Coulometric Titration of Polonium at the Microgram Level R . C . Propst Savannah River Laboratory, E. I . du Pont de Nemours & Co., Aiken, S. C . 29801
RECENT ADVANCES IN SPACE TECHNOLOGY have stimulated interest in polonium-210 as a heat source (144 watt/gram) for thermoelectric power generation and as a source of ions (1.67 X 1014a/sec-gram) for propulsion in space. Tentative plans for the production of this isotope require the distillation of kilogram amounts from irradiated bismuth. The product will be > 9 5 z pure with bismuth as the major contaminant. Conventional analytical procedures for the separation of bismuth and polonium (1) involve the spontaneous deposition of the polonium on less noble metals such as silver, copper, and nickel. The polonium is then determined from an alpha count of the deposit. Although this is an extremely sensitive technique for polonium, the need to determine the bismuth impurity required the use of scanning coulometry ( 2 ) and made attractive the investigation of a companion coulometric method for polonium that would utilize the same containment box and instrumentation. The high sensitivity of scanning coulometry made it attractive for minimizing the interference due to radiolysis of the solvent because only 1pg (4.5 X curie) of zlOPowould be required for a titration. No procedure has been published for the direct coulometric determination of polonium. Indeed, radiolysis effects have limited the precision with which the various polonium couples can be measured, and opinions differ concerning the oxidation states in solution (3). Therefore, the electrodeposition of 2lOPo was studied to develop a direct coulometric method. Perchloric acid was chosen for the electrolyte because it is more resistant to radiolytic degradation than nitric and hydrochloric acids. EXPERIMENTAL Reagents. The zlOPosolutions were prepared by dissolving the freshly distilled metal in 4M nitric acid. These solutions
were standardized by alpha counting. A stock solution of bismuth was prepared by dissolving a weighed amount of the metal in a minimum volume of 5M nitric acid, and diluting to the required volume with 1 M nitric acid. Reagent grade chemicals and demineralized laboratory-distilled water were used. All solutions were sparged with copious amounts of helium that had previously been bubbled through distilled water. Equipment. The scanning coulometer was described earlier (2); both the controlled-potential and scanning modes were used. All titrations were initiated at the equilibrium potential of the electrode with respect to the solution. For electrolysis at controlled potential, the titration was initiated at the equilibrium potential and the potential was advanced manually to the electrolysis potential. This precaution was necessary to prevent amplifier overload. Background compensation was not utilized. The cell (2) used is shown in Figure 1. The side arm was provided so that the cell could be filled and emptied without disassembly. (1) H. T. Millard, Jr., ANAL.CHEM., 35, 1017 (1963). (2) R. C. Propst, Ibid.,35, 958 (1963). (3) H.W. Moyer, Polonium, U.S. At. Energy Comm. Rept. TID5221 (1956).
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ANALYTICAL CHEMISTRY
1
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Magnetic Stirring Bar Polyethylene Covered
Figure 1. Titration cell for zloPo
The 1-inch diameter, cylindrical gold electrodes were fabricated from 99.95z gold foil (20 mil) with a surface area of 36 cm2; a gap of about '14 inch was left in the side of the cylinder. The edges of the gap were offset approximately lis inch along the diameter (in the direction of rotation of the stirring bar) to allow free circulation of the solution on both sides of the foil. The mercury-mercurous sulfate (1M sulfuric acid) electrode (MSE) had a potential of 0.418 d= 0.003 V cs. the saturated calomel electrode All experiments were performed in a conventional glove box. In those experiments where the deposition or stripping of 210Powas followed by plancheting aliquots of the solution, alpha counting was done in a conventional methane-flow, proportional counter. Procedure. The general titration procedure described earlier ( 2 ) was followed with the exception that the polonium solutions were freed from accumulated radiolytic degradation products by deposition at -0.4 V us. MSE, discarding the solution and redissolving into a fresh 10-ml portion of 0.1M perchloric acid. In a few cases, a few drops of 1M sulfamic acid was required to destroy the degradation products before the polonium could be deposited. RESULTS AND DISCUSSION
The continuous production of redox species in the titration cell from the intense alpha bombardment of the solution limited most of the titrations to less than monolayer amounts (0.3 pg/cm2) of 210Po. Thus, it was reasonable to anticipate undervoltage effects and to interpret the results in terms of
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Figure 2. Deposition stripping curves for z l o P in ~ 0.1M HCIO4 at the gold electrode
monolayer deposition-stripping processes (4,5). Because previous results obtained in this laboratory by scanning coulometry for the deposition-stripping of silver (6) at the gold electrode agreed with the treatment of Rogers and Stehney (4,the half-wave potential and values for RT/nF reported were calculated by the modified Nernst equation given by these authors. Electrodeposition Behavior. Typical coulograms for the titration of 210Poin 0.1M perchloric acid at the gold electrode are shown in Figures 2 and 3. The curves in Figure 2 show two processes for the deposition-stripping of 210Poin less than monolayer amounts : the irreversible deposition-stripping of 210Poin amounts less than 0.04 pg/cmZ,and the coulometrically reversible deposition-stripping of 210Po,observed when the 210Podeposit exceeded about 0.1 pg/cm2. The primary Po4+-PoO dissolution wave shown in Figure 3 could not be utilized for analytical purposes because of the spontaneous dissolution of the deposit. The half-wave potential for this process was f0.04 i 0.01 V us. the MSE and the average value for RTinF was 0.016 V. The theoretical value of RT/nF for a 4e change is 0.015 V cs. the MSE. The irreversibility of the first process was indicated by a 0.23-V separation between the deposition and stripping steps. The half-wave potentials for the deposition and stripping steps were +O.Ol and f0.24 V, respectively, and the corresponding RTInF values were 0.13 and 0.05 V (apparent 1/2e and l e steps in the monolayer formation). Alpha activity assay of the solution at several potentials agreed with the coulombpotential curves. The correlation between millicoulombs consumed and the amount of 210Poadded, Table I, indicates a 6e transfer. This transfer, together with the apparent and l e steps, which implies that the mechanisms for monolayer formation involve ionic aggregates of several polonium atoms. Because the most probable oxidation state for polonium in dilute acid is 4+(7), the irreversible process apparently corresponds to the deposition-stripping of a polonide. The reversible process also corresponds to a 6e transfer (Table I). The half-wave potential for this process was 0.005 V, an the calculated values for RT/nF ranged 0.165 from 0.028 to 0.055 V (apparent 2e and l e steps, respectively). Thus, it appears that the reversible process also involves the
*
(4) L. B. Rogers and A. F. Stehney, J . Electrochem. Soc., 95, 25 ( 1949). ( 5 ) J. T. Byrne and L. B. Rogers, Ibid.,98,457 (1951). (6) R. C. Propst, J . Electroanal. Chem., in press. (7) K. W. Bagnall, “Chemistry of the Rare Radioelements,” 1st ed., Academic Press, London, 1957, p. 53.
formation of a polonide at the electrode surface. Although the waves obtained for the reversible process were reasonably well defined, and quantitative information was obtained from the combined wave heights for the irreversible and reversible waves, the reproducibility of the ratio of the wave heights at a given 210Poconcentration was poor. This absence of reproducibility appeared to be related to the history of the electrode and the amount of 210Pothat had been deposited during previous titrations. Because of this phenomenon and the interference from radiolysis products, attempts to develop a family of curves to elucidate the mechanism of monolayer formation were unsuccessful. The transition from monolayer formation to the primary 210Podeposition process was also poorly defined as shown by the curve for 7.8 pg in Figure 2. This titration involves the deposition of about 3 0 z more 210Pothan that required for monolayer formation. The results, however, correspond to a 6e transfer; hence, polonide formation. In order to obtain reproducible results for the amount of 210Po involved in monolayer formation, it was first necessary to deposit considerably more than the amount required to form a monolayer and then strip the excess at 0.0 V 6s. the MSE. That an excess had been deposited was confirmed by the appearance of a primary dissolution wave near -0.06 V (Figure 3). Approximately 6 pg of 210Poremained on the electrode following the stripping step. This amount, when corrected for an aver-
Table I.
Sample size, p g
Per cent Deposit
0.40 0.79 1.19 1.56 1.56 1.56 3.95 3.95
103 102 93 92 94 101 93 100 97 4.6Z
Av Re1 std dev
Analytical Results 210Po
found Strip
107 104 101
92 94 97 101 98 99
5.oz
n-Value Deposit Strip 6.4 6.4 6.0 5.5 5.6 5.9 6.1 5.9 6.0
6.2 6.3 5.5 5.5 5.6 6.1 5.7 6.0 5.9
~
VOL 40, NO. 1, JANUARY 1968
0
245
age surface roughness factor of 1.1, corresponds to the deposition of about 0.15 wg/cm2 or about half the estimated amount required to form the monolayer. The surface roughness factor was extrapolated from prior experience with silver (6). Thus each zlOPoatom appears to be associated with two atoms of gold in the monolayer, an observation that supports, but is not conclusive evidence for, the postulated polonide formation. We are not prepared to offer an explanation for the observed undervoltage behavior of Z1OPo at this time. Evidence for the Po4+-Poo couple was obtained in experiments designed to determine the amount of 210Po involved in monolayer formation. The potential for this couple, +0.04 f 0.01 V cs. MSE, was calculated from the data by means of the Nernst equation for the dissolution of a metal from a surface of like atoms. The RT/nF values so obtained indicate a 4e transfer for this reaction. This result, together with the equation of the wave, are strong evidence that this was the primary dissolution wave for polonium metal. Further evidence was sought by alpha counting techniques. However, the coulometric results were always low because of the rapid spontaneous dissolution of the deposit. Spontaneous dissolution was indicated by the rapid increase in count rate of the solution with a corresponding drift in the “open circuit” potential of the electrode to positive values. Because of this rapid spontaneous dissolution, the half-wave potential and RT/nF values reported for this couple are only approximate. In addition, this wave has little value for quantitative coulometric purposes. It should be noted, however, that more precise results for the half-wave potential of this couple might be obtained by chronopotentiometry, thinlayer, or other fast scan techniques. The slow scan rate of the scanning coulometer was the limiting factor in this instance. Radiolysis Effects. Alpha bombardment in solutions of 21OPo results in the production of about 7.9 x lo-” mole of hydrogen peroxide per microgram-second, equivalent to a continuous faradaic current of 2.6 pA at f0.5 V ES. MSE. Typical difficulties encountered in this study due to radiolysis effects are also illustrated in the coulograms shown in Figure 2. The distortion of the stripping curve for 7.8 pg of 210Pois
246
ANALYTICAL CHEMISTRY
attributed to the simultaneous oxidation of hydrogen peroxide during the stripping scan. Thus, about 0.1 pg of 21OPo/cm2 is about the maximum that can be determined by stripping without excessive error. Interference from the spontaneous dissolution of 210Pois evidenced in the deposition curve for 7.8 pg by the rising slope following the deposition. Interference from this source is not immediately apparent until polonium is deposited in greater than monolayer amounts. A clean separation of the 210Pofrom radiolysis products is very important. The coulogram for 4 pg of 210Po,Figure 2, illustrates the difficulty encountered when radiolysis products are carried over from the plating step. Analytical Results. Results for the determination of 210Po in 0.1M perchloric acid at the gold electrode are given in Table I. Precision ( 5 %) was relatively poor because radiolysis products interfered; 1-2% is normal for other metals at these concentrations. Sources of Error. Two possible sources of error were investigated; loss of zlOPo during the initial separation from accumulated radiolysis products, and bias due to the presence of bismuth in the freshly distilled polonium. Neither of these sources made detectable contributions to the observed precision. Experiments involving the deposition of 0.4 to 56 pg of Z1OPoshow that greater than 99.9% of the 210Pois deposited at -0.4 V cs. MSE. The zloPothat remained in solution following the deposition was determined by alpha counting. Thus a negligible error is involved in separating the 210Pofrom accumulated radiolysis products. The deposition of bismuth in less than monolayer amounts at the gold electrode from 0.1M perchloric acid produces a wave at -0.2 V cs. MSE; however, this wave is well separated from the polonium waves and did not interfere in the concentration range under study. Indeed, when the deposited 21OPo approached monolayer amounts, bismuth could not be deposited, even at -0.4 V. The precise determination of bismuth impurity in 210Pois currently under study. RECEIVED for review September 18, 1967. Accepted October 30,1967. Information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U. S. Atomic Energy Commission.
