J Radioanal Nucl Chem (2014) 301:703–709 DOI 10.1007/s10967-014-3201-3
Spectrophotometric determination of microamounts of thorium with thorin in the presence of cetylpyridinium chloride as surfactant in perchloric acid Muhammad Haleem Khan • Muhammad Hafeez Syed Manzoor Hussain Bukhari • Akbar Ali
•
Received: 12 November 2013 / Published online: 8 June 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract A simple and more sensitive spectrophotometric method is developed for determination of thorium using thorin as a chromogenic reagent in the presence of cetylpyridinium chloride (CPC) in perchloric acid. The reaction was instantaneous and complex was found stable for 168 h. A significant bathochromic shift was noted in the presence of CPC. The determination range was enhanced from 25 to 30 lg mL-1 with molar absorptivity of 2.95 9 104 L mol-1 cm-1 at 25 ± 5 °C. Sandell’s sensitivity was calculated to be 6.8 ng cm-2 at 581 nm. Relative standard deviation was reduced from 4.25 to 2.5. The interference of Ni2?, Mn2?, Sn4?, phosphate, EDTA, sulphate and tartrate has been reduced significantly in the presence of surfactant. The validity of the proposed method was tested by determining thorium in Certified Reference Materials. Keywords Spectrophotometric determination Thorium Thorin CPC Perchloric acid
Introduction The use of micelles in analytical chemistry is increasing due to the beneficial alteration of metal–ligand complex spectral properties via surfactant association [1–12]. The utility of micelles in spectroscopic measurements is derived from several possible effects upon the system of M. H. Khan (&) M. Hafeez S. M. H. Bukhari Department of Chemistry, University of Azad Jammu & Kashmir, Muzaffarabad, Pakistan e-mail:
[email protected] A. Ali Chemistry Division, Pakistan Institute of Nuclear Science & Technology, Nilore, Islamabad, Pakistan
interest, such as shift of wavelength due to formation of ternary complex [13]. The other merit of the presence of a surfactant in the system is the capacity to solubilize an insoluble complex or ligand [14–18]. These effects show the advantage of such surfactant systems in the development of new spectrophotometric methods for determining micro amounts of metal ions, anions, biological compounds and pesticides [19–21]. Besides various industrial applications, thorium is very important as a nuclear reactor fuel element. The role of various organic reagents for spectrophotometric determination of Th(IV) is well known. Among these, arsenazo-III [22], xylenol orange [23], methylthymol blue [24], 5,8dihydroxy-1,4-naphthoquinone [25], 1-amino-4-hydroxyanthraquinone [26], 4-(2-triazolylazo) resatophenone [27], 1-(2-thiazolylazo)-2-naphthol [28] and thoron [29] are commonly reported in the literature. Nonionic surfactants in the presence of pyrogallol red [30], pyrocatechol violet [31], galleon and tetrachlorogallene [32], have been used, whereas chromeazural S [33], eriochromecyanin R [23] and arsenazo-III [34] were also used in the presence of anionic surfactants. Cationic surfactants such as cetylpyridinium bromide (CPB) were also employed for the determination of thorium using phenylfluorone [35]. The determination of thorium with thorin as a chromogenic reagent in perchloric acid as a medium of determination has been reported in our earlier communication [36]. The use of surfactant in the spectrophotometric determination of thorium with thorin has not been cited in the literature so far. In the present communication, an improved method for determination of thorium in the presence of surfactant is reported. In this method, the effect of various surfactants on the determination of thorium with thorin in perchloric acid has been studied. The cetylpyridinium chloride (CPC) was selected as the most suitable
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surface active agent for thorium determination. The parameters affecting the sensitivity and selectivity were optimized and the procedure so developed has been applied for the determination of thorium from Certified Reference Materials (CRMs) and the results were found in good agreement with the reported values. The main objective of the proposed method is to enhance the colour intensity and the determination range of the metal complex with chelating dye in the presence of a surfactant. Because of the wide range of determination and higher sensitivity, the proposed method enables chemists to determine toxic elements like thorium; even if present in low concentrations; in ordinary labs for subsequent elimination.
J Radioanal Nucl Chem (2014) 301:703–709
Experimental
again to near dryness. After cooling, the contents were dissolved in 10.0 mL of 3 M HClO4 solutions and transferred to a 100.0 mL volumetric flask. The volume was made up to the mark with 3 M HClO4. Thorin is readily soluble in dilute perchloric acid and its stock solution (0.1 % w/v) was prepared by dissolving 0.1 g of the reagent in 3 M HClO4 in 100.0 mL volumetric flask and diluted up to the mark. The working solutions of thorin ranging from 0.01 to 0.09 % (w/v) were prepared by appropriate dilution of the stock solution with 3 M HClO4. The CPC solution (1.0 9 10-2 mol L-1) was prepared by dissolving 0.7160 g of salt in doubly distilled water. The contents were shaken well and diluted to the mark with doubly distilled water in 100 mL measuring flask. The various solutions of CPC, ranging from 1 9 10-5 to 1 9 10-3 mol L-1 were prepared by appropriate dilution of the stock solution with doubly distilled water.
Apparatus
General determination procedure
A UV–Visible spectrophotometer (UV-1601) of Shimadzo, Japan with a fixed slit width of 0.5 nm and x–y recorder was used for measurement of optical density of the complexes. An Orion research Model (601A) ion analyzer fitted with a combined glass electrode was used for pH measurement. Electric balance of Sartorius H110 (Germany) was used for weighing purpose. Heater bath (GFL) was used during experimental work. Micropipettes (Socorex ISBA S.B.) of different capacities were used for volume measurement. Mechanical shaker, centrifuge oven and water bath of Gallenkamp (Germany) were used.
A sample solution containing thorium ranging from 1 to 30 lg mL-1 was transferred to a 25.0 mL measuring flask, then 1.0 mL of CPC (1.0 9 10-2 mol L-1) solution was added then followed by adding 2.5 mL of (0.05 %) thorin solution in 3 M HClO4 and diluted up to the mark with 3 M HClO4. The contents were mixed thoroughly and allowed to stand for 10 min. The optical density was measured at 581 nm in 1.0 cm quartz cell against the reagent blank prepared in the same way without adding metal ions. The same procedure was adopted for determination of thorium in the absence of surfactant. All experiments were carried out at room temperature (25 ± 2 °C).
Reagents and solutions Preparation of CRMs solutions Thorium nitrate of E. Merck (Germany), O-arsonophenylazo2-hydroxynaphthalene-3,6-disulfonic acid disodium salt (thorin) of BDH and CPC of Aldrich were used in this study. Various surfactants namely, CPC, CPB, cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium dioctylsulfosuccinate (SDSS), sodium dodecylbenzenesulfonic acid (SDBS), benzyldimethyltetradecyl ammonium chloride, polyoxyethylene-p-t-octylphenol (Triton X-100) and polyoxyethylenesorbitanmonolaurate (Tween-20) of Aldrich, were used in these studies. The CRMs were provided by Chemistry Division, PINSTECH, Islamabad, Pakistan. All other reagents and chemicals used were of analytical reagent grade. Thorium standard solution (1.0 9 103 lg mL-1) was prepared by dissolving 1.229 g of thorium nitrate in 25.0 mL of 1.0 mol L-1 nitric acid and diluted up to the mark with nitric acid. Working solutions of thorium were prepared by taking 10.0 mL of thorium stock solution in a 100.0 mL Pyrex glass beaker and heated to near dryness. The contents were dissolved in 10.0 mL of 3 M HClO4 and heated
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A known weight (1.0 g) of CRMs (NBL-79-A, granite) were taken separately in 250.0 mL Teflon beakers, moist with doubly distilled water and heated near to dryness with a mixture of acids consisting of 10.0 mL concentrated nitric acid, 10.0 mL of hydrofluoric acid (48 %) and 5.0 mL of perchloric acid (70–72 %) [37]. The process was repeated thrice and then 10.0 mL nitric acid was added and heated near to dryness. The contents were digested with 50.0 mL nitric acid (1:1) at room temperature and then heated (below boiling point) for 2 h. Then cooled slowly, filtered and again heated near to dryness and residue was re-dissolved in 0.5 M nitric acid as CRMs stock solution. Thorium was extracted with 30 % DBSO in xylene prior to its determination using thorin (0.5 %) as chromogenic reagent in 3 M HClO4 [37]. The accuracy of the method reported herein has been checked by added-found method (known amount of thorium was added in CRMs solution).
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Fig. 2 Absorption spectra of thorium complexes; (curve A) Th4?– thorin complex, and (curve B) Th4?–thorin–CPC complex
Fig. 1 Effect of surfactants on sensitivity and selectivity of spectrophotometric determination of thorium with thorin
Results and discussion Role of surfactant Nine selected surfactants were studied for their effect on thorium determination using thorin as a complexing agent. An appropriate volume of sample solution ranging from 1 to 30 lg mL-1 of thorium was added to each surfactant solution [1 9 10-2 mol L-1 of CPC, CTAB, SDS, SDSS, SDBS, CPB, and 1.0 % (v/v) of Triton X-100 and 2.0 % (v/v) of Tween-20] separately. One mL of 3 M HClO4 and thorin (0.05 %) solutions were also added to each solution before measuring the optical density. The volume was made up with doubly distilled water and absorbance of each solution was measured using 1.0 cm quartz cell and the results are depicted in Fig. 1. It is observed that cationic surfactant (CPC) recorded the maximum absorbance. It was further noted that surfactant–thorium–thorin complex (rose red) formed instantaneously. On the basis of maximum absorbance, wide determination range and sharp contrast of the resulting complex, CPC was selected as most effective surfactant for the proposed method. Surfactants have a property to adsorb onto the interfaces of the system when present in low concentration and alter interfacial free energy significantly. Surfactants form aggregates or micelles which increase the adsorption at the interfaces of the complex for removing hydrophobic groups from contact with water. This help in decreasing solubility of surfactant in water than in the organic solvent and also cause closer packing of surfactant molecules at interfaces, thus, increases the tendency of surfactant to adsorb on the interface or form micelles. The interfaces of the complex is made hydrophobic by the use of cationic surfactant like
CPC, because a cationic surfactant is most suitable for adsorption onto the interfaces of the complex with its positively charged hydrophobic head due to electrostatic attraction and its hydrophilic group will be oriented away from the surface and thus make the surface water repellent [1, 38]. Like most of the natural complexes, Th(IV)–thorin is a negatively charged complex which form neutral complex with CPC [39]. This property of CPC facilitates in increasing the surface area for reaction and cause increasing in determination range, bathochromic shift, sensitivity and selectivity of the existing determination method. Absorption spectra of thorium complexes Thorin in perchloric acid medium is most selective and sensitive reagent for thorium determination. It readily reacts with thorium forming a coloured, water soluble complex. The spectra for a binary neutral complex of Th(IV)–thorin (curve A) and ternary Th(IV)–thorin–CPC complex (curve B) in perchloric acid is shown in Fig. 2. The binary complex shows the absorption maximum at 544 nm while the ternary complex with CPC was recorded at 581 nm. Thus, a significant change in the location and intensity of the absorption band in the presence of CPC was observed. The absorption peak in the presence of surfactant was further shifted towards longer wavelength (581 nm). The comparison of two spectra showed a significant bathochromic shift in case of ternary complex as compared to binary complex and a distinct hyper chromic effect was achieved along with increase in determination range of thorium. Byrd and Banks [39] have studied the nature of the Th(IV)–thorin complex in detail by Job’s method and indicated that the predominant species was one having a ligand to metal ratio of 3:2. They failed to get identical molar absorptivity in an excess of either reactant and suggested the presence of several complexes and the charge on all the complexes were appeared to be negative. The negatively charged Th(IV)–thorin complex suggests the formation of neutral Th(IV)–thorin–CPC complex in
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the presence of cationic surfactant, which may be the reason for large batho-chromic shift and high absorbance in the presence of CPC. The role of surfactant is already explained earlier. Effect of concentration of perchloric acid In order to establish the optimum conditions for the determination of thorium with thorin, in the presence of CPC, the effect of perchloric acid was investigated at 581 nm. It was found that absorbance increases from 1 to 5 M HClO4, beyond that a small decrease in absorbance was observed. This may be due to decomposition of the complex because of the high acid concentration [36]. Therefore, 3 M HClO4 was selected as optimum concentration for further studies. Effect of concentration of thorin and CPC solution To determine the optimum concentration of thorin in perchloric acid necessary for maximum absorbance, 0.01, 0.05 and 0.1 % (w/v) solutions were examined for the colour development using fixed amount of thorium and 1.0 mL of CPC solution (1 9 10-2 mol L-1). The best results were obtained with 0.05 % thorin in 3 M HClO4 solution. The effect of CPC concentration on the formation of complex was studied over the range of 1 9 10-5–1 9 10-3 mol L-1. It was noted that maximum absorbance was obtained with 1 9 10-2 mol L-1 of CPC. Stability of the complex The addition of various reagents was studied by changing the order of mixing of reagents and it was noted that complex formation is not affected by the sequence of reagent addition. However, the order of adding thorium– thorin followed by CPC and perchloric acid was practiced in the general procedure. The ternary complex formed immediately after the addition of reagents however for completion of reaction, absorbance was taken after 10 min. The complex stability was carefully observed for 168 h and absorbance was found quite stable at room temperature. No temperature effect was observed in the colour intensity up to 45 °C. The effect of cations and anions on the determination of thorium The selectivity of the present method was investigated by analyzing the samples containing 5.0 lg mL-1 of thorium in the presence of many fold excess amount of foreign ions. The effect of various ions on thorium determination was checked individually. The cations were added in about
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J Radioanal Nucl Chem (2014) 301:703–709 Table 1 Interference of foreign ions in the determination of thorium in the absence and presence of CPC [Th4?] = 5.0 lg mL-1 Foreign ions
Absorbancea without CPC
Nil
0.425
0.0
0.637
0.0
Al3?
0.445
4.7
0.656
3.0
Cr3? Co2?
0.439 0.430
3.3 1.2
0.652 0.639
2.4 0.3
Pb2?
0.441
3.8
0.647
1.6
Mg2?
0.428
0.7
0.640
0.5
Cu2?
0.439
3.3
0.641
0.6
K?
0.434
2.1
0.644
1.1
Cd2?
0.436
2.6
0.647
1.6
2?
Be
0.447
5.2
0.656
3.0
Ag?
0.439
3.3
0.639
0.3
Fe3?
0.448
5.4
0.660
3.6
Hg2?
0.437
2.8
0.652
2.4
Sb3?
0.441
3.8
0.659
3.5
3?
Bi
0.436
2.6
0.625
-1.9
Ni2?
0.465
9.4
0.677
6.3
Sn4?
0.463
8.9
0.664
4.2
Zn?2 Mn2?
0.448 0.463
5.4 8.9
0.644 0.667
1.1 4.7
Zr4?
0.466
9.6
0.660
3.6
U6?
0.463
8.9
0.654
2.7
Nitrate
0.439
3.3
0.657
3.1
EDTA
0.465
9.4
0.668
4.9
Carbonate
0.441
3.8
0.652
2.4
Sulfate
0.403
-5.2
0.600
-5.8
% Deviation
Absorbancea with CPC
% Deviation
Phosphate
0.463
8.9
0.659
3.5
Oxalate
0.439
3.3
0.647
1.6
Acetate
0.434
2.1
0.644
1.1
Citrate
0.421
-0.9
0.622
-2.4
Tartrate
0.457
7.5
0.666
4.6
a
Based on the average of duplicate results
70–80-fold excess as their nitrate and chloride salts whereas anions were present at 100-fold excess as their sodium and potassium salts where necessary (Table 1). The optical density in the absence of foreign ions is listed against the observed absorbance in the presence of diverse ions. A ±3 % variation in the absorbance was taken as tolerable limit within experimental error. It was found that cations such as Al3?, Pb2?, Be2?, Fe3?, Ni2?, Mn2?, Sn4?, Zn2?, Zr4? and U6? interfere in the determination of thorium in the absence of surfactant whereas this interference have been minimized in the presence of surfactant (Table 1). All other cations studied at 70–80-folds in excess could be minimized in the reported method. However, Ni2?, Sn4?, Fe3? and Zr4? ought to be avoided or masked prior to the determination of thorium.
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The interference of anions such as EDTA, sulphate and tartrate has been decreased in the presence of surfactant. These anions interfere due to strong bonding with thorium beyond the tolerable limit. The decrease in interference is may be due to the use of the perchloric acid as medium of determination where volatile anionic interference has been minimized by the repeated slow heating of the test material in 3 M HClO4 prior to the colour development. A serious interference has been reported for thorium determination methods developed in basic, aqueous as well as in organic medium [36]. It was further noted that action of surfactant is independent of the presence of foreign ions.
Table 3 Accuracy of the spectrophotometric determination method of Th(IV) with thorin in the presence of surfactant (added and found procedure) Thorium added (lg mL-1)
Thorium found (lg mL-1)
±% Deviation
300.0
298.5
0.5
250.0
249.0
0.4
200.0
199.0
0.5
150.0
149.0
0.6
100.0
99.5
0.5
50.0
50.0
0.0
10.0
10.0
0.0
Effect of metal ion concentration The effect of metal ion concentration was studied in the presence as well as in the absence of surfactant. The addition of CPC followed the Beer’s law over the concentration range of 1–30 lg mL-1 of thorium as compared to 1–25 lg mL-1 in the absence of CPC [36]. A significant increase in the range was noted. The linear regression equation of y = 0.0838x ? 0.0062 with correlation coefficient 0.9993 in the absence of CPC and y = 0.1272x ? 0.0013 with correlation coefficient of 0.9994 in the presence of CPC was obtained showing a &2-fold enhancement in the sensitivity of the proposed method. Molar absorptivity and Sandell’s sensitivity of the proposed method was found to be 2.95 9 104 L mol cm-1 and 6.8 ng cm-2, respectively. The comparison of proposed method is summarized in a tabular form elsewhere. The relative standard deviation for four replicate samples containing 5.0 lg mL-1 of Th4? was 2.52 % in the presence of CPC. Analytical application of the proposed method The proposed method was applied for the determination of thorium in various CRMs. The results were found in good agreement with the certified value, indicating the accuracy of the developed method. The recovery of thorium and relative standard deviation has been calculated from the standard addition procedure which confirmed the accuracy of the method reported herein (Table 2). The reference value for standard reference material (SRM) sample (NBLTable 2 Determination of thorium in Certified Reference Materials S. no.
CRMs
Proposed methoda (lg mL-1)
1
NBL-79-A
1.03 ± 0.03
2
MALINIM-202 83-G (granite)
48.7 ± 2.15
a
Based on the average of triplicate results
Certified values (lg mL-1) 1.00 51.0
79-A) was reported to be 1.0 lg mL-1 and the recovered amount was noted to be 1.03 ± 0.03 lg mL-1. The measured and certified values for thorium were found within ±3 % deviation. The data testifies the accuracy and precision of the reported method. Accuracy of the proposed method The accuracy of the reported method developed herein was checked by determining the concentration of standard thorium nitrate solution in the determination range and the results are listed in Table 3. These results showed that percent deviation is negligible and did not depend upon the concentration within the determination range. However, it decreases with decreasing concentration of thorium. At low concentration the deviation was found negligible. The added found method indicated that thorium can be determined from 1 to 30 lg mL-1 in the presence of surfactant. The accuracy of the proposed method has also been verified by analyzing the pre-concentrated water samples after addition of known amounts of thorium. By using the method reported herein, a good agreement was obtained between the added and found amount of thorium. The recovery of thorium and percent deviation were calculated from standard addition method. The method was found quite precise and accurate within a given range of determination. Only less than 1 % handling error was noted in some measurements. These studies showed that thorium in the presence of CPC can be easily determined in the range of 1–30 lg mL-1. For more concentrated samples appropriate dilution method can be used. The significance of reported method The determination method for thorium in the absence of CPC reported earlier followed the Beer’s law over the concentration range of 1–25 lg mL-1 [36] whereas, thorium determination range is significantly enhanced
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Table 4 Determination of thorium with thorin in the absence as well as in the presence of CPC as surfactant Parameters
Comparison of data Existing In the absence of CPC
Improved In the presence of CPC
Wavelength, kmax (nm)
544
581
Reaction medium
Perchloric acid (3.0 mol L-1)
Stability (months) Temperature (°C)
2.0 Room temperature (preferably 20 ± 5)
3.0
1.0–25.0
1.0–30
Beer’s law range (lg g-1) -1
Molar extinction coefficient (L mol
-1
cm )
-2
1.94 9 10
4
2.95 9 104
Sandell’s sensitivity (ng cm )
11.9
6.8
Regression equation
y = 0.0838x ? 0.0062
y = 0.1272x ? 0.0013
Correlation coefficient
0.9993
0.9994
Relative standard deviation (%) [Th4?] = 5.0 lg mL-1, n = 5
4.25
2.52
(1–30 lg mL-1) in the presence of CPC (present work). Molar absorptivity was also increased up to 2.95 9 104 L mol-1 cm-1 at 581 nm. Sandell’s sensitivity was noted to be 6.8 ng cm-1 as compared to 11.9 ng cm-1 in the absence of surfactant. The relative standard deviation for four replicate determinations of samples containing 5.0 lg mL-1 of thorium was reduced to 2.52 % as compared to 4.25 % in the absence of CPC. The achievements of the proposed method are summarized in Table 4. The proposed method has been compared with many other existing methods reported in the literature [39–42]. The comparison showed that stability, range of determination, molar extinction coefficient, Sandell’s sensitivity has been improved significantly. None of the existing methods referred earlier [36, 39–42] have the significance over the proposed method. The foreign ions interference in this method has been minimized with the use of perchloric acid as a medium of determination. The results agreed well with our previous work reported for Th(IV) and U(VI) determination in perchloric acid medium [43–45].
Conclusion The method using thorin in perchloric acid for determination of thorium in the presence of CPC is simple and selective with high complex stability. A significant increase in the molar absorptivity of Th(IV)–thorin complex was observed due to the micellar sensitization in the proposed method. The introduction of surfactant has reasonably improved the determination limit of the method, minimized the common anionic and cationic interferences as well as relative standard deviation. The method is quite suitable for determination of thorium in SRMs at lg mL-1 level. The method is rapid, simple, easy in operation, better in
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sensitivity and applicable in the field as well as in common labs as compared to many of the reported methods. Acknowledgments The investigations reported herein are the part of Research Project (No. 20-3/2nd phase/R&D104/51) being run under the Grant provided by Higher Education Commission (HEC) Government of Pakistan. The author for correspondence is thankfully acknowledges the sponsorship and support of HEC, Government of Pakistan.
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