Neena Nashine ⢠M.K. Deb ⢠R.K. Mishra. Spectrophotometric determination of thorium in standard samples and monazite sands based on the floated complex of ...
Fresenius J Anal Chem (1996) 355: 34-36
© Springer-Verlag 1996
ORIGINAL PAPER
Neena Nashine • M.K. Deb • R.K. Mishra
Spectrophotometric determination of thorium in standard samples and monazite sands based on the floated complex of thorium with N-hydroxy-N,N'-diphenylbenzamidine and thorin
Received: 19 April 1995 /Revised: 21 June 1995 /Accepted: 23 June 1995
Abstract A selective and sensitive spectrophotometric method for the determination of Th(IV) has been based on the reaction with thorin and subsequent extraction of the red-orange coloured complex with N-hydroxyN,N'-diphenylbenzamidine (HDPBA) in benzene as floated complex at pH 2.2. The complex in ethanol exhibits a maximum absorbance at 495 nm, with a molar absorptivity of 6.0 x 10 4 1 mol - t cm 1 , with a Sandell's sensitivity of 3.9 x 10 3 [tg cm Z . The method follows Beer's law up to 3.0 µg Th(IV) ml of the common cations and anions tested interfere. The detection limit of the method is 0.04 tg Th(IV)ml -1 , the RSD (n =10) is 1.4%. The method has been successfully employed for the determination of thorium in various standard and monazite samples. -
Therefore an effort has been made to develop a selective, reproducible and sensitive method for the determination of Th(IV), based on the selective extraction of thorium with HDPBA in the presence of thorin as an ion-associated floated complex in benzene-ethanol mixture at pH 2.2. Due to the extraction of Th(IV) with HDPBA most of the common ions which interfere in the other methods do not cause interferences in the present procedure. Table 1 is a comparative chart indicating the figures of merit of the present method over the above mentioned methods.
Experimental Apparatus and reagents. A Carl-Zeiss Jena SPEKOL spectro-
Introduction
Many spectrophotometric methods for the determination of thorium have been reported, using various reagents and dyes such as 2-hydroxy-l-naphthaldehyde-iso-nicotinoylhydrazone [1], N-phenylbenzohydroxamic acid [2], 4-methoxy-2'-chlorodibenzoylmethane [3], molybdothorophosphoric acid [4], 4-(2-pyridylazo) resorcinol [5], aniline blue [6], catechol violet [7], eriochrome cyanine R [8], gallein [9], thorin [10], and others [11]. But most of these methods are tedious, need rigid control of pH, temperature and reagents, and are not selective as most of the common ions and anions associated with Th(IV) interfere. N. Nashine • M.K. Deb (®) • R.K. Mishra School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur-492010, M.P., India Presented at the 31st Annual Convention of Chemists, 1994, Banaras, India
photometer equipped with matched 1 cm quartz cells was used for the absorbance measurements. A Systronics pH meter type 335 was employed for pH-measurements. All chemicals used were of analytical-reagent grade (BDH or SD fine Chem.). The standard Th(IV) stock solution was prepared by dissolving 0.6329 g thorium nitrate in 0.09 mol/1 HNO 3 . The working standard solutions were prepared by appropriate dilution. N-Hydroxy-N,N'-diphenylbenzamidine was synthesised according to literature [12] and its 0.0035 mol/l (0.1%, w/v) solution in benzene was employed for the extraction. A 0.0017 moll (0.1%, w/v) thorin solution was prepared in glass distilled water for colour development. HC1-KC1 buffer solution [13] (pH, 2.2) was used.
Procedure. Take an aliquot of the solution containing up to 30 µg Th(IV) in a 125 ml separatory funnel. To this add 1 ml thorin solution and adjust the pH of the solution to 2.2 by addition of HCI-KCl buffer solution in a total volume of 10 ml aqueous phase. Mix a 3 ml volume of 0.1% HDPBA solution in benzene into the above solution and shake it vigorously for 2 min. Discard the aqueous phase, wash the red-orange coloured complex formed at the interface twice with 2 ml of buffer solution and then reject the washings. Dissolve the floated complex in 5 ml of ethanol and transfer the solution into a 10 ml volumetric flask. Make up the solution with ethanol and measure the absorbance at Xmax of the complex against benzene-ethanol (3:7) mixed solvent.
35 Table 1 Comparison of some spectrophotometric methods for Reagent the determination of thorium 2-Hydroxy-lnaphthaldehyde isonicotinoylhydrazone N-Phenylbenzohydroxamic acid + thorin 4-Methoxy-2'-chlorodibenzoylmethane Molybdothorophosphoric acid N-(p-Methoxyphenyl)2-furylacrylohydroxamic acid + PAR Aniline blue
Acidity/pH
Amax
Reference
Remark
nm
l mol ' cm -1
Acidic medium
420
2.18 x 10`
Selective but less sensitive
(1)
0.2 mol/l HCl
545
1.7 x 10 4
Zr, Mo, V, W and Cu interfere
(2)
pH 2.5-3.5
390
1.5 x 10 4
Less sensitive
(3)
pH 5.1
400
2.5 x 10 3
Poor sensitivity
(4)
pH 3.6-5.5
500
3.9 x 10 4
Less sensitive
(5)
pH 4.9
660
3.16 x 10 3
Zr, Mo, Fe and PO interfere seriously EDTA, F , tartrate and citrate interfere
(6)
-
Catechol violet + hexadecyltrimethylammonium ions Eriochrome cyanine R + zephiramine Gallein
pH 7-7.4
665
4.6 x 10
pH 4.4
585
7.8 x 10 4
pH 4.1
600
2.56 x 10 4
Classical thorin
pH 0.2-1.0
545
1.5 x 10 4
Thorin + N-hydroxyN,N'-diphenylbenzamidine
pH 1.0-5.5
495
6.0 x 10 4
4
-
-
(7)
Al, Ga, In, Sc, Y (8) and U interfere Al, Fe, Pb, Cu, La, (9) Bi and Ca interfere U, F , C 2 O4 , (10) PO and SO interfere seriously and should be removed Zr and U interfere (PM) but effectively masked -
PM-Present Method
Results and discussion The absorption spectra of the Th(IV)-Thorin-HDPBA complex formed in benzene-ethanol mixture exhibits a maximum absorbance at around 495 nm against reagent blank. It was observed that the absorbance of the Th(IV)Thorin-HDPBA complex remains constant between pH 1.0 and 5.5. Therefore a pH of 2.2 was kept constant throughout the experiment using HCl-KC] buffer. The effect of various polar and non-polar organic solvents on the formation of floated Th(IV)-ThorinHDPBA complex was studied. The values of molar absorptivity at the wavelength of maximum absorption of the complex in different solvents are: 1-pentanol (E495 = 50000), ethyl acetate(E490-495 = 52000), isomethyl butyl ketone(E490 = 47000), butanol(8 490 = 55000), = 43500), toluene(s 495 = 58000) chloro nzene 490495 (8 _ and benzene 495 nm= 600001 mo1 1 cmOn -t ). dis solution of the floated complex in ethanol, maximum absorbance is observed with benzene as compared to
the other solvents. Benzene' was chosen as solvent for the extraction of Th(IV) due to its selectivity, high colour intensity and complete extraction (> 99.8 %) of the complex. The results obtained indicated that at least 2.78 x 10 -3 -3.47 x 10 -2 mol/l HDPBA in benzene is sufficient for complete extraction of 15 tg of Th(IV) as a floated complex. Therefore a 3.47 x 10 mol/l HDPBA in benzene was used for extraction work throughout the experiment. The studies on the effect of concentration of thorin indicated that a concentration range of 8.5 x 10 -5 -2.5 x 10 -4 mol/l thorin solution was sufficient for maximum colour development of Th(IV)-Thorin-HDPBA complex. In practice, a 1.7 x 10 -4 mol/1 thorin solution was kept constant throughout the experiment. The calibration curve for the determination of Th(IV) was prepared by analysing 3, 5, 10, 15, 20, 25 ( ) p py y g 'Warning: benzene is a carcinogenic agent
36 Table 2 Determination of thorium in standard and monazite samples
Samples
Th(IV) found by the present method
AAS
Relative standard deviation of the present method",
USGS a b BHVO-1 G-2 BCR-32
0.78 24.0 3.2
0.784 24.2 3.0
1.5 1.6 1.5
7.2 8.4
7.1 8.35
1.7 1.6
Monazite` A B
n=6 ' United States Geological Survey Standards. Certified Th(IV) content in ppm: BHVO-1 = 0.783; G-2 = 24.2; BCR-32 = 3.0 e Concentration expressed in ppm ° Concentration expressed in % A and B are two different monazite sand samples obtained from different locations at Travancore coast, Kerala, India
and 30 µg of Th(IV) per 10 ml in the aqueous phase. The system obeys Beer's law up to 3.0 µg Th(IV) ml' with a correlation coefficient value of 0.99. The Ringbom's plot [14] gave a linear dynamic range of the method of 0.3-3.0 µg Th(IV) ml - '. The molar absorptivity of the Th(IV)-Thorin-HDPBA complex calculated in terms of Th(IV) is 6.0 x 10 4 1 mol t cm at max 495 nm. The Sandell's sensitivity of the ternary complex is 3.9 x 10 - 3 tg cm - 2 . The relative standard deviation (n = 10) for the determination of 15 µg Th(IV) is 1.4%. The detection limit of thorium is 0.04 pg ml -1 The stoichiometry of the complex was determined by plotting log distribution ratio [D = Ae q /(Amax - Ae q )] of the metal versus log molar concentration of thorin or log molar concentration of N-hydroxy-N,N'-diphenylbenzamidine (HDPBA). The results indicated the formation of a 1: 3 : 2 Th(IV)-Thorin-HDPBA ion-association complex. The most probable reaction mechanism can be expressed as: .
Th 4+ + 3H 3 R 2[Th(H3R)3]2[Th(H 3 R) 3 ] 2- + 2HDPBA o + 2H + ±{[Th(H 3 R) 3 ] 2(H 2 DPBA)} o where subscript o denotes organic phase and R is the thorin anion. The following diverse ions do not interfere in the determination of 15 pg Th(IV) 10 ml - ': Fluoride, Zr 4+ U 4 + , F.e 3+ , Pb 2+ , Cu 2+ V s+ , Ni 2+ Cd 2+ Zn 2+ , Sn 2+ in the range (0.02-1.2).mg; Nb5+ Mo5+ W6,Re7+ Pd 2 + , Ga 2+ , A1 3+ , Sb 3+ , Co 2+ , Cr3+ Hg2+, Mn z+ , Mg z + in the range (3.5-8.0) mg; acetate, tartrate, oxalate, thiourea, ascorbic acid in the range (15.0-20.0) mg. Fe(III) interferes when present in a 30fold excess. However, its reduction-to Fe(II) by ascorbic acid (1 ml, 2%) increases the tolerable amount up to 0.8 mg. Interferences due to U(IV) and Zr(IV) were avoided by oxidation of U(IV) to U(VI) with perchloric ,
,
,
acid [10] and masking with tartaric acid (0.2 ml, 10%), respectively. The present method was applied to the determination of thorium in standard and monazite samples. The standard samples were prepared according to the literature [15]. The Th(IV) content in monazite samples was determined using decomposition by fusion with sodium peroxide. Thorium along with the lanthanides was precipitated as oxalate by the addition of oxalic acid. The oxalates were then decomposed by heating with nitric acid. The excess of nitric acid was removed by treatment with hydrochloric acid and the solution made up to a known volume with distilled water [11]. The data obtained were compared by analyzing the sample by AAS [16]. The results obtained (shown in Table 2 were comparable and in good agreement).
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Kavlentis E (1988) Microchem J 38:188 Agrawal YK, Dayal U (1985) J Radioanal Nucl Chem 90:303 Sharma SC, Tyagi MP, Purohit DN (1990) Asian J Chem 2:214 Balon M, Munoz MA (1987) Anal Chim Acta 193:325 Abbasi SA (1988) Anal Lett 21:1723 Singh NSB, Thankachan TS, Balasubramanian GR (1980) J Radioanal Chem 60:395 Tikhonov VN, Pavlova OK (1982) Zh Anal Khim 37:1809 Valero J (1987) An Quim Ser B 83:106 Mori I, Fujita Y, Fujita K, Koshiyama Y, Nakahashi Y (1987) Anal Lett 20:1567 Thomason PF, Perry MA, Byerly WM (1949) Anal Chem 21:1239 Busev Al, Tiptsova VG, lvanov VM (1981) Analytical chemistry of rare elements, Mir Publishers, Moscow Satyanarayana K, Mishra RK (1974) Anal Chem 46:1609 Lange NA (1967) Handbook of chemistry, McGraw Hill, New York Ringbom A (1932) Fresenius Z Anal Chem 115:332 Shapiro L (1975) Rapid analysis of rock and minerals, USGS Kirkbright GF, Johnson HN (1973) Talanta 20:433
