding oxalic acid to give an oxalate concentration of 0.1M and a p H of about 2 was found t o approach but not exceed the initial p H on electrolysis for 60 minutes at 1.0 A. The recovery of plutonium was 93 under these conditions. An evaluation of the effect of oxalate ion and anode shape on the electrodeposition efficiency for environmental plutonium is given below. Medium 1M ( N H M 0 4 1M (NH4)~S04-0,01M HGOa
Shape of anode Rod
Disk
98 Z
96 72
94 72
84 %
The values are the percentages recovered in the first of two successive electrodepositions conducted for 90-minute periods a t 1 . 1 A. The electrolytes contained the amount of iron normally present in 1 d m 3of sea water. In view of the lower deposition rate and lower recovery in the presence of oxalic acid, the chemical separation procedure should have provision for reducing the iron content t o less than 0.1 mg, thus obviating the need for oxalic acid. Planchets polished t o a mirror finish are essential for lowlevel determinations by alpha spectrometry to avoid a n in(9) R . KO, U.S. A t . Etiergy Comm., HW-32673, Richland, Wash., Sept. 7, 1954.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
creased and variable background caused by scattered counts of degraded alpha particles. The improvement is slight when the alpha activity is determined by measurement of gross count rate. Although the data for this study were obtained with abrasive-polished planchets, electropolishing has the advantages in routine work of a saving in time and of the assured cleanliness of the surface. Electropolished planchets are highly reflective but tend to have an orange-peel texture which reduces the effective surface area inasmuch as deposition is heavier at the high points. An improvement in spectrometric resolution was noted for “weightless” plutonium standards but not for plutonium separated from environmental samples when deposited on electropolished planchets. The mean overall recovery of 236Putracer in the analysis of twelve 1-gram soil samples by an ion exchange method (10) was 98 when the separated plutonium was electrodeposited over a 90-minute period on abrasive-polished planchets. The mean overall recovery from 125 soil samples using a 60-minUte period and electropolished planchets was 94 %. Figure 3 shows the deposition curves obtained for 200-pCi amounts of actinides by the proposed procedure and cells. The y-axis represents the percentage of undeposited nuclide as determined by a second 120-minute electrodeposition. Aside from a minute amount which escaped the cell as mist, essentially quantitative recovery was obtained in the two successive electrodepositions. Mean total recoveries of 101.3 and 99.8% were found for plutonium and americium, respectively, in the 40-, 80-, and 120-minute electrolysis periods when compared with independent calibrations of these two nuclides. The curves suggest a n inverse square relationship of depletion with electrolysis time. The deviations during the first 15 minutes of electrolysis are due primarily to increasing temperature and current. Because the power supply was incapable of delivering more than 10 volts, the maximum available current at the beginning of electrolysis was 0.8 ampere. The current increased with increasing temperature and was maintained manually at 1.2 amperes after the first 10 minutes of electrolysis. The later decrease in deposition rate is attributable to the sum of such causes as holdup of spray, delay in desorption from the cell wall, and equilibrium (10) N. A. Talvitie, ANAL.CHEM., 43, 1827 (1971).
between deposition and dissolution at the cathode. When electrolysis is continued beyond 120 minutes, the deposit tends to redissolve because of increasing acidity and increasing concentration of the electrolyte. Inasmuch as high recoveries are obtainable, one calibrated actinide can be used for the spectrometric standardization of another codeposited actinide, especially if quantitative recovery of both is ensured by a second electrodeposition. The efficiency of the second electrodeposition was consistently higher than that of the first. This was attributed to a carrier (11) effect of trace impurities in the reagents as evidenced by a more pronounced stain on the second planchet. A self-carrier effect was noted in that 238Puand 239Pu standards deposited with higher efficiencies than shown for 236Pu. The mass of the 2 3 S Pused ~ in deriving the data for Figure 3 is equivalent to that of 0.02 pCi of 239Puwhich is the detection limit of the alpha spectrometer for a 1000-minute count. The electrodeposition efficiencies for plutonium separated from environmental samples agreed with those obtained using standard solutions. Efficiencies of 99.1 to 99.5% for deposition periods ranging from 90 to 120 minutes were found for 200-pCi amounts of 238Puseparated from six I-dm3 sea water samples by an ion exchange concentration-solvent extraction method. Like amounts of 238Puseparated from four 250-cm3 urine samples by ion exchange deposited with 98.1 to 99.5% efficiency in 90 minutes. An efficiency range of 96.0 to 97.7% in 40 minutes was found for 4-pCi amounts of 236Puseparated from eight 1-gram siliceous soil samples by ion exchange. Efficiencies of 97.5 and 99.8 % were found for 260- and 4000-pCi amounts of 239Pu separated from two 10gram samples of coral limestone soil by ion exchange and electrodeposited for 120 minutes. RECEIVED for review June 17, 1971. Accepted September 24, 1971. Presented in part at the 24th Annual Northwest Regional Meeting, American Chemical Society, Salt Lake City, Utah, June 1969. Mention of trade names of products or sources of supply does not constitute endorsement of the products by the Environmental Protection Agency. (11) M. Y. Donnan and E. K. Dukes, ANAL.CHEM., 36,392(1964).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
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