Research Journal of Recent Sciences ______ ______________________________ ______ ____ ___ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 47 - 50 (201 3 ) Res. J. Recent . Sci. International Science Congress Association 47 Development of Sensitive Voltammetric Method for Determination of Thorium in Waste Waters Dubey S. 1 and Kherwa V. 2 1 MVJ College of Engineering, Bangalore, Karnataka, INDIA. 2 Department of Chemistry, C.D.L.University, Sirsa 125055, Haryana, INDIA. Availa ble online at: www.isca.in Received 06 th June 2012 , revised 18 th Dcember 2012 , accepted 26 th December 2012 Abstract An indirect voltammetric determination method of thorium based on the displacement reaction between T h (IV) and Zn (II) - EDTA complex in ammonium acetate buffer medium is described. The limit of quantification of 0.005 ppm was observed using differential pulse polarography. The possibly associated metal ions did not interfered in measurements. The method has been successfully applied for the analysis of thorium in waste water samples. Keywords : Thorium, differential Pulse polarography , waste waters analysis . Introduction The tremendous uses of thorium in nuclear reactors as a substitute of uranium has made it immensely important element. It can also be used in refractory materials and incandescent gas mantles. Other applications of thorium include in heat resistant ceramics, in gas tungsten arc welding electrodes, additives in alloys and as a catalyst in petroleum cracking. In regard to toxicity, the radioactivity of thorium overrides other toxicity considerations. Acute exposure can lead to cause cancer and dermatitis 1 . Therefore, it is appropriate to develop a simple analytical method for the determin ation of thorium. Most applications of chemical analysis to the samples of nuclear technology important elements involve trace determination at very low concentration. High sensitivity of the employed method must further be accompanied by sufficient sele ctivity, precision and accuracy. In this way several methods such as spectrophotometry 2 - 4 , mass spectrometry 5 , AAS 6,7 , XRD 8 , ICP - AES 9 , NAA 10 and X - ray fluorescence 11 are reported in determination of microgram level of various metal ions. In most cases proc edures requires preliminary treatment of the sample (in addition to any concentration step) to bring thorium to the proper step for measurement by preoxidation or reduction. The voltammetric methods are considered more suitable because due to certain selec tivity of the redox potential, these can identify and determine different forms of an element as in case of thorium, which shows three oxidation states of +2, +3 and +4. Differential pulse polarography (DPP) and anodic stripping voltammetry have proved use ful in such determinations 12 . Thus it is of interest to investigate the suitability of these methods in determination of thorium. The electrochemical data on thorium reveals that tetravalent thorium is the only ionic species that has been characterized in aqueous solutions. Further, reduction potential of Th 4+ is too negative ( - 1.83V vs NHE) to permit thorium wave before reduction of hydrogen ion or other cation of supporting electrolyte 13 . It makes voltammetric determination of thorium rather difficult. M any authors have therefore employed supporting electrolytes which makes a complex with thorium so that reduction potential might sufficiently be shifted and measurable. Gritzner etal 14 have shown polarographic behavior of thorium in dimethyl formamide and dimethyl sulphoxide. Mosrafa etal 15 have used Th (IV) complex with alanines in these observations. Electrochemical studies of Th (IV) in dimethyl sulphoxide and propylene carbonate are reported by Duyckaerts and Degueldre 16 . Cassaure and coworkers 17 have d escribed electrochemical behaviour of thorium (IV) in LiCl - KCl eutectic melts. Sindhu etal 18 have reported polarographic determination of thorium (IV) with 3 - hydroxypyridine - 2 - thiol. Linear sweep oscillographic polarography and pulse polarographic determin ation of thorium is given by Plock and Vasquez 19 , and Das etal 20 , respectively. Li and Coworkers 21 have described adsorptive voltammetric study of thorium - alizarin complexon at a carbon paste electrode. The anodic stripping voltammetric determination of th orium has also been shown by Liu etal 22 . Herein we suggest an indirect differential pulse polarographic determination method of thorium. It is based on the displacement reaction between Th (IV) and Zn (II) – EDTA complex in ammonium acetate buffer medium. The DPP determination of tellurium 23, 24 , gallium 25 , osmium 26 and silver 27 were reported earlier. Material and Methods Instrumentation: A microprocessor based pulse polarographic analyzer (Model CL – 362) in combination with a drop – timer assembly, all from Elico Limited, Hyderabad, India, was used for polarographic measurements. Current voltage curves were recorded by an Epson printer (Epson – LX – 300+II). The instrumental settings for DPP were as follows: a dropping Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 47 - 5 0 (201 3 ) Res. J. Recent. Sci International Science Congress Association 48 mercury electrode (DME) as the working ele ctrode; pulse amplitude, 50 mV; pulse duration, 57ms; clock time of pulse, 0.5s; scan rate, 12mV/sec and charging current compensation, 20%. Potentials were measured against a saturated calomel electrode (SCE). Platinum wire was used as auxiliary electrode while ammonium acetate buffer was the supporting electrolyte. A systronics UV - vis spectrophotometer (Model UV – 108) with a wavelength range of 190 - 900nm was also used for sample analysis. A tungsten – halogen deuterium lamp and wide range photomultiplie r were used as the light source and detector, respectively. The spectral band width was 0.5nm. The pH studies were made by a systronics digital pH meter (Model – 355). Sample Preparation: Samples of industrial waste from common nala in Basni Industrial A rea of Jodhpur were collected in polyethylene containers. These were filtered to separate any suspended particulate matter and acidified with HCl to pH≈2 for storage. 50ml aliquot of the sample was treated with 1 ml of an oxidizing mixture of acids to dest roy biological materials 28 . The treated samples were spiked with certain concentration of thorium and were taken as test solution for analysis work. Chemicals : Chemicals used were of reagent grade purity. Standard solution of thorium was prepared from tho rium nitrate of B.D.H. Other reagents employed in different steps, as described in the text, include: nitrates of lead, zinc and nickel, vanadium dioxide, EDTA disodium salt and ammonium acetate. Test solutions were deaerated for 20 minutes by passing pu rified nitrogen. All of the experiments were carried out in an air - conditioned laboratory where the temperature was maintained at 25 ±1°C. Results and Discussion The voltammetric determination of thorium was studied in complexing media of Zn - EDTA in sodiu m acetate buffer (pH 4.03) because of its more negative reduction potential. The method is based on the displacement of Zn 2+ equivalent of Th 4+ and indirect measurement of thorium by DPP. A sharp DP peak was obtained at - 1.06V as shown in figure - 1 Peak cu rrent increases linearly with the concentration of thorium up to 10ppm. DP polarograms at different concentration of thorium are recorded in figure - 2. The calibration curve drawn in Fig 3 shows following characteristics: Slope, 0.0360; Intercept, 0.0208; C oefficient of correlation (r), 0.9990. Interference : The major ions commonly present in industrial wastes such as copper and lead were tested for interference during thorium determination .The reduction of Cu 2+ occurred at positive potential (�0.0V), and did not interfere. Similarly, the DP peak of Pb 2+ was noticed at - 0.45V which was well - separated from that of Th 4+ at - 1.06V as shown in figure - 3. Figure - 1 DP Polarogram of Zn (II) equivalent to Thorium in Acetate buffer (pH 4.03) (a) Blank solution of A cetate buffer (pH 4.03) (b) Zn (II) equivalent to Thorium (IV) in Acetate buffer (pH 4.03) Th = 7.5 ppm, Zn – EDTA = 1 x 10 – 4 M Figure - 2 DP Polarograms of Thorium (IV) at different concentrations in acetate buffer (pH 4.03) Minimum concentration = 0.01 ppm, maximum concentration =10 ppm Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 47 - 5 0 (201 3 ) Res. J. Recent. Sci International Science Congress Association 49 Figure - 3 The plot of calibration curve, concentration in (ppm) vs peak current (µA) Minimum concentration = 0.01 ppm, maximum concentration =10 ppm DPP Parameters: modulation amplitude, 25 mV; pulse duration, 57 ms; drop time, 0.5s; scan rate, 12mV/s; deaeration time, 20 min; W. E = DME; h = 65 cm. Uranium might also be present with thorium due to their wide use in nuclear reactors. U 6+ in presence of Zn - EDTA showed distinguishable DP peak at - 0.237V, illustrating no interference. Peak potential of metal ions is shown in table - 1. Table - 1 Peak potentials of metal ions in presence of Zn – EDTA complex Supporting electrolyte: 0.01M ammonium acetate buffer S.No. Metal ion - Ep(V) vs SCE 1 Copper (II) +0.005 2 Uranium ( VI) - 0.237 3 Lead (II) - 0.448 4 Cadmium (II) - 0.617 5 Thorium (IV) - 1.060 Limit of Determination The minimum amount of thorium which could be determined under these experimental conditions was 0.005 ppm. Analytical Applications: The voltammetric obse rvations on Th 4+ were applied to analyze thorium in industrial wastes. The prepared samples were taken into the complexing polarographic medium of Zn - EDTA in ammonium acetate buffer and DP polarograms were recorded from - 0.8 to - 1.2 V. The peak currents we re noted at - 1.06V after making blank corrections. The concentrations were determined by standard addition method 29 . Thus results of determination of thorium are presented in table2 . Table - 2 DPP determination of thorium in industrial waste samples Sample T horium(IV) concentration (ppm) S.D (±) C.V (%) Present Found 1. 3.6 3.58 0.026 0.687 2. 3.0 2.94 0.113 3.826 3 2.8 2.78 0.026 0.943 4 2.0 1.90 0.047 2.480 Validation : The UV - Vis spectrophotometric method based on reaction of thorium with thorin in aqueous medium was used to compare the results of DPP determination of thorium. The comparative data are included in table - 3 . It revealed that DPP measurements were quantitative in terms of average recovery of 97.48 %. Table - 3 Comparison of results of thorium determination by DPP and UV – Vis spectrophotometry Sample Thorium concentration(ppm) DPP Recovery (%) UV – Vis 1 (3.6) 3.58 99.44 3.52 2 (3.0) 2.94 98.00 2.91 3 (2.0) 1.90 95.00 1.85 Average Recovery 97.48 n = number of determinations = 3 , Values shown in parentheses are thorium concentration in ppm. Conclusion From the present work it was concluded that in presence of Zn - EDTA complex the limit of voltammetric determination of thorium by using DPP technique could be lowered down from 0 .01µg/mL to 0.005 µg/mL as reported in our earlier work 30 . The suggested method of thorium determination is further specific, rapid and convenient due to simple sample preparation and no interference from major ions particularly of uranium. Acknowledgem ent Financial supports from CSIR, New Delhi is gratefully acknowledged by S.Dubey in receipt of CSIR - NET JRF Fellowship. References 1. Emsley J., The Elements, Oxford University Press, Oxford, (1988) 2. 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