considerable increase in the operating distribution coefficient, because in the regions useful for separations many distribution coefficients change very rapidly with acetone concentration. An increasing coefficient can lead to quite an appreciable increase in tailing. This has been effectively avoided by closing the eluting agent reservoir with a rubber stopper through which passes a narrow glass tube drawn into a capillary. The second one is caused by the tendency of acetone to polymerize in acid solutions. This leads to the formation of nonvolatile organic matter, which often has to be destroyed before a determination can be carried out. Because the formation of
organic matter increases with hydrochloric acid concentration and is only negligible in dilute acid, the addition of some water before the evaporation is started is very profitable in the case of fairly concentrated acid solutions. It is also advisable to equilibrate columns with water-acetone mixtures before use and leave eluting agents standing for some time after mixing, replacing any acetone lost by evaporation. This avoids bubble formation in the resin column.
RECEIVED for review November 2, 1970. Accepted February 16,1971.
Cation Exchange Separation of Metal Ions with Potassium Chloride-C helating Age nt-0 rganic Solvent Medium Kil Sang Lee, Dai Woon Lee, and Sam Woo Kang Department of Chemistry, Yonsei University, Seoul, Korea The cation exchange behavior of several metal ions toward 50w-x8, Kf form has been investigated in KCI-methanol or KCI-chelating agent-organic solvent medium as an eluent. Distribution coefficients and volume distribution coefficients in these media have been measured. By the addition of a chelating agent, such as salicylic and sulfosalicylic acid, and glycine, severalseparations Of the synthetic mixtures were performed successfully.
chelating agent, such as salicylic or sulfosalicylic acid, was helpful in preventing the tailing, hydrolysis, and reduction of Fe(III), and for separating the mixtures including Fe(II1). By using a KC1-glycine-methanol (or acetone) medium, separations such as Mg(I1)-Zn(II), Cu(I1)-Ni(II), and Cu(I1)Zn(II)-Ni(II) which could not be separated by elution with KC1-methanol and KCl-saficylic acid-methanol solution, were accomplished successfully.
A CONSIDERABLE AMOUNT of study has been devoted to the cation exchange separation of metal ions as the chloride complex by elution with hydrochloric acid medium. The distribution coefficients for 43 cations at various hydrochloric acid concentrations using AG 50W-X8 resin have been obtained and the separation of cadmium from uranium, cobalt, nickel, zinc, copper, titanium, and other metals has been carried out by elution with 0.5M hydrochloric acid by Strelow ( I ) . By eluting with acetone-water-hydrochloric acid solution, Fritz and Rettig ( 2 ) separated cadmium(II), iron(III), copper(II), manganese(II),. and nickel(I1) from each other. It has been observed that upon addition of an organic solvent, such as acetone, the metal ions which form chloride complex strongly are eluted more rapidly than they would be in the absence of the organic solvent because of the enhanced complex formation (2). The aim of the present work was to study the application of potassium chloride-chelating agent-organic solvent as an eluent in the separation of metal ions. The distribution coefficients of metal ions for both Dowex 1-X8 and Dowex 50W-X8in various KC1-methanol solvents were calculated and volume distribution coefficients, which should be useful in determining optimum elution conditions for separations, were measured in KCl-chelating agent-organic solvent. The results of the experiments on the effect of organic solvent are presented. Evidence for the formation of chloride complexes of metals, such as zinc(I1) and cadmium(II), was obtained by measurement of the distribution coefficient with Dowex 1-X8 and Dowex 50W-X8 resin. The addition of a
(3). Ion Exchange Column. The column, which was made from borosilicate glass tube with an inner diameter of 7 mm was furnished with Teflon (Du Pont) fittings and a porous Teflon bottom. The resin bed was adjusted to 60 cm and rinsed with water-methanol solvent by a Beckman Accu-Flo pump. Distribution Coefficients. One gram of air-dried cation or anion exchange resin is weighed in a 100-ml glass-stoppered flask and 1.0 ml of 0.2M metal salt solution and 49 ml of the 0.5M KC1-H20 or 0.5M KC1-MeOH solution are pipetted into the flask. The flask is then mechanically shaken for about 24 hours. An aliquot taken from the supernatant liquid is then analyzed by titration with EDTA.
(1) F. W. E. Strelow, ANAL.CHEM., 32, 1185 (1960). (2) J. S. Fritz and T. A. Rettig, ibid., 34, 1562 (1962).
(3) H. A. Flaschka, “EDTA Titrations,” 2nd ed., Pergamon Press, New York, N. Y., 1964.
876
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
EXPERIMENTAL
Ion Exchange Resin. Dowex 50W-X8 resin, 200- to 400mesh (K+ form), and Dowex 1-X8 resin, 200- to 400-mesh (Cl- form) are used in the measurements of distribution coefficients. These resins must be purified by the routine method. Metal Salts. Stock solutions of metals were prepared by dissolving guaranteed reagent metal chlorides or nitrates in demineralized water. The solutions of metal ions contain 10 mg of metal in 1.0 ml of solution. Eluents. KC1-chelating agent-MeOH solutions are prepared so that the amount of methanol is expressed as percentage by volume and chelating reagent as percentage by weight per volume, and the potassium chloride as molarity. Volume changes due to mixing are disregarded. EDTA. 0.100M EDTA stock solutions and 0.010M EDTA solutions were prepared from analyzed reagent grade salt and standardized according to the general procedures
Distribution coefficients, Kd, were calculated using the following relation: Kd =
meq metal on resin/gram dry resin meq metal in solution/ml solution
(1)
The dry weight of the resin was calculated from the air-dried weight and the moisture content of the resin. The moisture content was 18.6% for cation exchange resin and 12.8% for anion exchange resin. Volume Distribution Coefficients. The peak elution volumes (0) were determined from a great number of runs in various eluents with a single metal ion. From these values the volume distribution coefficients (Dv)were calculated from the equation ( 4 ) : o / X = Dv
+
E
(2)
where X is the corrected column volume and E the void fraction of the column. The value E = 0.379 for Dowex 50W-X8, 200- to 400-mesh was measured by Materova. Separation Procedure. The column was washed with deionized water and then with a water-methanol solvent identical with the eluent solvent. The surplus watermethanol solvent was drained from the column until the liquid level was about 1 mm above the surface of the resin. Aliquots of metal solution, ranging from 0.2 ml to 0.5 ml of the stock solution, were added and then forced into the resin bed using nitrogen gas; then more solvent was added. A supply of the appropriate eluent as chosen from the Dv data was then connected to the column. During the elutions the flow rate was adjusted to 0.5 to 1.0 ml per minute by the Accu-Flo pump. The aliquots of the effluent fractions were collected, and the amount of metal was determined by titration with 0.01M EDTA or by spectrophotometric determination. The average recovery for all separations was 99.5%. The standard deviation for all data was 1.0%. RESULTS AND DISCUSSION
Distribution Coefficients. Measurements of the batch distribution coefficients of metal ions on Dowex 50W-X8 are presented in Table I. The order of metal ions by Kd value, as shown by distribution coefficients at 0.5M KCl-HI0 (Table I), is as follows: Cd(I1) < Mg(I1) < Zn(I1) < Cu(I1) < Ni(I1). This order for metal ions, except for Mg(II), is in agreement with the result previously reported for hydrochloric acid by Strelow, though qualitative comparison only could be made because of the lack of uniform experimental conditions. Work by Strelow ( I ) showed that with 0.5M hydrochloric acid the Kd values of Cd(II), Zn(II), Cu(II), Ni(II), and Mg(I1) are 6.5, 64, 65, 70, and 88, respectively. It has been known that a metal ion can be removed from a cation exchange resin by simple mass action replacement with another cation or by metal-chloride complexing and ion pair formation by chloride. In order to investigate the probability of the formation of metal-chloride complex in KCl medium, measurement of the distribution coefficients of metal ions on Dowex 1-X8, anion exchange resin, was performed, and some of the results are shown in Table 11. The Kd values of metal ions other than Cd(I1) and Zn(II), which could be held strongly by the anion exchange resin as metal-chloride complex, were too small to measure in 0.5M KCl-MeOH solution. This means that in this medium the metal-chloride complexes are very unstable. (4) 0. Samuelson, “Ion Exchange Separations in Analytical Chemistry,” Almquist and Wiksell, Stockholm; Wiley, New York, 1963, p 126.
Table I. Distribution Coefficients of Several Metals on Dowex 50W-X8, K+ Form, 200- to 400-Mesh Metal Medium Cd(I1) Mg(I1) Zn(I1) Cu(I1) Ni(I1) 4.20 27.2 31.4 35.6 42.0 0 . 5 M KCl-Ht0 19.8 23.2 30.9 0.5MKC1-15zMeOH 4.13 19.1 17.1 23.1 13.2 0.5MKC1-3097,MeOH 3.85 12.2 5.94 8.52 11.2 13.8 0.5MKC1-4597,MeOH 3.58 Table 11. Distribution Coefficients of Cd(I1) and Zn(I1) on Dowex 1-X8, C1- Form 200- to 400-Mesh Metal Medium Cd(I1) Zn(I1) 0.5M KCl-HZO 540.7 62.80 0.5M KCl-15 MeOH 687.9 157.4 0 . 5 M KC1-3Oz MeOH 1346 410.0 0.5M KC1-45 MeOH 2442 687.9
z
The effect of methanol upon the distribution coefficient of metal ions on cation exchange and anion exchange resin may be seen in Tables I and 11. In all cases, distribution coefficients decrease with increasing methanol percentage in 0.5MKC1-MeOH solution in the case of cation exchange resin (Table I). These decreases in the case of Zn(I1) and Cd(I1) are suggested by the fact that the metal-chloride complex formation is enhanced with increasing concentration of organic solvent. On the other hand the distribution coefficients in the case of anion exchange resin increase with increasing percentage of methanol. This increase is also attributed to the metal-chloride complexing. In the present work the effect of the methanol upon the Kd values of Mg(I1) and Ni(I1) is quite different from the effect of acetone-water-hydrochloric acid (2). This difference between potassium chloride and hydrochloric acid solution as an eluent is a very interesting phenomenon to be studied in the near future. The variation in the methanol percentage was unfortunately limited to 45% because of the solubility of potassium chloride. The results of the experiments with Fe(II1) are not shown in Table I and I1 because the small portions of Fe(II1) were reduced to Fe(I1). A qualitative experiment to investigate the reduction of Fe(II1) to Fe(I1) was tested. When the solution containing FeCb and resin in the potassium form was shaken and the supernatant liquid was tested with 1.IO-phenanthroline, the test was positive. On the other hand this reduction did not occur in the case of the resin in the hydrogen form. This unexpected phenomenon will be investigated more carefully in the next paper. The reduction of Fe(II1) was exhibited also in the work by Fritz and Abbink (5) and by Fritz and Karraker (6). All data shown in Table I and I1 were measured at a fixed concentration of potassium chloride (0.5M) which is the optimum condition for the separations of metal ions in this work. Volume Distribution Coefficients (Dv). Volume distribution coefficients were measured to predict the elution behavior and the optimum condition for separation of metal ions. The elution order of metal ions of Dowex 50W-X8 presented (5) J. S. Fritz and J. E. Abbink, ANAL.CHEM., 37, 1274 (1965). (6) J. S. Fritz and S.K. Karraker, ibid.,32, 957 (1960). ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
877
I
I
I
I
I
200
300
400
500
cdm 0,8
-
I
0.6
‘00
f
$06-
t
-3
1
2
Ob
Lc
Y
3fi
0.4
._ E
-
%
-(I
3.,
I
5
I
H -3
5.
0.2
-
ao
i
s
j
0.2
I
0.0 100
300
200
400
100
0
Eluate volume, ml
Eluate volume, ml
Figure 1. Elution curve for Cd(I1)-Mg(I1)-Ni(11) separation
--:
.
. . . a
0.5M KC1-15% MeOH, flow rate: 0.90-0.95 ml/min 0.5M KC1-30% MeOH, flow rate: O . W . 9 5 ml/min
Table 111. Volume Distribution Coefficients of Metals on Dowex 50W-X8, K+ Form, 200- to 400-Mesh Flow Rate: 0.90-0.95 ml/min Eluent 0.5M KCl-% MeOH Metal ion 1 M KCl-H,O 15 % 30 % 45 % 2.22 1.56 0.70 0.70 Cd(I1) Zn(I1) 5.47 12.3 6.55 2.22 8.37 4.17 6.12 14.3 Cu(I1) MgW) Ni(I1) Fe(II1) 5
6.12 1.85 12 (?)
10.0 17.8 .,.
22.1a 6.77 11.5 15 (?)
2.44 5.78
...
0.5M KNO3-3O% MeOH.
Table IV. Volume Distribution Coefficients of Metals on Dowex SOW-XS, Kf Form, 200- to 400-Mesh Eluent 0.5MKCl- 0.5MKCl- 0.5MKCl- O.5MKClMetal 0 . 3 %SA0 . 3 % SSA- 0.3%SA- 0 . 3 %SSAion 30% MeOH 30% MeOH 15% MeOH 15% MeOH 1.35 2.00 1.79 Cd(I1) 1.35 12.4 11.8 6.55~ 5.90 Zn(I1) 7.42 14.1 13.0 8.28 Cu(I1) 10.0 9.58 6.33 6.12 MgUI) 10.2 16.9 15.9 10.7 Ni(I1) 2.65 2.00 4.60 3.00 Fe(II1) Flow rate: 0.80-0.85 ml/min.
as the Dv value (Table 111), is in agreement with the Kd values of metal ions shown in Table I, except for the small difference between Mg(I1) and Zn(I1). In the case of Fe(III), the exact measurement of the volume distribution coefficient was impossible because of the tailing and reduction phenomena of Fe(II1) during the elution on the column. It has been observed that the Dv values decrease with increasing methanol percentage, affirming the effect of methanol upon the formation of metal-chloride mentioned previously. 878
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
Figure 2. Separation of the mixture, Cd(I1)-Fe(II1)-Zn(I1)Ni(I1) by elution with 0.5M KC1-0.3 SA-15 % MeOH. Flow rate: 0.90-0.95 ml/min
z
The effect of the chloride anion on the elutions of metal ions was investigated by the comparison of the value of Dv when 0.5M KC1-30z MeOH and 0.5M KNO8-30% MeOH were used as eluents. For example, in the case of Cu(II), a large difference was noticed (Table 111). In the present work the KC1-methanol solution containing salicylic or sulfosalicylic acid, which reacts with Fe(II1) to form a colored ferric salicylate or ferric sulfosalicylate complex, was tried as the eluent to reduce the tailing and reduction of Fe(II1) on the resin by the complex formation. The elution behavior of ferric salicylate and sulfosalicylate complexes on the anion exchange resin was studied in the separation of organic acids (7). The effect of salicylic and sulfosalicylic acid upon the elution of Fe(II1) and other metal ions is shown in Table IV. For example, the D E value of Fe(II1) dropped from 15 in 0.5M KC1-30z methanol solution to 3.00 in 0.5M KC1salicylic acid-30 methanol medium. This large decrease in Du value is attributed to the formation of the ferric salicylate complex which is held weakly by the resin. As shown in Table IV, there are no appreciable changes in Dv value of other metal ions with the addition of salicylic and sulfosalicylic acid, indicating that the reactions between these metal ions and the chelating agents may be negligible in that elution condition. Consequently, it is suggested that these eluents containing the complexing agent can be very useful to separate Fe(II1) from the mixtures of metal ions. Separations of Mixtures. From the results presented in Table 111, the separation of the synthetic metal mixtures by KC1-methanol medium has been considered. The amounts of the metals to be separated were 1.0 mg to 3.0 mg. In the 1M KC1-H20 medium only Cd(I1) could be separated from Zn(II), Cu(II), Mg(JI), and Ni(I1). A s o u p including Cd(I1)-Zn(I1)-Ni(I1) could be separated from each other by elution with 0.5M KC1-15z methanol or 0.5M KC1-30 methanol. A Cd(I1)-Mg(I1)-Cu(I1) group could only be separated in the 0.5M KC1-15z methanol.
z
z
~
(7) K. S. Lee, D. W. Lee, and E. K. Lee, ANAL.CHEM., 42, 554 (1970).
Volume Distribution Coefficients of Metals Flow Rate: 1.0 ml/min Eluent Glycine-3Oz MeOH 0 . 5 M KCI-O. 3 PH
Glycine-30 PH
7.2 1.22 6.77 4.82
7.2 1.14 3.95 13.9
Table V.
Metal ion
cum z&j Ni(I1) M g(I1)
0 . 5 M KC1-O. 3
6.3 1.31 6.98 9.40 6.24
z
9.0 1.22 1.31 1.70 6.24
...
On the other hand, the components of a Cd(I1)-Cu(I1)Ni(I1) mixture could be separated from each other only in the case of 0.5M KC1-30z methanol medium. A Cd(I1)Mg(I1)-Ni(I1) group could be separated with both eluents of 0.5M KC1-15z methanol and 0.5M KC1-30z methanol as shown in Figure 1. With 0.5M KC1-45z methanol, the only two metal ions, such as Cd(I1)-Cu(II1) and Cd(I1)Ni(II), could be separated from each other because of the overlapping of the eluate volumes of Mg(II), Cd(II), and Cu(I1) or Ni(I1). When Fe(II1) was added to a mixture of metal ions, for example, Cd(I1)-Cu(I1)-Ni(II), separation was impossible using the 0.5M KC1-30 methanol eluent because of the tailing of Fe(II1) from 230 to about 500 ml. As shown in Table IV, successful separations of the mixtures of metals including Fe(II1) were possible by the addition of a chelating agent, such as salicylic acid (SA) and sulfoSA-30 salicylic acid (SSA). With the 0.5M KC1-0.3 MeOH eluent, the separations of some groups, such as Cd(I1)-Fe( 111)-Cu(II), Cd(I1)-Fe( 111)-Ni( 11), Cu(11)-Mg( 11)Ni(I1) and Cd(I1)-Zn(I1)-Ni(II), could be performed. On the other hand a group of Fe(II1)-Zn(I1) [or Mg(II)]-Ni(I1) could be separated by elution with 0.5MKCl-0.3 SSA-30 MeOH. An elution curve of a four-component mixture, Cd(I1)-Fe(II1)-Zn(I1)-Ni(I1) is shown in Figure 2. KC1-Glycine-Methanol (or Acetone) Medium. To separate the mixture of Cu(II), Zn(II), Ni(II), and Mg(I1) which was not possible in KC1-methanol and KCl-SA (or SSA)methanol medium, the addition of glycine is necessary to effect the separation of Cu(I1)-Zn(I1)-Ni(I1). Comparison of the Du value when KC1-methanol and KCI-glycine-methanol (or acetone) are used as eluents shows a marked difference in Table I11 and V. This difference in Du value is due to the formation of metal-glycine complexes in the elution. The effect of pH upon the Do of metal ions was investigated and the results are presented in Table V. It is known that in general two molecules of glycine react stoichiometrically with one metal ion, such as Cu(II), Zn(II), and Ni(II), to give the deep blue colored complex, and in the basic solution Cu(I1) can react with glycine to form a positive, neutral, or negative charged copper-glycinate complex. In the elution of Cu(I1) the change of the Du values with increased pH of the eluent was negligible, presumably because the Cu(I1)-glycine complex had a stronger complex formation constant than Zn(I1)- and Ni(I1)-glycine and it was held less strongly by the cation resin. On the other hand, the Du value of Zn(I1) and Ni(1I) decreased with increasing the pH. It is suggested that this decrease is attributable to the fact that Zn(I1)- and Ni(I1)-glycine complexes are affected by the change of pH. For example, at pH 6.3, Zn(I1) was eluted from 130 ml to 195 ml with a 160-ml peak elution volume with 0.5M KC1-30z MeOH; Zn(1I) was eluted from 70 ml to 190 ml and its peak elution volume was 170 ml with 0.5M KC1-0.3z glycine-3Oz MeOH. The long
z
z
z
6.3 1.18 4.17 13.9
I 0 . 5 ~ 1 c ~ 1 - 0 .cjtycine-25YoMeOH 5%
+aE 0.8
I?
Acetone 9.1 1.05 1.36 1.31
I
0.5M KCI -30% MeOH
5E t
6 2
04
%
J
5
0.2
z
z
0.0
Eluate volume, ml
Figure 3. Separation of the mixture, Cu(I1)-Zn(I1)-Ni(I1) tailing in the initial portion of the elution curve for Zn(J1) in the 0.5M KC1-0.3z glycine-3OZ MeOH medium could be explained by the suggestion that Zn(I1)-glycine complex formed in this medium was unstable and would be interfered with the formation of zinc chloride complex in the same medium. In the alkaline solution (pH 9.0), Zn(I1) and Ni(I1) were eluted rapidly as shown by the Dv values in Table V, indicating that at this pH Zn(I1)- and Ni-glycine complexes are stable and nonabsorbable on the resin. From the Du values of metal ions in Table V, some separations were investigated. The separation of Mg(I1) from Cu(II), Zn(II), and Ni(I1) could be carried out in the 0.5MKC1-0.3 Z glycine3 0 z MeOH with pH 9.0. A group of Cu(I1)-Zn(I1)-Ni(I1) could be separated from each other by elution with 0.5M KC1-0.3z glycine-30z acetone, pH 6.3 or 7.2. It was found that a Cu(I1)-Zn(I1)-Ni(I1) separation could be successfully performed following elution scheme: Cu(II), 80 ml0.5M KC1-0.5 glycine-25 % MeOH, Zn(I1) and Ni(II), 320 ml 0.5M KC1-30z MeOH. This elution scheme is shown in Figure 3. ACKNOWLEDGMEh'T
The authors are grateful to Miss Jung Soon Choi for her help in the laboratory work of this paper. RECEIVED for review October 22, 1970. Accepted February 8, 1971. This work was financially supported in part by grants from the United Board for Christian Higher Education in Asia and the Research Committee of Yonsei University for which the authors express their gratitude. ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
879
