Potentiometric titration in a low volume of solution for

Potentiometric titration in a low volume of solution for rapid assay of uranium: application to quantitative electroreduction of uranium(VI) P. Sahoo, C. Mallika, R. Ananthanarayanan, Falix Lawrence, N. Murali & U. Kamachi Mudali Journal of Radioanalytical and Nuclear Chemistry An International Journal Dealing with All Aspects and Applications of Nuclear Chemistry ISSN 0236-5731 Volume 292 Number 3 J Radioanal Nucl Chem (2012) 292:1401-1409 DOI 10.1007/s10967-012-1622-4

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Author's personal copy J Radioanal Nucl Chem (2012) 292:1401–1409 DOI 10.1007/s10967-012-1622-4

Potentiometric titration in a low volume of solution for rapid assay of uranium: application to quantitative electro-reduction of uranium(VI) P. Sahoo • C. Mallika • R. Ananthanarayanan • Falix Lawrence • N. Murali • U. Kamachi Mudali

Received: 30 December 2011 / Published online: 17 January 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012

Abstract A simple, inexpensive PC based potentiometric titration technique for the assay of uranium using low volumes of sample aliquot (25–100 lL) along with all reagents (total volume of solution being less than 2.5 mL) is presented. The technique involves modification of the well known Davies and Gray Method recommended for assay of uranium(VI) in nuclear materials by introducing an innovative potentiometric titration device with a mini cell developed in-house. After appropriate chemical conditioning the titration is completed within a couple of minutes with display of online titration plot showing the progress of titration. The first derivative plot generated immediately after titration provides information of end point. The main advantage of using this technique is to carry out titration with minimum volumes of sample and reagents generating minimum volume of wastes after titration. The validity of the technique was evaluated using standard certified samples. This technique was applied for assay of uranium in a typical sample collected from fuel reprocessing laboratory. Further, the present technique was deployed in investigating the optimum conditions for efficient in situ production of U(IV). The precision in the estimation of uranium is highly satisfactory (RSD less than 1.0%).

P. Sahoo (&) R. Ananthanarayanan N. Murali Innovative Instrumentation Section, Electronics and Instrumentation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India e-mail: [email protected] C. Mallika F. Lawrence U. Kamachi Mudali Reprocessing Research and Development Division, Reprocessing Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamil Nadu, India

Keywords Potentiometric titration Assay of uranium Pulsating sensor Mini cell Electro-reduction of U(VI)

Introduction The potentiometric titration method for the quantitative determination of uranium proposed by Davies and Gray [1], based on the reduction of U(VI) to U(IV) and subsequent titration with potassium dichromate is widely used for uranium bearing materials in the nuclear fuel cycle. This method involves addition of several reagents into the sample matrix at different intervals before carrying out the redox titration. Still it is considered as a novel technique for assay of uranium since this method permits the determination of uranium in the presence of large quantities of impurities. To apply this technique for accurate determination of uranium in different types of samples received from fuel reprocessing plant, several investigators reported the modification of the original method either to improve the sensitivity of detection in the end point or in scaling down the analytical technique [2–6]. Bickel [7] reported an inter laboratory comparison study for the assay of uranium in nuclear fuel samples using Davies–Gray method with potentiometric titration for the detection of end point. In order to avoid manual addition of several reagents before titration which is extremely cumbersome, some authors [8–12] reported towards either semi-automation or fully automation of the entire titration process. Though a lot of modified techniques have been proposed for Davies–Gray method the volume of solution taken for titration varies from 30 to 300 mL. In order to employ the technique in radioactive laboratory, where handling of radioactivity as well as waste disposal are critical issues, it is desirable to modify the technique for minimizing the volume of solutions taken for titration without sacrificing the precision and accuracy in analysis.

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This will minimize the waste volume substantially during analysis of samples in the reprocessing laboratory. Potentiometric titrations with extremely small volume of solution in a poly(dimethylsiloxane)-based microfluidic device has been reported by Ferrigno et al. [13]. The device is used to carry out titrations on-chips. Though about less than 10 lL volume of titrant is sensed by the electrodes each time it needs continuous flow of titrant and sample solution through two external reservoirs. Hence about 3–5 mL of reagents is required for each titration. To determine unknown sample concentrations this device requires preliminary calibrations. The technique is a complicated one and instead of giving direct volume versus potential plot to get information of end point, a computed plot is generated from the experimental plot (channel number vs. potential). In spite of these limitations, the accuracy of the technique is also low and the technique was not yet tried for uranium assay. Except this technique no direct potentiometric titration technique has yet been reported to carry out titration in very low volume of solution. In the present work, an attempt is made to address this problem by using an in-house built instrumentation with pulsating sensors. Pulsating sensors developed in the authors’ laboratory [14–17] were already deployed in many physico-chemical and analytical applications. A rapid redox titration technique for assay of uranium in less than 2.5 mL solution including all reagents using a pulsating type potentiometric titration device with a mini cell developed in-house is reported in this study. After successful deployment of this low volume titration technique for assay of uranium in some typical uranium rich samples, this method was employed for the study of electro-reduction of U(VI) to U(IV). Use of in situ electroreduction of U(VI) and Pu(IV) is an advantageous choice for valency control prior to partitioning of Pu from U during reprocessing of spent FBR fuel by PUREX process. The reduction is carried out in a bank of two-compartment flow type cells. It is necessary to study the yield of U(IV) in the cathode compartment as functions of time and current density. As an appropriate facility for on-line monitoring of this uranium bearing species is not available, the in-house developed PC based rapid and automated titration facility, which is powerful enough in carrying out titrations in very small volumes of solutions, was adopted for assay of U(IV) at frequent intervals without interruption of the electroreduction process.

Experimental Chemical reagents All chemical reagents used in this work were of analytical reagent grade. High purity Millipore water (conductivity less than 1.0 lS cm-1) was used throughout. The Certified

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Reference Materials NIST U3O8 primary standard and uranyl nitrate hexa hydrate (99.99% purity), Sigma Aldrich were used to examine the validation of the proposed technique.

Potentiometric titration facility for titration with low volume The schematic diagram of the titration facility using mini cell deployed for assay of uranium is illustrated in Fig. 1. The concept of design and fabrication of this titration facility is briefly mentioned as follows. The hardware consists of a three-channel signal routing cum power supply unit. For potentiometric titration only Channels 2 and 3 are used. A nonconventional conductance based reagent dispenser [18] made out of a disposable medical syringe is used to deliver the reactant to the sample solution placed in a mini titration cell. In some cases a Hamilton syringe is used for delivery of the reactant in small volume aliquots. Addition of each drop is preceded by momentary generation and communication of fixed frequency to the PC in second channel as the event marker. The second channel provides information regarding volume of reactant dispensed during the titration, while the third channel is dedicated for real time display of titration plot (time vs. potential). The potential generated from the electrochemical cell is converted to digital pulse frequency using a specially designed V to F converter, which is powered by 12 V DC. This pulse frequency (f), a linear function of emf is communicated to the PC through an interfacing cable connecting the power supply cum signal routing unit with the PC parallel port without any add on cards. The redox electrode couple placed in the solution monitors potential in every 0.3 s as the reaction progresses in case of automated titration whereas in every 1 s in case of semi automated titration with manual addition of the reactant. The fourth channel, which is generated immediately after the end of titration by offline processing of data, presents the actual titration plot in the volume domain. Titrations in most of the cases are completed within 100 s. Offline analysis of data to get end point is completed immediately after titration. An interactive software package developed either in quick basic or in C does the following operations during the course of titration: (i) simultaneous counting of pulses from both channels for the determination of frequencies, (ii) graphical display of online data in time domain for both channels, (iii) presentation of titration plot in volume domain by appropriate processing of the data after the end of the titration and (iv) determination of end point of the titration through first derivative plot. Further, the titration plots can be saved for future review. The mini cell capable of holding about 2.5 mL solution for titration consists of a pair of tiny wire type electrodes (Pt and W).

Author's personal copy Potentiometric titration in a low volume of solution

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Fig. 1 Schematic diagram of titration facility

Estimation of uranium Uranium assay by redox titration using Davies–Gray method [1] needs strict kinetic control. To maintain excellent accuracy before the onset of titration, sequential addition of reagents, mixing of reagents and time duration for each addition should be strictly followed. Hence, though Davies–Gray method is a very well known technique, the appropriate recipe with systematic procedure is mentioned here to carry out miniaturized titration in order to obtain accurate and precise results. The chemical conditioning involving addition of very small volume of reagents used in the present work is presented below. (1) (2)

(3)

(4)

(5)

(6)

In a 10 mL beaker 25–100 lL sample containing 0.25–1 mg U(VI) was taken. To the sample, 50 lL 1.5 M sulphamic acid was added followed by the addition of 500 lL ortho phosphoric acid. The solution was mixed thoroughly to ensure no phase separation of viscous phosphoric acid layer. Thorough mixing was done after adding 50 lL of 0.5 M ferrous ammonium sulphate solution and 200 lL of 4 M nitric/sulphamic acid mixture. One hundred microlitre of 1% (w/v) ammonium molybdate was added, mixed well for about 30 s and the mixture was allowed to stand for at least 7 min before proceeding to the next step. Subsequently, 50 lL of 10% (w/v) vanadyl sulphate and 1 mL of 2 M H2SO4 were added in sequence, mixed properly and transferred to the titration cell. Within 2–3 min, the solution containing U(IV) was titrated using appropriate standard K2Cr2O7 solution.

The steps 3 and 4 were omitted for the determination of U(IV) and the entire operation including sample processing and titration, took less than 5 min for the assay of U(IV).

Fig. 2 Schematic diagram of the electrolysis cell

Flow type cell for electro-reduction The schematic of the electrolysis cell is shown in Fig. 2. The cell was made of poly acrylic acid. A separator made out of the same material isolated the cathode and the anode compartments. A small gap at the lower end of the separator provided a conducting path between the anode and cathode compartments. The base as well as the inner walls of the cathode compartment was provided with Ti liner which acts as the cathode, whereas a rectangular shaped Pt foil was used as the anode. The surface area of the cathode and anode were of 64 and 9.7 cm2 respectively. The anode was positioned such that it was about 5 mm above the electrolytically conducting connection, to minimize the oxidation of generated U(IV), in the event of oxygen evolution at the anode. Electro-reduction The flow type electrolytic cell shown in Fig. 2 was used for electro-reduction of U(VI). A stock solution of U(VI) (about 10 g U/L) was prepared by dissolving appropriate

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amount of U3O8 in 1 M HNO3. The concentration of hydrazine in the stock solution was maintained as 0.2 M. Seventy-five millilitres of this solution taken in each run for electrolysis was allowed to circulate in the electrolysis cell at a flow rate of about 3.5 mL/min. The cell was connected to a 30 V DC power supply and electrolysis was carried out in constant current mode. Experiments were conducted with 0.8, 1.6, 2.3, 3.1 and 12.5 mA cm-2 as the cathodic current density in order to optimize the conditions for quantitative yield of U(IV). The electrodes were mildly shaken intermittently to minimize the bubble over voltage. During the course of electrolysis, sample aliquots (about 0.1 mL) were withdrawn from time to time for assay of U(IV).The reactions taking place during electrolysis are:At cathode þ 4þ UO2þ þ 2H2 O 2 þ 4H þ 2e ! U

ð1Þ

2Hþ þ 2e ! H2

ð2Þ

þ

2H þ

NO 3

! H2 O þ

NO 2

ð3Þ

At anode þ N2 Hþ 5 ! N2 þ 4e þ 5H

ð4Þ

Results and discussion Estimation of uranium in uranyl nitrate solution using the present technique The basic principle of determination of uranium by Davis– Gray titration method involves the reduction of U(VI) to U(IV) by Fe(II) in ortho phosphoric acid medium in the presence of sulphamic acid, destruction of excess Fe(II) using nitric acid in the presence of Mo(VI), and titration of U(IV) with K2Cr2O7 using a suitable redox indicator. Fig. 3 Online potentiometric titration plot for the assay of uranium in a very low volume of solution (0.1 mL standard U(VI) solution was titrated against 0.01 M K2Cr2O7 after adequate chemical treatment. End point corresponds to 0.99 mg U)

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Hence, human errors associated with detection of end point affect the precision of the technique. The use of the present titration facility minimized volume of sample and reagents significantly. Initial experiments were conducted with standard uranyl nitrate solutions in order to demonstrate the efficacy of titration and the quality of titration plot generated online. Only 0.1 mL uranyl nitrate solution containing 1 mg U was treated with Davies–Gray reagents described in ‘‘Experimental’’ section. The total volume of the solution after chemical treatment was 2.5 mL. It was titrated against standard dichromate solution and the online titration plot obtained is shown in Fig. 3. The real time change in frequency, a direct function of emf, during titration is given in channel-3 whereas channel-2 provides information about drops of reagents dispensed during titration. Channel-4 shows volume versus frequency plot and its corresponding first derivative plot, generated immediately after completion of titration, which provides information of end point, whereas channel-1 was kept as dummy. Titration was completed within 1 min and the RSD in 10 sets of measurement was 1.0% with excellent reproducibility. The jump in potential at end point was very sharp and quite distinct. The use of Pt–W couple as bimetallic electrode has been recommended in certain redox systems [19]. It is observed that in the present work the use of bimetallic electrode couple (Pt–W) helps to give a shift in potential better than that using Pt-SCE electrode couple. The proposed potentiometric titration facility has several specific features such as (i) high speed in titration which helps to complete the entire titration process including analysis of data within a couple of minutes (generally less than 2 min in most of the cases), (ii) titration using very low volume (about 2–2.5 mL solution), (iii) high precision and excellent resolution in potential measurement, (iv) easy conversion of emf from the electrochemical system to digital


pulse frequency using minimum hardware components, (v) transmission of pulse frequency from sensor end to PC without using any add on card, (vi) excellent noise immunity, (vii) transmission of signal to a long distance, even about 200 m without any loss of signal quality, (viii) the entire operation during titration under software control and (ix) delivery of reactants either through reagent dispenser or in some cases manual addition of reagent through Hamilton syringe. The rapidity in titration was possible due to the designing skill of instrumentation, software package and automated drop dispenser. A specially designed V to F converter generates very stable frequency which is directly proportional to emf and is immediately transmitted to PC. Counting of data, plotting the data in time domain and quick offline analysis to get information on end point are done within a couple of minutes using the in-house built software package written in C or quick basic. Influence of hydrazine In process solutions containing uranium, hydrazine is normally present in the sample matrix since it is used as a valency stabilizer reagent. Hence, the effect of hydrazine towards assay of uranium was investigated. For this purpose, two sets of titrations of U(VI) with/without hydrazine were carried out and the results are listed in Table 1. From the results it is evident that 0.2 M hydrazine present in the sample does not affect the end point. Validation of the present technique with standard reference materials In order to ensure the reliability of the technique for various applications in fuel cycle, the performance of the technique was examined by analyzing the uranium content in standard solutions prepared by using reference materials (NIST U3O8 primary standard, uranyl nitrate hexa hydrate, 99.99% Table 1 Influence of 0.2 M hydrazine in U(VI) sample matrix on end point Volume of 0.01 M K2Cr2O7 consumed at end point (mL) Aliquots of U(VI) sample without hydrazine Aliquot-1

0.832

Aliquot-2

0.847

Aliquot-3

0.832

Aliquot-4

0.832

Aliquots of U(VI) sample containing hydrazine Aliquot-1

0.832

Aliquot-2

0.832

Aliquot-3

0.847

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purity, Sigma-Aldrich make). Sample aliquots containing 0.5–1.0 mg U were taken for analysis. Results are shown in Table 2. The precision obtained from this investigation lies between 0.004 and 0.006 mg in 0.5–1.0 mg U whereas the accuracy is between 0.125 and 0.42%. Assay of uranium in samples received from fuel reprocessing laboratory A batch of typical synthetic samples containing both uranium and plutonium were analysed using the present technique. The same samples were also analysed using two other independent techniques—(i) Modified Davies–Gray Method [4] following ASTM procedure [20] and (ii) Spectrophotometry [21]. Results obtained from the above experimental campaign, listed in Table 3 show excellent agreement. The advantage in the present technique over conventional Davies–Gray Method is minimization of the volume of sample aliquot and reagents from several millilitres to a few microlitre taken for titration and minimum production of waste volume after titration besides rapidity in measurement with distinct equivalent point. A slight deviation in spectrophotometry result from the other two approaches is possibly due to the error associated with dilution of the original samples by thousand times in order to bring into analytical measurement range. The experience gained from the synthetic sample analysis gives enough confidence to use the present miniaturized titration technique for assay of uranium in different process samples such as (i) samples for estimation of U in the lean organic after partitioning cycle, (ii) determination of U for the purpose of nuclear material accounting and (iii) final products for assay of uranium after PUREX campaign. Application towards studies on electro-reduction of U(VI) The electro-reduction of U(VI) using a compartmental cell was taken up in order to evaluate suitable operating conditions for quantitative reduction of U(VI). The information obtained from this study will help to design large scale facility for in situ electro-reduction of U(VI) in the context of developing facility for electro partitioning of U and Pu from fast breeder reactor’s spent nuclear fuel reprocessing solution. Electro-reduction experiments were conducted at different cathodic current densities for a fixed duration in the specially designed diaphragmless cell. During electrolysis 50–100 lL sample solution was withdrawn periodically from the cathode compartment for chemical analysis without disrupting the electrolysis process. Since the reduction efficiency of the process was calculated based on the results obtained by using the present analytical

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Table 2 Validation of the analytical technique using standard reference materials Experiment

Identification of reference samplea

Amount of U taken (mg)b

Amount of U found (mg)c

% Deviation

1

A

1.0025

1.001 ± 0.004

0.15

2

A

0.5013

0.503 ± 0.005

0.34

3

A

0.8020

0.803 ± 0.004

0.125

4

B

0.9999

1.002 ± 0.005

0.21

5

B

0.7999

0.798 ± 0.006

0.24

6

B

0.4999

0.502 ± 0.004

0.42

a

A NIST U3O8 primary reference standard, B uranyl nitrate hexa hydrate (99.999% purity) Aldrich-Sigma make

b

Computed from microlitre volume of stock reference solution using Hamilton syringe

c

Number of determinations: five per experiment

Table 3 Comparison of results obtained for synthetic sample (values in g/L) Sample identification numbera

Present methodb

Davies–Gray method (following ASTM procedure)c

Spectrophotometry

S1

9.98

9.92

9.64

S2

15.13

15.22

15.78

S3

4.06

3.98

3.87

S4

10.55

10.55

10.28

a

S1 and S2 contained plutonium whereas S3 and S4 were free from Pu

b

Total volume of solution, including all reagents, did not exceed 2.5 mL

c

Total volume of solution, including all reagents, was 77 mL

Table 4 Material balance for two sets of electrolysed samples collected from two independent experimental campaigns; volume of sample withdrawn for assay of U(IV) and total U content: 0.1 mL Description of sample

Amount of U(IV)/(mg) (titration without adding Fe(II)) (A)

Total U content (mg)a (titration after adding Fe(II)) (B)

Amount of U(VI)/(mg) (B - A)

I. Campaign-1 (sample withdrawn after 1 h electrolysis)

0.36

1.08

0.72

II. Campaign-1 (sample withdrawn after 2 h electrolysis)

0.70

1.07

0.37

III. Campaign-1 (sample withdrawn after 4 h electrolysis)

0.96

1.08

0.12

IV. Campaign-2 (sample withdrawn after 1 h electrolysis)

0.34

1.07

0.73

V. Campaign-2 (sample withdrawn after 2 h electrolysis)

0.73

1.08

0.35

VI. Campaign-2 (sample withdrawn after 4 h electrolysis)

0.98

1.07

0.09

a

-1

Total uranium content before the onset of electrolysis: 10.7 mg mL

technique only, the reliability of the technique was confirmed by adopting the following procedure. During electrolysis a series of samples were collected at different time intervals. Each sample was divided into two aliquots. One aliquot was analysed for the assay of U(IV) without adding Fe(II) and the other was analysed for total uranium content by adding Fe(II). The difference between the two values gave the quantity of U(VI) present in the sample. The analytical results of the entire series of samples are given in Table 4. Prior to electrolysis the total uranium content was determined. Results from the above investigation showed excellent material balance.

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It was observed that during electrolysis gas evolution occurred at Pt anode and with increase in applied current the rate of evolution was found to increase. When electrolysis was conducted at 800 mA applied current, the volume of gas evolved near anode was large and it spread through out the solution. With increase in applied current the cell voltage also increased. The voltage (after attaining steady value) built-up during electrolysis from 50 to 200 mA was between 2.6 and 3.4 V, whereas at 800 mA it was appreciably high (about 6.0 V). Further, in each run, during first phase of electrolysis (first 20 min), gas evolved at the anode was found to accumulate on the surface of Pt


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Fig. 4 Formation of U(IV) as a function of time at different applied cathodic current density

Fig. 5 The percentage yield of U(IV) during electrolysis with respect to current strength (electrolyte: U(VI) solution (10.7 g U/L) in 1 M HNO3/0.2 M hydrazine)

electrode giving rise to bubble over voltage. This disappeared and maintained a steady value by slightly shaking the Pt electrode. After 20 min of electrolysis, such temporary built-up of voltage was not experienced. Hence, it was essential to adopt such a precaution in the initial stage to avoid bubble over voltage in order to achieve quantitative production of U(IV). The formation of U(IV) as a function of time at various constant applied currents is shown in Fig. 4. In all the experimental runs, the percentage conversion to U(IV) gradually increased and it attained almost a plateau above 90%. Curves 2, 3 and 4 in Fig. 4 revealed that 90% conversion to U(IV) could be achieved in about 4.5 h, whereas curve 1 shows that more than 7 h was required to get the same yield of U(IV). From the basic data, the amount of current consumed per litre of electrolyte during electrolysis was computed. Percentage yield against the quantity of current, for each constant applied current, is plotted in Fig. 5. Curves 1 and 2 (at 50 and 100 mA respectively) of this figure, indicate that the current input for the generation of about 90% U(IV) was 4.6 Ah/L, whereas in order to obtain similar yield of U(IV), it required about 9 and 11 Ah/L at 150 and 200 mA of applied currents respectively. At higher cathodic current densities (above the limiting

current density), the cell voltage was found to increase proportionately leading to the setting in of concentration and kinetic polarization in the cell, which in turn decreased the reduction efficiency. Hence, to achieve quantitative yield of U(IV) with better energy efficiency, one has to choose about 100 mA as the applied current, i.e. the threshold current density window for the reduction process may be chosen well below the limiting current density. Reproducibility checks of the results (depicted in Fig. 6) was done by estimating the amount of U(IV) produced in 2 independent sets of electrolysis campaigns at an optimum applied current of 100 mA. The percentage yield of U(IV) estimated at different time intervals was highly reproducible in both the runs. This corroborates the effectiveness of the analytical technique deployed to evaluate U(IV) assay. The optimized conditions to achieve near quantitative reduction of U(VI) to U(IV) are listed in Table 5. During the reduction of U(VI) at the cathode, nitrogen gas was evolved at anode due to the oxidation of hydrazine as per Eq. 4. At high voltage, when the electrode potential reached the decomposition potential of water, oxygen evolution occurred at the anode. Standard reduction potentials for the couples (UO22?/U4?), (N2/N2H5?) and (O2/H2O) are 0.33, -0.23 and 1.23 V respectively. Hence, if electrolysis is carried out below oxygen evolution

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Fig. 6 Reproducibility check in the electro-reduction of U(VI) to U(IV) Table 5 Optimum conditions for the electro-reduction of U(VI) solution Electrolyte

0.1 M U prepared by dissolving appropriate amount of U3O8 in 1 M HNO3/0.2 M hydrazine

Volume of electrolyte

75 mL

Anode Cathode

Platinum (rectangular shaped foil) Titanium (as liner in the inner wall and bottom of cathode compartment)

Anode area

9.7 cm2

Cathode area

64 cm2

Applied current

100 mA

Cathodic current density

1.6 mA cm-2

Anode current density

10.3 mA cm-2

Cell voltage (steady value)

About 3.0 V

potential, quantitative reduction of U(VI) occurs at the cathode and nitrogen evolution occurs at the anode. At high current densities, the cell voltage increases and exceeds oxygen evolution potential, thereby liberating oxygen along with hydrazine decomposition. The presence of oxygen leads to the oxidation of U(IV) to U(VI) in the vicinity of anode. The behavior observed in the present

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work while carrying out electrolysis at an applied current of 800 mA upheld this phenomenon of oxidation of a part of U(IV) and decreasing the reduction efficiency. Typical values obtained for the percentage yield of U(IV) when 800 mA current was passed through the cell were 38.5, 35 and 33 in 2, 3 and 4 h respectively after the start of electrolysis. Such behaviour was not encountered when electrolysis was carried out at lower applied currents (50, 100, 150 and 200 mA). Hence, oxygen evolution can be eliminated by adjusting current density lest it would lead to oxidation of a part of U(IV) to U(VI) thereby reducing the efficiency of the process. The investigations on electrochemical reduction of U(VI) reported in the present work had been compared with a similar work reported by Wei et al. [22] using glassy carbon as working electrode. In their work, the authors recommended 0.075–0.015 M hydrazine in 6 M HNO3 for high yield of U(IV) (almost 100%). Controlled addition of hydrazine helped to suppress the reduction of HNO3. In the present work, as 0.15–0.2 M hydrazine in 1 M HNO3 was found to help in getting high yield of U(IV), high concentration of HNO3 (6 M) is not recommended. Since it was possible to estimate U(IV) content during electrolysis using a few microlitre sample within 1–2 min duration in much simpler way by using the rapid miniaturized titration technique, the U(IV) yield with respect to initial hydrazine concentration taken in electrolyte bath was investigated. Based on these data, it was found that a feed concentration of 0.15 M hydrazine was enough to produce maximum yield of U(IV). Since during electrolysis there would be a continuous reduction of hydrazine concentration, it was preferred to select a slightly higher concentration of hydrazine (0.2 M) as the optimum concentration. The U(IV) yield (90–95%) as obtained in the present work was derived by quick assay of U(IV) and total uranium content in the electrolyte solution at different time intervals during electrolysis. The % yield of U(IV) is slightly less than that reported by Wei et al. They used spectrophotometry and ICP-AES for assay of U(IV) and total uranium respectively. These analytical techniques are best suited for chemical analysis at trace levels. Hence, in order to apply these techniques to evaluate % yield, they might have used appropriate dilution factors before measurements in order to bring in the range of analysis specified for such analytical techniques. In the present study, the analysis is straight forward and it has been performed by adopting a primary technique (titrimetry) using the mini titration facility. The current input to obtain maximum yield of U(IV) in the present work is 4.6 Ah/L which is almost half of the value reported in earlier work. The data from the present work could not be compared with the work reported by Wei et al. owing to lack of information about volume of electrolyte used by those authors.


Conclusion The evolution of a very handy and high precision analytical technique for quick assay of U(IV) as well as U(VI) was brought out in the present work. The advantage of the present technique is to carry out volumetric analysis of uranium using very low volume of solution and handling minimum volumes of reagents. The technique is extremely convenient for routine assay of uranium in a reprocessing laboratory. In addition to this, the technique helped in evaluating suitable conditions for the electro-reduction of U(VI) to U(IV) in a much simpler way and the experience gained would aid to design the plant-scale electrolysis facility for in situ reduction of U(VI) for effective U/Pu partitioning. Since, these processes need to be carried out in highly radioactive environment, a compact signal processing-cum-data storage unit, instead of PC will add to the operational convenience during analysis in less accessible areas. The online unit would process the pulsed signals from the titration cell and from the drop dispenser. It would store the data for further offline processing using a PC. In order to achieve this objective, an embedded pulse processor was designed, developed and its functioning was demonstrated successfully for the deployment of this unit in fuel reprocessing facility. Acknowledgments The authors are deeply indebted to Shri R. Natarajan, Director, Reprocessing Group and Shri S.A.V. Satyamurty, Director, Electronics and Instrumentation Group, Indira Gandhi Centre for Atomic Research for their keen interest and constant encouragement throughout the course of this work. The authors express their gratitude to Dr. Tom Mathews, Materials Science Group for designing the electrolysis flow cell.

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