T h e thin layer technique is well suited t o t h e s t u d y of chemical reactions coupled t o t h e electrochemical re-
in the electrode cavity would lead t o a transition time corresponding t o the equilibrium amount of B initially present. If the current is turned off for a measured time and then a second anodic chronopotentiogram recorded, the transition time would be a measure of the rate a t which A is converted to B. An experimental application of these ideas to a study of the rate of formation and dissociation of metal chelates is in progress.
action. For euample, if species -4 is not electroactive, b u t in slow equilibrium with a n electroactive species B , t h e complete ouidztion of species B
(1) dnson, F. C., (1961). ( 2 ) Zbzd., p. 1838.
tions between intermediate electrode reaction products and the starting material, producing substances which can be further oxidized to cause the stoichiometrp of the reaction to become complicated ( 5 ) . This difficulty is largely eliminated in thin layer chronopotentiomet ry. Application t o
Kinetics Studies.
LITERATURE CITED
CHEM.33, 1198
a1~.4~.
(3) Anson, F. c . , J . Am. U ~ e m .floc. (4)83,Carslaw, 2387 (lg61). H. S., Jaeger, J. C., “Conduction of Heat in Solids,” 2nd ed., p. 112, Oxford Univ. Press, London, 1959.
(’1 F. S., Adams, ’., T.Tihite, R. N., *J‘ . ,4m. 84, 3065 (1962).
Chem. Ro’vlalld! SOC.
( 6 ) Kuwana, Theodore, Ph.D. Thesis, University of Kansas, Lawrence, Karl., (19j7).
RECEIVED for review September 26, 1962. hccepted December 20, 1962. Work )\as supported by the U. S. Army Research Office under Grant No. D.l-ARO(D)-31124-0315. Contribution KO. 2891 from the Gates and Crellin Laboratories of Chemistry.
Cation Exchange Separation and Spectrographic Determination of Soluble Silicon in Plutonium CHARLES E. PlETRl and ALBERT W. WENZEL U. S. Atomic Energy Commission, New Brunswick, N. J.
b A cation exchange separation of silicon from plutonium i s made prior to spectrographic determination i o reduce the health hazard and spectral interference from plutonium. Boiler cap electrodes in conjunction with a sodium carbonate buffer double the sensitivity normally obtained without caps in the same direct current arc methods. The lower limit of detection is 1 p.p.m. silicon. The over-all relative standard deviation of the method was within 19%.
T
HE
SPECTROGRrlPHIC
DETERBIINA-
of impurities in plutonium is complicated by its complex spectrum and radiological health hazard, T o reduce plutonium spectral interference other investigators have adapted the carrier distillation method for uranium to plutonium (3). This method requires about 1 gram of powdery plutonium dioxide for a complete analysis (e), and the spread of this material upon excitation creates a potential safety hazard e l e n when all operations are performed in a closed gloved bou. Direct spark methods (5) or chemical separation of the impurities from plutonium ( I , 10) prior to spectrographic analysis have been used to minimize the health problem as well as spectral interference. Existing anion exchange separation methods (2, 6, 2 1 ) employing nitric or hydrochloric acid media are useful because of the large number of elements that can be separated. Silicon, however, may either precipitate or be adsorbed with plutonium on anion resins depending upon the nature and concentration of the acid as well as TIOS
other conditions. Accordingly, the purpose of this study was to investigate a n ion exchange separation of silicon from plutonium suitable for subsequent spectrographic analysis. -4cation exchange separation process was chosen since Pu(II1, IV) is adsorbed on strong sulfonic acid type resins in nitric acid concentrations less than 1-11 (S), while silicon as an anion should pass unadsorbed into the effluent. For the spectrographic determination the use of boiler cap electrodes (4) t o increa.de the spectrographic sensitivity of silicon was studied. EXPERIMENTAL
Apparatus and Reagents.
GLOVED
BOXES. T h e toxic nature of plutonium requires t h a t i t be handled in gloved boxes during chemical separations and spectrographic excitation. SPECTROGRSPHIC EQUIPMENT..k Baird-Atomic Model GX-1 spectrograph mas modified for use with gloved box operations (11). It was used with a n Eagle mount, 3-meter, concave grating having 15,000 lines per inch for a first order reciprocal dispersion of 5.53 A. per mm. The instrument was equipped with a seven step, step sector having a n exposure ratio of the steps of 1 to 2, the first step of which is 100% exposure. A Baird-Atomic Spectrosource, Model LW-1, was used to supply direct current for electrode excitation. ELECTRODES.n’ational Carbon Co. electrodes as follows were used: Lower, Anode Cap, No. L-4030; Upper, Cathode, KO.L-4036; Pedestal, to fit above anode cap; Boiler Cap, Grade SPK, No. L-3715. Only the grade or density of the boiler cap specified above was satisfactory for high sensitivity and precision. Other grades may not give
similar results. Prior to use, the anode caps were sealed with a solution of Kel-F grease ( 2 grams per 100 ml. of CC14) * ION EXCHANGE CoLuhxN. h simplified Tompkins design (9), continuous flow, self leveling glass column (1-em. i.d.) was loaded with 6 em. of wet cation resin washed free of “fines” (Dowex50W, X-10, 200 to 400 mesh, hydrogen form, obtained from Bio-Rad Laboratories, Richmond, Calif.). A plug of fine mesh Dacron cloth a t the bottom of the column mas used to support the resin bed. Liquid in the column will adjust automatically to the level of the tip. The height of the liquid remaining above the resin bed may be controlled by the length of auxiliary tip used. The flow rate is adjusted by means of a small hosecock clamp. The column was washed with 30 ml. of 0.2N HK03 prior to use. SODIUMCARBOSATE SOLUTION.Reagent grade anhydrous KazCOjwas used to prepare a n aqueous solution containing 15 mg. of Na2C03per ml. STAKDARDSOLUrIONS. Reagent grade Ka2Si03 9H20 was used to prepare solutions in 0 . 2 5 HKO3 by suecessive dilution to give the following concentrations: 70, 40, 20, 10, 4, 2 , and 1 /ig. of Si per ml. Analytical 1% orking curves were prepared using these standards; the silicon concentrations were coiiverted to parts per million (p.p.m.) based upon a 100-mg. plutonium saniple, Figure 1. For the experimental work the SBMP solutions were used to prepare plutonium samples (containing approximately 3 , 7, and 17 pg. of Si per 100 mg. of P u per nil. of solution) from silicon-free plutonium solutions in 8-V nitric, 6 X hydrochloric, and 0.2N sulfuric acid. Procedure. Pipet a n aliquot of solution containing not more t h a n 500 mg. of plutonium into a 125-ml. VOL. 35, NO. 2, FEBRUARY 1963
209
Teflon dish. (If t h e aliquot is 1 ml. or less, add 5 ml. of 0 . 2 s H S 0 3 to t h e dish.) Carefully evaporate the solution to a minimum volume making sure i t does not PO to dryness. Dilute the solition wiih 10 ml. of 0 . 2 5 HS03-O 05S ?;H20H HC1 solution snd heat a t about 70" C. for 20 minutes. Cool the solution and add it to the prepared ion exchange column. Maintain the flow rate a t about 0.3 ml. per minute. When the level of the solution reaches the top of the resin bed, wash the silicon through with 35 ml. of 0 . 2 5 H K 0 3 . Collect all the effluent in another Teflon dish, evaporate to about 1.5 ml., transfer to a 25-ml. platinum crucible, and evaporate to dryness. Bake the residue for an additional 15 minutes. cool, add 0.5 ml. of ?;a2C03 solution, and eraporate to dryness. Fuse the residue a t 850" C. for 2 to 3 minutes in a furnace. Cool the crucible and dissolve the residue in 0.5 ml. of water. Pipet 100-pl. aliquots of the solution onto prepared
Table I. Wavelengths of Spectral lines for Silicon and lower limits of Detection
T,ower limit of detection, p.p.m .a
Wavelength, A.
2506 90 2881,578 pg. of Si per gram of Pu.
5 1
Table II. Determination of Silicon Added to Plutonium Solution
Silicon found, 1i.p.m __ Silicon, Silicon Plutonium prepared standards samples value, not through through p.p.m.Qjd column column Ih,d IIC I*>d IIC 36
68
36 38
68 63
40d 38d 36 e
41 38
63d 68d 64e
63 6%
3Gd
SO" 386 X6~ 42f 38f 3 68 6'id 09e
70" 6P 63f 721 648 688
167
165 162
160d liOd 163'
160 165
160d 163' 1T o e 1GOe
.IO"
A-,
16Of 1628 fig. of
Si per gram of Pu.
* Colorimetric analysis.
Spectrographic analysis, this method. Prepared from 0.2,Y HSOI solutions. e Prepared from SAY HNO, solutions. Prepared from 6 S HC1 solutions. Prepared from 0.2.V H2S04solutions.
f
Q
210
ANALYTICAL CHEMISTRY
N YI
1500
>
1300
z
I100
L
2000
i.
z
900
CI
2
700
Y
?
500
U
de
300 IO0 - 1 00
4
001
IO
too
I
1000
C O N C E N T R A T I O N OF S I L I C O N , P . P . ~ .
Figure 1 . for silicon
Analytical working curves
electrodes, dry under heat lamps, and cap the electrodes with boiler caps. Run a blank determination of residual impurities in reagents by following the above procedure without the sample. Excite the electrodes under the folloning conditions: pre arc, none; eyposure, 30 secondq; source, 20 d.c. amperes; analytical gapl 4 mm.; entrance slit, 25 microns. Use Eastman SA-1 plates to record the spectrum over a spectral region of 2200 to 3400 A. in the first order. Process the spectrographic plates using standard procedures, densitometer the analytical lines (Table I), determine the relative log intensity of each line, and read the concentration of silicon in the sample directly from the analytical working curve. Remove the plutonium from the resin by nashing the column with 5 . 7 s "03. RESULTS A N D DISCUSSION
The ~vavelengths of the spectral lines used for the determination of silicon in plutonium and the lower limits of detection are shown in Table I. Plutonium samples with added silicon impurity were run through the cation exchange separation-spectrographic method and compared with the silicon standards. For additional substantiation of the method, the separation procedure was repeated, the silicon in the ion exchange effluent after carbonate fusion n-as determined colorimetrically (?), and the results were compared with standard values. Results for samples over three concentration ranges of silicon are shown in Table 11. The spectrographic results compare well with those obtained colorimetrically and with the theoretical value of t h e standards. The over-all precision of the spectrographic method gave a relative standard deriation \Tithin &g% for 24 analyses, The recovery of silicon in standard samples averaged \Tithin i10% of the amount present. This procedure is suitable for the determination of soluble silicon in
hydrochloric, nitric, or dilute sulfuric acid plutonium solutions. The data in Table I1 indicate no appreciable difference in results obtained for samples prepared from these acids. The method may be applicable to plutonium metal or compounds such as sulfates and oxides. I n cases where the silicon is in an insoluble form it may be brought into solution by digestion, fusion, or other standard chemical means. Although plutonium(1V) is adsorbed more readily than plutonium(II1) at low acidities on Dowex-50 cation resin, it is more difficult t o remove. Accordingly, hydroxylamine hydrochloride was used to reduce plutonium to the +3 oxidation stat'e in the sample. The addition of hydroxylamine to t h e dilute nitric acid elutriant was unnecessary in maintaining plutonium(II1) on the resin bed during the washing step when plut,onium was initially adsorbed a t the rate of 100 mg. per day for a t least 4 days. Plutonium elution was accomplished without dificulty and resulted in 99.9% recovery. Radiometric analysis of the effluent after the column had been loaded in excess of 400 mg. of plutoniiim showed
