BARC/2012/E/001 BARC/2012/E/001
COULOMETRY FOR THE DETERMINATION OF URANIUM AND PLUTONIUM: PAST AND PRESENT by M.K. Sharma, J.V. Kamat, A.S. Ambolikar, J.S. Pillai and S.K. Aggarwal Fuel Chemistry Division
2012 BARC/2012/E/001
GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION BARC/2012/E/001
COULOMETRY FOR THE DETERMINATION OF URANIUM AND PLUTONIUM: PAST AND PRESENT by M.K. Sharma, J.V. Kamat, A.S. Ambolikar, J.S. Pillai and S.K. Aggarwal Fuel Chemistry Division
BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2012 BARC/2012/E/001
BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT (as per IS : 9400 - 1980)
01 Security classification : Unclassified
02 Distribution : External
03 Report status : New
04 Series : BARC External
05 Report type : Technical Report
06 Report No. : BARC/2012/E/001
07 Part No. or Volume No. :
08 Contract No. :
10 Title and subtitle : Coulometry for the determination of uranium and plutonium: past and present
11 Collation : 34 p., 2 figs., 7 tabs.
13 Project No. :
20 Personal author(s) : M.K. Sharma; J.V. Kamat; A.S. Ambolikar; J.S. Pillai; S.K. Aggarwal
21 Affiliation of author(s) : Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai
22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai - 400 085
23 Originating unit : Fuel Chemistry Division, BARC, Mumbai
24 Sponsor(s) Name : Department of Atomic Energy
Type : Government
Contd... BARC/2012/E/001
30 Date of submission : December 2011
31 Publication/Issue date : January 2012
40 Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai
42 Form of distribution : Hard copy
50 Language of text : English
51 Language of summary : English, Hindi
52 No. of references : 55 refs.
53 Gives data on :
60 Abstract : Precise and accurate determination of uranium (U) and plutonium (Pu) in nuclear fuels is an essential requirement in nuclear fuel cycle for chemical quality assurance of these materials. The redox based electroanalytical methods viz. Potentiometry and Biamperometry are capable of meeting the requirements of high accuracy and precision using milligram amounts of the analyte. However, use of chemical reagents to perform redox reactions in these methods, generates radioactive liquid waste which needs to be processed to recover plutonium. The analytical waste generated by the controlled-potential coulometric (CPC) method is clean as the change in the oxidation state of the analyte is done by the electrolytic reaction. Therefore, the determination of U and Pu in nuclear fuel materials by controlled-potential coulometry is an attractive option instead of biamperometry and potentiometry. In this report, the work carried out to develop CPC employing indigenous coulometers is discussed. The coulometric results for both U and Pu using indigenous coulometers agreed within ± 0.2% with the biamperometric values. The results indicate that indigenous coulometers are suitable for U and Pu determination in nuclear fuel materials and the CPC method can be employed for nuclear fuel samples.
70 Keywords/Descriptors : VOLTAMETRY; URANIUM OXIDES; FUEL CYCLE; PLUTONIUM OXIDES; QUANTITATIVE CHEMICAL ANALYSIS; REDOX PROCESS; RADIOACTIVE EFFLUENTS; QUALITY ASSURANCE
71 INIS Subject Category : S11
99 Supplementary elements :
Coulometry for the Determination of Uranium and Plutonium: Past and Present
1
Contents
Sr. No. Title Page No.
ABSTRACT 1. INTRODUCTION 2. TECHNIQUES AND INSTRUMENTATION 2.1 Galvanostatic or constant-current coulometry 2.2 Potentiostatic or controlled-potential coulometry 3. DETERMINATION OF URANIUM 3.1. Coulometric Titration of Uranium 3.2. Controlled-Potential Coulometry of Uranium at Mercury Pool Working Electrode 3.2.1. International Laboratories 3.2.2. Fuel Chemistry Division, BARC 3.3. Controlled-Potential Coulometry of Uranium at Solid Working Electrode 3.3.1. International Laboratories 3.3.2. Fuel Chemistry Division, BARC 3.4. Development and Performance Evaluation of Indigenous Controlled-Potential Coulometer 4. DETERMINATION OF PLUTONIUM 4.1. Coulometric Titration of Plutonium 4.2. Controlled-Potential Coulometry of Plutonium 4.2.1. International Laboratories 4.2.2. Fuel Chemistry Division, BARC 4+ 3+ 2+ + 5. INVESTIGATIONS ON REDOX BEHAVIOUR OF Pu /Pu and UO2 /UO2 REDOX COUPLES ON NANOPARTICLES MODIFIED ELECTRODES ACKNOWLEDGEMENTS REFERENCES
Figures Fig. 1 Controlled-potential coulometric setup Fig. 2 Coulogram for determining the working electrode potential for irreversible redox couple
Tables Table 1. Multiple oxidation states of uranium in aqueous solution Table 2. Redox potentials of various redox couples of uranium Table 3. The E0’ values for Pu (IV)/Pu (III) and Fe (III)/Fe (II) in various acids
Table 4. Determination of Pu in chemical assay standard [K4Pu (SO4)4] by controlled-potential coulometry in two different supporting electrolytes Table 5. Determination of Pu in alloy samples by controlled potential coulometry Table 6. Determination of Pu in (Pu, U) mixed carbide samples by controlled potential coulometry Table 7. Results of the paired t-test for the data obtained by the two methodologies
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Abstract: Precise and accurate determination of uranium (U) and plutonium (Pu) in nuclear fuels is an essential requirement in nuclear fuel cycle for chemical quality assurance of these materials. The redox based electroanalytical methods viz. Potentiometry and Biamperometry are capable of meeting the requirements of high accuracy and precision using milligram amounts of the analyte. However, use of chemical reagents to perform redox reactions in these methods, generates radioactive liquid waste which needs to be processed to recover plutonium. The analytical waste generated by the controlled-potential coulometric (CPC) method is clean as the change in the oxidation state of the analyte is done by the electrolytic reaction. Therefore, the determination of U and Pu in nuclear fuel materials by controlled-potential coulometry is an attractive option instead of biamperometry and potentiometry. In this report, the work carried out to develop CPC employing indigenous coulometers is discussed. The coulometric results for both U and Pu using indigenous coulometers agreed within ± 0.2 % with the biamperometric values. The results indicate that indigenous coulometers are suitable for U and Pu determination in nuclear fuel materials and the CPC method can be employed for nuclear fuel samples.
Keywords: Controlled potential coulometry, Plutonium, Uranium, Pt, Hg, Graphite, Nanoparticles
1. INTRODUCTION
Coulometry is based on the measurement of coulombs (unit of electrical charge named in honor of Charles-Augustin de Coulomb) and is the name given collectively to the electrochemical techniques used for quantitative determination of the analyte by measuring the charge consumed/produced when the analyte undergoes reduction/oxidation during exhaustive electrolysis at the large surface area electrode. Fundamental requirement of a coulometric analysis is that the electrode reaction should proceed with 100% current efficiency. Coulometry is an absolute technique since it is based on fundamental physical quantities. Faraday’s first law of electrolysis is used to calculate the amount of analyte
where, ‘m’ is the mass of the analyte, ‘M’ is the molar mass of the analyte, ‘F’ (= 96487 C) is the Faraday constant, ‘n’ is the number of electrons change in the redox process, ‘Q’ is the amount of electrical charge consumed/produced, ‘I’ is the current passing through the electrical circuit for time ‘t’.
The coulometric techniques can be grouped into two categories: (1) Galvanostatic or constant- current coulometry and (2) Potentiostatic or controlled-potential coulometry (CPC).
2. TECHNIQUES AND INSTRUMENTATION
2.1 Galvanostatic or Constant-Current Coulometry
In this technique, the current passing through the working electrode is kept constant throughout the reaction. The working electrode potential changes during the course of the reaction. As a result, simultaneous undesired charge transfer processes can occur if the impurities are present in the sample solution. Moreover, an additional end-point detection method is required. In the early stages of the development of coulometric methods for analytical applications, this technique was more popular than controlled-potential technique because of the requirement of simple instrumentation (a constant current source - galvanostat).
In constant current coulometric titration, a constant current is applied to a working electrode at which another electroactive substance (reagent precursor), added in relatively large concentration to the test solution, reacts to generate a reagent which in turn reacts with the analyte in a definite stoichiometric way. The reaction of the reagent precursor at the electrode limits the potential of the working electrode and prevents the occurrence of irrelevant reactions. Classical techniques (potentiometry, amperometry, colorimetry, etc.) can be used to detect the end-point. The charge passed is measured simply by multiplying the constant current value with the time required to reach the end-point. This titration is much faster (typically about 5 minutes) than controlled- potential coulometric analysis and is capable of yielding very high precision and accuracy for pure solutions.
2.2. Potentiostatic or Controlled-Potential Coulometry
In this technique, the potential of the working electrode is held constant with respect to a reference electrode by a potentiostat throughout the electrochemical reaction. Controlled- potential coulometry is the preferred technique because undesired interfering electrode reactions are prevented by fixing the potential of the working electrode to a value at which the desired electrochemical reaction can be quantitatively accomplished with 100% current efficiency. Further, the detection of the end-point is easy. The end-point is attained when the current passing through the electrical circuit is close to the blank/background value. Controlled-potential coulometry is selective and is well-suited to the analysis of mixtures. However, slow rate of current decay at the later stage of the exhaustive electrolysis leads to a relatively longer analysis time (typically 30 minutes to 1 hour). Since the non-Faradaic (blank) charge due to back-ground current is almost always present, therefore, the accuracy depends on reliable correction for blank value. In the beginning, controlled-potential coulometry was not extensively used in routine analysis, though it is highly versatile and selective, because complex electronic circuitry was involved in its design and development. This prevented the commercial production of cheap controlled- potential coulometers. Earlier, some coulometers with a simple manual control circuit were built and investigated for the analysis where the potential of the working electrode was prevented from exceeding the control potential value by changing the constant current applied to the working electrode in discrete increments. This was known as coulometry by “potential limit” method. This potential-limit method prevented the undesired rapid changes in the working electrode potential and thus did not affect the analytical results, since the average potential of the working electrode was kept well below the chosen controlled potential value. The manual control circuit was replaced by a simple automatic circuit to make the current integration more accurate and precise. Later on, a controlled-potential coulometer with a completely electronic potentiostat and an integrator circuit was developed by Booman in 1957 [7] to carry out controlled-potential oxidation or reduction, and simultaneous integration of the electrolysis current. These authors investigated the coulometric determination of U(VI) at a controlled potential by reducing U(VI) to U(IV) at a controlled-potential mercury cathode in potassium citrate-aluminum sulphate or in sulphuric acid electrolyte [8].
In coulometry, the speed of electrolysis and the time required for quantitative reduction or oxidation of the analyte depends on the potential applied at the working electrode, rate of stirring of electrolyte solution, temperature, active surface area (A) of the working electrode, and the solution volume (V) in the cell. The larger the A/V ratio, the higher is the current and shorter is the time for quantitative electrolysis [24].
°
Here, ‘I(t)’ is the current at any time ‘t’, ‘Io’ is the initial current at t = 0, ‘a’ is a constant.
3. DETERMINATION OF URANIUM
Uranium exists in multiple oxidation states in aqueous solution as given in Table 1. In solution, 2+ 4+ UO2 is the most stable species. Another less stable oxidation state is +4 (U ), which can be 2+ formed by reducing UO2 using either a suitable reducing agent or by bulk electrolysis. In 2+ + aqueous solution, one electron reduction of UO2 gives UO2 , which is unstable and undergoes 2+ 4+ disproportionation to yield UO2 and U .
+ + 2+ 4+ 2UO2 + 4H UO2 + U + 2H2O
+ The kinetics and mechanism of disproportionation of UO2 have been investigated by many researchers. The reaction is second order with respect to U(V) concentration, first order with respect to the acid concentration, and is dependent on the total ionic strength and nature of the anions present [15, 4]. The redox potential values for various redox couples of uranium are given in Table 2. There are a number of analytical methods for uranium determination, but choice of the method is governed by the nature and the quantity of sample available and the accuracy required. Methods such as gravimetry [22], spectrophotometry in the visible region [52], fluorescence measurement [37], X-ray fluorescence (XRF) [34], inductively coupled plasma mass spectrometry (ICP-MS) [35], thermal ionization mass spectrometry (TIMS) [50], polarography [43] and nuclear methods such as gamma-ray spectrometry [39], α-spectrometry [38], neutron activation analysis [36] and fission track [19] are widely used for the determination of uranium. Some of these methods are inherently not capable of giving precision and accuracy better than about 1%, are subjected to many interferences and are time consuming and expensive. Though high precision and accuracy is obtainable in the determination of uranium by the redox titrimetric methods such as potentiometry [25] and biamperometry [53] on both micro and macro scales, but there are certain limitations of these methods. These include the necessity of standard reference materials, addition of chemicals for reduction and oxidation, interferences from easily oxidisable and reducible impurities and nitrate removal, and in some cases, slow redox reactions requiring high titration temperatures and poor end point behaviour on aged inactive electrode surface. The isotope-dilution method using thermal ionization mass spectrometer is specific, but requires a longer analysis time, expensive equipment and needs spike of the analyte to be determined.
3.1. Coulometric Titration of Uranium
Prior to the development of controlled-potential coulometry, indirect coulometric titration methods were developed using electrolytically generated Ce(IV) or bromine which oxidized U(IV) [5, 1]. These methods needed a preliminary reduction of U(VI) to U(IV) by chemical reductant, and the freedom from easily oxidisable species and nitrate ions. A direct constant- current coulometric procedure was developed for U(VI) determination by reduction of U(VI) to U(IV) using electrolytically generated titanous ion [2, 6]. The accuracy was good for high concentration of uranium but required an elevated titration temperature (850 C) to increase the rate of reduction of U(V) with Ti(III). Any easily reducible ion would cause bias in the uranium determination. Tanaka et al. studied the current efficiency in the coulometric generation of Ti(III) as a function of electrolyte composition, current density and electrode (platinum, mercury and graphite) material [26]. Platinum and mercury were found to be suitable for uranium determination with high precision and accuracy. The precision and the error obtained were comparable for both platinum and mercury, but the use of mercury as an electrode material required complete removal of chloride from the supporting electrolyte.
3.2. Controlled-Potential Coulometry of Uranium at Mercury Pool Working Electrode
3.2.1. International Laboratories
The development of controlled-potential coulometer by Booman [7, 8] was the first study on coulometric determination of U(VI) at controlled potential. The U(VI) was reduced to U(IV) at a controlled-potential mercury cathode in potassium citrate-aluminum sulphate or in sulphuric acid electrolyte. Complete reduction was accomplished in 5 to 10 minutes at room temperature by using a 5-mL capacity electrolysis cell. The method could tolerate Hg(II), Cu(II), Fe(III) and large amounts of nitric acid. Controlled-potential coulometric method showed excellent precision with results reliable to better than 0.1% standard deviation in the range from 75 to 0.75 mg uranium in the sample and the sensitivity was extended to a few micrograms of uranium. Pre- reduction at a controlled-potential, more positive than that required for U(VI) reduction, removed the interference of substances which can be reduced at a more positive potential than uranium. Goode and Herrington introduced differential controlled-potential coulometry for the
determination of uranium in uranium standard by reducing U(VI) in 0.5 M H2SO4 at a mercury electrode. A higher precision was attainable than controlled-potential coulometry [18]. Further, Lingane [20] introduced a new technique known as ‘controlled-potential coulometric titration’ for the determination of uranium which combined the merits of both controlled-potential coulometry and constant-current coulometric titrations. The requirement of this method is that the analyte should be capable of undergoing stepwise reduction or oxidation in at least two stages at well separated potentials. In a typical controlled potential coulometry of uranium, reduction of U(VI) to U(IV) is carried out in aqueous acidic medium at mercury electrode. In this new method, potentiostat fixes the working electrode potential to a value at which analyte undergoes both the steps of reduction or oxidation simultaneously, so that the product of second step is generated and reacts with the original analyte to give the product of the first stage. Instead of carrying out the controlled potential electrolysis to exhaustion, the titration is stopped at a discrete end-point (signaled potentiometrically, amperometrically, or by other means) corresponding to the completion of only the first stage, and the quantity of substance reacted is evaluated from the number of coulombs required to reach this end-point. The controlled- potential coulometric titration of U(VI) to U(IV) was carried out via the U(III) produced during the reduction of U(VI) at – 0.7 V vs. NHE at a mercury cathode. The U(III) rapidly reduces the U(VI) to U(V) and then to U(IV). The redox potential of the solution undergoes a very sharp decrease of about 0.6 V when the original U(VI) is completely reduced to U(IV), and this large abrupt potential change serves precisely to signal an “end-point”. The time required to reach the end-point is typically 6 to 10 minutes, comparable to that required in constant-current coulometric titration. Kihara investigated the reduction and the oxidation processes of uranium in chloride solution and plutonium in perchlorate solution at the carbon electrode by using column coulometry [23].
3.2.2. Fuel Chemistry Division, BARC
In our laboratory, the determination of U in various types of nuclear fuel materials is carried out by redox titrimetry methods (potentiometry and biamperometry) [27, 28]. Though these redox titrimetry methods give high precision and accuracy, but they have some limitations, like the need of standard reference material of uranium and addition of chemicals for reduction or oxidation which generates radioactive liquid waste and recovery of Pu from such waste is a cumbersome process. Controlled-potential coulometry was, therefore, developed as an alternate method for the determination of uranium as it gives high precision and accuracy, does not require primary standard and generates clean radioactive analytical waste [9,10,15,29,30,40,41,51].
In the early stages of controlled-potential coulometry developments in our laboratory, coulometers procured from EG&G Princeton Applied Research Corporation, USA were used. An overview of this coulometric developmental work at BARC is available in the DM-COUL- 2005 proceedings [46].
The electrolysis cell contains three electrodes: working electrode, platinum wire as counter electrode and saturated calomel electrode (SCE) as the reference electrode as shown in Fig. 1. Counter and reference electrodes are kept in the separate compartments to avoid direct contact 2+ 4+ with the analyte solution. The reduction of UO2 to U is not a reversible process. Therefore, 2+ 4+ the selection of working electrode potential for irreversible redox couples (UO2 /U ) is not as simple as in case of reversible redox couple (Fe3+/Fe2+ and Pu4+/Pu3+). The working electrode 2+ potential for UO2 reduction is determined by plotting coulogram (charge or current vs. potential). The plot of charge or current against working electrode potential is S-shaped (Fig. 2). The potential required for the attainment of maximum charge or current is chosen as the working 2+ electrode potential. From the coulogram for UO2 reduction, pre-reduction at 0.085 V and reduction at -0.325 V (vs. SCE) were found to be the most suitable potentials to carry out precise
and accurate determination of uranium in 1 M H2SO4 with 100 % current efficiency. In the beginning, stirred mercury pool was used as the working electrode because of the electronegative 2+ 4+ potential of UO2 /U redox couple. In a cleaned electrolytic cell, nearly 10 mL of purified
mercury and the same volume of 1 M H2SO4 was taken. The mercury pool was vigorously stirred
and Iolar grade high purity (N2/Ar) gas was continuously purged over the electrolyte solution during the course of electrolysis. Prior to carrying out uranium determination, the blank determination was carried out by performing pre-reduction at 0.085 V and final reduction at
– 0.325 V vs. SCE in 10 mL of 1 M H2SO4. End-point of CPC electrolysis was determined by following the electrolysis current to a value of 10 µA. This process was repeated twice or thrice to achieve reduction of the interfering impurities present in the electrolyte and mercury. After blank determination, uranium determination was carried out by employing successive additions. In the successive addition technique, a fresh aliquot of the U(VI) sample is added to the same electrolyte solution in which earlier aliquot of U(VI) sample was analyzed by reducing U(VI) to U(IV). Gopinath et al. standardized the successive addition technique for coulometric determination of uranium in our laboratory [30]. The uranium content of the fresh aliquot analyzed in the same electrolyte containing U(IV), accumulated from previously analyzed aliquots, showed positive bias with increase in the time delay, but this bias reduced with the increase in sulphuric acid concentration. The positive bias was eliminated when the electrolyte containing small amounts of U(VI) was electrolyzed at - 0.320 V vs. SCE prior to the addition of the next aliquot for analysis.
3.3. Controlled-Potential Coulometry of Uranium at Solid Working Electrode
3.3.1. International Laboratories
Due to the toxicity associated with mercury, solid electrodes of silver, gold, platinum, graphite and activated platinum were investigated for the coulometric determination of uranium. Milner et al. studied the controlled-potential coulometric determination of uranium by reduction of U(VI) at a silver gauge working electrode [16]. This was found to be suitable for quantitative determination of 1 mg or more of uranium by CPC. The main interfering elements were Bi, Cu, Mo and Hg. Uranium determination by oxidation of U(IV) to U(VI) at a platinum electrode was 4+ 2+ investigated by Boyd et al. [13]. The oxidation of U to UO2 is kinetically slow due to 2+ uranium-oxygen bond formation in UO2 and a very large positive potential (~ 1.4 V) is required to achieve complete oxidation in a reasonable time. The reduction of U(VI) to U(IV) was investigated on Pt electrode at 0.05 V in 4.5 M HCl containing small amount of Bi(III) (< 0.01 M) [21]. The complex of Bi(III) with chloride ions gets chemisorbed on the platinum surface and reduces the hydrogen overvoltage on the working electrode. A monoatomic layer of metallic bismuth, formed by the underpotential deposition (UPD) of Bi(III) on the platinum surface, also reduced the rate of hydrogen evolution reaction.
3.3.2. Fuel Chemistry Division, BARC
Sharma et al. studied the coulometric determination of uranium employing a graphite electrode [33, 40]. Graphite electrode offered the advantage of simultaneous coulometric determination of uranium and plutonium from the same aliquot on a single electrode. A graphite electrode in the shape of a beaker showed satisfactory performance for the quantitative reduction of U(VI) to U(IV) and Pu(IV) to Pu(III) and also for the quantitative oxidation of Pu(III) to Pu(IV). The U(VI) reduction was a slow process. Therefore, to reduce the analysis time, predictive technique of charge evaluation was investigated and used to calculate the uranium content. An accuracy and precision of better than 0.5 % were obtained for uranium determination in samples containing only uranium. Determination of uranium in presence of plutonium was also studied on graphite electrode. A positive bias of 0.83 % was observed for uranium determination with a precision of better than 0.5 %. Subsequently, the reduction of U(VI) on activated platinum wire gauze electrode was investigated for the controlled-potential coulometric determination of uranium [40]. A large enhancement in the active area of a hollow cylindrical platinum wire gauge electrode was achieved by repetitive multilayer growth of oxide followed by its reduction. Growth of oxide was obtained by a combination of chemical and electrochemical treatments. A six-fold decrease in the time for quantitative reduction of U(VI) to U(IV) at a controlled potential of – 0.15 V vs. SCE in 1 M H2SO4 was observed compared to the untreated electrode. The enhanced activity of the electrode was found to decay on ageing. Determination of uranium was carried out by successive addition technique. Precision and accuracy of better than ± 0.2 % were obtained at 5 to 10 mg levels of uranium. This demonstrated the applicability of a platinum electrode, instead of the conventionally used Hg electrode, for the determination of uranium by controlled-potential coulometry. Joshi et al. also studied the controlled potential coulometric determination of uranium in presence of iron and plutonium using Pt wire gauge electrode [32].
3.4. Development and Performance Evaluation of Indigenous Controlled-Potential Coulometer
Recent advancement in electronics coupled with the enhancement in workmanship in Indian electronics industry for manufacturing precision electronic circuits increased the availability of indigenous controlled-potential coulometers. The new generation of controlled-potential coulometers, employed for the determination of U and Pu in our laboratory, were indigenously built by Santronics India Ltd., Mumbai [44]. The instrument incorporates a high sensitivity potentiostat capable of maintaining a constant potential to within 100 µV, a reference source of 1 ppm stability and a power amplifier with an output swing up to ± 30 V. The working of the controlled-potential coulometer is controlled by software. Sharma et al. carried out the performance evaluation studies of indigenous controlled potential coulometer for the determination of uranium using chemical assay standard solutions of uranium [9, 10, 15]. The coulometric results obtained for uranium determination in Rb2U(SO4)3 showed insignificant difference when compared with the biamperometric results at 95% and 99.9% confidence levels. Therefore, the indigenous controlled-potential coulometers were found suitable for uranium determination.
4. DETERMINATION OF PLUTONIUM
In aqueous solution, plutonium can exist simultaneously in different oxidation states viz. Pu3+, 4+ + 2+ Pu , PuO2 and PuO2 as given in the Table 1. The formal redox potentials of Pu(VI)/Pu(V) and Pu(IV)/Pu(III) couples are electropositive and are electrochemically reversible in non- complexing solution. A number of publications describe the use of coulometry on routine basis for precise and accurate determination of Pu.
4.1. Coulometric Titration of Plutonium
Prior to the development of controlled-potential coulometer, indirect coulometric titration methods were developed for plutonium determination. Furman et al. studied the coulometric titration involving Pu(IV)/Pu(III) couple using the electrolytically generated Ce(IV) ions and determining the end-point by potentiometric method [3]. The method was not suitable for the determination of microgram amounts of Pu because of the significant interference from iron present in traces in reagents. Carson et al. developed a coulometric titration method for rapid and accurate determination of plutonium by reducing Pu(VI) to Pu(IV) using electrolytically generated ferrous ions [9]. The method could be applied to determine small quantity of plutonium (as low as 3 µg) without any interference from iron.
4.2. Controlled-Potential Coulometry of Plutonium
Determination of Pu by CPC can be carried out either by primary coulometric or secondary coulometric technique. In primary coulometry, the Pu is oxidized or reduced directly at the working electrode and is based on Pu(IV)/Pu(III) couple. The limitations in employing primary coulometry for Pu determination is the interference from iron, especially in sulphuric acid medium, and the presence of organic impurities which get adsorbed on the working electrode surface, thus reducing its electrochemical activity. The interference from iron can be nullified using the secondary coulometry for the Pu determination [12]. In secondary coulometry, Pu and iron in the aliquot are first quantitatively oxidized to Pu(VI) and Fe(III), respectively, by using chemical reagents. After selectively destroying excess of chemical oxidant, the Pu(VI) and Fe(III) are completely reduced to Pu(III) and Fe(II), respectively, by controlled-potential coulometric reduction at 0.3 V. In final step, the Pu(III) and Fe(II) are completely oxidized to Pu(IV) and Fe(III) by controlled-potential coulometric oxidation at 0.7 V.
Pu and Fe (in solution) + oxidant Pu(VI) and Fe(III) Chemical oxidation Pu(VI) + Fe(III) + 4e- Pu(III) + Fe(II) CPC at 0.3 V Pu(III) + Fe(II) Pu(IV) + Fe(III) + 2e- CPC at 0.7 V In first CPC reduction step at 0.3 V, 4 electrons are involved and in the subsequent CPC oxidation step at 0.7 V, 2 electrons are involved. The subtraction of the oxidation reaction from the reduction reaction gives the net reaction: Pu(VI) + 2e- Pu(IV) Therefore, the subtraction of the oxidation coulombs from the reduction coulombs is equivalent to the coulombs obtained from two electron reduction of Pu(VI) to Pu(IV).
4.2.1. International Laboratories
Subsequent to the development of controlled-potential coulometer, the controlled-potential coulometric determination of Pu employing Pu(IV)/Pu(III) couple was investigated by many research groups. Unlike U(VI)/U(IV) couple, the Pu(IV)/Pu(III) couple is reversible and electrochemical reduction-oxidation of Pu(IV)/Pu(III) couple can be carried out with current efficiency of 100 % on platinum electrode in acidic solution. A number of studies on the controlled-potential coulometric determination of plutonium from various laboratories of U. S. Atomic Energy Commission and U. K. Atomic Energy Authority were reported [33,34,35,52]. An excellent review entitled ‘Applications of Controlled-Potential Coulometry to the Determination of Plutonium’ by Shults [17] gives the procedure for the determination of total Pu, Pu(III), Pu(IV), Pu(VI), mixture of various oxidation states of Pu, ionic Pu in presence of
polymeric Pu and polymeric Pu. In HClO4, HCl and HNO3 supporting electrolyte solutions, Pu(VI) is only partially reduced to Pu(III) leading to inaccurate results when Pu(VI) is present in appreciable amounts. In H2SO4, Pu(VI) is quantitatively reduced to Pu(III) and, therefore, H2SO4 is commonly used as the supporting electrolyte for Pu determination by CPC. However, the E0’ values for Pu(IV)/Pu(III) and Fe(III)/Fe(II) redox couples in 1 M H2SO4 are close to each other. Therefore, iron interferes in controlled-potential coulometric determination of Pu. The E0’ values for Pu(IV)/Pu(III) and Fe(III)/Fe(II) in various acids are given in Table 3. International Atomic Energy Agency (IAEA) [41] reported a controlled-potential coulometric method, based on Pu(IV)/Pu(III) redox couple, for Pu determination in nuclear grade plutonium nitrate solution in nitric acid medium using a gold (99.99% purity) wire gauge working electrode. An overall uncertainty of 0.1% was reported for Pu determination between 4 to 15 mg of Pu in samples.
Iron present at 500 ppm level in samples showed a positive bias of 0.1% in HNO3. Interferences from elements like Np, U, Au, Ir, Pd and Pt were also observed. The matrix effects of organic materials and anions were minimized by fuming the sample with sulphuric acid.
4.2.2. Fuel Chemistry Division, BARC
In our laboratory, controlled potential coulometric determination of Pu in nuclear fuel materials 2 is carried out using Pt wire gauze electrode (area ~ 20 cm ) in 25 mL of 1 M H2SO4, involving Pu(IV)/Pu(III) redox couple, by successive addition method. The aqueous solution of Pu contains Pu in different oxidation states. Therefore, Pu in the solution is first reduced to Pu(III) by applying a potential of +0.3 V (vs. SCE) and then, in the second step, Pu(III) is oxidized to Pu(IV) at +0.7 V (vs. SCE). The charge obtained in the oxidation step is used to calculate the amount of Pu. The sulphate anions are known to stabilize Pu(IV) more than Pu(III) because of the tendency of Pu(IV) to form much stronger complexes with sulphate. Therefore, the reduction of Pu(IV) to Pu(III) in 1 M sulphuric acid medium is sluggish and more time consuming than the oxidation of Pu(III) to Pu(IV). The repeated use of Pt wire gauge electrode leads to the passivation of the platinum surface resulting in the slower reduction and oxidation of the analyte. This translates to increased time for the complete electrolysis giving rise to higher blank value which results in higher uncertainty in the quantitative determination. Previously, the regeneration of the active electrode surface was carried out by keeping the electrode overnight in freshly prepared 1:1 HNO3 solution. This process generated the radioactive liquid waste. An electrochemical method of activation was, therefore, developed for regeneration of active electrode surface. The electrode was oxidized at 1.5 V for 3 minutes followed by reduction at -
0.25 V for 12 minutes in 1 M H2SO4. This redox cycle was repeated thrice and the Pu
determination was carried out on the next day in the same 1 M H2SO4.
The use of graphite as a working electrode for the determination of Pu by CPC was also reported from our laboratory [33,40]. A graphite electrode in the shape of a beaker showed satisfactory performance for the quantitative reduction of Pu(IV) to Pu(III) as well as for quantitative oxidation of Pu(III) to Pu(IV). However, the time required for the quantitative electrolysis was more with graphite electrode compared to that in the case of Pt electrode. Thus, the utility of other carbon-based electrodes like glassy carbon (GC) was explored to study the kinetics of Pu(IV) to Pu(III) reduction. The performance of the indigenous controlled potential coulometer was evaluated for the determination of Pu using chemical assay standard solutions of plutonium by Sharma et al. [10,15]. The experiments showed a significant difference between the coulometric value and the expected value. In view of this, more studies for the determination of
Pu in chemical assay standard solutions of plutonium were carried out using 1 M HClO4 as the supporting electrolyte and results were compared with those obtained with 1 M H2SO4. The perchlorate ion is non-complexing compared to sulphate ion which complexes Pu4+ ion strongly.
Table 4 shows the results of Pu determination in K4Pu(SO4)4 by controlled potential coulometry
in 1 M H2SO4 and in 1 M HClO4. The results in both the supporting electrolytes agreed within 0.2 % with those obtained by biamperometry. However, the agreement with biamperometry
value was better in 1 M HClO4 as a supporting electrolyte. The coulometric method was also applied for the determination of Pu in Pu alloy and (U,Pu) mixed carbide samples and the results were compared with those from biamperometry. The results are summarized in Tables 5 and 6 which demonstrated that the coulometric values agreed within 0.2 % with those obtained by biamperometry. It can be seen that the mean variation of the data obtained by the two methodologies is close to unity for the two samples.
To evaluate the results statistically, paired t-test was performed on the data. The paired t-test is useful when the observation in one set of data is directly related to the corresponding observation in the other set. Since the data are on different samples, the paired t test was found to be suitable for the evaluation of the results obtained by the two methods. The results of the paired t-test are given in Table 7. It can be seen that the results obtained by the two methods match with a confidence interval of 95%.
Kasar et al. also studied the controlled potential coulometric determination of Pu in mixed oxide samples [29]. Controlled-potential coulometric method is also used for the accountability analysis of purified plutonium obtained from spent fuel reprocessing [48, 49].
4+ 3+ 2+ + 5. Investigations on Redox Behaviour of Pu /Pu and UO2 /UO2 Redox Couples on Nanoparticles Modified Electrodes
4+ 3+ Recently, we investigated the redox behaviour of Pu /Pu couple in 1 M H2SO4 on platinum nanoparticles-modified glassy carbon and platinum electrodes [54]. The experimental results clearly established the faster redox kinetics at Pt nanoparticles-modified electrode compared to 2+ + bare Pt and GC electrodes. We also carried out the electrochemical studies of UO2 /UO2 redox couple in saturated Na2CO3 solution at gold nanoparticles (AuNPs) embedded cellulose triacetate 2+ + (CTA)-modified electrode [55]. In this study, an electrocatalytic reduction of UO2 to UO2 in saturated Na2CO3 solution was observed at AuNPs-CTA-modified electrode with higher current density and faster heterogeneous electron-transfer kinetics than that of bare Au electrode. The standard heterogeneous rate constant, k◦, for the reduction process at AuNPs-CTA modified electrode was about 25 times higher than that of bare Au electrode. The AuNPs embedded in CTA membrane improved the interfacial electron-transfer properties of electrode, resulting in a better electrochemical response than the bare Au electrode. It is expected that these studies will lead to the development of better electrodes for the quantitative determination of plutonium and uranium by controlled-potential coulometry. Further studies are necessary to replace the conventional electrodes with the modified electrodes for routine analysis of nuclear fuel samples for determining different actinides using controlled-potential coulometry.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the contributions by a large number of colleagues (both past and present) of the Chemical Analysis Section and Electrochemistry Section at different stages of the development of Coulometric methods.
REFERENCES
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(a)
(b)
(c)
(d)
Figure 1. (a) Controlled-potential coulometric setup, (b) electrolysis Cell, (c) Potentiostat and (d) platinum wire gauze electrode and coulometric cell
Figure 2. Coulogram for determining the working electrode potential for irreversible redox couple.
Table 1. Multiple oxidation states of uranium and plutonium in aqueous solution
Oxidation State +3 +4 +5 +6 3+ 4+ + 2+ Uranium U U UO2 UO2 3+ 4+ + 2+ Plutonium Pu Pu PuO2 PuO2
Table 2. Redox potentials of various redox couples of uranium in acidic medium
Redox Couples E0 / V vs. NHE 2+ + UO2 / UO2 + 0.334 + 4+ UO2 / U + 0.612 U4+ / U3+ - 0.607 U3+ / U(s) - 1.798 2+ UO2 / UO2(s) + 0.45
UO2(s) / U(s) - 1.444
Table 3. The E0’ values for Pu(IV)/Pu(III) and Fe(III)/Fe(II) in various acids
E0’ vs. SCE (in Volts) Supporting electrolyte Pu(IV)/Pu(III) Fe(III)/Fe(II) HClO4 0.72 0.47 HCl 0.72 0.45 HNO3 0.60 0.50 H2SO4 0.49 0.45
Table 4. Determination of Pu in chemical assay standard [K4Pu(SO4)4] by controlled potential coulometry in two different supporting electrolyte
Supporting electrolyte : 1 M H2SO4
Pu determined Aliquot wt. Sample (g) Amount Concentration (mg) (mg/g) PS-01 0.5181 4.612 8.903 PS-02 0.3586 3.191 8.899 PS-03 0.5406 4.809 8.896 PS-04 0.3433 3.055 8.899 PS-05 0.5787 5.149 8.898 Average ± RSD 8.899 ± (0.03%) Deviation from biamperometry value 0.20 %
Supporting electrolyte : 1 M HClO4
Pu determined Aliquot wt. Sample (g) Amount Concentration (mg) (mg/g) PS-06 0.6459 5.752 8.905 PS-07 0.5781 5.152 8.911 PS-08 0.4179 3.736 8.940 PS-09 0.6699 5.971 8.913 PS-10 0.6239 5.561 8.912 Average ± RSD 8.92 ± (0.014%) Deviation from biamperometry value 0.01 %
Table 5. Determination of Pu in Pu alloy samples by controlled potential coulometry (Supporting electrolyte : 1 M H2SO4)
% of Plutonium Sample No. Coulometry (A) Biamperometry (B) A/B
1 98.27 ±0.30 98.36±0.16 0.9991
2 98.09±0.50 98.81±0.10 0.9927
3 97.55±0.05 97.73±0.01 0.9982
4 97.41±0.05 97.55±0.03 0.9986
5 95.32±0.01 94.5±0.10 1.0087
6 97.63±0.09 97.78±0.10 0.9985
7 98.32±0.02 98.04±0.10 1.0029
8 98.30±0.08 98.35±0.10 0.9995
9 98.45±0.01 98.32±0.12 1.0013
10 98.65±0.14 98.52±0.20 1.0025
11 96.55±0.10 96.49±0.03 1.0006
12 97.43±0.66 97.78±0.27 0.9964
Mean of mean Values (A/B) = 0.9998 ± (0.38 %)
Table 6. Determination of Pu in (Pu0.7U0.3)C mixed carbide samples by controlled potential coulometry (Supporting electrolyte : 1 M H2SO4)
% of Plutonium Sample No. A/B Coulometry (A) Biamperometry (B)
1 67.33 ± 0.01 67.08 ± 0.03 1.0037
2 68.27 ± 0.01 68.24 ± 0.10 1.0004
3 68.02 ± 0.01 67.81 ± 0.06 1.0031
4 67.54 ± 0.01 67.45 ± 0.11 1.0013
5 67.77 ± 0.12 67.92 ± 0.12 0.9978
6 68.30 ± 0.10 68.24 ± 0.07 1.0009
7 65.71 ± 0.01 65.27 ± 0.03 1.0067
8 67.27 ± 0.09 67.34 ± 0.04 0.9990
Mean of means (A/B) = 1.0016 ± (0.28%)
Table 7. Results of the paired t-test for the data obtained by the two methodologies
Mean of the No. of Sample difference T-value Probability degrees of Remarks Type between the two freedom groups
Alloy 0.2026 0.8431 11 0.0646 Results Obtained are within 95% confidence Carbide 1.6162 0.1501 7 0.1075