Translate this page into:
Determination of thorium(IV) with rifampicin in synthetic mixture and soil samples by spectrophotometry
⁎Corresponding author. Tel.: +91 2703515. lutfullah786@gmail.com (Lutfullah)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
A validated spectrophotometric method has been described for the determination of thorium(IV) in synthetic mixture and soil samples. The method is based on the complexation of thorium(IV) with rifampicin at room temperature which absorbs maximally at 525 nm. Beer's law is obeyed in the concentration range 1.16–23.2 μg mL−1 with apparent molar absorptivity and Sandell's sensitivity of 8.23 × 103 L mol−1 cm−1 and 0.0232 μg/cm2/0.001 absorbance unit, respectively. The influence of variables was investigated and optimized. Interference due to other metal ions was studied and the tolerance limit was achieved. The proposed method was applied to the analysis of thorium(IV) in a synthetic mixture containing various other metal ions and in soil samples. The results of analyses of the proposed method were statistically compared with the reference method showing acceptable recovery and precision.
Keywords
Spectrophotometry
Thorium nitrate pentahydrate
Rifampicin
Validation
1 Introduction
Thorium is a naturally occurring actinide element found in the environment or associated with other metal ions in different complex matrices, nuclear fission products, monazite sands and geological materials. Thorium is known to have acute toxicological effects on human and progressive and irreversible renal injury (Sahoo et al., 2004). The determination of thorium(IV) in the presence of various metal ions found in soil and rivers is of special interest. Various analytical techniques such as thin layer chromatography (Soran et al., 2005), gravimetry (Arora and Rao, 1981), titrimetry (Maiwal and Srinivasula, 1982), reversed phase liquid chromatography (Hao et al., 1996), fluorimetry (Pavon et al., 1989), potentiometry (Baumann, 1982; Chandra et al., 2005), X-ray fluorescence (Golson et al., 1983) and inductively coupled plasma mass spectrometry (Aydin and Soylak, 2007) have been reported for Th(IV) determination. These reported methods such as liquid chromatography, X-ray fluorescence, and inductively coupled plasma mass spectrometry are sensitive but expensive due to high cost and laborious cleanup procedure (Hao et al., 1996) required prior to the analysis of thorium(IV). Thorium(IV) has been determined spectrophotometrically based on the reaction with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol in the presence of sodium acetate-acetic acid buffer solution of pH 4.86 at 580 nm (Abu-Zuhri, 1984), sodium 4,8-diamino-1,5-dihydroxyanthraquinone-2,6-disulphonate in acidic pH at 685 nm (Navas and Garcia-Sanchez, 1979), sodium 5-(4-diethyl-amino-2-hydroxyl phenyl azo) 1,2,4-triazole-3-carboxylate in acidic pH at 535 nm (Thimmaiah et al., 1984) and bromocresol orange at 560 nm (Khalifa and Hafez, 1998). Two extractive spectrophotometric methods have been utilized for the estimation of thorium(IV) based on isoamyl alcohol extractable ion pair complex of thorium(IV) with 2-hydroxy-1-napthaldehyde isonicotinoyl hydrazone at pH 3 with λmax 420 nm (Kavlentis, 1988) and chloroform extractable ion pair complex with 8-quinolinol in the presence of acidic buffer at 390 nm (Goto et al., 1966). The present communication describes a simple and sensitive spectrophotometric method for the determination of Th(IV). The method is based on the reaction of Th(IV) with rifampicin resulting in the formation of a colored complex which absorbs maximally at 525 nm.
2 Experimental
2.1 Materials and reagents
All absorbance measurements were made on a spectronic 20D+ spectrophotometer (Milton Roy Company, USA) with 1 cm matched glass cells. An Elico model LI-10 pH meter (Hyderabad, India) was used to measure pH of the solutions.
All chemicals and solvents used were of analytical reagent grade. Glass distilled deionized water was used throughout the experiment.
-
1.0× 10−2 M thorium nitrate pentahydrate (CAS: 16065-92-2, M.W. = 570.13, Fluka Chemie AG, Darmstadt, Germany) was prepared in distilled water.
-
1.125 × 10−3 M (0.1%) rifampicin (CAS: 13292-46-1, M.W. = 822.94, Merck, USA) was prepared in methanol.
2.2 Procedure for the determination of thorium(IV)
Into a series of 10 mL volumetric flasks, aliquots of 0.05–1.0 mL of 1.0 × 10−2 M standard thorium nitrate solution were pipetted. To each flask 1.7 mL of 1.125 × 10−3 M rifampicin was added and diluted up to the mark with distilled water (pH 3.5). The contents of each flask were mixed well at room temperature (25 ± 1 °C) and the absorbance was measured at 525 nm against the reagent blank prepared similarly except thorium(IV) within the stability time period of 24 h. The concentration of thorium (IV) was calculated either from a calibration curve or regression equation.
2.3 Determination of thorium(IV) in synthetic mixture sample
The synthetic sample of thorium was prepared by taking 570.13 mg of thorium nitrate pentahydrate with 596 mg Pb(NO3)2, 580 mg ZrOCl2·8H2O, 523 mg Ba(NO3)2, 480 mg CrCl3·6H2O, 222 mg CaCl2·2H2O, 119 mg NiCl2·6H2O, 65 mg FeCl3 and 341 mg CuCl2·2H2O, in 100 mL standard volumetric flask and diluted up to the mark with distilled water. The amount of thorium(IV) was determined by the proposed procedure.
2.4 Determination of thorium(IV) in synthetic soil sample
Soil samples were taken from Aligarh district of Uttar Pradesh. The digested soil samples were analyzed for thorium (IV) but tested negative. Therefore, an air-dried finely powdered soil sample (500 mg) with 570.13 mg of thorium nitrate pentahydrate was digested with 2 mL of concentrated H2SO4 in a closed platinum crucible following the method reported by Hughes and Carswell (1970). After digestion, the content of the crucible was cooled and transferred to ice-cold water. The mixture was stirred until all the soluble matters had dissolved and then filtered through Whatmann No. 42 filter paper (Whatmann International Limited, Kent, UK) in 100 mL standard volumetric flask. The filtrate was diluted up to the mark with distilled water. A 20 mL portion of this solution was percolated through the column packed with Amberlite IR 400. The column was washed with 150 ml of 0.1 M H2SO4 to remove unadsorbed species. Thorium (IV) was eluted with 0.5 M H2SO4 at a flow rate of 2 mL per minute. The effluent was evaporated to dryness. After evaporation, 10 mL of distilled water was added and the pH of the solution was adjusted to 3.5 by the addition of ammonia. The final volume of the solution was maintained at 20 mL. The concentration of thorium (IV) was determined by the proposed procedure.
3 Results and discussion
A pink colored complex was obtained with λmax at 525 nm due to the interaction of thorium (IV) with rifampicin while the rifampicin in methanol-water medium showed an absorbance peak at 470 nm (Fig. 1). The reaction was carried out at room temperature. The absorbance measurement at 525 nm as a function of initial concentration of thorium (IV) was utilized to develop a rapid and selective spectrophotometric method for the determination of thorium (IV).
Absorption spectra: (a) 9.721 × 10−5 M rifampicin in methanol, (b) 2.066 × 10−4 M rifampicin in methanol + 7.0 × 10−3 M thorium(IV) in distilled water.
The stoichiometry of the reaction between thorium(IV) and rifampicin was studied by Job's method of continuous variations (Likussar and Boltz, 1971) using equimolar concentrations of 3.037 × 10−3 M. As can be seen from Job's plot (Fig. 2) that one mole of thorium(IV) reacted with one mole of rifampicin. Thus, the combining molar ratio between thorium(IV) and rifampicin is 1:1.
Job's plot for thorium-rifampicin complex.
The I.R. spectra of free rifampicin and Th(IV)-rifampicin complex are shown in Fig. 3(a) and (b), respectively. Rifampicin has –OH and –C⚌O potential sites for coordination with metal ions. Comparison of IR spectrum of the complex with those of free rifampicin indicates that a phenolic ν(C-OH) band appears at 2364 cm−1 in the free rifampicin while the complex does not show this band suggesting that there is coordination that occurs with Th(IV) at this potential site. The carbonyl band, ν(C⚌O) in the free rifampicin appears at 1746 cm−1 with a weak shoulder at 1663 cm−1, while the Th(IV)-rifampicin complex shows this band at 1648 cm−1 indicating that the complexation occurs through the coordination of oxygen atom. The phenolic O-H at carbon 9 in rifampicin is most deshielded (Sadeghi and Karimi, 2006). So, it is deprotonated followed by chelate formation through oxygen atoms of keto and phenolic C–O groups. The Th–O stretching vibration occurs at 416 cm−1 (Nakamoto, 1963). A tentative mechanism for the complexation between Th(IV) and rifampicin is given in Scheme 1.
IR spectra of (a) free rifampicin and (b) Th(IV)-rifampicin complex.
Reaction sequence of the proposed spectrophotometric method.
3.1 Optimization of variables
The optimization of variables was assessed by testing several parameters such as reaction time, concentration of rifampicin, and solvents.
The effect of the reaction time on the absorbance of the pink colored complex and its stability was investigated. The complex got stabilized immediately at 25 ± 1 °C after mixing the analyte and reagent. The complex remained stable for about 24 h.
The effect of the concentration of rifampicin on the absorbance of the colored complex was investigated in the range of 6.08 × 10−6–2.43 × 10−4 M rifampicin. The results (Fig. 4) showed that the highest absorbance was obtained with 1.70 × 10−4 M rifampicin and remained constant up to 2.43 × 10−4 M. Therefore, 2.07 × 10−4 M rifampicin was taken as the optimum concentration for the determination of thorium(IV).
Effect of the concentration of rifampicin on the absorbance of the product.
The effect of solvents such as acetone, acetonitrile, ethanol, dimethylsulfoxide, 1,4-dioxan, methanol and water were investigated on the absorbance of the colored complex. The results are shown in the bar graph (Fig. 5). As can be seen from the figure that the complex showed maximum absorbance in demineralized water and the pH of the complex in this medium is found to be 3.5. Therefore, all measurements were done in demineralized water (DMW) at pH 3.5.
Effect of solvent on the absorbance of the colored complex.
3.2 Effect of foreign ions
A study of the effect of competing metal ions showed that the determination of Th(IV) was not subsequently affected by a range of metal ions, i.e. Pb2+ (25-fold excess), Zr4+ (25-fold excess), Ba2+ (22-fold excess), Cr3+ (20-fold excess), Cu2+ (14-fold excess), Ca2+ (9-fold excess), Ni2+ (5-fold excess) and Fe3+ (2.8-fold excess). The results are summarized in Table 1. However, metal ions such as Mn2+, Zn2+, Hg2+, Cd2+, Mg2+ and Sr2+ interfere in the determination of Th(IV). The interference caused by metal ions can be removed through a column packed with Amberlite IR 400 resin. Most of the metal ions except Th4+ and UO22+ are washed out on passing 0.1 M H2SO4, whereas Th(IV) was eluted with 0.5 M H2SO4.
Metal ions
Added as
Tolerance limit (mg mL–1)
Pb2+
Pb(NO3)2
0.596
Zr4+
ZrOCl2·8H2O
0.580
Ba2+
Ba(NO3)2
0.523
Cr3+
CrCl3·6H2O
0.480
Cu2+
CuCl2
0.341
Ca2+
CaCl2
0.222
Ni2+
NiCl2·6H2O
0.119
Fe3+
FeCl3
0.065
3.3 Validation
The intra- and inter day precisions were evaluated within the same day and on five consecutive days. The results are summarized in Table 2. It can be seen from the table that the values of RSD (%) were in the range of 0.341–3.028%.
Parameters
Intra day assay
Inter day assay
Concentration taken (μg mL−1)
2.3200
11.600
23.200
2.3200
11.600
23.200
Concentration found (μg mL−1)
2.3204
11.596
23.229
2.3250
11.596
23.234
Standard deviationa (μg mL−1)
0.065
0.051
0.079
0.070
0.056
0.100
Recovery (%)
100.015
99.97
100.20
100.216
99.97
100.15
Relative standard deviation (%)
2.79
0.438
0.341
3.028
0.482
0.429
Standard analytical error (%)
0.029
0.023
0.035
0.029
0.025
0.045
Confidence limitb
0.080
0.063
0.098
0.080
0.069
0.124
The accuracy of the proposed method was evaluated by performing recovery experiments through the standard addition technique. The recovery of thorium(IV) in the synthetic sample at two concentration levels was investigated and the results are summarized in Table 3. It is evident from the table and the graph (Fig. 6) that the linearity of the regression line of the standard addition method was good. This indicated that the proposed method is precise and accurate. a Mean for five independent analyses. b Confidence limit at 95% confidence level and four degrees of freedom (2.776).
Concentration (μg ml−1)
Coefficients of linear regression equation of standard addition
Recovery (%)
Synthetic
Standard
Nominal
Intercept
Slope
ra
6.96
0, 1.16, 2.32, 4.64, 6.96
6.962
0.30031
0.04313
0.99996
100.04
13.92
0, 1.16, 2.32, 4.64, 6.96
13.94
0.60022
0.04303
0.99998
100.20
Recovery of thorium(IV) from the synthetic mixture sample by the standard addition technique: (a) 6.963 and (b) 13.949 μg mL−1.
The ruggedness of the proposed method was established by deliberately changing the concentration of the reagent as:
-
volume of 1.215 × 10−3 M rifampicin, 1.7 mL (±0.3 mL).
Under the optimal conditions, the thorium (IV) solution containing 23.2 μg mL−1 thorium(IV) (synthetic sample) was analyzed by the proposed method. The results showed the mean percent recovery and RSD (%) of 100.13% and 0.341%, respectively. The results of the analysis indicated the ruggedness of the proposed method.
The calibration curve was constructed by plotting absorbance against the concentration of thorium(IV) in μg mL−1. Beer's law was obeyed over the concentration range 1.16–23.2 μg mL−1 with molar absorptivity and Sandell's sensitivity of 8.23 × 103 L mol−1 cm−1 and 0.023 μg/cm2/0.001 absorbance unit, respectively. The calibration data (n = 9) were treated statistically and the results are summarized in Table 4. ±tSa and ±tSb are confidence limits for intercept and slope, respectively.
Parameters
Proposed method
Reference method
Wavelength (nm)
525
390
Beer's law limit (μg mL−1)
1.16–23.2
1.0–20
Molar absorptivity (L mol−1 cm−1)
8.23 × 103
9.70 × 103
Sandell's sensitivity
0.0232 μg/cm2/0.001 absorbance unit
0.0239 μg/cm2/0.001 absorbance unit
Linear regression equation
A = 5.917 × 10−4 + 4.31 × 10−2 C
A = 4.685 × 10−4 + 4.18 × 10−2 C
±tSa
1.88 × 10−5
1.722 × 10−3
±tSb
1.32 × 10−5
1.440 × 10−4
Correlation coefficient (r)
0.99999
0.99999
Variance (S02) of calibration line
1.69 × 10−6
1.369 × 10−6
Detection limit (μg mL−1)
0.099
0.092
Quantitation limit (μg mL−1)
0.301
0.280
The applicability of the proposed method for the determination of thorium (IV) in the synthetic mixture and soil samples has been tested. The results of the proposed method were statistically compared with those of the reference spectrophotometric method (Goto et al., 1966) using point and interval hypothesis tests. The t- (paired) and the F-values at 95% confidence level were calculated and found to be less than the tabulated t- (2.036 at υ = 8) and the F-values (6.39 at υ = 4.4) at 95% confidence level (Mendham et al., 2002), thus confirming no significant difference between the performance of the proposed method and the reference method (Table 5). It is also clear from the table that the bias evaluated by the interval hypothesis test (Hartmann et al., 1995) by means of lower limit (θL) and upper limit (θU) was in the range of 0.98–1.02. Thus, the proposed method is suitable for routine analysis of thorium(IV) in nuclear process laboratories where time and economy are essential.
Thorium samples
Proposed method
Reference method
Paired t- and F valuesb
θLc
θUc
Recovery (%)
RSDa (%)
Recovery (%)
RSDa (%)
Synthetic mixture sample⁎
99.97
0.438
100.18
0.492
t = 0.699, F = 1.123
0.989
1.006
Soil sample
100.05
0.498
100.18
0.492
t = 0.403, F = 1.024
0.990
1.007
4 Conclusions
The proposed method is simple and accurate for the determination of Th(IV). The proposed method has the advantages of having low limit of detection (0.099 μg mL−1) and low cost of analysis. In addition, the proposed method is based on the complexation of Th(IV) with rifampicin at room temperature showing the involvement of one reagent only and there is no use of buffer solution. Hence, the proposed method can be used for the routine analysis of Th(IV) in real samples as the interferences caused by some metal ions can be easily removed by anion exchange resin.
Acknowledgements
The authors are grateful to Aligarh Muslim University, Aligarh, India and Higher College of Technology (Ministry of ManPower), Muscat, Sultanate of Oman for facilities.
References
- Microchem. J.. 1984;29:345-349.
- Ind. J. Chem. A. 1981;20:539-540.
- Talanta. 2007;72:192-197.
- Anal. Chim. Acta. 1982;138:391-396.
- Ind. J. Chem. A. 2005;44:2060-2063.
- Chem. Geol.. 1983;38:225-237.
- Anal. Chem.. 1966;38:493-495.
- J. Chromatogr. A. 1996;749:103-113.
- Anal. Chem.. 1995;67:4491.
- Analyst. 1970;95:302-303.
- Microchem. J.. 1988;38:188-190.
- Talanta. 1998;47:547-559.
- Anal. Chem.. 1971;43:1265-1272.
- J. Ind. Chem. Soc.. 1982;59:395-396.
- Mendham, J., Denney, R.C., Barnes, J.D., Thomas, M., 2002. Statistics: Introduction to Chemometrics, In Vogel's Textbook of Quantitative Chemical Analysis, 6th ed., Pearson Education, Singapore, p. 137.
- Nakamoto, K., 1963. Infrared Spectra of Inorganic and Coordination Compounds, John Wiley & Sons, New York, p. 149.
- An. Quim.. 1979;75:511-513.
- Anal. Chim. Acta. 1989;224:153-158.
- Chem. Pharm. Bull.. 2006;54:1107-1112.
- Nucl. Tech. Rad. Protection. 2004;19:26-30.
- J. Liq. Chromatogr. Rel. Technol.. 2005;28:2515-2524.
- Curr. Sci.. 1984;53:645-646.
