Simultaneous spectrophotometric determination of thorium and rare earth metals with pyrimidine azo dyes and cetylpyridinium chloride

Talanta 54 (2001) 611– 620 www.elsevier.com/locate/talanta

Simultaneous spectrophotometric determination of thorium and rare earth metals with pyrimidine azo dyes and cetylpyridinium chloride Alaa S. Amin *, Talaat Y. Mohammed Chemistry Department, Faculty of Science, Benha Uni6ersity, Benha, Egypt Received 7 July 2000; received in revised form 6 December 2000; accepted 8 December 2000

Abstract Thorium and rare earth elements (REE) react with 5-(2%,4%-dimethylphenylazo)6-hydroxypyrimidine-2,4-dione (I) and 5-(4%-nitro-2%,6%-dichlorophenylazo)6-hydroxypyrimidine-2,4-dione (II) in the absence of cetylpyridinium chloride (CPC) to form red complexes. The molar absorptivity and Sandell sensitivity were calculated in absence of CPC. In its presence, REE — complexes are not formed due to miceller masking, whereas Th4 + has a sensitive reaction with the studied reagents I and II, with enhancement of the color intensity of the complex. Most of the foreign ions are tolerated in considerable amounts; 150–2400-fold amounts of rare earth do not interfere with the determination of thorium. The optimum experimental conditions of the complex formation reactions and the compositions of thorium complexes are described. A simple method is proposed for simultaneous determination of thorium and rare earth element without previous separation. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Spectrophotometry; Azo dyes; Thorium and rare earth determination; Cetylpyridinium chloride

1. Introduction Thorium and rare earth often coexist in their minerals, products and even in waste water. Due to their similar behavior, determination of thorium and/or rare earth is a problem in analytical chemistry. Although strong claims are made for the specificity and sensitivity of NAA, ICP-AES and ICP-MS, some of the interference causes a problem using these methods [1]. Therefore, spectrophotometric methods for thorium and rare * Corresponding author. Tel./fax: + 20-13-225494.

earth continue to be of interest. The methods reported for the spectrophotometric determination of thorium involve reactions with xylenol orange [2], semixylenol orange [3], methylthemol blue [4], semimethylthymol blue [5], arsenazo III [6,7], thoron I [8], 5,8-dihydroxy-1,4naphthoquinone [9], 1-amino-4-hydroxyanthraquinone [10] and 4-(2%-triazolylazo)resatophenone [11]. However, these methods lack sensitivity and/ or selectivity. The enhancement of the color intensity of metal complexes with chelating dyes by the presence of surfactants provides an inexpensive alternative to

0039-9140/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 6 7 9 - 2

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atomic spectroscopy methods, for meeting the present demands for determination of ever lower concentrations of elements. The colored complexes formed in micellar media are characterized by high molar absorptivities and high stability over a wide pH range, and usually by a large bathochromic shift caused by addition of surfactants to the binary complex formed in water [12,13]. Surfactants and micellar systems are currently used in the spectrophotometric determination of metal ions because of their unique properties in controlling solubility, reactivity, sen-

Fig. 3. Effect of pH on the complexes of 5.0 mg 25 ml − 1 of Th4 + [1,2] and Ce4 + [3,4], in the absence of CPC, CR = 2× l0 − 4 M.

Fig. 1. Absorption spectra in the absence of CPC for [1,2], reagent I and II (2× l0 − 4 M) against buffer as blank; [3,4], Th-I and-II complexes, CTh = 5.0 mg − 1 25 ml − 1; [5,6], Ce-I and II complexes, CCe = 5.0 mg 25 ml − 1.

Fig. 4. Effect of reagent concentration (2 ×l0 − 3 M) on complexes of 5.0 mg 25 ml − 1 of Th4 + [1,2] and Ce [3,4], in the absence of CPC.

Fig. 2. Absorption spectra in presence of CPC for: [1,2], reagents I and II (3.2 ×l0 − 4 M) using buffer as blank; [3,4], their thorium (5.0 mg 25 ml − 1) complexes.

sitivity and/or selectivity [14]. Pyrogallol red [15], pyrochatechol violet [16], gallein [17], glycin cresol red [18], tetrachlorogallein [19], have been used in presence of non-ionic surfactants, whereas chromeazural S [20], eriochrome cyanine R [21], arsenazo III [22], 2-(2%-thiazolylazo)-5-dimethylaminophenol [23], have been used in presence of anionic surfactants. Cationic surfactants as CPB was used for thorium determination using pheny-

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lfluorone [24] and bromocresol orange [25], whereas CPC was introduced using m-carboxychlorophosphonazo [26]. In order to continue the search for new sensitive and selective chromogenic reagents for the simultaneous determination of thorium and REE and studying the optimum conditions for their determination. We have studied the spectrophoto-

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metric behavior of some pyrimidine azo compounds I and II as a chromophoric reagents for complexation with Th4 + and Ce4 + (as well as other rare earth) in the presence and absence of cetylpyridinium chloride (CPC). A rapid method is proposed for the simultaneous determination of thorium and rare earth without previous separation.

2. Experimental

2.1. Reagents

Fig. 5. Effect of reagent concentration on the absorbance of complexes (5.0 mg 25 ml − 1 Th4 + ) using reagents I and II in the presence of CPC.

All the solutions were prepared with bidistilled water and/or pure ethanol. All the chemicals used were analaR grade. Surfactants and protective colloids were used as received without further purification. The standard stock solution of thorium was prepared by dissolving thorium nitrate in a given volume of water, and the solution was standardized by EDTA titration. The working standard solution (10 mg ml − 1) was prepared by further dilution. The standard stock solutions of rare earth were prepared by dissolving the oxides of rare earth (Aldrich products) in hydrochloric acid, whereas the cerium oxide (99.99%) was prepared in a mixture of sulphuric acid and hydrogen peroxide, evaporating the excess of acid and diluting with hydrochloric acid to a given volume. The solutions were then standardized by EDTA titration with xylenol orange as indicator. The working standard solutions were prepared by diluting as required with HCl (1:100). The standard solution of total rare earth (5.0 mg ml − 1) was prepared by mixing the rare earth solutions in the following proportions La2O3:CeO2:Nd2O3:Y2O3 = 30:40:20:10 [27]. Cetylpyridinium chloride solution (0.5%) was prepared by dissolving 0.5 g of CPC ( Fluka) in 100 ml of water.

2.2. Synthesis of reagents Fig. 6. Effect of 0.5% CPC add to 25 ml of complexes for I and II with 5.0 mg 25 ml − 1 Th4 + .

Pyrimidine azo dyes were prepared using the conventional diazotization and coupling method

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Table 1 Effect of various surfactants on the absorbance of Th4+ complexes with reagents I and II (CTh =5.0 mg 25 ml−1)a Surfactant

– SAS SLS Tween 40 Tween 60 Tween 80 a

Absorbance

Surfactant

I

II

0.175 0.105 0.098 0.185 0.164 0.155

0.160 0.088 0.092 0.178 0.170 0.135

Triton X-100 CTAB CPC CPB PVA Gelatin

Absorbance I

II

0.197 0.244 0.305 0.270 – –

0.180 0.296 0.3612 0.325 – –

Average of four measurements.

Fig. 7. Absorbance of reagent I and II complexes of rare earth at 563 and 557 nm, respectively; 5.0 mg 25 ml − 1 of REE, CR =2 ×l0 − 4 M.

[28]. The prepared compounds were purified by crystallization from a mixture of acetone and water. After recrystallization from acetone and water, there purifies were confirmed by the sharp melting point 298 and 290°C for reagent I and II, respectively. The IR spectra showed a band for wN = N = 1414 cm − 1, wC = O =1660 cm − 1 and wOH =3430 cm − 1 for reagent I, whereas for reagent II wN = N =1410 cm − 1, wC = O = 1670

cm − 1 and wOH = 3410 cm − 1. Studying 1H-NMR spectra in DMSO showed a peak at l= 10.8 (1H, d, J =9.90 Hz OH), 7.8 (2H, d, J= 9.90 Hz, NH) for reagent I and l= 10.6 and 7.8 for H of OH group and 2H of the 2 NH groups. The elemental analysis for reagent I was found to be C, 55.65; H, 4.48; n, 21.25; whereas for reagent II it was C, 35.05; H, 1.31; N, 19.95; and C1, 20.20. The calculated values for reagent I (C12H12N4O3): C,

A.S. Amin, T.Y. Mohammed / Talanta 54 (2001) 611–620

Fig. 8. Job’s continuous variation plots for Th4 + [1,2] and Ce4 + [3,4], complexes in the absence of CPC.

55.39; H, 4.62; N, 21.54 and for reagent II (C10H5N5O5Cl2) C, 34.68; H, 1.45; N, 20.23; Cl, 20.52% confirming the following structure.

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A stock solution of reagents I and II of concentration 2×10 − 3 M was prepared by dissolving an accurately weighed amount of the purified reagent in ethanol. More dilute solutions were prepared by appropriate dilution. Borate buffer solutions of different pH values ranging from 5.5 to 10.5 were prepared as recommended [29].

Table 2 Characteristics of reagents I and II complexes of Th4+ and Ce4+ Complex

pH

umax (nm)

CPC

M:R

Beer’s mg 25 ml−1

Ringbom mg 25 m (×10−5) l mol−1 cm−1 ml−1

SS ng cm−2

Th-I Th-II Ce-I Ce-II Th-I Th-II Ce-I Ce-II

8.5 9.5 8.5 9.5 8.5 9.5 8.5 9.5

563 557 563 556 592 586 – –

Absent Absent Absent Absent Present Present Present Present

1:2 1:2 1:1, 1:2 1:1, 1:2 1:4 1:4 – –

0.0–20 0.0–25 0.0–18 0.0–22 0.2–8.5 0.2–9.4 – –

0.5–17.5 0.5–22.0 0.5–15.5 0.5–19.5 0.5–8.0 0.5–9.0 – –

1.67 1.81 1.20 1.59 6.50 5.52 – –

2.03 1.86 1.17 0.875 5.17 6.11 – –

Table 3 Straight line equations of calibration graphs Procedure

Metal determination

Straight line equationa

Correlation coefficient

(AI) (AII) (BI) (BII) (BI) (BII) (BI) (BII)

Th4+ Th4+ Ce4+ Ce4+ REEb REEb Th4+ Th4+

A= 0.061C+0.005 A =0.072C+0.008 A= 0.033C−0.007 A =0.025C+0.006 A =0.032C+0.009 A= 0.024C+0.007 A= 0.024C−0.009 A= 0.022C+0.004

0.9998 0.9996 0.9992 0.9994 0.9988 0.9993 0.9998 0.9990

a b

A is absorbance, C is mg in 25 ml solution. Total rare earth elements.

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Table 4 Tolerance limits (5.0% error maximum) for foreign ions in the determination of 5.0 mg of thorium and cerium in absence and in presence of CPC Foreign ions

Na+, K+, NH+ 4 Ca2+, Sr2+, Mn2+ 2+ 2+ Mg , Ba , Zn2+ Co2+, Pb2+, Ni2+ Ag+, Cu2+, Fe2+ La3+, Sc3+, Y3+ Al3+, Fe3+, Ce3+ Yb3+, Ce4+, Eu3+ 4+ Zr4+, UO2+ 2 , Ti 5+ 5+ Nb , V 2− − B4O− 7 , S2O3 , ClO4 − 2− 3− NO3 , SO4 , PO4 Silicate Fluoride a b

Presence

Absence

Tolerance limits mg

Ion/Th W/W

Tolerance limits mg

Ion/Th W/W

40 000a 30 000a 20 000a 12 000 7500 6000 3000b 1750 1200 750 50 000 12 000a 6000 750

8000 6000 4000 2400 1500 1200 600 350 240 150 5000 2400 1200 150

15 000 12 000 8000b 50 30 25 12 5 2.5 2.0 15 000 15 000 2500 2000

3000 2400 1600 10 6 5 2.4 1.0 0.5 0.4 30 000 3000 250 400

Not to the maximum amounts. Add 5.0 ml of 0.5% CPC.

2.3. Apparatus Absorption spectra and absorbances were recorded and measured with a Perkin Elmer l3B double beam UV– Vis spectrophotometer with 10 mm quartz cuvettes. An Orion research model 601 A/digital ionalyzer fitted with a combined glasscalomel electrode was used for the pH measurements. The IR spectra of the prepared reagents was carried out using a Beckman IR spectrometer, whereas 1H-NMR spectra was carried out using a Varian EM-390 (90 M Hz) spectrometer with DMSO-d6 as solvent.

on using reagents I and II, was measured against a reagent blank solution prepared similarly. 2. To the mixed solution of Th4 + and Ce4 + (or other REE), 5.0 ml buffer of the optimum pH values, 2.5 ml 2× 10 − 3 M reagent solution and 10 ml water were added in a 25 ml calibrated flask. The mixture was dilute to the mark with water and the absorbance (A2) for the sum of Th4 + and Ce4 + (or other REE) complexes at umax 563 and 557 nm, using reagent I and II, respectively, was measured against the reagent blank.

2.4. General procedure

2.5. Procedure for waste water samples

1. To the mixed solution of Th4 + and Ce4 + (or other REE), 5.0 ml buffer solution of the optimum pH values, 3.0 ml of 0.5% CPC and 4.0 ml of 2 ×10 − 3 M reagent solution were added successively in a 25-ml calibrated flask. The mixture was diluted to the mark with water and stand for 5.0 min. The absorbance due to Th4 + complex (A1) at 592 and 586 nm

Different water samples from cities with potential thorium pollution were collected in polyethylene containers, filtered and adjusted to the optimum pH values by HCl or NaOH. An appropriate aliquot of the sample was placed in a 25-ml calibrated flask and then the above-described general procedure (A) and (B) was followed for the determination of the metal ions.

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3. Results and discussion

3.2. Optimization

3.1. Absorption spectra

In the absence of CPC, borate buffer solutions of different pH values were examined to achieve the optimum pH value for complexation. The pH values 8.5 and 9.5 were chosen as the best media for the complexes of I and II, respectively (Fig. 3). Moreover 5.0 ml of the optimum pH value gave the highest and most constant absorbance values of Ce4 + and Th4 + complexes. Also, the reagent concentration was examined to achieve maximum color intensity (Fig. 4). Addition of 2.5 ml of reagent (2× 10 − 3 M) is chosen for maximum absorbance and highest stability. All complexes were formed instantly and their absorbances were stable for at least 12 h. In the presence of CPC, the optimum buffer media is the same as mentioned above, whereas the optimum reagent concentration is varied and

In the absence of CPC, reagent I and II exhibits an orange color with maximum absorbance at 451 and 495 nm (Fig. 1), whereas cerium and thorium complexes present red color with maximum absorption peaks at 563 nm using I, and 557 nm using II, in borate buffer solution of pH 8.5 and 9.5, respectively. On addition of CPC, the absorption band of reagents I and II is shifted to 464 and 508 nm, respectively (Fig. 2); micellar masking [30] prevents the formation of a Ce4 + -reagent complexes, while the Th4 + complexes give a higher color (pink) intensity with higher absorbance band at longer wavelength at 592 and 586 nm using I and II, respectively.

Table 5 Analysis of waste-water samples Location

Amount added mg ml−1

Amount founda mg ml−1 Th4+

Shobra El-Kema – 3.0 Enshaz

– 3.0

10th Ramadan

– 3.0

6th October

– 3.0

Helwan

– 3.0

Abou-Zaabal

– 3.0

a

Ce4+

I

II

References [7]b

I

II

Ref [37]c

1.55 4.60 (t= 1.17)d 2.50 5.60 (F= 2.17) 0.90 3.95 (t= 1.78) 0.25 3.20 (F= 2.31) 1.15 4.20 (t= 1.36 1.95 5.00 ( f= 2.48)

1.60 4.50 (F= 2.56)d 2.51 5.45 (t =1.54) 0.85 3.80 (F= 3.11) 0.22 3.25 (t =1.44) 1.10 4.15 (F =3.20) 2.05 4.90 (t=1.25)

1.52 4.50

3.60 6.50 (F= 3.03)d 2.85 5.80 (t =1.39) 1.65 4.50 (t =1.83) 1.30 4.40 (F= 2.37) 3.15 6.20 (F=3.17) 2.45 5.50 (t=1.48)

3.63 6.70 (t = 1.66)d 2.90 5.95 (F= 2.40) 1.70 4.60 (F= 2.73) 1.25 4.20 (t= 1.08) 3.05 6.00 (t= 1.72) 2.60 5.45 (F= 2.98)

3.50 6.60

2.60 5.50 0.95 3.92 0.20 3.30 1.20 4.25 2.00 5.05

Average of six determinations. After extraction by PMBP-acetobutyric ester, spectrophotometric determination with arsenazo III. c Using p-acetylarsenazo as chromogenic reagent. d Theoretical values of t- and F- tests at 5 d.f. and 95% confidence limits are 2.57 and 5.05, respectively. b

2.80 5.90 1.60 4.65 1.40 4.30 3.10 6.10 2.50 5.55

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found to be 4.0 ml of reagent I or II ( 2× 10 − 3 M) (Fig. 5). Studying the CPC optimum concentration showed that below 2.5 ml of 0.5% CPC added to 25 ml of the final solution cause turbidity and the absorbance decrease gradually as the concentration decrease. Thus, 3.0 ml of 0.5% CPC is, therefore, the best concentration of surfactant to achieve higher absorbance and sensitivity (Fig. 6). The order of addition of CPC (before or after complex formation) did not influence the results. The Th4 + - reagent complex formation was completed within 5.0 min of mixing and it remained stable for at least 12 h. All the solutions became turbid after standing for longer periods.

3.3. Effect of surfactants The effect of some surfactants and protective colloids on the color reaction of thorium with reagents I and II have been investigated. The surfactants utilized were sodium alkylbenzene sulfonate (SAS), sodium lauryl sulfate (SLS) [anionic], polyoxyethylene sorbitan monopalmitate (Tween 40), polyoxyethylene sorbitan monostearate (Tween 60), polyoxyethylene sorbitan mono-oleate (Tween 80) and polyoxyethylene pter-octylphenol (Triton X– 100) [non ionic] and cetyltrimethyl ammonium bromide (CTAB), benzyldimethyl tetradecyl ammonium chloride (zephiramine), cetylpyridinium chloride (CPC) and cetylpyridinium bromide (CPB) [cationic], whereas the examined protective colloids were polyvinyl alcohol (PVA) and gelatin. Anionic surfactants (SAS, SLS) decrease the absorbance of the complex (Table 1). However, protective colloids and surfactants of the cationic and non-ionic types enhance the absorbance of the complexes. Maximum absorbance of Th4 + - reagent complex is obtained in the presence of the cationic surfactant CPC. Consequently CPC has been selected to improve the sensitivity of the formed complex, in addition to dissociate the cerium complex completely due to micellar masking.

3.4. Beha6ior of other rare earth elements The absorbances of the complexes of the other rare earth (5.0 mg each) were also measured at 563

and 557 nm using reagents I and II, respectively, against a reagent blank in the absence of CPC. Fig. 7 shows that the complexes of cerium subgroup rare earth and yttrium (the major constituents in rare earth mixtures) give higher absorbances. The absorbances from gadolinium to lutetium (the minor constituents) decrease slightly with an increase in atomic number. In the presence of CPC, cerium, as well as other rare earth are completely masked. This behavior enables other earth to be determined along with cerium.

3.5. Composition of the complexes The stoichiometry of the Th4 + - reagent (I or II) complex formed in the absence of CPC was ascertained by Job’s method of continuous variations and molar ratio methods. The results indicated that the complexes have (1:2) (Th4 + :R) molar ratio. In the presence of CPC, the stoichiometry of the complex was found by the same methods to be 1:4 (Th4 + :R). It is because more of the negative charged reagent ions are adsorbed and concentrated on the positive–charged surface of CPC micelle that the formation of a higher order complex is favored [31]. The stoichiometry of the Ce4 + - reagent complex (Fig. 8) in the absence of CPC is (1:1) and (1:2) (M:R). After the addition of CPC, the complexes were completely dissociated due to micellar masking. The characteristics of the complexes mentioned above are listed in (Table 2).

3.6. Quantification Under the optimum experimental conditions employed in the procedure (A), cerium, as well as other rare earth are completely masked in the presence of CPC, while the absorption of Threagent complex is greatly enhanced via bathochromic shift in umax and higher absorbance values. Under the conditions for procedure (B), the absorbance of thorium and cerium (as well as other rare earth) complexes are additive. This property can be utilized to create a method for the simultaneous determination of thorium and REE. Their amounts were obtained by the following expressions [32].

A.S. Amin, T.Y. Mohammed / Talanta 54 (2001) 611–620

CTh =

A1 m´Th

CREE =





A2 −A1mTh/m%Th mREE

(1) (2)

Where A1 and A2 are the absorbances read in the presence of CPC (procedure A) and in the absence (procedure B), respectively. m%Th and mTh are the molar absorptivities of thorium found in the presence and absence of CPC. mREE is the molar absorptivity of rare earth found in the absence of CPC. In the presence of CPC, the calibration graph for thorium was constructed according to procedure (A). Beer’s law was obeyed for 0.2–8.5 and 0.2–9.4 mg of thorium in a 25 ml solution at 592 and 586 nm using reagent I and II, respectively. The apparent molar absorptivity and Sandell sensitivity were found to be 5.17×105 l mol − 1 cm − 1 and 6.5 ng cm − 2, using reagent I, and 6.11×105 l mol − 1 cm − 1 and 5.5 ng cm − 2, using reagent II. Eight replicate analysis of a test solution containing 5.0 mg of Th4 + gave a coefficient of variation of 0.95 and 0.80% on using reagents I and II, respectively. In the absence of CPC, calibration graphs of cerium, total rare earth and thorium were constructed according to procedure (B). Beer’s law was followed for 0– 18 mg of cerium, 0– 15 mg of the total rare earth and 0– 20 mg of thorium in 25 ml of final solution at 563 nm on using reagent I. On using reagent II, Beer’s law was followed for 0–22, 0–20 and 0– 25 mg of cerium, total rare earth and thorium, respectively, in 25 ml of the final solution at 557 nm. For more accurate analysis, Ringbom optimum concentration ranges were calculated and recorded in Table 2. The apparent molar absorptivities of cerium and thorium at 563 nm were 1.17× 105 and 2.03× 104 l mol − 1 cm − 1 in using reagent I, whereas on using reagent II, 8.75 ×104 and 1.86× 104 l mol − 1 cm − 1 were obtained, respectively. The Sandell sensitivities were 1.20 and 1.67 ng cm − 2 on using reagent I, whereas on using reagent II, they were 1.59 and 1.81 ng cm − 2 for cerium and thorium, respectively. The coefficients of variation for the determination of 5.0 mg of cerium or thorium (eight determinations each) were 1.4 and 1.1%,

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respectively. The straight line equations of the calibration graphs obtained by a least squares treatment are listed in Table 3. The difference in absorbance, obtained for equal weights of metals, between the calibration graph for cerium and that for total rare earth element is usually not more than 4.2%, hence the former can be employed for the determination of total rare earth.

3.7. Effect of foreign ions Solutions containing 5.0 mg of Th4 + or Ce4 + and various amounts of foreign ions were prepared and determined by procedure (A) and (B), respectively. The tolerance limits (5.0% error maximum) are listed in Table 4. As shown in Table 4, 350-fold amounts of cerium, europium and ytterbium do not interfere with the determination of 5.0 mg of thorium, which is an advantage over the published methods [33–36].

3.8. Simultaneous determination of thorium and rare earth in waste water The proposed method was applied to the determination of thorium and REE (expressed as cerium) in six industrial waste-water samples taken from chemical works. Thorium values were in the range 0.22–2.51 mg ml − 1 and cerium values in the range 0.24–3.63 mg ml − 1. The coefficients of variation ranged from 0.5–1.5% for thorium and from 0.7–1.9% for cerium. The recoveries of thorium and cerium found by standard addition of 3.0 mg of thorium or cerium to each determination averaged 99.2 and 98.8%, respectively, with ranges of 97.5–102.2 for thorium and 96.5– 101.8% for cerium. For Th4 + determination, the performance of the proposed methods was assessed by comparison with spectrophotometric method depending on complexation with arsenazo III after extraction of Th4 + by PMBP- acetobutyric ester [7]. For Ce4 + and REE, they compared with that method depending on complexation with p-acetylarsenazo in chloroacetic acid medium of pH 3.1 [37]. Mean values were obtained with Student’s tand F-tests at 95% confidence limits for 5 d.f. [38].

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