Indian Journal of Chemical Technology Vol. 16, September 2009, pp. 437-441
Spectrophotometric determination of uranium (VI) via complexation with piroxicam Lutfullaha*, Farheen Khana, Nafisur Rahmana & Syed Najmul Hejaz Azmib a
Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India Department of Applied Sciences, Chemistry Section, Higher College of Technology, P. O. Box 74, Al-Khuwair-133, Muscat, Sultanate of Oman Email:
[email protected]
b
Received 20 January 2009; revised 29 June 2009 An optimized and validated spectrophotometric method has been described for the quantitative analysis of uranyl ion in soil samples. The method is based on the chelation of uranyl ion with piroxicam to produce a yellow complex in 1,4-dioxan–water medium at room temperature which absorbs maximally at 390 nm. Beer’s law is obeyed in the concentration range of 6.75 × 10-2 – 9.45 × 10-1 µg mL-1 with apparent molar absorptivity and Sandell’s sensitivity of 4.114 × 105 L/mol/cm and 0.00066 µg/cm2/ 0.001 absorbance unit, respectively. Interference due to various non-target ions was also investigated. The proposed method was successfully applied to the analysis of uranyl ion in synthetic soil samples. The validity of the proposed method was checked by applying the standard addition technique in addition to a comparison with the results obtained from other earlier reported methods. Keywords: Spectrophotometry, Uranyl nitrate hexahydrate, Piroxicam, Validation
Uranium finds extensive applications as nuclear fuel in power plants. The main sources of uranium are soil, rocks, plants, sand and water. Uranyl ion is the species of interest which is soluble, stable and mobile in aqueous phase and found in soils near nuclear establishments. Uranium compounds are carcinogenic and hence there has been some interest in the development of low cost rapid techniques for measuring uranium in soil samples1,2. Eventhough there are many techniques for the analysis of uranium spectrophotometry3-13 is widely used due to its simplicity, low cost and adaptability. Hence, it is decided to exploit this technique for the estimation of uranyl ion in soil samples. The literature survey revealed that existing spectrophotometric methods are time consuming, employing many reagents to develop the colour4,5 and requiring extraction of uranium complex into organic solvent3-6. Therefore, there is a need for the development of a simple and selective spectrophotometric method for the estimation of uranium in soil samples. The proposed method is based on the reaction of uranyl ion with piroxicam in 1,4-dioxan–water medium to form a yellow complex which absorbs maximally at 390 nm.
Experimental Procedure Apparatus
Spectral runs and all absorbance measurements were made on Spectronic 20 D+ spectrophotometer (Milton Roy Company, USA). An Elico model Li-10 pH meter (Hyderabad, India) was used to measure pH of the solutions. Reagents and Standard solutions
All chemicals and solvents used were of analytical reagent grade. A uranyl nitrate solution (2.5 × 10-5 M) was prepared by dissolving uranyl nitrate hexahydrate in distilled water (CAS: 1352083-7, M.W.=502.13, Fluka Chemie AG, Darmstadt, Germany). 6.036×10-3 M (0.2%) piroxicam (CAS: 36322-90-4, M.W=331.35, Merck, USA) was prepared in 1,4-dioxan. Proposed method for the determination of uranyl ion
Aliquots (0.05–0.7 mL) of standard uranyl nitrate solution (2.5×10-5 M) were added to a series of 10 mL standard volumetric flasks along with 1.6 mL of 0.2% piroxicam and made up to 10 mL with 1,4-dioxan. The contents of each flask were mixed well at room temperature (25±1ºC) and the absorbance was measured at 390 nm against the reagent blank
438
INDIAN J CHEM. TECHNOL., SEPTEMBER 2009
prepared similarly. The concentration of uranyl ion was calculated either from a calibration curve or regression equation. Determination of uranyl ion in soil
Soil samples tested negative for uranium were chosen. These soil sample were air dried and 500 mg of the sample along with 12.55 mg of uranyl nitrate hexahydrate was placed in a closed platinum crucible. The sample was digested with 2 mL of concentrated H2SO4 following the method recommended by Hughes and Carswell14. The content of the crucible was cooled and transferred to ice-cold water. The mixture was stirred until all the soluble matter had dissolved and then filtered through Whatmann No. 42 filter paper (Whatmann International Limited, Kent, UK) in 1000 mL standard volumetric flask and was diluted up to the mark with distilled water. 20 mL of this solution was percolated through the column packed with Amberlite IR 400. The column was washed with 0.1 M H2SO4 to remove unadsorbed species. Uranyl ion was eluted with 2 M H2SO4 at a flow rate of 2 mL per min. After evaporation 10 mL of distilled water was added. The pH of the solution was adjusted to 4 by the addition of ammonia and the final volume of the solution was maintained to 20 mL. The concentration of uranyl ion was determined by the proposed procedure and compared with the data obtained by adopting the reference method.6 Validation
The proposed method has been validated for selectivity, linearity, precision, accuracy, and limits of detection and quantitation. For evaluation of selectivity, 0.945 µg of uranyl ion was determined in presence of metal ions such as Ni2+ , Ca 2+, Mg2+, Mn2+ and Zn2+. The intra- and interday precisions were assessed at three concentration levels: 0.135, 0.540 and 0.945 µg mL-1. The accuracy of the proposed method was determined by standard addition technique. Under this method 2 or 4 mL of sample solution were spiked with known amounts of uranyl nitrate. The linearity of the proposed method was evaluated at eight concentration levels in the range of 6.75×10-2 – 9.45×10-1 µg mL-1. The limits of detection (LOD) and quantition (LOQ) were calculated using the equation: LOD =3.3×So/b and LOQ = 10 ×So/b (where So is the standerd deviation and b is slope of the calibration line). The robustness of the method was assessed by determining 0.945 µg mL-1 uranyl ion by varying the volume of piroxicam (1.6 ± 0.2 mL) and temperature (25 ± 3˚C).
Point and interval hypothesis tests15 have been employed to evaluate the bias, based on the mean recovery value of proposed method and the reference method. Results and Discussion A yellow coloured complex was obtained with λmax at 390 nm due to the interaction of uranyl ion with piroxicam while the piroxicam in 1, 4-dioxan–water medium showed negligible absorbance (0.07) at 390 nm and itself showing its λmax at 355 nm (Fig. 1). The reaction was carried out at 25 ± 1oC and the absorbance of the coloured complex was measured immediately at 390 nm. Therefore, the absorbance measurement as a function of initial concentration of uranyl ion was utilized to develop a spectrophotometric method for the determination of uranium (VI). Stoichiometry and Mechanism
The stoichiometry of the yellow coloured complex between uranyl ion and piroxicam was studied by Job`s method of continous variation. It was found that the molar combining ratio between uranyl ion and piroxicam was 2:1. The three functional groups such as -OH, -NH, and –CONH in piroxicam are responsible for chelation with metal ions. The existence of the –SO2 group in the drug is expected to increase the stability constant16 of UO22+ and ZrO22+. It is reported that piroxicam chelates with U(VI), Cu(II) and Fe(III) to form metal ligand complexes17,18. It is observed that 2 mol of uranyl ion chelates with 1 mol of piroxicam to form 1:2 (piroxicam: UO22+) yellow complex
Fig. 1—Absorption spectra: (a) 6.036×10-3M piroxicam in 1,4-dioxan (b) 1.932 × 10-3M piroxicam in 1,4-dioxan + 3.500× 10-6 M uranyl nitrate hexahydrate in distilled water
LUTFULLAH et al.: SPECTROPHOTOMETRIC DETERMINATION OF U(VI)
which absorbs maximally at 390 nm. The apparent formation constant and standard Gibb`s free energy (∆G˚) were calculated and found to be 1.563×108 and -36.40 KJ mol-1, respectively. Therefore, based on the literature background and present experimental findings, the reaction sequence of the proposed method is given in Fig. 2. Optimization
The effect of the concentration of piroxicam on the absorbance of the product was studied in the range of 1.207 × 10-4 M–2.173 × 10-3 M. The maximum absorbance was observed in the range of piroxicam concentration of 1.690 × 10-3 M–2.173 × 10-3 M. Therefore, a concentration of 1.932×10-3 M piroxicam was chosen as optimal using this system. The influence of solvent such as ethanol, methanol, acetone, acetonitrile, dimethylsulfoxide and 1,4dioxan on the absorbance of product was investigated. The highest absorbance was obtained with 1,4-dioxan.
439
Therefore, 1,4-dioxan was selected as the solvent for complex formation between uranyl ion and piroxicam. Validation
The tolerance limit in mg mL-1 for non-targetted metal ions with 0.945 µg/mL uranyl ion is summarized in Table 1. This shows that the method is selective for the determination of uranyl ion in soil samples in presence of Ni2+, Ca2+, Mg2+, Mn2+ and Zn2+. However, the method was found to be less selective in the presence of Cd2+, Zr4+, Fe3+, Al3+ and Cu2+. Beer`s law range, molar absorptivity, Sandell`s sensitivity, regression equation, correlation coefficient, LOD values were calculated19,20 (Table 2). A linear relationship was found between absorbance at λmax and the concentration of uranyl ion in the range of 0.067-0.945 µg mL-1. The high value of correlation coefficient (r=0.9999) indicated excellent linearity. The high molar absorptivity (4.11×105 Lmol-1cm-1) of the resulting coloured complex indicates the high sensitivity of the method. Table 1— Effect of various metal ions on the determination of 0.945 µg mL–1 uranyl ion Metal ions Added as Tolerance limit (mg mL–1) 2+ Ni NiCl2.6H20 0.71 Ca2+ Ca(NO3)2.4H2O 0.11 Mg2+ MgCl2.6H20 0.20 Mn2+ MnCl2.5H20 0.20 Zn2+ ZnCl2.6H20 0.86
Fig. 2—Reaction sequence for the proposed method
Table 2 — Optical and regression characteristics of the proposed method Parameters Proposed method Wavelength (nm) 390 Beer’s law limit (µg/mL) 0.0675 – 0.945 Molar absorptivity (L/mol/cm ) 4.114 × 105 Sandell’s sensitivity 0.0007 µg/cm2 /0.001 absorbance unit Linear regression equation A = 5.55 × 10-4 + 1.52 C ±tSa 3.377 × 10-3 ±tSb 5.995 × 10-3 Correlation coefficient (r ) 0.99999 Variance (S02) of calibration line 4.244 × 10-6 Detection limit (µg/mL ) 0.005 Quantitation limit (µg/mL) 0.014 ±t Sa and ±t Sb are confidence limits for intercept and slope, respectively.
Table 3 - Test of precision of the proposed method Parameters Intra day assay Inter day assay Concentration taken (µg/mL) 0.1350 0.5400 0.945 0.1350 0.5400 0.945 Concentration found (µg/mL) 0.1352 0.5403 0.946 0.1349 0.5402 0.946 Standard deviationa (µg/mL) 0.003 0.004 0.01 0.003 0.004 0.01 Recovery (%) 100.12 100.05 100.07 99.92 100.03 100.14 Relative standard deviation (%) 2.28 0.675 1.057 1.911 0.801 1.189 a Mean for five independent determinations. b Confidence limit at 95% confidence level and four degrees of freedom (t = 2.776).
INDIAN J CHEM. TECHNOL., SEPTEMBER 2009
440
Table 4 — Test of accuracy: recovery of uranyl ion by standard addition technique Coefficients of linear regression equation of standard addition Soil sample Standard Nominal Intercept slope ra 0.270 0, 0.0675, 0.135, 0.2025, 0.270 0.2705 0.4112 1.520 0.99999 0.540 0, 0.0675, 0.135, 0.2025, 0.270 0.5406 0.8210 1.519 0.99998 a Coefficient of correlation. b Mean for five independent analyses. Concentration (µg mL-1)
Recoveryb (%)
100.19 100.12
Table 5 — Evaluation of bias: point and interval hypothesis tests at 95% confidence level for the determination of uranyl ion from soil samples with proposed and reference methods Sample of uranyl ion Proposed method t-valueb Reference method t-valueb F valuec θLd θUd
Soil sample 1
Recovery (%) 100.07
RSDa, (%) 1.06
0.224
Recovery (%) 99.86
Soil sample 2 100.14 1.19 0.202 100.05 a Mean for 5 independent analyses. b Theoretical t-value (γ= 4) at 95 % confidence level is 2.776. c F-value (γ= 4, 4) at 95 % confidence level is 6.39. c A bias, based on recovery experiments, of ± 2% is acceptable.
RSDa (%) 1.86
0.164
3.063
0.988
1.007
1.47
0.064
1.596
0.982
1.016
Table 6 — Comparison of the proposed method with reported UV-visible spectrophotometric methods for the determination of uranyl ion Reagents References Beer’s law limit Molar absoptivity Analysis time λmax (L/mol/cm) (min) (nm) (µg/mL) 8-Quinolinol 380 2 - 40 1.50 × 104 10 [3]a 4 4-(2-pyridylazo)resorcinol 530 0-7 3.87 × 10 immediately [4] Chromazurol S 625 0-2 9.9 × 104 15 [5] Anthranilic acid and rhodamine 6G 575 0.04 - 4 6.25 × 104 15 [6]b p-Carboxychlorophosphonazo 714 4 - 12 1.78 × 105 immediately [8] SnCl2.H2O and NH4SCN 365 5 - 60 6.25 × 103 immediately [10] Piroxicam 390 0.0675 – 0.945 4.114 × 105 immediately at 25 ± 1ºC This work a. Extractive method. b. Reference method
The intra- and interday precision of the proposed method was investigated at three concentration levels: 0.135, 0.540 and 0.945 µg mL-1 .The results of analysis are summarized in Table 3. These results show the accuracy and reproducibility of the assay. Thus, it was concluded that there were no significant intraday and interday differences for the assay. The accuracy of the proposed method was also evaluated by determining the U(VI) in synthetic soil sample using the standard addition technique. The data indicate that the method has good accuracy (Table 4). The robustness of the proposed method was assessed by deliberate small variation of the experimental parameters such as volume of 0.2% piroxicam (1.6±0.2 mL) and temperature (25±3˚C). Under these conditions, content of uranyl ion (0.945 µg mL-1) in soil sample was determined. The percent recovery ±RSD was found to be 100.14±1.19. It is concluded that the method is robust.
The content of uranyl ion in soil sample was determined by proposed method and a reference method6. The results of proposed method were compared statistically to these obtained by refernce method. It is evident from Table 5 that there is no significant difference between the performance of the methods compared. The performance of the proposed method was compared with that of other existing UV-visible spectrophotometric methods (Table 6). It is clear from the table that the proposed method is simple and requires less time to complete the analysis. Moreover, the proposed method is more sensitive with low limit of detection. Conclusion The proposed method has the advantage of having high molar absorptivity (4.114 × 105 L/mol/cm) with low limit of detection (0.005 µg mL-1). The method is sensitive, accurate and useful due to high tolerance limits from cations and anions. Therefore, the
LUTFULLAH et al.: SPECTROPHOTOMETRIC DETERMINATION OF U(VI)
proposed method is an effective method for the quantitative analysis of uranyl ion in soil samples.
8
Acknowledgements The authors are grateful to Aligarh Muslim University, Aligarh, India and Ministry of ManPower (Higher College of Technology) Muscat, Sultanate of Oman for facilities.
10 11 12 13 14 15
References 1 2 3 4 5 6 7
Arruda A F, Campiglia A D, Chauhan B P S & Boudjouk P, Anal Chim Acta, 396 (1999) 263. Moulin C, Decambox P, Mauchein P, Pouyat D & Couston L, Anal Chem, 68 (1996) 3204. Motojima K, Yoshida H & Izawa K, Anal Chem, 32 (1960) 1083. Florence T M & Farrar Y, Anal Chem, 35 (1963) 1613. Leong C L, Anal Chem, 45 (1973) 201. Ramakrishna T V & Murthy R S S, Talanta, 27 (1980) 442. Perez Pavon J L, Moreno Cordero B, Rodriguez Garcia E & Hernandez Mendez J, Anal Chim Acta, 230 (1990) 217.
9
16 17 18 19 20
441
Ru Y, Yan L, Guilan S, Tao W & Jiaomai P, Anal Chim Acta, 314 (1995) 95. Murty B N, Jagannath Y V S, Yadav R B, Ramamurty C K & Syamsundar S, Talanta, 44 (1997) 283. Currah J E & Beamish F E, Anal Chem, 19 (1947) 609. Smith W B & Drewry J, Analyst, 86 (1961) 178. Abe S & Weisz H, Mikrochim Acta, 58 (1970) 550. Dubey S C & Nadkarni M N, Talanta, 24 (1973) 266. Hughes K C & Carswell D J, Analyst, 95 (1970) 302. Hartmann C, Smeyers-Verbeke J, Pinninckx W, Heyden Y V, Vankeerberghen P & Massart D L, Anal Chem, 67 (1995) 4491. Sadeghi S, Mohammadzadeh D & Imampur J S, Anal Bioanal Chem, 383 (2005) 261. El-Khateeb S, Abdel Fattah S, Abdul Razeg S & Tawakkol M, Anal Lett, 22 (1989) 101 El-Ries M A, Anal Lett, 31 (1998) 793. Nalimov V V, The Application of Mathematical Statistics to Chemical Analysis (Pergmon Press, Oxford), 1963, 167. Mendham J, Denney R C, Barnes J D & Thomas M, Statistics: Introduction to Chemometrics, In Vogel’s Textbook of Quantitative Chemical Analysis, 6th edn (Pearson Education, Singapore), 2002, 137.
