536
ANALYTICAL CHEMISTRY
extract it quantitatively from the butylated hydroxyanisole. Qualitative tests for the detection of propyl gallate, nordihydroguaiaretic acid, butylated hydroxyanisole, and gum guaiacum are given in a previous publication (4). Since nordihydroguaiaretic acid and gum guaiacum cannot be quantitatively separated from butylated hydroxyanisole, this method fails if these materials are present in the sample. If gum guaiacum is present in the butylated hydroxyanisole extract, the following effects will be observed. Gum guaiacum will react with the ferric chloride-1,l'-bipyridine reagent to produce a color of approximately the same intensity per unit weight as that obtained with butylated hydroxyanisole. However, on reaction with the 2,6-dichloroquinonechloroimide-borax reagent the color produced is relatively faint. As a result the 620 mp/515 mp ratio for gum guaiacum is of the order of 0.12, which is lower than the ratio for pure 2-tert-butyl-4-hydroxyanisole (Figure 2). Thus the presence of gum guiacum results in the overestimation of the total butylated hydroxyanisole and under estimation of the proportion of 3-ferf-butyl-4-hydroxyanisole
ACKNOW LEDGNIENT
The authors wish to express their thanks to C.W. Dunnett for carrying out the statistical analysis of the data and also to the Griffith Laboratories, Ltd., the Tennessee Eastman Corp., and the Universal Oil Products Co. for samples used in this study. LITERATURE CITED
(1) Dugan, L. R., Jr.,Hoffert, Eugenia,Blumer, GraceP.,Dabkiewicz,
Irene, and Kraybill, H. R., J . Am. Oil Chemists SOC.,28, 4935 (1951). (2) Kraybill, H. R., Beadle, B. W., Vibrans, F. C., Swartz, VeN., Wilder, 0. H. M., and Rezabek, Helen, Am. Meat Institute Foundation, Bull. 2 (April 1948). (3) Kraybill, H.R., Dugan, L. R., Jr., Beadle, B. W., Vibrans, F. C., Swartz, VeN., and Reaabek, Helen, J . Am. OiE Chemists SOC., 26,449 (1949). (4) Mahon, J. H.,and Chapman, R. A., ANAL. CHEM., 23, 1116 11951). (5) I b k , p.1120. (6) Rosenwald, R. H.,and Chenicek, J. A,, J . Am. Oil Chemi8b SOC., 28, 185 (1951). RECEIVED for review June 17, 1931. .4ccepted September 25, 1951
Quantitative Electrodeposition of Plutonium HAROLD W. MILLER AND RICHARD J. BROUNS General Electric Co.,Richland, Wash.
A procedure was desired for the preparation of thin, uniform, and adherent films of plutonium on platinum disks. Radioactive films meeting these requirements were to be used in instrumental analysis, where the self-absorption and straggling of alpha particles might present a serious source of error. An electrodeposition technique was found to be very satisfactory and a quantitative procedure was developed, whereby uniform, adherent films of plutonium dioxide were deposited on platinum disks. Quantities of plutonium, from tracer amounts to 9 mg., have been deposited completely from solu-
A
1ICTEIOD was desired for the preparation of uniform, adherent films of plutonium on platinum disks. Disks possessing these characteristics are ideal for use in alpha-energy annlysis, fission counting, and the preparation of geometry standards in ordinary counting where losses due to self-absorption would be serious. Various procedures for preparing disks were studied. The slurry spreading technique, direct evaporation, tetraethylene glycol spreading, and electrodeposition aere all investigated. These procedures are discussed in detail in an excellent paper by Hufford and Scott ( 2 ) . The first three procedures were found to be less satisfactory than the electrodeposition method. The slurry spreading technique was rejected because the sample evaporated in ridges, which resulted in an alpha-particle absorption of an order of magnitude of from 10 to 50%. Direct evaporation techniques were subject to the same disadvantages as the slurry spreading method. The tetraethylene glycol procedure was not quantitative and suffered the additional disadvantage of producing a poorly adherent deposit of plutonium. A great deal has been written concerning the electrodeposition of plutonium (2-4), and as this seemed to be the most promising procedure, experiments were started along this line. Miller ( 4 )
tions in 2 hours' time. The application of improved ionization chambers, range chambers, and fission counters to radiochemical research necessitates the utiliz'ationof thin, uniform, radioactive films. Films which are thick and clumped absorb alpha particles to the extent that erroneous results may be obtained. Alpha-energ-?-analysis gave evidence that there is a minimum of self-absorption and straggling in films prepared by electrodeposition. Such films are particularly suited for isotopic analysis, by range chamber and fission chamber methods, as well as for preparation of counting standards.
described a procedure whereby an electrolyte, consisting of 0.125
N potassium hydroxide, was used for the electrodeposition of plutonium with a current density of 10 ma. per sq. c p . Yields of from 85 to 90% were claimed. This laboratory mas unable to reproduce the yields obtained by Miller. Hufford and Scott ( 2 ) reported that the potassium hydroxide electrodeposition of plutonium was troublesome and not thoroughly understood. Using the procedure described by Miller, they obtained yields which were as low as 2%. This is consistent with results obtained in this laboratory. An upper limit of 0.1 mg. per sq. cm. was reported (8, 3 ) . This paper describes a procedure for the quantitative electrodeposition of plutonium oxide on platinum disks. Uniform, adherent films containing as much as 1 mg. of plutonium per square centimeter have been obtained. The procedure used was patterned after that used by Miller ($, 4). The essential differences were the method utilized to oxidize the plutonium and the normality of the plating solution. The plutonium was first oxidized to the plutonyl ion by the use of ozone, The solution was adjusted to a normality of 1 to 2 with potassium hydroxide and electroplated on a platinum disk which served as the cathode in the cell. The disk was then removed, washed,,and ignited to convert
V O L U M E 2 4 , NO. 3, M A R C H 1 9 5 2 the plutonium hydroxide deposit to a more adherent plutoiiiuin oxide film. APPARATUS AND PROCEDURE
The electrodeposition of plutonium was carried out in the cell shown in Figure 1. The apparatus consisted of a 6-volt storage battery, an electrolysis cell, and a constant-speed dirring motor. The electrodeposition control unit consisted of an on-off switch, a 10-ohm rheostat, and a 0-200 millianimetei, shunted so that 0-200 ma., and 0-2 ampere scales were available. The electrodeposition cell \vas made from a wide-mouthed bottle, with a Bakelite screw cap, by sawing off the bottom and using the bottle in the inverted position. The bottle size was selected so that the screir- cap n ould just hold the disks, which were 22 mni. in diameter. A copper disk of that size was placed in the screw cap and the platinum disk was placed on this copper disk. +A neoprene gasket was added and the bottle was then screwed down tightly o n the gasket. The negative electrode connection was made to a w r e Lvhlch was placed through a hole drilled in the bottle cap and was soldered to the back side of the cop er disk. The stirrer consisted of a 12mm. platinum disk wel&d to the end of a platinum rod. The stirrer, which also served as the anode, was inserted in the solution to within 2 cm. of the cathode. The stirring speed was 200 r.p.m. For electrodeposition on disks of a larger diameter a correspondingly larger cell must be used; the stirrer should be as large as is consistent with the diameter of the cell. The electrodeposition cell designed by Cohen and Hull (1, 2) was used and found satisfactory., although less convenient than the one dexribed aboie.
Y
W
O
D
-GLASS
E
CELL WALL
BAKELITE SCREW CAP
NEOPRENE GASKET PLATINUM DISC COPPER DISC (CATHODE)
Figure 1.
’
Electrodeposition Cell
Procedure. 4 sample of plutonyl ion, which is prepared by oxidizing plutonium with ozone in 2 S hydrochloric or nitric acid, is pipetted to the electrodeposition cell containing 10 ml. of potassium hydroyide solution. The normality is adjusted to between 1 and 2 with potassium hydroxide. T h e current is adjusted t o about 40 ma. per sq. cm. and the anode stirrer speed to about 200 r.p.m. Electrolysis is continued for about 2 hours. Samples may be taken from the solution occasionally t o determine the completeness of electrodeposition. -4t the end of the desired period, the solution is siphoned out and the cell is filled with distilled water three times, and siphoned dry after each filling. The stirrer and the current are left on during the washing process. The disk is removed, rinsed with acetone, dried under an infrared lamp, and flamed over a burner. EXPERIMENTAL
Three important chemical factors determine the efficiency and reliability of the electrodeposition: Completeness of the oxidation of plutonium t o the plutonyl state. Absence of impurities which form insoluble hydroxides that carry or adsorb plutonium. Alkalinity of the electrolyte. The first factor is the preparation of the plutonyl ion. The electrodeposition step must start with the plutonyl ion, a8 reduc-
537
tion takes place a t the cathode and the hydrous hydroxide of tetravalent plutonium is deposited. This is illustrated by the following equations:
+ 2H20 + 2 e - +P u + + + ++ 4OH+ p U + + +++ ii20+i’u(oH)a.tl r r 2 0
PuOn++ .WH-
72
The hL-drous hydroxide is then ignited to the oxide 1
P ~ ( o H )H?O ~ . ~-3 PUO,
+ ( a + 2 ) r-rno
Any plutonium (111) or ( I T ) preserit a t the beginning of tlic electrolysis will precipitate from the alkaline solution. This precipitate will be adsorbed upon the cell wdls or stirrer and thereby result in a low yield. It can be seen from these equations that the efficiency of electrodeposition is a function of the efficiency of the oxidation procedure. .is a result, an investigation of various oxidation procedures n-as undertaken. Several methods of oxidizing plutonium to the plut.,nyl ion in an acid solution were investigated and an ozonation proc,edure was selected as the moat satisfactory. Oxygen gas containing ozone was bubbled through a 2 11- hydrochloric acid solution of plutonium until oxidation n-as complete. The ozone was [!repared by passing acid-scrubbed, alkali-scrubbed, and dried oxygen through a modified ozonator of the type described t),v \lXx-d and Merritt ( 5 ) . AIoisture mu,ct be scrupulously excluded from the ozonator. The advantages of this procedure for the oxidation of plutoniuni are that the oxidation is quantitative, no impurities ale introduced into the solution, and the procedure is convenient. The time of oxidation will vary with the concentration of plutonium, rate of flon. of ozone, efficiency of ozonator, and orifice size. The osidation of the last few per cent of the plutonium required more ozonation time than that of the major part. It was therefore common to discontinue the ozonation before completeness. Most ozonations were carried to a t least 98% completion. The length of time that plutonium will remain in the plutonyl state is dependent upon its concentration. The intense alpha activity forms peroxides in aqueous solutions which will reduce plutonyl ion to the tetravalent st,ate. The analytical procedure vihich Tvas used to determine conipleteness of oxidation is as follows: About a 100-pl. aliquot was taken and diluted t o 2 ml. wit 11 2r\. hydrochloric acid. Fifty microliters of a solution containing 5 mg. of L a + + +per ml. were added to this solution. After 6 minutes, 14 drops of concentrated hydrofluoric azid, which had becn treated with dichromate t o destroy all reducing agents present, were added. The suspension was stirred, allowed t o digest 5 minutes, and then centrifuged. The precipitate was then washed with a 1 S hydrofluoric-nitric acid mixture and mounted on a platinum disk for counting. The activity which is carried should . which is not is considcred t o be all PuTC+ and/or P u + ~ + +This be PuOp ion. ++
The above procedure is standard for the determination of plutoniuin(II1, IV). It serves as a rather accurate measure of t h e completeness of oxidation, because the minor constituent ( t h e nonosidized part) is measured and a relatively small number is subtracted from the total plutonium concentration. I n practice, a known volume of plutonium stock solution w s oxidized and aliquots taken from it after oxidation were analyzed for total plutonium and nonoxidized plutonium. The total plutonium concentration was determined by counting direct amounts of samples large enough to be counted accurately. From this analysis of the solution a suitable aliquot was selected for electrodeposition. The electroplated disk was counted and the percentage yield was based upon the known plutonium(T’1) content of the aliquot taken for electrodeposition. The second factor, which was shown to be of importance for efficient electrodeposition, concerns the amount of certain impurities present in the solution. I t was shown conclusively t h a t traces of lanthanum interfered. I n one experiment 0.5 mg. of
A N A L Y T I C A L CHEMISTRY
538 lanthanum was added to the cell solution and the plutonium was electrodeposited in a manner which normally resulted in 95y0 yield. The result of this experiment was a plating yield of 0.1%. The mechanism of this interference may be a carrying phenomenon whereby the plutonium is adsorbed or occluded on the lanthanum hydroxide. Because most of the hydroxides of the lanthanide and actinide rare earth series have been shown to be isomorphous with one another, it can be safely inferred that these elements will interfere when present, even in trace amounts. Other elements which precipitate in alkaline solution and adsorbed plutonium mill also interfere. Some of these are scandium, yttrium, iron, silver, and chromium. Hufford and Scott had observed the interference of silver and chromium, but a t that time they were unable to explain this phenomenon.
Table I.
Efficiency of Plutonium Electrodeposition
uon, AT
Pu Plated, %
0.3 0.4 0.4 0.4 0.44 0.45 0.48 0.98 0.99 1.00 1,60 1.80
34 35 54 64 70 82 88 90 95 99 100 100
A study of the completeness of electrodeposition as a function of the normality of the cell solution with respect to potassium hydroxide was then undertaken. The results of this investigation show that the plating yield approached 100% as the normality approached 1. Xormalities as high as 2 were used with quantitative electrodeposition. Some typical results are listed in Table I. The normalities of the cell solutions in experiments 1 through 4 were not known exactly. The percentages obtained a t normalities of 0.98 and higher have been checked several times with varying concentrations of plutonium. In most cases the plating time was 2 hours. The solutions of lower normality were plated for longer periods in an unsuccesqful attempt to improve the electrodeposition yield.
Table 11.
Electrodeposition of Plutonium on Platinum Disks
Amount Taken, C./M. 96.0 126.0 251.0 279.0 990 3,182 1,202 6,319 7,867 9,229 9,936 9,488 11,771 110,166
Amount Plated, C./M. 96.0 126.0 252.0 280.0 991 3,182 1,190 6,300 7,867 9,200 10,000 9,464 11,700 110,871
Recovery,
Time Counted, Min.
100.0 100.0 100.4 100.4 100.1 100.0 99.0 99.7 100.0 99.7 100.6 99.8 99.4 100.6
60 60 60 60 60 60 5 5 5 5
%
Table 111. Electrodeposition of Milligram Quantities of Plutonium Expt.
D/bl/Disk
Mg. P u / Disk Approx.
Di-k Diam., Arm.
KOH, iV
Time
Hour;
Plutonium had not been analyaed for P u O P + +in this case. ably not completely oxidized. Q
Plated,
%
I t JFaa prob-
-. .
Some typical examples of disks of high counting rate which have been prepared are listed in Table 111. The counting rate is given in decompositions per minute (D/M). A number of other factors affecting electrodeposition were investigated. Rate of Stirring. Various stirring rates were used during electrodeposition. The most uniform films were obtained a t a stirring rate of 200 r.p.m. when the electrodes were 2 cm. apart. Temperature. Plating proceeds better a t room temperature than a t higher temperatures, although the temperature is not a critical factor. Time. Xost of the samples were electroplated in 2 hours. Gasket Material. The gasket between the disk and the cell should be made from neoprene. Rubber is unsatisfactory because of the difficulty of removing impurities from the rubber which gets on the disk and hinders plating. Disk Surface. The best films are prepared on smooth polished platinum disks. Scratched and etched platinum surfaces result in uneven plating and a straggling of the alpha-particle energies. Purity of Potassium Hydroxide. If the potassium hydroxide used contains a trace of iron, it will plate out over the plutonium, leaving a dark stain. The stain can be removed with dilute hydrochloric acid, but this will result in a small loss of plutonium. CONCLUSION
Quantitative electrodeposition of plutonium has been achieved in experiments in which 0.4 microgram to more than 1 mg. have been deposited per square centimeter of platinum disk. These disks are covered by a uniform, adherent yellow deposit of plutonium dioxide, spread out in as thin a layer as is consistent with the amount of plutonium deposited from solution. Many of the disks which were prepared were analyzed by the alpha-energy analyzer. The peaks obtained were sharp and free from irregularities. This is excellent evidence that self-absorption of alpha particles from the samples is a t a minimum. ACKNOWLEDGMENT
The authors wish to thank L. C. Schwendiman for his suggestions, especially relating to the design of the electrolysis cell.
5
5
LITERATURE CITED
5 5
Some examples of disks prepared by the procedure outlined above are given in Table 11. The osonated solutions were carefully standardized by alpha counting, using direct mountings of the samples on platinum disks. All counting data reported in the table are for a 50.5% geometry. The time of counting given in the table is for the electroplated disk and is reported here to enable the reader to calculate the precision of the count from counting statistics. The “amount taken” refers to PuO2++ only. In most cases ozonation had been carried to more than 98% of completion.
Cohen, B., and Hull, D., Columbia University R e p t . A-1234 Part I1 (Aug. 28, 1944), (Secret). Hufford, D., and Scott, B., “The Transuranium Elements,” National Nuclear Energy Series, Vol. 14B, Paper 16.1, New York. McGraw-Hill Book Co., 1949. (3) Jones, W., Goldstein, J., and Knesel, C., M-2350 (March 28, 1945). (4)
Miller, M., Supplement to Los Alamos Report LAMS-100 (July
11,1944). (5) Willard, H. H., and Merritt, L., ISD.ENG.CREM., ANAL.ED.,14, 486 (1942).
RECEIVED for review May 21, 1951. Accepted November 19, 1951. Research carried out under the auspices of the Atomic Energy Commission. Presented before the Pittsburgh Conference on Analytical Chemistry a n d Applied Spectroscopy, Pittsburgh, Pa., March 5 to 7, 1951.
