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Analytica Chimica Acta, 278 (1993) 317-320 Elsevier Science Publishers B.V., Amsterdam
Liquid-solid extraction system based on mixed polymers of polyethylene glycol2000 and Triton X-100 without organic solvents Buhai Li, Xiaomei Sun, Guihong Wei and Zizhuan Ma Departmentof Chemistty, South-Central Colkge for Nationalities, khan,
Hubei 430074 (China)
(Received 10th June 1992, revised manuscript received 18th January 1993)
Abatnlct On mixing of aqueous solutions of polyethylene glycol2000 and Triton X-100 (polyoxyethylene alkylphenol) and addition of ammonium sulphate, a polymeric solid phase and an aqueous phase are formed. Suitable phase separation conditions were studied in addition to the distribution of the Arsenaxo III complexes of U(VI), S&II), BifIII), Z&V), FeUII) and PbfII) between the two phases. The separation of zr(IV) from Fe(II1) and the determination of the former was achieved by properly adjusting the acidity of the solution. Keywords: Sample preparation; WV-Visible spectrophotometry; Metal ions; Polymer extractants; Zirconium
Liquid-solid extraction systems of / some water-soluble polymers, e.g., polyvinylpyrrolidone (PVP) [1,21, Tween 40 [3] and Tween 80 141,have been reported. These systems have an advantage over the two-phase aqueous system of polyethylene glycol (PEG) 2000 [5], and have been applied to the separation of various metals by using water-soluble chromgenic reagents as extractants. Aqueous solutions of mixed polymers give rise to liquid and solid phases after addition of ammonium sulphate. Under suitable conditions some metal ions can be selectively transferred into the polymeric solid phase, while others remain in the aqueous phase. Because the salt effect, which decreases the stability of the complexes of some metal ions, is lower than in a system with a single polymer, extraction in a system of mixed polymers is more convenient.
Correspondence to: Buhai Li, Department of Chemistry, South-Central College for Nationalities, Wuhan, Hubei 430074 (China). 0003-2670/93/$06.00
In this paper, a liquid-solid extraction system based on mixed polymers of PEG 2000 and Triton X-100 (polyoxyethylene alkylphenol) is reported.
EXPERIMENTAL
Apparatus and reagents A Shanghai Model 721 spectrophotometer was used for spectrophotometric measurements. Aqueous 32% solutions of PEG 2000 and Triton X-100 were used. A 1.29 X 10m3 mol 1-l Arsenazo III chromogenic reagent solution was prepared by dissolving the appropriate amount in water. Standard 1.0 mg ml-’ solutions of U(VI), Fe(II1) and Pb(II1 in 0.15 mol I-’ nitric acid, BKIII) in 0.75 mol 1-i nitric acid, Zr(IV) in 10% hydrochloric acid and Sc(II1) in 0.1 mol I-’ hydrochloric acid were prepared from UOJNO,), - 6H,O, pure lead, pure bismuth, ZrOCl, +8H,O and scandium
Q 1993 - Elsevier Science Publishers B.V. All rights reserved
B. Li et aL /Anal. Chim. Acta 278 (1993) 317-320
318
oxide, respectively. More dilute solutions were obtained by appropriate dilution. Chloroacetic acid buffers (pH 1.5-3.0) and acetic acid buffers (pH 3.5-6.5) were prepared by adding 40% sodium hydroxide to 0.7 mol l- ’ chloroacetic acid and 0.7 mol 1-l acetic acid, respectively. All other reagents were of analytical-reagent grade.
4
\
. Procedure
A 2.0-ml volume of 32% PEG 2000 solution, 3.0 ml of 32% Triton X-100 solution, a given volume of chromogenic reagent and metal ion solution were placed in a 25ml test-tube with a stopper. The pH of the solution was adjusted from 1.5 to 6.5 with chloroacetic or acetic acid buffers, and the acidity of the strong acid solution was adjusted with HCl. A given amount ammonium sulphate was added, then the solution was diluted to 10.0 ml with distilled water and shaken for 3 min. Five minutes later, a liquid and a solid phase were formed. The liquid phase was carefully decanted directly from the tube. The polymeric solid was washed two or three times with a saturated solution of ammonium sulphate. The separated liquid was combined with the wash solution. The solid polymeric phase was dissolved in distilled water. The concentration of the extracted species in the dissolved solid or the separated liquid was determined spectrophotometritally. The percentage extraction (E) was calculated as the ratio of the amount of species extracted by the solid phase to the total amount of the species. The conditions used for determination of the metal ions with Arsenazo III as chromogenic reagent are given in Table 1.
RESULTS AND DISCUSSION
Phase separation conditions for polymer solution
At fixed acidity (pH 4.5) and total concentration of the polymers (16%), the concentration of ammonium sulphate necessary for phase separation decreases with increasing the PEG 2000/ Triton X-100 ratio. When this ratio was fixed at
100 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
PH Fig. 1. Relationship between pH and ammonium sulphate concentration necessary for phase separation. Polymer concentration, 16%; PEG 2000 to Triton X-100 ratio = 2: 3.
2: 3, the necessary salt concentration decreases with increase in the total concentration of the polymers from 5 to lo%, and then remains constant. Figure 1 shows that the necessary salt concentration also depends on the pH. In the pH range 0.8-2.9 it decreases with an increase in pH and remains unchanged at higher pH. The reason is that at low pH the sulphate ion is protonated to HSO; and the salting-out ability is reduced. Dktribution of chromogenic reagents
The distribution of some water-soluble reagents between the polymeric solid phase and aqueous phase was determined in the pH range 1.1-6.2. The reagents examined containing different chelating groups, were Arsenazo III, Chromazurol S, Xylenol Orange and Bromopyrogallol Red. The E values for these reagents, as shown in Fig. 2, depended on the pH of the solution. The E values of Arsenazo III were the highest (90-96%), so this reagent was chosen as the extractant. Because of the dependence of E for Arsenazo III on pH, it is necessary that the polymeric phase that contains Arsenazo III at the same pH is used as the reference when determining spectrophotometrically the E values of metal ions with Arsenazo III as extractant.
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B. Li et al. /AnaL Chim. Acta 278 (1993) 317-320
50FC 25Pb 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0
PH Fig. 2. Extraction acidity curves for various chromogenic reagents. Extraction conditions as in Fig. 1. (a) Arsenazo III; (b) Chromazurol S; (c) Xylenol Orange; (d) Bromopyrogallol Red. Concentration of ammonium sulphate in the various acidity as in Fig. 1.
1.0
2.0
3.0
4.0
5.0
6.0
7.0
PH Fig. 3. Extraction acidity curve for the metals. Added amount of each metal, 50 fig; Arsenazo III concentration, 1.29 x 10m4 mol I-‘. Other conditions as in Fig. 1. 100
Extraction of metals by wing Arsenazo ZZZa.~ extractant EfSect of PH. Figure 3 shows the dependence of E for U(VI), S&II), BXIII), Zr(IV), Fe(II1) and Pb(I1) on pH in the presence of Arsenaxo III. The stability of the complexes of UWI) and SdIII> can be increased by raising the pH from 0.8 to 3.5 and 4.4, respectively, so E is correspondingly increased. The maximum E value for U(VI) is at pH 3.5 and for S&II) at pH 4.4, but then remains unchanged on further increase in pH. The E values for Bi(II1) and Fe(II1) increase with increasing pH from 1.6 to 3.6 and 3.8 respectively, and then decrease with a further increase in pH, probably owing to hydrolysis and other side-reactions of the metals at high pH. In contrast to UWI), S&II), BXIII) and Fe(III), the effect of pH on E for ZrW) is less. Because Pb(I1) and SOi- form an insoluble compound, PM111 can hardly be extracted into the polymeric phase, and the E values are lower.
d 75
50
25
0 Oil
1.b
2.0
3.0
Lr(xXTmd/l)
Fig. 4. Effect of ksenazo III concentration on extraction of the metals. (E) Extraction acidity, U(VI), Bi(III), Fe(III) and Pb(II) pH 3.5; S&II) and Zr(IV) pH 4.5. Other conditions as in Fig. 3.
Effect of Arsenuzo ZZZconcentration. At fixed pH the stability of the complexes of UWI), Sc(III), ZrW) and Fe(III), and the corresponding E
TABLE 1 Spectrophotometric
determination
4.0
of metals
Parameter
UwI)
sc(III)
IwIII
Zrw)
Bi(II1)
FefIII)
Wavelength (nm) Acidity
650 pH 2.0
660 pH 3.0
655 pH 5.0
665 0.1 M HCl
620 pH 2.0
622 pH 3.5
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B. L.iet aL/And. Chim.Acta 278 (1993)317-320
TABLE 2 Recovery results for extractive separation between zr(IV) and Fe(II1) Added (re)
ZIW) 50.0 50.0
20.0 10.0 10.0 10.0 5.0 5.0
Found (/& Fe(II1) 5.0 10.0 20.0 50.0 100.0 1000.0 1000.0 5000.0
zr(IV) 49.1 48.1 18.9 10.5 9.5 10.2 5.0 4.8
Recovery (%) Fe(III) 5.0 10.0 19.1 47.7 94.5 945.0 954.0 4659.0
Zrtnr) 98.1 96.3 94.5
Average S.D. R.S.D.
105.0 95.0 102.0 100.0 %.O 98.4 3.7 3.8%
Fe(II1) 100.0 100.0 95.5 95.4 94.5 94.5 95.4 93.2 96.5 2.4 2.5%
values, can be increased when the concentration of Arsenazo III is increased up to 6.45 x lo-’ or 1.29 X low4 mol l-l, and then E remains constant with further increase in arsenazo III concentration, as shown in Fig. 4. For the same reason as indicated above, the E value for PMII) is small. Figure 4 also indicates that without Arsenazo III all the E values for the studied metals are zero. This is the same as for a single polymer system, which mean that the uncomplexed metal ions cannot be extracted into the mixed polymer phase.
The data in Table 2 show that this mixed polymer-(NH,),SO,-H,O system can be applied to the extractive separation of various metals. The experiments also show that the reproducibility of the separation and determination of the metals is excellent. For example, for the same sample the relative standard deviation is less than 4.4% (n = 3). The sensitivity and linear dynamic range for the determination of the metals after separation are similar to those for the direct determination of the metal ions with Arsenazo III as chromogenic reagent.
Recovery tests for quantitative separation of Zr(N) and Fe(III) Figure 3 shows that in the lower pH range (0.8-1.21, the E value for Zr(IV) is higher than 80%, but that for Fe(II1) is zero. After a first extraction with Arsenazo III as extractant, we can put the mixed polymer and the Arsenazo III solutions into the extracted solution, which is shaken for 3 min. The mixed polymer solid is salted out again, and the complex of Zr(IV) with Arsenazo III quantitatively enters the mixed polymer solid, but Fe(III) remains in the solution. The separation results are also satisfactory (see Table 2).
This study was supported by the National Natural Science Fund of China.
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