Simple Ion Exchange Separation of Magnesium from Calcium and Other Metal Ions Using Ethyleneglycol-Bis(2-Aminoethylether)Tetraacetic Acid as a Complexing Agent Milan Marhol and K. L. Cheng Department of Chemistry, University of Missouri-Kansas C i t y , Kansas City, Mo. 64110 The calculated conditional stability constants at pH 2 to 10 for the ethyleneglycol-bis(2-aminoethy1ether)tetraacetic acid (EGTA) complexes of calcium, magnesium, and other metals indicate that no significant amount of magnesium is complexed by EGTA at pH below 7. The distribution coefficients for magnesium, calcium, and strontium ions in an EGTA solution at various pH values were estimated on Dowex 50 W-X8, Chelex 100, and Amberlite IRC-50 in sodium form and on Dowex 1-X8 in chloride form. Magnesium can be selectively adsorbed on cation exchangers from an EGTA solution at pH 6.5 to 7.0 while calcium and many other metal ions form negatively charged EGTA complexes which are not adsorbed under these conditions. Satisfactory results are reported for recovering known amounts of magnesium from synthetic sea water mixtures.
SEVERAL PAPERS HAVE APPEARED on the separation of magnesium from calcium using ion exchange chromatography. The separation can be carried out on a strongly acidic cation ion exchanger by eluting magnesium ion with hydrochloric acid ( I , 2). Some complexing agents have been used for the separation of magnesium in anion exchange chromatography (3). Good separations were reported by using various cation or anion exchangers in acid-alcohol mixtures (4-6). This paper reports a simple, selective and rapid method of separating magnesium from calcium and other metal ions. Large differences in the stability of magnesium and calcium EGTA complexes (log K C , = 11.0, log K M , = 5.2) and of magnesium and other polyvalent metal EGTA complexes offer a theoretical basis for a successful separation of these ions. Ringbom and Nymark (7) stated the possible separation of magnesium in the presence of EGTA from calcium on Dowex 50 in sodium form. The conditional constants for magnesium EGTA complex and other EGTA complexes have been calculated as a function of pH (Figure 1). It indicates that below pH 7 polyvalent metal ions except magnesium are complexed by EGTA. Povondra and Pribil (8) reported the ion exchange separation of magnesium using EGTA and carried out the separation over the pH range 7.5 to 8.5. We (1) N. D. Campbell and C. T. Kenner, ANAL. CHEM.,26, 560 (1964). (2) T. A. Ferraro, Ion Exchange Analysis of Magnesium Alloys (AD-682 741) U. S. Department of Commerce Clearinghouse for Scientific and Technical Information, December 1968. (3) F. W. E. Strelow, Y. C. R. van Zyl, and C. R. Nolte, Anal. Chim. Acta, 40, 145 (1968). (4) M. Tanaka, J. Chem. Soc. Japan, Pure Chem. Sect., 85, 117 (1964). (5) F. W. E. Strelow and C. R. van Zyl, Anal. Chim. Acta, 41, 529 (1968). (6) J. S. Fritz and H. Waki, ANAL.CHEM.35, 1079 (1963). (7) A. Ringbom and F. Nymark, unpublished results; cf. A. Ringbom, “Complexation in Analytical Chemistry,” Interscience, New York, New York, pp. 236-7, 1963. (8) P. Povondra and R. Pribil, Talanta, 10, 713 (1963). 652
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
found the optimum pH range to be 6.5 to 7.0, which is in good agreement with the calculated values. EXPERIMENTAL
Ion exchange resins: Dowex 50 W-X8 (50-100 mesh), Amberlite IRC-50 (50-100 mesh), Chelex 100 (1OG-200 mesh), all in sodium form and Dowex 1-X8 (100-200 mesh) in chloride form were used. The resins were treated with 3N hydrochloric acid, 0.2N sodium hydroxide, and 0.05M EGTA solutions and dried in air at room temperature. The moisture contents of the air-dried resins were determined by drying to constant weight over phosphorus pentoxide in a vacuum desiccator. Ion exchange columns : Conventional ion exchange columns of i.d. in 5.0 or 10.0 mm fitted with a sintered glass were used. The resin beds were 7 or 10 cm long. The resins were equilibrated with an acetate buffer at pH 6.7 and rinsed with water. Immediately before use the resin bed of the column was treated with 0.03M EGTA solution at pH 6.7. Stock solutions of calcium, strontium, magnesium, and other elements were prepared by dissolving appropriate amounts of nitrates of analytical reagent grade in water. Analytical Methods. The metal content was determined by EDTA (ethylenediaminetetraacetic acid) titration using murexide, Erio T, or Xylenol Orange as indicators. The metal contents in the EGTA complexes were destroyed by repeated evaporation of the solution with mixture of nitric acid and perchloric acid before the titration with EDTA. Measurement of distribution coefficient: The distribution coefficients, K O , were measured by the batch method. Exact amounts of the air-dried resin in sodium form (Dowex 50, 0.294 gram; Chelex 100, 0.297 gram; Amberlite IRC-50, 0.164 gram of dried resin) were equilibrated with a solution containing 15.0 ml of 0.05MEGTA solution, 2.5 ml of 0.252M calcium nitrate, magnesium nitrate, or strontium nitrate solution. The solution was adjusted to appropriate pH values with 2N hydrochloric acid or 0.05Nsodium hydroxide solution. The sodium concentration was kept constant by the addition of 0.05M sodium nitrate solution, approximately 0.1M. The total volume of 50.0 ml was occasionally shaken in 60-ml polyethylene bottles for 120 hours at room temperature. The equilibrium concentration of the metals was determined in the solution. The adsorption is expressed by the distribution coefficient, K D , which is given by
KD =
Amount of metal per gram of dry resin Amount of metal per ml solution
Separation Procedure. The mixture of different ratios of magnesium and calcium or other metal ions and a slightly excess of EGTA was adjusted to pH 6.7 and allowed to pass through the column at a flow rate of approximately 2 ml per minute. The column was washed with a sufficient amount of 0.03M EGTA solution at the same pH and rinsed with water. Magnesium was eluted from the resin by 2N hydrochloric acid solution.
P c u 16
KD 16 400-
\
14
14
\
350-
f 4
h
12
P2" 12 300-
10
10
250I
0
m
X
8
8
U
-
x
r
200-
:.
bp
x,
6
6
150-
4
4
100-
2
2
50-
0
0 2
4
a
6
10
PH
Figure 1. The parameter, aH,of EGTA and the conditional stability constants of some EGTA complexes
0 ' 2
I
4
5
6
7
9
8
pH (Equilibrated)
Figure 2. Adsorption of calcium, magnesium, and strontium on Dowex 50-X8from EDTA solution.
I
RESULTS AND DISCUSSION
The formation of EGTA complexes is affected by the hydrogen ion concentration; increasing the acidity of the solution will decrease the concentration of metal EGTA complex. The amount of metal ion adsorbable on the ion exchanger will also increase. Careful control of pH is very important for a successful separation. From the values of conditional constants for different metal complexes shown in Figure 1, magnesium exists as a cation which can be adsorbed quantitatively on cation exchangers, while many other metal ions are transformed into unadsorbable anionic complexes. The over-all results of batch experiments for three different cation exchange resins in the system Me(I1)-EGTA at various pH values are summarized in Figures 2-4. By using Dowex 50-X8in sodium form the adsorption of calcium at pH 6.5-7.0 is practically zero while magnesium is adsorbed quantitatively. An optimum pH value between 6.5 and 7.0 obtained from the batch experiments is in good agreement with the calculated value for the separation of magnesium from calcium using EGTA as shown in Figure 1. The results of Figure 2 show that strontium can be separated from magnesium at pH 7.0-7.3 and from calcium at pH 5.86.1. Using a slightly acidic exchanger Amberlite IRC-50 (see Figure 3) or the chelating resin Chelex 100 (Figure 4) the adsorption decreases in the order of Ca < Sr < Mg in accordance with the relative stability constant values of their EGTA complexes. The adsorption curves on both exchangers have maxima resulting from two factors--i.e., before reaching the maximum -increasing adsorption ability of the exchanger with increas-
I
3
KD
700-
600
IRC-50
-
500-
400
-
300
-
200 -
100
-
0
3
4
5
I
I
6
I
I
a
;
pH (Equilibrated)
Figure 3. Adsorption of calcium, magnesium, and strontium on Amberlite IRC-50 in sodium farm from EGTA solution ing pH and after passing the maximum, decreasing adsorption due to decreasing amount of positively charged free metal ion. Obviously pH plays an important role in the competing equilibria (9). (9) J. S. Fritz and G . R. Umbreit, Anal. Chim. Acta, 19,509 (1958). ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
0
653
I
KD
2ol
Amberlite IRC.50
100 800
150
100
1
700
600
0' 3
I
I
I
1
4
5
6
I
600 8
9
pH (Equilibrated) 400
Figure 4. Adsorption of calcium, magnesium, and strontium on Chelex 100 in sodium form from EGTA solution 300
KD 200
800-
700
-
600
-
"
I
0 500
-
I
0
100
-
V
I
I
I
60
Figure 6. Adsorption of magnesium from EGTA solution containing organic solvent on Amberlite IRC-50 in sodium form Methanol
0 Isopropanol
300-
-
I
40
Percent of Organic Solvent
400 -
200
I
20
0
Acetone
A 1,4-Dioxane X Tetrahydrofurane V Dimethylformamide
Table I. Analysis of Synthetic Mixtures 0
I
I
I
I
I
I
I
200
400
600
800
1000
1200
1400
Dowex 50-X8, sodium form Taken, mg Found, mg
Initial Sodium Concentration MG per 50 M L
Figure 5. Adsorption of magnesium from EGTA solution on different ion exchange resins at various initial sodium chloride concentration 0 Dowex50 0 Amberlite IRC 50 0 Chelax 100 A large difference between the sorption of magnesium and calcium ions on Amberlite IRC-50 in the range of pH 6.7-7.5 makes this resin also suitable for their separation. The optimum p H range for the separation is approximately about 0.5 unit higher than Dowex 50. Using Chelex 100 the separation of magnesium from calcium is also possible a t p H between 7.0 and 7.5. But its exchange capacity, in comparison with Dowex 50 and Amberlite IRC-50, is too low to be of practical use. For magnesium to be adsorbed by a cation exchanger, it must exist as a cation, however, its adsorption by a chelating 654
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
Calcium Magnesium Calcium Magnesium Calcium Magnesium Calcium Magnesium Other MetalsG Magnesium
49.6 42.7 49.6 17.4 3.00 25.8 1.1 0.9
49.6 42.7 49.5 17.2
17.4
17.4
3.00
25.8 1.1 0.9
Amberlite IRC-50.sodium form Calcium 49.6 49.5 Magnesium 1.8 1.8 Calcium 3.0 3.0 Magnesium 29.9 29.7 Other metals' Magnesium 0.9 0.9 Approximately 0.1 mole each of calcium, copper, cadmium, zinc, cobalt, and nickel.
Ion exchanger Sea water (ml) Total vol. (ml) Resin bed (cm) Flow rate ml/min Mg taken (mg) Mg found (mg) Difference (mg) No sulfur present.
Table 11. Separation of Magnesium from Artificial Sea Water Samplesn Amberlite Amberlite Amberlite Dowex Dowex IRC 50 IRC 50 IRC 50 50 50 100 130 21.5 X 1 1.1 130.9 130.1 -0.8
100 180 22.5 X 1 1.2 130.9 129.1 -1.8
50 80 20.0 x 1 1.3 65.5 65.4 -0.1
100 160 25.0 X 1 1.1 130.9 127.4 -3.5
50 160 20.0 x 1 0.9 65.5 65.3 -0.2
Dowex 50 50 85 20.0 x 1 0.9 65.5 64.5 -1.0
(I
resin requires a higher pH because the significant chelation of magnesium by the chelating resin takes place only at pH above 7. Furthermore, the kinetics might well be slow with Chelex 100. A separation method has been developed and applied to the analysis of synthetic mixtures containing known amounts of magnesium and calcium or other metal ions. The results of the separations on Dowex 50 and Amberlite IRC-50 are presented in Table I. Separations are sharp and the method is applicable for very high ratios of magnesium to calcium. In order to find the effect of sodium chloride on the adsorption of magnesium, the KI, values at different initial sodium chloride concentrations were measured. The data in Figure 5 show that at higher concentration of sodium chloride Dowex 50 is most affected. At a concentration of 30 grams per liter of sodium chloride, only about 15 %, 50%, and 42 of the initial amount of magnesium were adsorbed by Dowex 50, Amberlite, and Chelex 100, respectively. The absorption of Mg-EGTA and Ca-EGTA complexes on Dowex 1 in chloride form in the range of pH 4.5-11.5 gave poor results. Maximum KI, for the calcium complex was 10 and no significant adsorption for the magnesium complex was observed. This is probably because the ionic size of the complexes is too large to allow an effective exchange with the adsorbed chloride ion. The behavior of magnesium and strontium in organic-waterEGTA systems on Amberlite IRC-50 was also investigated. Six organic solvents were used from which the K D values were obtained as shown in Figure 6. Generally the separation factors C Y : : in mixed systems are higher than in water solutions (see Figure 3). The desirability of the use of organic
solvents for separation falls in the order: Methanol > 2propanol > acetone > 1,Cdioxane > tetrahydrofurane > dimethylformamide. This also follows the order of their dielectric constants. PROCEDURE The following procedure is recommended for the quantitative separation and determination of magnesium. A sample solution containing a slight excess of EGTA (approximately 10 % more than that required to complex the interfering metal ions) is adjusted to pH 6.5-7.0 on Dowex 50-X8 or 6.5-7.3 on Amberlite IRC-50. The solution is passed through a column containing 10 ml of resins previously treated with 0.03M EGTA solution at pH 6.5-7.0. A flow rate of 2-4 ml per minute is recommended. The column is washed with 30-50 ml of0.03MEGTA at pH 6.5-7.0 and with 50-100 ml of water, magnesium is eluted by a sufficient amount of 2-3M hydrochloric acid or a known amount of EDTA. The magnesium in the eluate is titrated complexometrically. By this way magnesium can be separated quantitatively from calcium, strontium, barium, iron, and many other polyvalent metals. RECEIVED for review November 12, 1969. Accepted February 27, 1970. Presented at Pacific Conference on Chemistry and Spectroscopy, October 1969, Anaheim, California. Work supported by Office of Saline Water, U. S. Department of Interior, Grant No. USDI 14-01-0001-1643. Dr. Marhol is on leave from Nuclear Research Institute, Czechoslovak Academy of Sciences, Rez, Czechoslovakia.
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