Analyst, November 1995, Vol. 120 2759

Sequential Separation and Determination of Thorium(iv) in Calixarene Hydroxamic Acids

Yadvendra K. Agrawal and Mallika Sanyal Chemistry Department, School of Sciences, Gujarat University, Ahmedabad 380009, India

A new calixarene hydroxamic acid, 25,26,27,28-tetra- hydroxy5,11,17,23- tetrakis(N-p-chlorop henyl)calix[4] - arene hydroxamic acid (CPCHA) is used for the extraction and spectrophotometric determination of thorium(1v). The molar absorptivity of the thorium(rv)-CPCHA-SCN complex is 2.2 X 104 m2 mol-1 at 450 nm. The system obeys Beer's law over the range 1.3-13.2 ppm of thorium. The thorium-calixarene hydroxamate ethyl acetate extract was directly subjected to ICP-AES measurements which increases the sensitivity by fifteen-fold. Thorium can be estimated at the 0.1 ppm level. The method is applied for the trace determination of thorium(rv) in geological samples at the low ppb level even after a 1 : 100 dilution (dissolution). Keywords: Spectrophotometry ; thorium; calixarene hydroxamic acid

Introduction There is considerable interest in the behaviour of actinides at trace levels in the environment. Thorium (particularly 232Th) is a nuclear fuel and its determination is of significance. Among the many spectrophotometric methods cited in the literature for the determination of thorium, those based on azo compounds containing arsonic acid groups are the most important.'-17 Determination by AAS (dinitrogen oxide-acetylene flame) and also by ICP, show poor sensitivity because of thorium's good chemical stability and high melting properties. However, the combination of liquid-liquid extraction and ICP-AES has proved to be an effective technique for trace analysis18 of thorium, primarily because of the increased element concentra- tion, but also because8 nebulization of organic solvents can produce a more disperse aerosol.19

In the present investigation, the calixarene with hydroxamic acid functional grouping, 25,26,27,28-tetrahydroxy- 5,11,17,23-tetrakis(N-p-chlorophenyl)calix[4]arene hydrox- amic acid (CPCHA) (Fig. 1) is used for the liquid-liquid ex- traction and spectrophotometric determination of thorium(1v) from the same solution. The thorium(1v) is extracted as ThtV-

Wgc, O O H -

Fig. 1

calixarene hydroxamate-thiocyanate complex into ethyl acetate and determined spectrophotometrically. The extract is also directly aspirated for ICP-AES measurements which enhances the sensitivity fifteen-fold.

Calixarenes can transport metal ions and molec~les.2*~3 They are amenable to chemical modifications and have proven to be effective ionophores that can selectively bind a range of transition me ta l~ .~&~9 The hydroxamic acids are bidentate ligands and have remarkable versatility for organic and inorganic analysis .30-36 These acids have achieved tremendous importance as analytical reagents for the separation and determination of a large number of metal ions.3-0 Thorium'" has been determined using N-phenylbenzohydroxamic acid41 and other substituted derivative^.^^

Experimental Apparatus A Hitachi (Tokyo, Japan) 3210 UV/VIS spectrophotometer and 10 mm quartz cells were used for spectral measurements.

A Labtam (Australia) Plasma Scan Model 710 sequential inductively coupled plasma atomic emission spectrometer with a Labtam Plasma Scan multi-tasking computer and peristaltic pump were used.

Instrumental conditions Radiofrequency (rf) 27.12 MHz; incident power, 2000 W Labtam GMK nebulizer; sample concentration, 0.1 pg cm-3; rf power, 5 W, observation height, 14 mm; argon-coolant flow rate, 10 dm-3 min-1; argon-carrier flow rate, 1 dm-3 min-l; intergraph period, 10 s; resolution, 0.004 nm; peristaltic pump flow rate, 1 cm3 min-l; wavelength, 283.73 nm.

Reagents All chemicals used were of analytical-reagent grade, obtained from E. Merck (Bombay, India). Distilled, de-ionized water was used throughout. A 0.2% solution of the ligand CPCHA was prepared in ethyl acetate. A standard thorium(1v) solution was prepared by dissolving 2.5 g of thorium nitrate tetrahydrate in 1 dm3 of water and 15 cm3 of hydrochloric acid. Its final concentration of 4.5 10-3 mol dm-3 was determined spectrophotometrically.2 This solution was further diluted as and when required. A 20% solution of KSCN was prepared in distilled water. A buffer solution of pH 6 was prepared by mixing 0.01 mol dm-3 HCl and 0.01 mol dm-3 N h 0 H solution.

Standard Geological Samples In a 200 cm3 Teflon beaker, 1 g of the sample was digested on a steam-bath with 10 cm3 of 1 X 10-4-1 X 10-3 mol dm-3 hydrofluoric acid and evaporated to dryness. Then, 25 cm3 of concentrated (1 mol dm-3) hydrochloric acid was added and

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2760 Analyst, November 1995, Vol. 120

transferred into a 200 cm3 Pyrex beaker and digested on a sand- bath for 1 h to dryness. The residue was redissolved in 10-15 cm3 of concentrated hydrochloric acid along with 0.5 g ammonium persulphate, and centrifuged at 3000 rpm. The solution was finally diluted to 100 cm3 with distilled water.

Extraction Procedure An aliquot of sample solution containing 32.5-33.0 ppm of thorium was transferred into a 60 cm3 separating funnel. The pH was adjusted with 5 cm3 of buffer solution. A volume of 10 cm3 of 0.2% CPCHA solution in ethyl acetate was added. The mixture was shaken for 5 min and the organic phase was separated. To ensure complete recovery of thorium, the extraction was repeated with 5 cm3 of CPCHA solution. The combined organic phase was shaken with 10 cm3 of 20% KSCN solution for 2 min. The organic extract was collected, dried over 1 g of anhydrous sodium sulfate and transferred into a 25 cm3 calibrated flask. The sodium sulfate was washed with 2 X 2 cm3 of ethyl acetate. The extract and washings were diluted up to the mark with ethyl acetate. The absorbance was measured against the reagent blank at 450 nm.

For ICP-AES determination, the thorium-calixarene hydrox- amate ethyl acetate extract was introduced into the plasma by peristaltic pump.

Results and Discussion Spectral Characteristics and Sensitivity The absorption spectrum of thorium(1v)-CPCHA-SCN com- plex recorded against a reagent blank has a maximum absorbance at 450 nm. The molar absorption coefficient of this complex at 450 nm is 2.2 X lo4 m2 mol-l. The system obeys Beer's law in the range 1.3-13.2 ppm of thorium. The regression analysis represents concentration = 4.069 X absorbance -0.021 (r = 0.999).

Choice of Extracting Solvent Chloroform, carbon tetrachloride, benzene, toluene, ethyl acetate, isoamyl acetate and nitrobenzene were used as extracting solvents for thorium(1v). Of these, ethyl acetate was found to be most suitable for quantitative extraction of thorium (Table 1). No extraction occurs in chloroform and carbon tetrachloride; however, the extraction is incomplete with isoamyl acetate, nitrobenzene, etc.

Effect of pH and Time of Equilibration The percentage extraction and formation of chelate is influ- enced by the pH of the medium. Maximum extraction of thorium with CPCHA was obtained in the pH range 3.5-5.0 (Fig. 1). The data given in Table 2 show that at higher and lower

Table 1 Effect of solvents on the extraction of thorium(1v)-CPCHAXNS complex. Thorium, 5.2 ppm; CPCHA, 10 ml(O.2% in ethyl acetate); KSCN, 10 ml (20%); pH, 4.0; A,,,, 450 nm. NE = no extraction occurs

Solvent Molar absorptivity/

Extraction (%) m2 mol-1 Chloroform NE - Carbon tetrachloride NE - Benzene 33.4 7.3 x 103 Toluene 31.7 7.0 x 103 Ethyl acetate 100.0 2.2 x 104

Nitrobenzene 73.2 1.6 x 104 Isoamyl acetate 80.4 1.8 X 104

pH, the extraction is incomplete. It was observed that 5 min of equilibration is sufficient for quantitative extraction of tho- rium.

Effect of CPCHA Concentration The influence of CPCHA was studied by extracting thorium with different amounts of CPCHA, keeping the concentration of thiocyanate fixed. It was observed (Table 3) that 10 cm3 of 0.2% reagent CPCHA is adequate for complete extraction of thorium. Lower concentrations of CPCHA reduce the percentage extraction while an excess of the reagent can be used without any difficulty.

Effect of Thiocyanate Concentration The influence of thiocyanate was studied by extracting thorium with different amounts of thiocyanate, keeping the concentra- tion of CPCHA fixed. It was observed (Table 4) that 10 cm3 of 20% KSCN solution is adequate for obtaining maximum absorbance of the extracted complex.

Stoichiomeby of the Complex To establish the stoichiometry of thorium(1v)-CPCHA-SCN complex, the method of slope ratio was employed,43.44 i.e., by plotting a graph of the log of the distribution coefficient of the

Table 2 Effect of varying pH on the extraction of thorium(1v)-CPCHA- CMS complex. Solvent, ethyl acetate; thorium, 5.2 ppm; CPCHA, 10 cm3 (0.2% in ethyl acetate); KSCN, 10 cm3 (20%); A,,,, 450 nm. Thorium in the aqueous phase was determined by ICP-AES

Extraction Molar absorptivity/ PH (%I m2 mol-' 1 .o 2.0 3.0 3.3 3.5 4.0 4.5 5.0 5.5 6.0 7.0

56.8 69.4 76.2 90.1

100.0 100.0 100.0 100.0 89.2 71.2 61.4

1.2 x 104 1.5 x 104 1.7 X 104 2.0 x 104 2.2 x 104 2.2 x 104 2.2 x 104 2.2 x 104 2.0 x 104 1.6 x 104 1.4 X 104

Extraction (%) = lOOD/D + (vaq/vorg), where va9 and vOrg are the volume of the aqueous and organic phases and the distnbution coefficient, D = concentration of thorium in the organic phase/(total thorium taken- thorium in organic phase).

Table 3 Effect of varying the concentration of CPCHA for the extraction of thorium(Iv)-CPCHA-CNS complex. Solvent, ethyl acetate; thorium, 5.2 ppm; KSCN, 10 cm3 (20%); pH, 4.0; A,,,, 450 nm. Thorium in the aqueous phase was determined by ICP-AES

CPCHA (mol dm-3 x 10-4) 3.69 4.47 5.90 7.38 8.86

10.80 14.80 22.10

Thorium/mol dm-3 -log [CPCHA] [Th],, x [Th],, x 10-5 3.43 0.64 1.66 3.35 0.77 1.53 3.28 0.98 1.32 3.13 1.13 1.17 3.05 1.27 1.03 2.97 1.42 0.88 2.83 2.30 < 0.01 2.66 2.30 < 0.01

log D -0.41 -0.30 -0.13 -0.01

0.09 0.2 1

> 2.37 > 2.37

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Analyst, November 1995, Vol. 120 276 1

Table 4 Effect of varying the concentration of thiocyanate for the extraction of thorium(1v). Solvent, ethyl acetate; thorium, 5.2 ppm; CPCHA, 10 cm3 (0.2% in ethyl acetate): pH, 4.0; A,,,, 450 nm. Thorium in the aqueous phase is determined by ICP-AES

Thio- cyanate/ mol dm-3 0.05 1 0.103 0.25 1 0.515 1.029 2.058 2.501

-log W N I

1.290 0.987 0.600 0.288

-0.012 -0.3 13 -0.398

Thorium/mol dm-3

[Thl,, x 1.36 1.63 1.87 2.02 2.12 2.3 2.3

~~~

[Th],, x 10-5 0.94 0.67 0.43 0.28 0.18

< 0.01 < 0.01

log D 0.16 0.39 0.64 0.86 1.07

> 2.37 > 2.37

Table 5 Effect of foreign ions on the extraction of thorium(1v)-CPCHA- CMS complex. Solvent, ethyl acetate; thorium, 5.2 ppm; pH, 4.0; A,,,, 450 nm

Ion

Be2+ Pb2+2 Mn2+2

cu2+ Zn2+ Cd2+ Hg2+ Pd2+ Ga3+3

Sn2+ Sb3+ La3+* Ce4+*

ZF+* Mo6+t u022+

V5+ W042- Fez+ Cr3+ Mg2+

Ag+

Ni2+2

AP+

Ti4+t

Amount added /mg 80 80 80 60 60 40 60 40 40 40 60 60 80 80 60 60 40 40 40 60 40 60 80 60 80

* Stripped with 0.1 % tartaric acid. t Stripped with 0.3 mol dm-3 sodium oxalate.

Method of recovery of thorium (ppm)

Spectro-

metry 5.18 5.18 5.18 5.17 5.17 5.16 5.17 5.16 5.16 5.16 5.19 5.19 5.2 5.21 5.17 5.19 5.17 5.16 5.16 5.19 5.05 5.16 5.19 5.18 5.21

photo- I C P - A E S 5.214 5.201 5.230 5.198 5.204 5.211 5.192 5.200 5.214 5.197 5.206 5.210 5.212 5.220 5.180 5.232 5.230 5.193 5.189 5.208 5.216 5.203 5.197 5.193 5.220

Table 6 Determination of thorium in standard USGS geological samples

Thorium determined by present method* (ppm)

Certified Th value Spectrophoto-

Sample (PPW metry I C P - A E S Basalt, BCR-1

(52/19) 6.0 6.03 f 0.06 6.01 f 0.05 Diasase, W-1 2.5 2.43 k 0.05 2.52 f 0.03 Andesite, AGV- 1

(74/19) 6.4 6.43 f 0.07 6.37 f 0.04 Granite, G-2

(1 08/15) 21.8 22.20 f 0.05 21.75 k 0.06 Shale 12 11.8 k 0.04 11.7 k 0.06

* Average of 10 determinations.

metal (log D ) against the negative log of the ligand concentra- tion (-log ligand). Two sets of extractions were carried out. The first, by taking fixed amounts of thorium and thiocyanate solution and varying amounts of CPCHA, and the second, by taking fixed amounts of thorium and CPCHA solution and varying amounts of thiocyanate.

The plot of log D versus -log (CPCHA) gave a straight line of slope 1.3, which indicates the composition of the complex (Th : CPCHA) is (1 + 1). Thus, 1 mole of CPCHA is required for 1 mole of thorium. Similarly, the plot of log D versus -log(SCN) gave a straight line of slope 0.7, which indicates the composition of the complex with respect to SCN is (1 + 1). Thus, 1 mole of thiocyanate is required for 1 mole of thorium(1v). Therefore, thorium(1v) : CPCHA : SCN is (1 + 1 + 1). The plausible reactions are as follows:

Tho:! i- 4L + [ThL4] + 2H20

ThL4 + SCN- + [ThL3SCN] + L

The CPCHA has four groups of hydroxamic acids (4L) and, hence, forms a (1 + 1) complex with thorium(1v). On addition of thiocyanate, one hydroxamate group is replaced by SCN and gives a (1 + 1 + 1) complex with thorium(1v).

Effect of Foreign Ions In order to examine the utility of this method in the presence of other commonly associated ions, a systematic study of their interferences was carried out. Moderate amounts (20-40 mg) of many commonly occurring metal ions with thorium were tolerated and did not interfere in the determination of thorium (Table 5). Interference was determined by measuring the absorbance of the extracted organic phase, and also by the measurement using ICP-AES of both the extract and the aqueous phase. The tolerance limit was set as the amount of a foreign ion causing a change of k0.02 absorbance or 2% error in the recovery of thorium. Titanium, molybdenum, zirconium and rare earths interfere in the determination of thorium. Titanium and molybdenum can be tolerated by stripping them out with 0.3 mol dm-3 of sodium oxalate. Interference of Zr"t+ and rare earths is eliminated by stripping them out with 0.1% tartaric acid. Thorium is determined in United States Geological Survey (USGS) geological samples by using this method (Table 6).

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Paper SIOl919B Received March 27,199s

Accepted July 17, I995

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