Coulometric Titration of Microgram Amounts of Americium at the Conducting Glass Electrode R. C . Propst Savannah River Laboratory, E. I. du Pont de Nernours and Co., Aiken, S. C. 29801
Coulograms of Am(lll) in 0.lM LiCI04-0.05M Na2COa electrolyte exhibit a well-defined wave for the Am(lll)Am(V) oxidation step at the conducting glass electrode. This wave provides the basis of a method for the quantitative titration of this element at the 30-microgram level with a precision of 1.5%. The method is restricted to pure solutions. Most cations interfere: Pu, Np, V, and Ru are reduced near the Am(lll)-Am(V) wave. Fe precipitates and scavenges Am. Radiolysis products from radioelements also interfere. S042-produces an anodic shift, and F- damages glass surfaces. CI- or NOa- do not interfere at 0.01M. Corrections are required for the partial oxidation of Am(V) to Am(VI) and the reduction of Am(VI) by water. The useful pH range is 10.0 to 11.5. The Am(lll)-Am(V) couple is indicated to be irreversible.
sentially 100% current efficiency. This method combined with precise alpha counting has been used to determine the half-life of 24*Am (1). EXPERIMENTAL
(1) L. C. Brown and R. C. Propst, J. Inorg. Nucl. Chem., 30, 2591
Reagents. The z4sAmstock solution was purified by ion exchange, solvent extraction, and finally by the method of Moore (5). Alpha pulse height analysis of the product showed it to be 98.8 alpha per cent 248Amwith less than 0.05 alpha per cent *'4Crn. The 24aAmsolution was standardized at 1.49 mg Am/ml by alpha pulse analysis and precision alpha counting in combination with mass spectrometry (I). The isotopic purity of the 24lAm solution was confirmed by mass spectrometry. All other solutions were prepared from reagent grade chemicals and demineralized laboratory distilled water. During titration, the solutions were purged continuously with helium that had been bubbled through distilled water. Apparatus. The scanning coulometer with automatic scan rate control has been described (6); both the controlledpotential and scanning modes were used. All titrations were initiated at the equilibrium potential of the working electrode with respect to the solution. For electrolysis at controlledpotential, the potential was advanced manually from the equilibrium potential to the desired electrolysis potential. This precaution prevented amplifier overload. The titration cell assembly (Figure l), was similar to those previously described (7) except for the conducting glass electrode (CGE) (4) which consisted of a layer of antimonydoped, tin oxide on the inner surface of the titration cell. A platinum wire sealed through the side of the cell made ohmic contact with the conducting coating. All potentials were measured with respect to the mercury :mercurous sulfate (1M &Sod) reference electrode (MSE). The MSE had a potential of 0.420 V (includes junction potentials) us. the saturated calomel electrode in 0.1M LiC1044.05M NazC03. The CGE's were fabricated from borosilicate glass. To assure positive contact between the platinum wire and the tin oxide coating, the inner platinum connection was spotted with Liquid Bright Platinum (Engelhard Industries, East Newark, N. J.). This coating was air dried and fired at 650 "C. The tin oxide coating was next applied by heating the cell to 600 "C and spraying the inside surface with a solution that was 2.8M in stannic chloride, 0.04M in antimony trichloride, and 1.2M in hydrochloric acid (8). A DeVilbiss 840 Nebulizer (DeVilbiss Co., Somerset, Pa.) was used for spraying. The air pressure was 5 psig, and the spraying time was two minutes. This technique produced essentially uniform and haze-free coatings. The uniformity was indicated by resistance measurements. Finally the completed CGE's were leached in concentrated HzSO4 for 24 hours and then rinsed thoroughly with demineralized water. During the electrolysis, any resistance inherent in the design of the cell
(1968). ( 2 ) G. Koehly, AnaI. Chim. Acta., 33, 418 (1965). (3) J. R. Stokely and W. D. Shults, Paper presented at the Eleventh Conference on Analytical Chemistry in Nuclear Technology, Gatlinburg, Tenn. October 10-12, 1967. (4) T. Kuwana, R. K. Darlington, and D. W. Leedy, ANAL.CHEM., 36,2023 (1964).
(5) (6) (7) (8)
AMERICIUM, a byproduct of nuclear fuel processing, is an important intermediate in the production of the transamericium elements via reactor irradiation. Therefore, analytical methods for accountability and control must be precise. Although coulometric methods would be ideal for this application, the high oxidation potential of the Am(II1)-Am(V) couple has prevented the development of a successful coulometric method involving the electrochemical oxidation of Am(II1). As a consequence, most assay methods have been based on alpha counting methods which are limited by uncertainties in the nuclear data (I). Previous work on the coulometric titration of Am is due to Koehly (2) and Stokely (3). Koehly oxidized Am(II1) to Am(V1) by exhaustive electrolysis at the platinum electrode and then determined the americium by coulometrically reducing Am(V1) to Am(V). This method requires excessively long electrolysis times (several hours) to oxidize the americium quantitatively. Stokely (3) shortened the procedure by replacing the electrolysis with a chemical oxidation. However, the excess oxidant must be destroyed, and electrolysis is still required to ensure complete conversion to the Am(V1) state before coulometric reduction to Am(V). Neither of these authors attempted to determine Am(V) by coulometric oxidation, presumably because the required oxidation potential was beyond the useful working range of the platinum electrode. Kuwana (4) has reported, and we have confirmed, a wide working range for the tin oxide semiconducting electrode as compared to conventional electrode materials. We have developed a successful coulometric method for Am based on the use of this electrode and on the stability of Am(II1) and Am(V) in carbonate electrolyte. The direct electrochemical oxidation of americium from (111) to (V) is achieved at es-
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ANALYTICAL CHEMISTRY
F. L. Moore, ANAL.CHEM.,35,715 (1963). R. C. Propst, ibid., p 958. Ibid., 40,244 (1968). J. M. Mochel, U. S. Patent 2,546,707 (1951).
47.10
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Figure 2. Coulograms of 29.8 0.OSMNazCOa
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Figure 1. Titration cell assembly effectively appears in series with the electrochemical impedance of the interface. Therefore the resistance of each cell was determined by substituting an equivalent volume of mercury (10 ml) for the sample solution, and recording current-voltage curves between the mercury pool and the platinum contact. The scan rate was 5 V per minute. Because the platinum contact was well above the level of the mercury, Figure 1, the measured resistance represented the sum of the resistances of the platinum-tin oxide contact, the tin oxide layer above the mercury pool and the tin oxidemercury interface all in series. The resulting current-voltage curves were linear, indicating no potential dependence for this measured resistance; the series resistance was typically 14 to 40 ohms. The CGE's were evaluated by recording cyclic voltammetric and chronopotentiometric curves for the ferrocyanideferricyanide couple in 0.1N potassium chloride as was done by Kuwana (4). The results for the cyclic voltammetric studies gave values for E, and (Epa- EPJ of $0.21 V us. SCE and 0.07 V, respectively. Thus the E, value is in good agreement with that reported by Kuwana (7), and the ( E EPE)value is in resaonable agreement with the theoretical value of 0.056 V for a one electron process. The chronopotentiometric studies also produced consistent results although the observed values for E114 and i T1'2/c probably reflect the influence of the nonuniform distribution of the current over the surface of the electrode (geometry effect). The observed value for E114 was +0.24 V US. SCE and values for i rLIz/Cwere reproducible to within *4%. Because of the geometry effect, these values could not be compared with calculated theroetical values. Procedure. The general titration procedure has been described (6), double-layer compensation was used, but residual current compensation was not required. The capacitance of the electrical double-layer at the CGE as estimated from the slope of the background coulogram was 130 pF/cm2. In this study, the electrolyte was preoxidized and reduced and background scans were recorded to assure that no americium remained in the cell from previous titrations. Carryover of electroactive contaminants from previous titrations was minimized by oxidizing the americium before discarding the solution. The cell was filled with 1 M nitric acid when not in use. Because cell resistance was low (
