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Determination of uranium(VI) in seawater by ion-exchanger phase absorptiometry with arsenazo III
Tolanta, Vol. 39, No. 5, pp. 523-521, Printed in Great Britain
1992
0039-9140/92 S5.00 + 0.00 Pergamon Press plc
DETERMINATION OF URANIUM(V1) IN SEAWATER BY ION-EXCHANGER PHASE ABSORPTIOMETRY WITH ARSENAZO III TOSHIONAKASHIMA* Department of Chemistry, Faculty of Education, Oita University, Dan-noharu 700, Oita 870-I 1, Japan KAZUHISAYOSHIMURA Chemistry Laboratory, College of General Education, Kyushu University, Ropponmatsu, Chuo-ku, Fukuoka 810, Japan TOMITSUGUTAKETATSU Kurume Institute of Technology, Kamitsu, Kurume 830, Japan (Received
16 July 1991. Revised 1 October 1991. Accepled 2 October 199 1)
Summary-An ion-exchange phase absorptiometric method with Arsenazo III has been developed for the determination of uranium(W). A flow cell with 0.1 ml of anion exchange resin was employed to achieve a detection limit for uranium of 0.16 pg/l. in 100 ml of a seawater sample. The sensitivity is about 300 times higher than for corresponding solution spectrophotometry.
For the determination of uranium in seawater, it is usually necessary to separate and/or concentrate uranium from a solution containing large amounts of background electrolytes. Among a number of preconcentration techniques reported, methods using ion-exchange or chelating resins ‘-8 have sometimes proven to be more effective. However, the uranium species sorbed on the solid phase has usually been eluted for succeeding measurement, leading to an undesirable dilution of the uranium. Ion-exchanger phase absorptiometryy-‘2 based on the direct absorptiometric measurements of adsorbed analytes on ion exchange resins, has been conveniently used for the determination of trace amounts of components present in natural waters: this method does not need any such preconcentration and/or undesirable dilution by elution. Neodymium” and cobalt(II)‘2 have been successfully determined recently by this method, the sensitivities being 260 and 1000 times higher than conventional solution
spectrophotometries, respectively. In this study ion-exchanger phase absorptiometry has been applied to uranium determination using Arsenazo III as a colouring agent.r3 The present method is highly sensitive and directly applicable to uranium analysis of sea-water without any other means of preconcentration. EXPERIMENTAL
Reagents
Analytical grade Bio-Rad AG l-X2 anion exchange resin (100-200 mesh, Cl- form) was used in the original wet state. EDTA solution (2 x 10m3M) was prepared by dissolving an appropriate amount of disodium salt of EDTA (Dojindo, Japan). Arsenazo III was purchased from Dojindo, Japan. All chemicals were of analytical grade. Apparatus
The attenuancest of the ion exchanger were measured with a double-beam Nippon Bunko spectrophotometer, Model UVIDEC-320 and a perforated metal disc with an attenuance of 2.0 was used to balance the light intensities. A small-volume flow cell was supplied by Nippon Quartz Glass Co. This cell was blacksided with a 0.5~cm path length and 0.035-ml
*Author for correspondence. tin ion-exchanger phase absorptiometry, the contribution of light scattering in a measurement is considerable, so the term “attenuance” is preferable to “absorbance”, although attemtance has essentially the same meaning as absorbance. 523
Tmo
524 to disposal syringe
NAKASHIMA et al.
km sxchqer
Intotand outM
C, , and 0.13 moles of sodium chloride was adjusted to 3.3 with formate. Then 5 x 10m6 moles of Arsenazo III and 2 x 10e6 moles of EDTA were added and the total solution volume was adjusted to 250 ml. After a short mixing period, 0.8 1 ml of Bio-Rad AG l-X2 was added. After one hour equilibration, the solution and the ion-exchanger were separated and the attenuance, AA,, of the ion-exchanger phase was measured. To the filtrate a further 0.81 ml of Bio-Rad AG t-X2 was added and the solution was stirred for one hour. The exchanger attenuance, AA*, was again measured and the distribution ratio, D, was calculated from mmoles of uranium sorbed per ml of exchanger D= mmoles of uranium per ml of solution = (C, - C,*AAJAA,)/0.81 (C, *AA2/AA,)/250
Fig. 1. Flow cell for ion-exchanger phase absorptiometry. Two transparent fused silica glass plates (0.1 cm in thickness), A, are pasted on both sides of black cell glass, B, of OS-cm thickness.
cell volume. The cell was fitted with a polypropylene filter tip, as shown in Fig. 1, to retain the ion exchange resin. Procedure for the determination of uranium(VZ) in seawater To 100 ml of seawater sample, 1 ml of 0.5M formate buffer solution (pH 3.2), 2 ml of 0.002% Arsenazo III, 1 ml of 2 x 10e3M EDTA were added. Then, 0.10 ml of anion exchanger slurry was collected with suction in a fused-silica tube (0.15 cm i.d,, 6.0 cm in length), fitted on one side with a polypropylene filter tip and connected with a disposable syringe [Fig. 2(A)], and then poured into the solution with a small amount of water. The mixture was stirred for 60 min at room temperature. After settling, the resin beads were collected in the fused-silica tube mentioned above and transferred to a flow cell with a syringe [Fig. 2(B)]. The attenuances at 665 and 800 nm were measured against air as reference. The difference in attenuances (A.A) at 665 nm and 800 nm, was used for the determination of uranium by means of a calibration graph. Distribution measurement The pH of about 230 ml of a water sample containing 1.05 x 10c6 moles of uranium(VI),
= 309(AA, /AA2 - 1)
(1)
AA, and AA, were found to be 0.87 and 0.024, respectively, giving 1.1 x 104 as the value of the distribution ratio.
RESULTS AND DI!SCUS!SION
Enhancement of sensitivity with a flow cell of small volume For an ion-exchanger layer prepared with V ml of ion exchanger which was previously eq~librated with P” ml of solution confining a sample component at CJt4, the net absorbance, A, of the ion exchanger phase due to the sorbed coloured complex can be expressed as: A=K,,lr
1 v 1+ l/P-D
(2)
where E and 1 are the molar absorptivity of the complex and the light path length in cm in the ion exchanger phase, respectively. The distribution ratio of the complex between the solid and solution phase is represented by D. This equation means that, if D is sufficiently large as in the case of the present system, higher sensitivity can be attained by using a large amount of sample volume and/or a small amount of ion exchanger within the limits of the resin capacity: the resin capacity measured by acid-base titration was 0.0554 4 0.0003 meq for 0.1 ml of the ion exchanger aliquoted with the device mentioned below. Therefore, a small amount of ion exchanger slurry could increase
Determination of uranium(VI) (d)
(A)
525
in seawater
(b)
(i) -
-
i.d. km)
(B)
o.d. km)
(a)
0.15
0.35
(c)
0.10
0.30
frame fithngs for PTFE
tubing
-
(ii)
l-l Zcm
to disposal syinge
(c) o.d., 0.2 cm
ia) i.d., 0.3cm
to disposal syringe
2cm in length
Fig. 2. Capillary fused silica tube; (A) for measuring a constant amount of ion-exchanger slurry; (B) for collecting colour-developed ion-exchanger beads; (a): fused silica tube; (b): silicone rubber tube, (c): PTFE tube; (d): polypropylene filter tip
the sensitivity if it could be handled reproducibly. To achieve this reproducibility, the fusedsilica tube apparatus shown in Fig. 2(A) was designed. Ion exchanger was collected into the tube from the left (i) and water was discarded with a disposal syringe. The ion exchange resin could then be added to a sample with a small amount of water. After equilibration, the ion exchanger was collected from the inlet (ii) and then introduced into the flow cell by connecting the frame fitting [Fig. 2(B)] to nut (i) of the flow cell. The relative standard deviation of the resin amounts collected by using this resin aliquoting method was 0.5% for 6 repeated measurements.
in anion exchanger phase should be done at the longest wavelength possible. Therefore 665 nm was selected as the wavelength for the determination of uranium(V1).
Optimization of conditions Absorption spectra in the anion exchanger. Arsenazo III forms a 1: 1 complex with uranyl ion in the pH range 1.5-2.0 in an aqueous solution. The purplish red colour complex has a maximum absorption at about 650 nm with a molar absorptivity of 4.4 x lo4 l.mole-‘. cm-’ in the presence of a large excess of Arsenazo III.13 This complex is easily sorbed on an anion exchanger, together with free Arsenazo III, because of its high anionic charge. Absorption spectra of the complex in an anion exchanger and solution are shown in Fig. 3. In order to make the contribution of free Arsenazo III low, the attenuance measurements of the complex
Wavelength, nm
Fig. 3. Absorption spectra of uranium(W)-Arsenaxo III complex in anion exchanger and aqueous solution. (a) Spectrum in anion exchanger phase: U(W): 5.25 x IO-‘&f; Arsenaxo III: 1 x lo-‘M; resin: 0.80 g; solution volume: 250 ml, pH 3.3. (b) Spectrum in solution: U(W): 5.25 x 10-5mmM;Arsenazo III: 1.0 x 10e5 mM; pH 3.3. (c) Spectrum in anion exchanger phase (Arsenaxo III in excess): U(W): 5 pg; Arsenaxo III: 1.3 x lo-’ mmoles; resin: 0.10 g; solution volume: 250 ml; pH 3.3.
Tos~lo NAKASHIMA et al.
526
06
I
06
-
d
60
Time, mh
Fig. 4. Effect of colour development on stirring time. Sample: 100 ml, pH 3.2,0.25 pg U(VI), 0.05 moles of NaCI; resin: 0.10 ml; cell length: 0.5 cm. -_O-_O20”; --O--O-40”; --A--A-56”.
Stirring
time and temperature
dependence.
The dependence of colour development on stirring time at different temperatures is shown in Fig. 4. Attenuance reached a constant value within one hour of stirring and colour development did not accelerate appreciably at higher temperatures. Accordingly, the solution-ion exchanger mixture was stirred for one hour at room temperature. The distribution ratio of the complex was 1.1 x 104, and 92% of the uranium in the sample solution could be recovered. pH Dependence of colour development in anion exchanger. The UO,-Arsenazo III complex is
strongly sorbed on an anion-exchanger in the pH range 2.5-5.0. Free Arsenazo III is also sorbed in this pH range but the reagent has a low constant absorption at 655 nm, as shown in Fig. 5. Therefore the pH of the sample solution was adjusted to 3-4. Dependence on sodium chloride concentration.
The effect of sodium chloride concentration on colour development is shown in Fig. 6. AA gradually increased with sodium chloride concentration up to 0.4M and reached a constant value at 0.6M, while AA for the reagent blank is almost constant at 0.25. The apparent AA increase may be due to contraction of the ion-exchanger and the same phenomenon was
d
0
I 04
I 06
I 06
M
Fig. 6. Effect of sodium chloride concentration. Sample: 40 ml, pH 3.2,0.4 pg U(W), 2.1 x lo-* moles of Arsenazo III; resin: 0.10 ml; stirring time: 60 min; cell length: 0.5 cm. O---O; AA for sample. l ---a; AA for reagent blank.
observed in the determination of cobalt by ion-exchanger phase absorptiometry.‘2 Consequently, uranium in seawater may be conveniently analysed without dilution of the salt matrix because sensitivity at 0.6M sodium chloride concentration is 40% higher than that in the absence of sodium chloride. Table 1. Effects of foreign ions on the determination uranium(V1)
Mg Ca Fe(II1) t
La(II1) Th(IV) EDTA
Fig. 5. Dependence of colour development on pH. Sample: 40 ml, pH 3.2,0.8 pg U(W), 2.1 x lo-* moles of Arsenazo III; resin: 0.10 ml; stirring time: 30 min; cell length: 0.5 cm.
I 02
[NaCa,
Type
06
I
Foreign ion amount, mgll. 1300 1100 1 10 0.01 0.01 7.5 x lo-St 5.0 x lo-‘? 1.0 x lo-37
0.556 0.535 0.536 0.029 0.467 0.562 0.544 0.551 0.575
of
U found, Mgll.
Relative error, %
10.2 9.8 9.8 0.5 6.7 10.3 10.0 10.0 10.5
+2 -2 -2 -95 -33 +3 0 0 +5
Solution: 100 ml, 1 peg U(VI), pH 3.2 (formate, 5.2 x lo-* moles of Arsenazo III, 0.055 moles of NaCl; Resin, Bio-Rad AG 1-X2 (Cl- form, 100-200 mesh), 0.1 ml; Stirring time, 60 min. *A = AA - AA (for the blank). TM.
Determination
of uranium(VI) in seawater
527
to about 300 times enhancement by the present method. The detection limit was measured in a matrix of a loo-ml blank solution containing 0.055 moles of sodium chloride. For 6 determinations, AA was 0.315 &-0.004. The detection limit (the concentration corresponding to an attenuance equal to twice the magnitude of the error) was 0.29 pg/l. Determination
Fig. 7. Calibration curve for the determination of U(V1). Sample: 100 ml, pH 3.2, 0.05 moles of NaCl, 2.1 x IO-* moles of Arsenazo III; resin: 0.10 ml; stirring time: 60 min; cell length: 0.5 cm.
E#ect of foreign
ions
Table 1 shows the effect of foreign ions on the determination of uranium. Below the concentration levels present in the seawater no heavy metals except uranium(V1) can complex with Arsenazo III in these experimental conditions, therefore no coexisting ions interfered. All the sample solutions contained 2 x 10P5M EDTA in order to mask foreign ions such as iron(II1) present in seawater. Further increase in EDTA concentration at least up to 5.0 x 10e4M did not interfere with the uranium determination. Therefore this procedure may also be applied to other solution systems which contain foreign ions up to 5 x 10-4M. Calibration
curve
A calibration curve for uranium(V1) is shown in Fig. 7. The linearity holds up to 10 pg/l. of uranium, but the curve became convex at more than 10 ,ug/l. of uranium(W). Sensitivity
and detection
limit
The uranium concentrations giving a final absorbance of 0.1 were 7.7 x 10e9M for the present method and 2.3 x 10e6M for conventional solution absorptiometry,‘3 corresponding
of uranium(VI)
in seawater
Seawater samples were analysed by the standard-addition method in order to check the recovery of added uranium. As shown in Table 2, the recovery of the spiked uranium was quantitative. In coastal seawater from Mitoma, Fukuoka (open sea), the uranium content was found to be 3.5 + 0.3 pg/l. The content was in accord with those reported previously.3*4 Recently, ion-exchanger phase absorptiometry has been extended to flow analysis.‘4*‘5 Using this method, trace levels of a target element can be determined in water only in fairly small volumes. However, the present system could not be applied to flow analysis, because the uranium-Arsenazo III complex is difficult to be desorbed from the ion exchanger.
REFERENCES 1. J. Korkisch and I. Steffan, Anal. Chim. Acta, 1975, 77, 312. 2. R. J. N. Brits and M. C. B. Smit, Anal. Chem., 1977,49, 67. 3. R. Kuroda, K. Oguma, N. Mukai and M. Iwamoto, Talanta, 1987, 34, 433. 4. R. Kuroda, Y. Hayashibe, K. Oguma and K. Kurosu, Z. Anal. Chem., 1989, 335, 404. 5. E. S. Gladney, R. J. Peters and D. R. Perrin, Anal. Gem., 1983, 55, 976. 6. A. A. Prange, A. Knochel and W. Michaelis, Anal. Chim. Acta, 1985, 172, 79. 7. K. H. Lieser and B. Gleitsmann, Z. Anal. Chem., 1982,
313, 289. 8. M. Ckhsenkuhn-Petropulu and G. Parissakis, ibid., 1985, 321, 581. 9. K. Yoshimura, H. Waki and S. Ohashi, Tulanta, 1976, 23,449.
Table 2. The standard addition method for determination of uranium(VI) in sea water U(YI)added,rcn
0
0
0.63
1.25
1.88
A+ U(V1) found, pg
0.185 0.96
0.160 0.75
0.302 1.44
0.410 2.10
0.565 2.80
Sample volume: 100 ml. Resin: 0.10 ml. *A = AA -AA (for the blank). TAL 39,~F
IO. K. Yoshimura and H. Waki, ibid., 1985, 32, 345. 1 I. K. Yoshimura and T. Teketatsu, Z. Anal. Chem., 1987, 328, 553. 12. T. Nakashima, K. Yoshimura and H. Waki, Tulanta, 1990, 37, 735. 13. H. Gnishi and Y. Toita, Bunseki Kagaku,
1969, 18, 592. 14. K. Yoshimura, Anal. Chem., 1987, 59, 2922. 15. K. Yoshimura, S. Nawata and G. Kura, Analyst, 1990, 115, 843.
1992
0039-9140/92 S5.00 + 0.00 Pergamon Press plc
DETERMINATION OF URANIUM(V1) IN SEAWATER BY ION-EXCHANGER PHASE ABSORPTIOMETRY WITH ARSENAZO III TOSHIONAKASHIMA* Department of Chemistry, Faculty of Education, Oita University, Dan-noharu 700, Oita 870-I 1, Japan KAZUHISAYOSHIMURA Chemistry Laboratory, College of General Education, Kyushu University, Ropponmatsu, Chuo-ku, Fukuoka 810, Japan TOMITSUGUTAKETATSU Kurume Institute of Technology, Kamitsu, Kurume 830, Japan (Received
16 July 1991. Revised 1 October 1991. Accepled 2 October 199 1)
Summary-An ion-exchange phase absorptiometric method with Arsenazo III has been developed for the determination of uranium(W). A flow cell with 0.1 ml of anion exchange resin was employed to achieve a detection limit for uranium of 0.16 pg/l. in 100 ml of a seawater sample. The sensitivity is about 300 times higher than for corresponding solution spectrophotometry.
For the determination of uranium in seawater, it is usually necessary to separate and/or concentrate uranium from a solution containing large amounts of background electrolytes. Among a number of preconcentration techniques reported, methods using ion-exchange or chelating resins ‘-8 have sometimes proven to be more effective. However, the uranium species sorbed on the solid phase has usually been eluted for succeeding measurement, leading to an undesirable dilution of the uranium. Ion-exchanger phase absorptiometryy-‘2 based on the direct absorptiometric measurements of adsorbed analytes on ion exchange resins, has been conveniently used for the determination of trace amounts of components present in natural waters: this method does not need any such preconcentration and/or undesirable dilution by elution. Neodymium” and cobalt(II)‘2 have been successfully determined recently by this method, the sensitivities being 260 and 1000 times higher than conventional solution
spectrophotometries, respectively. In this study ion-exchanger phase absorptiometry has been applied to uranium determination using Arsenazo III as a colouring agent.r3 The present method is highly sensitive and directly applicable to uranium analysis of sea-water without any other means of preconcentration. EXPERIMENTAL
Reagents
Analytical grade Bio-Rad AG l-X2 anion exchange resin (100-200 mesh, Cl- form) was used in the original wet state. EDTA solution (2 x 10m3M) was prepared by dissolving an appropriate amount of disodium salt of EDTA (Dojindo, Japan). Arsenazo III was purchased from Dojindo, Japan. All chemicals were of analytical grade. Apparatus
The attenuancest of the ion exchanger were measured with a double-beam Nippon Bunko spectrophotometer, Model UVIDEC-320 and a perforated metal disc with an attenuance of 2.0 was used to balance the light intensities. A small-volume flow cell was supplied by Nippon Quartz Glass Co. This cell was blacksided with a 0.5~cm path length and 0.035-ml
*Author for correspondence. tin ion-exchanger phase absorptiometry, the contribution of light scattering in a measurement is considerable, so the term “attenuance” is preferable to “absorbance”, although attemtance has essentially the same meaning as absorbance. 523
Tmo
524 to disposal syringe
NAKASHIMA et al.
km sxchqer
Intotand outM
C, , and 0.13 moles of sodium chloride was adjusted to 3.3 with formate. Then 5 x 10m6 moles of Arsenazo III and 2 x 10e6 moles of EDTA were added and the total solution volume was adjusted to 250 ml. After a short mixing period, 0.8 1 ml of Bio-Rad AG l-X2 was added. After one hour equilibration, the solution and the ion-exchanger were separated and the attenuance, AA,, of the ion-exchanger phase was measured. To the filtrate a further 0.81 ml of Bio-Rad AG t-X2 was added and the solution was stirred for one hour. The exchanger attenuance, AA*, was again measured and the distribution ratio, D, was calculated from mmoles of uranium sorbed per ml of exchanger D= mmoles of uranium per ml of solution = (C, - C,*AAJAA,)/0.81 (C, *AA2/AA,)/250
Fig. 1. Flow cell for ion-exchanger phase absorptiometry. Two transparent fused silica glass plates (0.1 cm in thickness), A, are pasted on both sides of black cell glass, B, of OS-cm thickness.
cell volume. The cell was fitted with a polypropylene filter tip, as shown in Fig. 1, to retain the ion exchange resin. Procedure for the determination of uranium(VZ) in seawater To 100 ml of seawater sample, 1 ml of 0.5M formate buffer solution (pH 3.2), 2 ml of 0.002% Arsenazo III, 1 ml of 2 x 10e3M EDTA were added. Then, 0.10 ml of anion exchanger slurry was collected with suction in a fused-silica tube (0.15 cm i.d,, 6.0 cm in length), fitted on one side with a polypropylene filter tip and connected with a disposable syringe [Fig. 2(A)], and then poured into the solution with a small amount of water. The mixture was stirred for 60 min at room temperature. After settling, the resin beads were collected in the fused-silica tube mentioned above and transferred to a flow cell with a syringe [Fig. 2(B)]. The attenuances at 665 and 800 nm were measured against air as reference. The difference in attenuances (A.A) at 665 nm and 800 nm, was used for the determination of uranium by means of a calibration graph. Distribution measurement The pH of about 230 ml of a water sample containing 1.05 x 10c6 moles of uranium(VI),
= 309(AA, /AA2 - 1)
(1)
AA, and AA, were found to be 0.87 and 0.024, respectively, giving 1.1 x 104 as the value of the distribution ratio.
RESULTS AND DI!SCUS!SION
Enhancement of sensitivity with a flow cell of small volume For an ion-exchanger layer prepared with V ml of ion exchanger which was previously eq~librated with P” ml of solution confining a sample component at CJt4, the net absorbance, A, of the ion exchanger phase due to the sorbed coloured complex can be expressed as: A=K,,lr
1 v 1+ l/P-D
(2)
where E and 1 are the molar absorptivity of the complex and the light path length in cm in the ion exchanger phase, respectively. The distribution ratio of the complex between the solid and solution phase is represented by D. This equation means that, if D is sufficiently large as in the case of the present system, higher sensitivity can be attained by using a large amount of sample volume and/or a small amount of ion exchanger within the limits of the resin capacity: the resin capacity measured by acid-base titration was 0.0554 4 0.0003 meq for 0.1 ml of the ion exchanger aliquoted with the device mentioned below. Therefore, a small amount of ion exchanger slurry could increase
Determination of uranium(VI) (d)
(A)
525
in seawater
(b)
(i) -
-
i.d. km)
(B)
o.d. km)
(a)
0.15
0.35
(c)
0.10
0.30
frame fithngs for PTFE
tubing
-
(ii)
l-l Zcm
to disposal syinge
(c) o.d., 0.2 cm
ia) i.d., 0.3cm
to disposal syringe
2cm in length
Fig. 2. Capillary fused silica tube; (A) for measuring a constant amount of ion-exchanger slurry; (B) for collecting colour-developed ion-exchanger beads; (a): fused silica tube; (b): silicone rubber tube, (c): PTFE tube; (d): polypropylene filter tip
the sensitivity if it could be handled reproducibly. To achieve this reproducibility, the fusedsilica tube apparatus shown in Fig. 2(A) was designed. Ion exchanger was collected into the tube from the left (i) and water was discarded with a disposal syringe. The ion exchange resin could then be added to a sample with a small amount of water. After equilibration, the ion exchanger was collected from the inlet (ii) and then introduced into the flow cell by connecting the frame fitting [Fig. 2(B)] to nut (i) of the flow cell. The relative standard deviation of the resin amounts collected by using this resin aliquoting method was 0.5% for 6 repeated measurements.
in anion exchanger phase should be done at the longest wavelength possible. Therefore 665 nm was selected as the wavelength for the determination of uranium(V1).
Optimization of conditions Absorption spectra in the anion exchanger. Arsenazo III forms a 1: 1 complex with uranyl ion in the pH range 1.5-2.0 in an aqueous solution. The purplish red colour complex has a maximum absorption at about 650 nm with a molar absorptivity of 4.4 x lo4 l.mole-‘. cm-’ in the presence of a large excess of Arsenazo III.13 This complex is easily sorbed on an anion exchanger, together with free Arsenazo III, because of its high anionic charge. Absorption spectra of the complex in an anion exchanger and solution are shown in Fig. 3. In order to make the contribution of free Arsenazo III low, the attenuance measurements of the complex
Wavelength, nm
Fig. 3. Absorption spectra of uranium(W)-Arsenaxo III complex in anion exchanger and aqueous solution. (a) Spectrum in anion exchanger phase: U(W): 5.25 x IO-‘&f; Arsenaxo III: 1 x lo-‘M; resin: 0.80 g; solution volume: 250 ml, pH 3.3. (b) Spectrum in solution: U(W): 5.25 x 10-5mmM;Arsenazo III: 1.0 x 10e5 mM; pH 3.3. (c) Spectrum in anion exchanger phase (Arsenaxo III in excess): U(W): 5 pg; Arsenaxo III: 1.3 x lo-’ mmoles; resin: 0.10 g; solution volume: 250 ml; pH 3.3.
Tos~lo NAKASHIMA et al.
526
06
I
06
-
d
60
Time, mh
Fig. 4. Effect of colour development on stirring time. Sample: 100 ml, pH 3.2,0.25 pg U(VI), 0.05 moles of NaCI; resin: 0.10 ml; cell length: 0.5 cm. -_O-_O20”; --O--O-40”; --A--A-56”.
Stirring
time and temperature
dependence.
The dependence of colour development on stirring time at different temperatures is shown in Fig. 4. Attenuance reached a constant value within one hour of stirring and colour development did not accelerate appreciably at higher temperatures. Accordingly, the solution-ion exchanger mixture was stirred for one hour at room temperature. The distribution ratio of the complex was 1.1 x 104, and 92% of the uranium in the sample solution could be recovered. pH Dependence of colour development in anion exchanger. The UO,-Arsenazo III complex is
strongly sorbed on an anion-exchanger in the pH range 2.5-5.0. Free Arsenazo III is also sorbed in this pH range but the reagent has a low constant absorption at 655 nm, as shown in Fig. 5. Therefore the pH of the sample solution was adjusted to 3-4. Dependence on sodium chloride concentration.
The effect of sodium chloride concentration on colour development is shown in Fig. 6. AA gradually increased with sodium chloride concentration up to 0.4M and reached a constant value at 0.6M, while AA for the reagent blank is almost constant at 0.25. The apparent AA increase may be due to contraction of the ion-exchanger and the same phenomenon was
d
0
I 04
I 06
I 06
M
Fig. 6. Effect of sodium chloride concentration. Sample: 40 ml, pH 3.2,0.4 pg U(W), 2.1 x lo-* moles of Arsenazo III; resin: 0.10 ml; stirring time: 60 min; cell length: 0.5 cm. O---O; AA for sample. l ---a; AA for reagent blank.
observed in the determination of cobalt by ion-exchanger phase absorptiometry.‘2 Consequently, uranium in seawater may be conveniently analysed without dilution of the salt matrix because sensitivity at 0.6M sodium chloride concentration is 40% higher than that in the absence of sodium chloride. Table 1. Effects of foreign ions on the determination uranium(V1)
Mg Ca Fe(II1) t
La(II1) Th(IV) EDTA
Fig. 5. Dependence of colour development on pH. Sample: 40 ml, pH 3.2,0.8 pg U(W), 2.1 x lo-* moles of Arsenazo III; resin: 0.10 ml; stirring time: 30 min; cell length: 0.5 cm.
I 02
[NaCa,
Type
06
I
Foreign ion amount, mgll. 1300 1100 1 10 0.01 0.01 7.5 x lo-St 5.0 x lo-‘? 1.0 x lo-37
0.556 0.535 0.536 0.029 0.467 0.562 0.544 0.551 0.575
of
U found, Mgll.
Relative error, %
10.2 9.8 9.8 0.5 6.7 10.3 10.0 10.0 10.5
+2 -2 -2 -95 -33 +3 0 0 +5
Solution: 100 ml, 1 peg U(VI), pH 3.2 (formate, 5.2 x lo-* moles of Arsenazo III, 0.055 moles of NaCl; Resin, Bio-Rad AG 1-X2 (Cl- form, 100-200 mesh), 0.1 ml; Stirring time, 60 min. *A = AA - AA (for the blank). TM.
Determination
of uranium(VI) in seawater
527
to about 300 times enhancement by the present method. The detection limit was measured in a matrix of a loo-ml blank solution containing 0.055 moles of sodium chloride. For 6 determinations, AA was 0.315 &-0.004. The detection limit (the concentration corresponding to an attenuance equal to twice the magnitude of the error) was 0.29 pg/l. Determination
Fig. 7. Calibration curve for the determination of U(V1). Sample: 100 ml, pH 3.2, 0.05 moles of NaCl, 2.1 x IO-* moles of Arsenazo III; resin: 0.10 ml; stirring time: 60 min; cell length: 0.5 cm.
E#ect of foreign
ions
Table 1 shows the effect of foreign ions on the determination of uranium. Below the concentration levels present in the seawater no heavy metals except uranium(V1) can complex with Arsenazo III in these experimental conditions, therefore no coexisting ions interfered. All the sample solutions contained 2 x 10P5M EDTA in order to mask foreign ions such as iron(II1) present in seawater. Further increase in EDTA concentration at least up to 5.0 x 10e4M did not interfere with the uranium determination. Therefore this procedure may also be applied to other solution systems which contain foreign ions up to 5 x 10-4M. Calibration
curve
A calibration curve for uranium(V1) is shown in Fig. 7. The linearity holds up to 10 pg/l. of uranium, but the curve became convex at more than 10 ,ug/l. of uranium(W). Sensitivity
and detection
limit
The uranium concentrations giving a final absorbance of 0.1 were 7.7 x 10e9M for the present method and 2.3 x 10e6M for conventional solution absorptiometry,‘3 corresponding
of uranium(VI)
in seawater
Seawater samples were analysed by the standard-addition method in order to check the recovery of added uranium. As shown in Table 2, the recovery of the spiked uranium was quantitative. In coastal seawater from Mitoma, Fukuoka (open sea), the uranium content was found to be 3.5 + 0.3 pg/l. The content was in accord with those reported previously.3*4 Recently, ion-exchanger phase absorptiometry has been extended to flow analysis.‘4*‘5 Using this method, trace levels of a target element can be determined in water only in fairly small volumes. However, the present system could not be applied to flow analysis, because the uranium-Arsenazo III complex is difficult to be desorbed from the ion exchanger.
REFERENCES 1. J. Korkisch and I. Steffan, Anal. Chim. Acta, 1975, 77, 312. 2. R. J. N. Brits and M. C. B. Smit, Anal. Chem., 1977,49, 67. 3. R. Kuroda, K. Oguma, N. Mukai and M. Iwamoto, Talanta, 1987, 34, 433. 4. R. Kuroda, Y. Hayashibe, K. Oguma and K. Kurosu, Z. Anal. Chem., 1989, 335, 404. 5. E. S. Gladney, R. J. Peters and D. R. Perrin, Anal. Gem., 1983, 55, 976. 6. A. A. Prange, A. Knochel and W. Michaelis, Anal. Chim. Acta, 1985, 172, 79. 7. K. H. Lieser and B. Gleitsmann, Z. Anal. Chem., 1982,
313, 289. 8. M. Ckhsenkuhn-Petropulu and G. Parissakis, ibid., 1985, 321, 581. 9. K. Yoshimura, H. Waki and S. Ohashi, Tulanta, 1976, 23,449.
Table 2. The standard addition method for determination of uranium(VI) in sea water U(YI)added,rcn
0
0
0.63
1.25
1.88
A+ U(V1) found, pg
0.185 0.96
0.160 0.75
0.302 1.44
0.410 2.10
0.565 2.80
Sample volume: 100 ml. Resin: 0.10 ml. *A = AA -AA (for the blank). TAL 39,~F
IO. K. Yoshimura and H. Waki, ibid., 1985, 32, 345. 1 I. K. Yoshimura and T. Teketatsu, Z. Anal. Chem., 1987, 328, 553. 12. T. Nakashima, K. Yoshimura and H. Waki, Tulanta, 1990, 37, 735. 13. H. Gnishi and Y. Toita, Bunseki Kagaku,
1969, 18, 592. 14. K. Yoshimura, Anal. Chem., 1987, 59, 2922. 15. K. Yoshimura, S. Nawata and G. Kura, Analyst, 1990, 115, 843.