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Journal of Radioanalytical and Nuclear Chemistry, Articles,. Vol. ... Department of Chemistry, Addis Ababa University, P.O. Box 1176, Addis'Ababa (Ethiopia).
Jointly published by Elsevier Science S. A., Lausanne and Akaddmiai Kiadr, Budapest

Journal of Radioanalytical and Nuclear Chemistry,Articles, Vol. 210, No. 1 (1996) 171-181

EXTRACTION A N D SPECTROPHOTOMETRIC DETERMINATION OF URANIUM(VI) WITH N-PHENYLCINNAMOHYDROXAMIC ACID B. S. CHANDRAVANSHI, TEMAM JUHAR

Department of Chemistry, Addis Ababa University,P.O. Box 1176, Addis'Ababa (Ethiopia) (Received JuIy 23, 1996) Uranium(VI) reacts with N-phenylcinnamohydroxamic acid to form an orange-yellow complex in the pH range 5.5-8.5. The orange-yellow complex, having the composition of 1 : 2 (metal : ligand), is quantitatively extractable into ethyl acetate. The spectrum of the complex e~hibits a m a x i m u m absorption at 400 nm wkh a molar absorptivity of 6500 h~l"-1 9 cm-1. The colottred system obeys Beer's law in the concentration range 2-40 ~ g . m1-1 of uratfiumCVI).The photometric sensitivity of the colour reaction is 0.037 ~tg 9 cm -2 of urarSumfVI). Most of the common ions do not interfere and the method has been fomld to be sknap!e, precise, and free from the rigid control of experimental conditions. The method has been appfied to the determination of uranium ha synthetic matrices and potable water.

The increasing demand of tu'anium ha nuclear energy programs requires a suitable method for its exlracfion, separation, and determination. A number of methods have been ~ported for tile sp~trophotomelric determination of uranium,i~ N-Phenylcinnamohydroxamic acid ,.~Pc'HA~ , has also been used for the extraction and spectrophotometric determination of several metal ions, H-~ including uranium(V1).~2 Eventhrough, PCHA has been reported for the extraction and Sl~trophotometric detemaina6on of umninm(VI), 12 the reaction of umnium(VI) With PCHA has not been studied in detail. The composition of the complex has not been established, the spectrophotometric parameters, precision and accuracy of the method have not been evaluated, the effects of foreign ions have not been studied, and the method has not been applied to the determination of twanium in real samples. Hence, the reaction of uranium(VI) with PCHA has been studied in detail finder different experimental conditions in the present investigation. This work has led to the development of a simple, precise, sensitive, and selective method for the extraction and spectrophotometric determination of uranium(Vi) with PCHA. The method has been applied successfully to the determinationof uranium in synthetic matrices and potable water with reasonable precision and accuracy.

0236-5731/96/US $15.0 Copyright 9 1996 Akaddmiai Kiadr, Budapest All rights reserved

B. S. CHANDRAVANSHI, TEMAM JUHAR: EXTRACTION AND SPECTROPHOTOMETRIC

Experimental

Apparatus: A Beckman Model 24 UV-Visible Spectrophotometer equipped with 1 cm quartz cells was used for the absorbance measurements. A Beckman Chem-Mate pH meter was used for tile measurements of pH. Reagents and chemicals: A stock solution of uranium(VI) was prepared by dissolving 0.6280 g of umnyl nitrate hexahydrate (BDH, AnalaR) in 250 ml of I% (v/v) nitric acid. The solution was standardized gravimetrically.21 Subsequent dilutions were made, as and when needed, from this stock solution. N-Phenylcinnamohydroxamic acid (PCHA) was prepared by the rep3rted method.22 A 0.01M solution of PCHA in ethanol was used for the reaction with uranium(VI). A 0.01M of PCHA in chloroform was used for the removal of interfering ions. Solutions of foreign ions were prepared by dissolving known quantities of reagent grade salts in distilled water to give 10 nag- ml-I of the ion in question. Stock solutions of synthetic matrices of composition similar to those solutions obtained by hydrometaUurgical treatment of ores, m';nerals, and to those of water samples were prepared by mixing solutions of the constituent ions. Dilute solutions of hydrochloric acid and ammonia were used for adjusting the pH of the solutions. Redistilled ethyl acetate and solvents were used for the extraction of uranium(VI) and other metal ions. All other chemicals used were of analytical grade. General procedure: An aliquot of the solution containing 0.05-1.0 mg of uranium(VI) was transferred to a 50 ml beaker and 5 ml of 0o01M solution of PCHA in ethanol was added to it. The solution was diluted up to 25 ml with distilled water and the pH was adjusted to 6-8 using 1M hydrochloric acid or IM ammonia solution. The mixture was transferred to a 100-ml separatory funnel and the beaker was washed with 25 ml of ethyl acetate. The washings were added to the funnel, the mixture was shaken vigorously for 2 minutes, and allowed to separate the two phases. The organic phase was collected in a 25 ml volumetric flask after drying over anhydrous sodium sulphate and diluted to volume with ethyl acetate. The absorbance of the coloured extract was measured at 400 nm against the reagent blank. For calibration, 0.1, 0.5, 1.0, 1.5 and 2.0 ml of the standard solution (500 ].tg U. 9 ml-l) were used throughout the procedure. Results and discussion

Solvents for extraction and absorption spectra Uranium(VI) was found to react with PCHA to form an orange-yellow coloured complex insoluble in aqueous ethanol. Several organic solvents such as chloroform, carbon tetrachloride, benzene, chlorobenzene, 1,2-dichlorobenzene, toluene, xylene and 172

B. S. CHANDRAVANS}iI,TEMAMJUI-IAR:EXTIL~=CTIONAND 8PEC'IROPHOTOMETRiC ethyl acetate were found to extract the U(VI)-~PCHA complex from the aqueous phase. Ethyl acetate was lbtmd to be the most suitable solvent because the quantitative extraction of the U(VI)-PCHA complex was xeadily accomplished in it. It was also preferred due to the higher sensitivity of the co~o~ re,action in it than in other solvents.

2.O

,~ 1.6

2

1./+ ~.2 1.0 0.8

().4 02 0

35O

z~30

450

5O0

550 6OO Wavelength =nrn

Fig, I, Absorptionspectraof 8.0. 10-3MPCHA(the reagentblank)haethylacetate(curve1)'and3.0.10-4M U(VI)-PCHA complexin ethylacetateagainstthe reagentblank(curve2)

The spectrum of U(VI)-~PCHA complex exhibited an intense absorption band at 400 nm (Fig. 1). The position of the absorption band was unaffected by the changes in the concentrations of uranium(VI) and PCHA and the pH of the solution over a wide range. The spectrum of the reagent blank (PCHA) showed negligible absorption in the region 700--500 nm, sfightly increasing absorption between 500 and 400 nm, and strong absorption below 400 nm (Fig. 1). Hence, a reagent blank is necessary for the precise measurements of absorbanee at 400 nm,

Effect of pH The extraction of uranium(VI) was studied in the pH range of 1-10. The optimal pH range for the quanitative exlraction of umnium(VI) was found to be 5.5-8.5. At lower pH the extraction of uranium(VI) decreases probably due to protonation of the ligand, 173

B. S. CHANDRAVANSHI, TEMAM JUHAR: EXTRACTION AND SPECTROPHOTOMETRIC

PCHA, while at higher pH the extraction of uranium(VI) decreases due to the precipitation of uranium(VI) as uranyl hydroxide.

Effect of reagent concentra~qen The extraction of a fixed amount of uranium(VI) was studied at varying amounts of PCHA. A 8-fold molar excess of the reagent was adequate for the quantitative extraction of uranium(VI) from the aqueous phase under optimum experimental conditions. A large excess of the reagent up to 100-fold molar excess had no adverse effect on the extraction of uranium(VI) from the aqueous phase.

Effect of amount of ethanol Uraniurn(VI) was found to be unextractable from the aqueous phase by the ethyl acetate solution of PCHA. This is due to the fact that the extraction of uranium(VI) as U(VI)-PCHA cornl~tex occurs in the pH range 5.5-8.5 and at this pH m-aniurn(VI) is precipitated as uranyl hydroxide in the absence of PCHA in the aqueous phase. Hence the reagent was introduced as an e~hanolic solution to the solution of uranimn(VI) and the U(VI)-PCHA complex precipitated in the aqueous phase was found to be readily extractable in ethyl acetate. The effect of amount of ethanol was studied by varying the percentage composition of e~anol in the aqueous phase at constant volumes of aqueous and organic phases. A 10% (v/v) of ethanol in the aqueous phase was necessary for the quantitative extraction of U(VI) and up to 30% (v/v) of ethar~ol in the aqueous phase, it was found that there was no adverse effect on the extraction of U(VI) and. on the extraction efficiency of the system. However; at higher percentages of ethanol in the aqueous phase ( > 30%, v/v), the ex~action of umnium(V'I) decreases due to increasing solubility of the U(VI)-PCHA complex in the aqueous phase and at 50% (v/v) aqueous ethanol there was no phase separation.

EfCectof ionic strength, ~emperature,and volume of aqueousphase The ahsorbance value of the coloured extract of U(VI)-PCHA complex and the extraction efficiency of the coloured system were not affected by the changes in ionic slrength of the aqueous phase between 0.05 and 1M with respect to potassium nitrate or ammonium nitrate. Variation ii~ the ternperamre between 20~ and 40 ~ did not produce any change in the absorbance value of the coloured extract of the complex or in the extraction efficiency of the system. It was found that the volume of the aqueous phase can be varied from 10 to 50 ml with respect to a fixed volume of 25 ml of the organic phase without any adverse effect on the extraction efficiency of the system. !74

B.

;S, CHANDRAVANSHI,TEMAMJUHAR:EXTRACTIONAND SPECTROPHOTOMETRIC

However, larger volumes of the aqueous phase decrease the rate of phase separation and at very large volume, i.e., 100 ml, the phase separation was very difficult, almost impossible.

Extraction time and stability of the complex The U(VI)-PCHA complex was quantitatively extracted into ethyl acetate within 2 minutes from the aqueous phase. The ethyl acetate exlract of the U(VI)-PCHA complex was stable for at least two months at 20 + 2 ~

Composition of the complex The composition of the U(VI)-PCHA complex was determined by the continuous variation method2s and the metal-to-ligand ratio in the complex was found to be 1 : 2.

Beer's law, molar absorptivity, detection limit, and sensitivity The U(VI)-PCHA complex obeyed Beer's law in the concentration range 2 to 40 I~g. m1-1 of uranium. The molar absorptivity, detection limit,24 and SANDELL sensitivity25 were found to be 6500 M-f- em-1, 0.36 ~tg- m1-1, and 0.037 ~tg. cm-2 of uranium, respectively.

Precision and accuracy The precision and accurac~r of the method were studied by analysing solutions containing known amounts of uranium. From ten repeated determinations with 20 lxg9 ml-l of uranium(VI), the coefficient of variation and relative error were found to be + 0.8% and + 1.0%, respectively.

Effect of foreign ions The effect of foreign ions on the extraction and determination of uranium(VI) was studied by adding a known quantity of the ion in question to a solution containing 500 ~tg of uranium(VI). The pH of the solution was adjusted to 6 and uranium was extracted and determined following the general procedure described above. The tolerance limits of foreign ions taken as amount (mg)which cause an error less than + 2% are given in Table 1. The results obtained indicate that many common ions such as K§ Na§ Sr2+, Ba2§ Ca2§ Li+, Mg2+, Be2§ Cd2§ TI+, La 3§ Ni 2§ Pb2§ Sb3§ Bi3§ Hg2§, Th4+, Sn2§, Zn 2§, 175

B. S. CHANDRAVANSHI, TEMAM JUHAR: EXTRACTION AND SPEC1ROPHOTOMETRIC TaMe 1 Effect of foreign ions on the determination of uranium with PCHA; uranium: 0.5 mg, pH: 6 Ion

Tolerance limit, mg

Ion

Tolerance limit, mg

K+ Na+ Sr2+ Ba2+ Ca2+

1000 575 550 350 250

Sn2+ Zn2+ AI3+ IVln2+ Cr3+

1 1 1 1 0.5

Li+ Mg 2+

175 150

Co2+ Cu2+

0.5 0.5

Be2+

45

~o~

1550

Cd2+

10

SCN-

1450

T1+ La3+ Ni 2+ Pb2+ Sb 3+ Bi3+ Hg 2+ Th4*

10 5 5 5 4 4 2 2

CH3COOSO42C1B2O2AsO3PO43FC202-

750 600 450 10 2 I 0.5 0.5

Table 2 Determination of urat-dv_mha presence of interfering ions with PCHA; uranium added: 500 ~tg

I~ ZrfIV) V(V) Ti(IV)

Amount added,b ~tg

Uraaim~ found,c btg

730 400 380

495 508 493

l~ W(VI) Mo(VI) Fe(IIl)

Amount added,b~tg

Umnium found,e ~g

1400 800 450

491 492 509

aIons removed by prior extmc:ion. bAmotmt greater than this was not removed and caused interference. eUranium found after the removal of interfering ions. dIons removed by prior precipitation.

AP § Mn2+, nitrate, thiocyanate, acetate, sulphate, chloride, borate, arsenate and phosphate do not interfere in the determination of uranium(VI) with PCHA. Titanium(IV), vanadium(V), zirconium(IV), molybdenum(VI), tungstenCr and iron(IlI) react with PCHA and interfere in the determination of uranium(VI) by the 176

B. S. CHANDRAVANSHI,TEMAMJUHAR:EXTRACTIONANDSPECTROPHOTOMETRIC recommended procedure. However, the interferences due to titanium(IV), vanadium(V) and zirconium(IV) were eliminated by extraction of these ions with 25 ml (10 + 10 + + 5 ml) of 0.01M solution of PCHA in chloroform from 8, 4 and 2M hydrochloric acid solutions, respectively, prior to the extraction of uranium(VI). Uranium(VI) remained in the aqueous phase under these conditions and it was extracted and determined by the general procedure after raising the pH of the aqueous phase to 6. The interferences due to molybdenum(VI), tungsten(~,q) and iron(Ill) were also eliminated by precipitation of these ions with excess amount (5 ml of 0.05M) of PCHA in ethanol from 1, 0.5 and 0.01M hydrochloric acid solutions, respectively, followed by filtration, centrifugation or extraction into benzene prior to the extraction of uranium(VI). Uranium(VI) remained in the aqueous phase under these conditions and it was extracted and determined by the general procedure after raising the pH of the aqueous phase to 6. The results are given in Table 2. Successive extraction and determination of vanadium(V), iron(Ill) and uranium(Vl) Vanadium(V) reacts with PCHA to form a bluish-violet coloured complex quantitatively extractable into chloroform from 4-8M hydrochloric acid solutions. 11 Iron(Ill) and uranium(VI) remain in the aqueous phase under these conditions. Iron(III) reacts with the ethanolic solution of PCHA to form an orange coloured complex insoluble in aqueous ethanol. The Fe(Ill)-PCHA complex is quantitatively extractable into benzene 16 at pH 1.5--2.0. Uranium(VI) remains in the aqueous phase under these conditions. Uranium(VI) reacts with the PCHA to form an yellow--orange coloured complex insoluble in aqueous ethanol at pH 5.5-8.5. The U(VI)-PCHA complex is quantitatively extracted by ethyl acetate. Thus it is po~ible to separate the three metal ions from each other by solvent extraction at different pH and determine the metal ions in the organic phase spectrophotomelrically. Hence the proposed method has been extended for the successive extraction and spectrophotometric determination of vanadiumfV), iron(Ill) and uranium(VI). An aliquot of the solution containing 20-200 I.tg of vanadium(V), 10-200 [tg of iron(Ill) and 59--1000 ~g of uranium(VI), was transferred into a 100-ml separatory funnel. Sufficient quantities of distilled water and concentrated hydrochloric acid were added to maintain the acidity to 4M and volume of the aqueous phase to 10 ml. A I0 ml aliquot of 0.005M solution of PCHA in chloroform was added, the mixture was shaken vigorously for 2 minutes, and allowed to separate the two phases. The organic phase was collected in a 25 ml volumetric flask after drying over anhydrous sodium sulphate and diluted to volume with chloroform. The absorbance of the coloured chloroform extract was measured at 540 nm against the chloroform as blank. For calibration, 0.2, 0.5, 1.0, 1.5 and 2.0 ml of the standard solution (100 ~tg V. m1-1) were used throughout 177

B. S. CHANDRAVANSHI,TEMAMJUHAR: EXTRACTIONAND SPECTROPHOTOMETR/C Table3 Determinationof vanadium,iron,and uraniumby successive extractionwithPCHA Amount taken, mg Vanadium

Iron

200 100 50

200 100 50

Amountfound,* mg

Uranium Vanadium 100 500 1000

196 101 49

Iron

Uranium

197 102 51

102 496 991

*Mean of three determinations.

the above procedure. The concenlration of vanadium(v) in the sample was deduced from the calibration curve. The aqueous phase left after extraction of vanadium(V) was transfen'ed into a 50 ml beaker and 5 ml of 0.01M solution of PCHA in ethanol was added to it. The pH of the solution was adjusted to 1.5-2.0 with 6M ammonia solution and the solution was diluted to 20 ml with distilled water. The solution was transferred to a 100 ml separatory funnel and the beaker was washed with 25 ml of benzene. The washings were added to the funnel, the mixture was shaken vigorously for 2 minutes, and allowed to separate the two phases. The organic phase was collected in a 25 ml volumetric flask after drying over anhydrous sodium sulphate and diluted to volume with benzene. The absorbance of the coloured benzene extract was measured at 440 nm against the reagent blank. For calibration, 0.1, 0.5, 1.0, 1.5 and 2.9 m! of the standard solution (100 pg Fe- m1-1) were used throughout the above procedure. The concentration of iron(III) in the sample was deduced from the calibration curve. The aqueous phase left after extraction of iron(fit) was transferred into a 100 ml beaker and 5 ml of 0.01M solution of PCHA in ethanol was added to it. The solution was diluted to 40 ml with distilled water and the pH of the solution was adjusted to 6 using 1M ammonia solution or 1M hych-ochloric acid solution and then proceeded as described in the general procedure for the extraction and determination of uranium(VI). The results are given in Table 3 .

Determination of uranium in synthetic matrices and potable water The analytical potential of the method was tested by applying it to the determination of uranium in synthetic malrices with a similar composition of leaching solutions resulting from the treatment of ores, minerals, and of water samples. The results are given in Table 4. 178

B, S. CHANDRAVANSHI, TEMAM JUHAR: EXTRAC'fTON AND SPECTROPHOTOMETRIC ~h

8 1.28

0.8 0.6 O.4

0.2

O0

10

20

30

40

Uranium concentration i Pg" mt -1 Fig. 2. Calibration curve for determination of uranium in potable water Table 4 Determination of uranium in synthetic matrices Matrix

Composition

Uranium found,* mg

A

0.5 m g U(VI) + 500 mg Na(I) + 500 mg K(I) + + 100 mg Li(I) + 100 mg Ca(II) + 100 mg Mg(II) + + 100 mg Sr(II) + I00 mg Ba(II) + 100 mg Be(lI)

0.502 :t: 0.008

B

0.5 mg U(VI) + 2 mg La(lll) + 2 mg Th(IV) + + 2 mg pb(lI) + 2 mg Cd(ll) + 2 mg Hg(II)

0.505 + 0.009

*Mean _+95% confidence limit of three determinations.

Table 5 Determination of uranium in potable water Uranium added, I . t g 100 250 500 750 1000

Uranium found,* ~tg 101 + 247 + 495 + 758 + 992 +

2 5 9 11 13

*Mean + 95% confidence limit of three determinations.

179

B. S. CHANDRAVANSHI, TEMAM JUHAR: EXTRACTION AND SPECTROPHOTOMETRIC

The method was also applied to the determination of uranium in potable water. The traces of iron in the water was removed by precipitation and extraction prior to the determination of uranium. The uranium in potable water was determined from the calibration curve (Fig. 2). The results are given in Table 5. The results of the analyses indicate that the method is precise and reliable for the determination of uranium in diverse samples.

Conclusion The proposed method provides a precise and reliable means of uranium determination by solvent extraction and spectrophotometry. The method is free from the rigid conlrol of experimental conditions. Hence, the method can serve as an alternative to other means of analysis for the precise and selective determination of uranium from the hydrometaUurgical treatment of ores, minerals, and of water samples. The method can also be used for the successive extraction and spectrophotometric determination of vanadium, iron and uranium in diverse samples.

The authors are grateful to the Chaimaan, Department of Chemistry, Addis Ababa University, Addis Ababa, Ethiopia, for providing laboratory facilities

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B. S. CHANDRAVANSHI, TEMAM JUHAR: EXTRACTION AND SPECTROPHOTOMETRIC 17. B. S. CHANDRAVANSHI, A. YENESEW, Z. KEBEDE, Anal. Chim. Acta, 172 (1985) 175. 18. S. AFEWORKI, B. S. CHANDRAVANSHI, Mikrochim. Aeta, II (1987) 143. t9. S. AFEWORKI, B. S. CHANDRAVANSHI, Bull. Chem. Soc. Ethiop., 1 (1987) 1. 20. A. FURA, B. S. CHANDRAVANSHI, Ann. China. (Rome), 78 (1988) 335. 21. J. BASSETI', R. C. DENNEY, G. H. JEFFERY, J. MENDHAM, Vogel's Textbook of Quantitative Inorganic Analysis, 4th ed., Longmans, London, 1978. 22. A. K. MAJUMDAR, N-Benzoylphenylhydroxylamine and its Analogues, Pergamon, Oxford, 1972. 23. P. JOB, Ann. Chim. (Paris), 9 (1928) 113. 24. ANALYTICAL METHODS COMMITTEE, Analyst, 112 (1987) 199. 25. E. B. SANDELL, Colorimetric Determination of Traces of Metals, 3rd ed., Intersr New York, 1959.

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