S q ua re-Wave Polarography of
PIutoniu m
KARL KOYAMA‘ Hanford laboratories Operation, General Elecfric Co., Richland, Wash.
b
The application of a square-wave polarograph to small amounts of plutonium in hydrochloric and nitric acid solutions was studied. Plutonium concentrations below 1 OP5 mole per liter were easily determined in these acids with a stationary platinum electrode. The derivative wave heights were proportional to concentration as predicted b y the square-wave theory. Half-wave reduction potentials obtained for the plutonium(1V) to (Ill) couple agree with accepted values. The method has higher sensitivity than conventional polarography, no effects from sample agitation, and negligible effects from irreversibly reduced materials, such as oxygen, on the measurement of plutonium ions.
T
information reported on the polarography of plutonium is mostly concerned with the measurement of oxidation-reduction potentials in various media (3, 10). Warren (IS), lion ever, obtained polarogaphic waves for many metal ions in the presence of plutonium(II1). H e observed that plutoniuni(II1) did not produce a reduction wave between 0.0 and -0.9 volt vs. S.C.E. in hydrochloric acid As these nieasuremmts n ere generally made by direct-current methods, the application of square+\ ave polarography to plutonium in hydrochloric and nitric acid solutioiis n as studied. Since the development of the squarenave polarograph by Barker and Jenkins (2)1 several papers on its application (4, ‘7, 9 ) and theoretical aspects (1, 9, 11) have been published. Hamm’s (9) polarograph is simpler electronically than the Barker and Jenkins instrument. He later developed a n instrument with higher sensitivity and better performance characteristics (8). The improved instrument was applied to the polarography of plutonium after slight changes in the circuitry to reduce the frequency of the square nave from 200 to 85 cycles per second and its amplitude to 10 mv. A reduction in frequency n a s required because the area of the solid electrode HE
1 Present address, John Jay Hopkins Laboratory for Pure & Applied Science, General Atomic Division of General Dynamics Corp., Box 608, San Diego 12, Calif.
LTas larger than the dropping mercury electrode n ith which the instrument \\as previously used. Polarograms were obtained by superimposing a square wave on a direct-current voltage scan of 50 mv. per minute. A glass polarographic cell 32 inm. in outside diameter and 23/4 inches high, n i t h appropriate entry ports for removal and addition of samples, n a s used. The cell had three openings at the top for introducing a saturated calomel and two platinum electrodes. The platinum electrodes were made by sealing 24gage mire into 6-mm. outside diameter lead glass and alloning 1 em. of the wire to protrude from the tubing to contact the solution. The ends of the electrodes n ere fire-polished to remove the roughness caused by cutting. A piece of porous Vycor as the salt bridge separated the calomel electrode from the electrolyte. -4 large capacitor (300 pf. electrolytic) was connected betn-een the calomel and one of the platinum electrodes to minimize resistance effects. The other platinum electrode served as the working electrode. K i t h this three-electrode system, the squarenave signal bypasses the impedance of the saturated calomel electrode 11 hile the direct-current potential of the platinum working electrode is in reference with the calomel electrode. This method shortens the time constant of the cell circuit sufficiently to allow the double-layer capacitance current t o decay to a negligible value before the faradaic current of the electrode process is measured.
30 c .-0
-
.-In
,220L L
-
O
c 0
IO -
+. 6
where
S n
= =
k
=
AE =
the square-nave current the number of electrons transferred in the electrode process a constant which depends upon grometrp, the amplification being used, the electrode size, and the temperature the square-n-ave amplitude used
t.8
Figure 1. Square-wave polarogram of pIutonium(1V) solution 7.5 X
mole per liter in 1M hydrochloric
acid
Table I.
Test o f Linearity with Concentration
Concn. of Pu(IS’), Mole/Liter 3.75 x 10-6 7 . 5 x 10-6 1.5 X 2 . 0 x 10-6
C”
=
D
=
f
=
RESULTS
The following modified form of the equation developed by Kambara ( 1 1 ) and experimentally verified by Hamm ( 8 ) for cadmium and indium reductions has been applied to the reduction wave obtained for plutonium :
+.7 V o l t s vs. S.C.E.
=
Kave Rave Height, Ht./Concn. Cm. x 10-6 2.4 4.7 9.3 12.3
64.0 62.7 62 0 61 5
the concentration of the reducible mecies in the bulk of the solutibn the diffusion coefficient of the reducible suecies the frequeniy of the square wave esp
nF [m ( E - E,,,)]
n-here ElIz is the half-wave potential and E the actual potential applied The typical square-wave polarogram of Figure 1 was obtained by a cathodic scan of a solution containing 7 . 5 X mole per liter of plutonium in 1JI hydrochloric acid. The circles shown on the curve \yere calculated by means of the above equation using the point a t the maximum t o evaluate 12 in the equation. The curve fits the theory remarkably well. Most of the “hash” visible on the polarogram comes from noise in the preamplifier. I n Table I, derivative wave heights VOL. 32, NO. 4, APRIL 1960
523
divided by concentration show the proportionality of the square-wave current n-ith concentration as predicted by theory. The half-wave reduction potential for plutonium(1V) to (111) was found to be +O.T1 volt us. S.C.E. in 1V hydrochloric acid and $0.66 volt in 2 V nitric acid. These values agree closely n i t h those obtained by Scott and Peekema (12) by controlled potential coulometry. DISCUSSION
I n nitric acid systems, the presence of nitrite introduces a very high background or residual current which makes it impossible to obtain reduction waves for low concentrations of plutonium. The addition of a small amount of sulfamic acid was found to remove the interfering effects of nitrite. I n general, the residual current for a nitric acid solution is higher than for hydrochloric acid solution. Interference from iron(III), which has a half-wave potential of about $0.45 volt us. S.C.E., is much more pronounced in nitric acid than in hydrochloric acid solutions. I n nitric acid very little separation b e h e e n the plutonium and iron waves exists when the iron content reaches about five times the plutonium concentration.
The continual scanning of plutonium, particularly in nitric acid solutions, apparently produces an oxide layer on the surface of the platinum electrode resulting in a high residual current which virtually masks the reduction nave of interest. Investigators have used various methods, such as an aqua regia or a hot cleaning solution treatment, in rejuvenating solid electrodes. All of these practices were unsatisfactory in cleaning the platinum used in this study; hom-ever, a periodic evolution of hydrogen, accomplished by applying a small negative potential to the electrode returned the electrode to a usable condition. Square-wave polarography has higher sensitivity than conventional polarography, derivative type waves which are easily interpreted, no effects from sample agitation, and negligible effects from irreversibly reduced materials, such as oxygen, on the measurement of reversibly reduced ions. The instrument is potentially capable of operating below the 10-6 mole per liter region.
ACKNOWLEDGMENT
The author thanks R. E. Connally for his assistance in the electronics of the instrument.
LITERATURE CITED
(1) Barker,
G. C., Faircloth, R. L., Gardner, 4. W., Brit,. Atomic Energy Research Establishment,, Rept. AERE C/R 1786 (Feb. 6, 1956). (2) Barker, G. C., Jenkins, I. L., Analyst 77, 685 (1952). (3) Cook, G. P., Foreman, J. K., Kemp, E. F., Anal. Chim.Acta 19, 174 (1958). ( 4 ) Ferrett, D. J., hlilner, G. W. C., Analyst 80, 132 (1955). (5) Ibid., 81, 193 (1956). (6) Ferrett, D. J., Milner) G. TV. C., J . Chem. SOC.1956, 1186. ( 7 ) Ferrett, D. J., Miher, G. W. C., Shalgosky, H. I., Slee, L. J., Analyst 81, 506 (1956). (8) Hamm, R. E., ANAL. CHEM. 30, 350 (1958). (9) Hamm,. R. E., U. S. Atomic Energy Commission, Rept. HW-52915 (Oct. 7, 1957) declassified. (10) Harvey, B. G., Heal, H. G., Maddock, A. G., Rowley, E. C., J. Chem. SOC.1947, 1010. (11) Kambara, R., Bull. Chem. SOC. Japan 27, 523, 527, 529 (1954). (12) Scot,t, F. A,, Peekema, R. ll., “Analysis for Plutonium by Controlled Potential Coulometry,” Proc. 2nd Intern. Coni. on Peaceful Uses of Atomic Energy 28, 573 (195S)> Paper P-914. (13) Warren, C. G., U. S.Atomic Energy Commission, Rept. LA-1843 (July 1953) unclassified. RECEIVED for review October 12, 1959. Accepted December 28, 1959. Work performed under Contract No. W-31109-Eng-52 bet-xeen the U.S. Atomic Energy Commission and General Electric Co.
CouIometric Titrations with EIectroIyticaIIy Generated Sulfhydryl Compounds Applications of Thiog lycollic Acid BARRY MILLER and DAVID
N. HUME
Department o f Chemistry and laboratory for Nuclear Science, Massachusetts institute of Technology, Cambridge
The feasibility of constant current coulometric generation of the sulfhydryl group is illustrated for thioglycollic acid, HSCH,COOH. Source of the reagent is the very stable mercuric complex, which is soluble at pH values above the pK, of the acid, 3.60. The response of a mercury p M electrode is generally used for end point indication. Amperometry at two mercury electrodes is also suitable. Determinations of mercury, gold, copper, and ferricyanide illustrate the utility of the generated sulfhydryl as a complexing agent for metals and as strong reducing agent in the neutral and alkaline regions.
524 *
ANALYTICAL CHEMISTRY
T
sulfhydryl group, through its affinity for metals and as a reducing agent, offers a wide range of applications to analysis. For metal determinations, sulfhydryl sources offer particular promise, because many of the metals whose affinity for sulfur is pronounced are those for which (ethylenedinitrilo) tetraacetic acid (EDTA) is least useful ( 1 2 ) . This group includes the platinum metals, silver, gold, and the thio-anion-forming elements of the acid hydrogen sulfide group. As a reductant, the sulfhydryl group has the advantage of being a strong reducing agent in the high p H range where there are fen- good reHE
39,
Mass.
agents available. For ordinary titrimetry, however, organically bound sulfhydryl suffers from the drawbacks of instability toward air oxidation and objectionable odor as well as possible difficulties due to limited solubility and excessive volatility. Internal generation by constant current titrimetry has made available many unstable but potentially useful reagents for chemical analysis ( 5 ) . Such a n approach has been taken for the generation of sulfhydryl and its applications have been explored. M E T H O D OF GENERATION
Free sulfhydryl may be readily pro-
