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Selective retention of alkali metals on cation-exchange resins
Talanta. 1965. Vol. 12. pp. 81 to 90. Pergamon Prela Ltd. Printed in Northern Ireland
SELECTIVE RETENTION OF ALKALI METALS ON CATION-EXCHANGE RESINS RAPID SEPARATION OF ALKALI
EUGENE D. Or_?&
METALS FROM OTHER METALS* and HAROLD R. SOBEL
Department of Chemistry, Franklin and Marshall College Lancaster, Pennsylvania, U.S.A. (Received 17 August 1964. Accepted 22 September 1964) Summary-The alkali metals, as a group, can be rapidly separated from multivalent metals by batchwise precomplexing of the multivalent metals with EDTA at pH 8 and passing the solution through a cation resin in the tetramethylammonium ion form. After isolation of the alkali metals on the resin, they can be rapidly eluted as a group and concentrated. Separations of the alkali metals from the alkaline earths and cadmium are illustrated. Variables affecting the efficiency of alkali metal uptake have been studied in detail. INTRODUCTION
THEREis a need for a simple, rapid method of separating the alkali metals, as a group, from other metal ions. In flame photometry, for example, many alkaline earths interfere with the analysis of the alkali metals,l and vice uer.sa.1~2 The high selectivity of various inorganic ion-exchange crystals for certain alkali metals in acidic solutions is becoming well known, and has been used very successfully for the separation of individual alkali metals.%’ A group separation of the alkali metals from the alkaline earths has been worked out on the ammonium form of zirconium phosphate, but the ammonium form has essentially no selectivity left for the alkali metals, and they are eluted as a group, ahead of the alkaline earths3 Likewise, in previously reported group separations of the alkali metals on organic ion-exchange resins, the heavier metals are preferentially held on the resin while the alkali metals are made to pass into the effluent. For example, Tsubota elutes sodium and potassium from a cation-exchange resin with dilute HCl or a formate buffer eluant, and then removed heavier metals with various formate buffer mixtures.* Schumacher preferentially eluted microgram amounts of potassium and rubidium from Dowex-50 ahead of the alkaline earths and rare earths .s Blaedel, Olsen and Buchanan held radiotracer amounts of most of the alkali metals on a cation resin while sequentially eluting groups of heavier metals, but the separation necessitated very low ionic strength eluants, and is applicable only to microgram amounts of metals10 Anion-exchange resins have been used to separate the alkali metals from certain other metal ions. Samuelson and SjiistrGm used Dowex-2 anion-exchange resin in a mixture of EDTA and acetate forms to absorb calcium and magnesium from solutions containing sodium and potassium .ll Samuelson, Sjiistriim and Forsblom separated the alkali metals from calcium, magnesium, vanadium(IV), iron(III), aluminium(III), l Abstracted in part from the Honors Dissertation of Harold R. Sobel, A. B., Franklin and Marshall College, l!X4. t Present address : Department of Chemistry, University of South Florida,Tampa, Florida, U.S.A.
81
82
EUGENE
D. OLSENand HAROLD R.
SOBEL
copper( nickel(II), cobalt(II), manganese(I1) and zinc(I1) by passing solutions of these ions through a column of Dowex-2 in the EDTA, acetate, and oxalate forms. The alkali metals pass into the effluent while the multivalent ions are retained on the resin.12 Although this is a satisfactory group separation, the alkali metals cannot be concentrated by this method. In contrast, the procedure described in this paper retains the alkali metals on a cation-exchange resin and allows the other metal cations to pass through. Thus, even though the alkali metals ordinarily possess the weakest exchange affinity of all the metal ions,13 and can readily be displaced by dilute hydrogen ion or ammonium ion,1° the selectivity can be partially reversed by batchwise precomplexing of the multivalent cations with EDTA and passing this solution through a cation resin in the tetramethylammonium ion form. Variables affecting the efficiency of alkali metal uptake have been studied in detail. EXPERIMENTAL
Apparatus and reagents The ion-exchange resin used in most of this work was Baker Analyzed Dowex SOW-X8, 100-200 mesh, having a total exchange capacity of 1.8 meq/ml in the water-swollen hydrogen ion form. Baker Analyzed Dowex 5OW-X12, 100-200 mesh, had a total exchange capacity of 2.2 meq/ml, and Dowex A-l resin, used in preliminary studies, was obtained from the Dow Chemical Company and had a total exchange capacity of 0.66 meq/ml. The breakthrough capacity of the resin (the number of meq of ions that can be. taken up quantitatively during column operations) varies with experimental conditions, and is expressed in this paper as the percentage of total exchange capacity. The resins were cleaned in large lots as described previously.1o Theglass columns used to hold the resin were 20 cm long by 12 mm inside diameter, closed at the lower end by a coarse sintered-glass disk upon which the resin bed rested, and fitted at the top with a bowl 8 cm in diameter to hold eluants. A resin volume of 10.0 ml (H-form) was used in most of this work, giving a bed height of about 9.5 cm in the hydrogen ion form, and 10.2 cm in the tetramethylammonium ion form. The column free volume is about 3.8 ml.” Five-ml siphon pipettes, used to collect 5- or lo-ml fractions of column effluent, were obtained from the Ace Glass Company, Vineland, New Jersey, U.S.A. and were recalibrated under the conditions of use. Reagent-grade EDTA (ethylenediaminetetra-acetic acid) was obtained in the acid form. Tetramethylammonium hydroxide was prepared according to the method of Peracchio and Meloche,ls using purified silver oxide (Fisher Scientific Co.) and highest purity tetramethylammonium chloride (Eastman Organic Chemical Co.). Highest purity tetraethylammonium hydroxide was obtained as a 10 % solution from Eastman Organic Chemical Co., but commercial tetramethylammonium hydroxide could not be used because the sodium ion content was over O.OSM. The nitrate or chloride salts of all metals were reagent-grade. Procedure Ten ml of Dowex 5OW-X8 in the hydrogen ion from was converted to the tetramethylammonium ion form by passing 05-0.8M tetramethylammonium hydroxide through the column at a flow rate of about 1 ml/min until the effluent was basic to litmus; then the column was washed with 20 ml of water. In preliminary experiments in which effluent pH was plotted against effluent volume, using an apparatus described previously, lo it was found that the column is completely converted with an equivalent amount of the reagent, indicating virtually 100’~ efficiency of conversion. When the conversion was attempted with l.OM tetramethylammonium chloride, it was found that even a ten-fold excess was insufficient for quantitative conversion, as indicated by a slow rise of efliluent pH. This finding is consistant with that of others.lB The sample solution, which may contain all the alkali metals at any concentration up to about O.lM, and heavier metals up to a total concentration of about O.O5M, was made 2% in EDTA (0.068M) and adjusted to pH 8.0 with tetramethylammonium hydroxide. The sample solution was then passed continuously through the column at a flow rate-of 1.0 ml/min and 5- or lO-ml fractions were collected until breakthrough equilibrium was achieved. After washing the column with 20 ml of water, the alkali metals were eluted with 5MHCI,2-ml fractions being collected in a graduated cylinder.
Retention of alkali metals on cation-exchange
a3
resins
Analysis A Beckman Model DU spectrophotometer with flame attachment was used for analysis of effluent fractions. An oxygen-hydrogen flame was used for barium, and an oxygen-acetylene flame was used for all other elements. At high concentrations, dilutions were performed to minim& selfabsorption. To enhance barium readings, 1: 1 dilutions with acetone were made. Standard solutions used for flame analysis contained the same concentration of EDTA as the effluent fraction analysed. Because the flame photometric analysis of magnesium and cadmium is not very sensitive,’ the behaviour of these elements was tested by running them individually, with only the alkali metals present, and effluent fractions were analysed by indirect titration of the excess EDTA with standard magnesium nitrate solution, using Eriochrome Black T indicator. I7 After elution of the resin with HCI, magnesium was qualitatively tested for with p-nitrobenzeneazoresorcinol,‘8 and cadmium was tested for polarographically. RESULTS
AND
DISCUSSION
Fig. 1 illustrates the separation that can be achieved when an ionic solution containing calcium, strontium, lithium, potassium, rubidium and caesium, each at a
"s *
0.60
f
040
Et = 0.20 z
0
20
40
60
60
Effluent volume,
FIG. l.-Breakthrough
100
I20
140
0
ml
of lo-‘M Li, Na, K, Rb, Cs, Ca and Sr on 10.0 ml of Dowex 5OW-X8, (CHJ)aN+ form.
concentration of 0.OlOM, is made 2 ‘A in EDTA at pH 8 with tetramethylammonium hydroxide and is passed continuously through 10 ml of Dowex 5OW-X8 in the tetramethylammonium form at a flow rate of 1.0 ml/min. Calcium and strontium breakthrough immediately, and rapidly rise to influent concentration, and no leakage of the alkali metals occurs until over 80 ml of solution have passed. Thus, the first 80 ml can be collected free from all alkali metals. Magnesium, barium and cadmium, tested individually, behave similarly to calcium and strontium. After washing the column with 20 ml of water, 5M HCl can be used to elute the alkali metals rapidly from the resin, as illustrated in Fig. 2. Only the lithium and caesium curves are shown, because the curve for sodium is similar to that of lithium, and the curve for rubidium is almost identical to that of caesium, with the potassium curve comming in between. No calcium or strontium can be detected in the HCl fraction. In samples containing magnesium and barium, 0.5 % of the total magnesium and about 1 o/0of the total barium appear in the HCl fraction. Cadmium, like calcium and strontium, is quantitatively complexed by the EDTA, and does not appear in the
84
EUGENE
D. OLSN and
HAROLD
R.
S~BEL
HCI fraction. The amount of magnesium and barium retained on the resin during the original passing of the sample solution is too small to give a measurable deer ease in the flame analysis of the EDTA effluent, and likewise the conta~nation of the 5M HCI fraction is too small to interfere in the ffame photometric analysis of the alkali metals. If it were desired to obtain the alkali metals completely free from magnesium and barium, this could be accomplished by elution of the column with @lM HCI instead of 5M HCl. Three hundred ml of 0-M HCl were found to be sufficient to elute all the alkalies quantitatively *, leaving magnesium and barium on the resin from which they
Effluent volume, mi
FOG.2.--Eiution
of alkali metals with SM HCl after passing 10~*I%?alkali metals through 10-O ml of Dowex 5OW-X8 until b~kthrough equiIib~um.
can be rapidly eluted with 5M HCI. No attempt was made to improve on these latter separation conditions. It can be calculated from Fig. 1 that a total of 4.74 meq of alkali metals are quantitatively retained on the resin up to breakthrough of each of the ions. This represents 26 % of the total exchange capacity of the resin. Since this breakthrough capacity is critically dependent upon a large number of variables, the next section is devoted to a study of these variables. Conditions afecting breakthrough capacity
In most of the following breakthrough studies, lithium, sodium and potassium were chosen to be representative of all the alkaiies, because rubidium and caesium can * Efutionof the alkali metals with either O*lOM HCI or @05M HCI results in mutuaily~veriapping elution curves, indicating that the various alkali metals are probably retained in a somewhat random distribution on the resin, rather than in sharply-defined bands. No separation of the individual alkali metals can be expected by this technique because of the excessive loading of the resin.
Retention of alkali metals on cation-exchange resins
85
be expected to show at least equal or greater uptake on the resin. Calcium was chosen to represent the alkaline earths because of its high sensitivity of detection with a flame phot0meter.l Type of resin: Previous studies in this laboratory indicated that a rather high degree of selective retention of the alkali metals in the presence of other metal ions could be accomplished on Dowex A-l, a chelating resin, when the resin is used in the
tetramethylammonium ion form and EDTA is used to complex the other metals into the anionic form.ls To compare the breakthrough capacities of Dowex SOW-X8 and Dowex A-l, breakthrough curves were determined for each kind of resin using an influent solution containing 0,OIM Li, Na, K and Ca, and 2% in EDTA at pH 7-O with tetramethylammonium hydroxide. * The breakthrough capacity on Dowex A-l was 8.0% of its total exchange capacity, whereas on Dowex SOW the breakthrough capacity was 26% of its total exchange capacity. It is interesting that in both runs the order of breakthrough is sodium, potassium, and then lithium. For Dower, A-l this order has been explained on the basis of chelation of lithium by the i~nodiacetic acid groups on the resin. Is With Dowex SOW this explanation would not be valid, but no satisfactory explanation has yet been found. Because of the superior breakthrough capacity of Dowex SOW, it is used in all the studies that follow. TABLE I.-EFFECT OF RESINCXXMTERION ON ALKALI METAL BREAKTHROUGH
CAPACITY
0.01~~ in Li+, Na+, KC, Ca*+; 2 % in EDTA at pH 7.0 with hydroxide of counter ion; Resin: Dowex SOW-X8, in counter ion form indicated; Flow rate: 1.0 ml/min.
Influent
:
Breakthrough capacity Counter ion
(% of total capacity)
g&N+ (‘GH,),N+
5-6 26 28
Type of counter ion: Table I summarises the effect of using Dowex SOW-X8 in the ammonium ion and te~aethylammonium ion forms as compared to the tetramethylammonium ion form. It was of considerable interest to compare the effect of substituting ammonium ion in all places in the procedure where tetramethylammonium ion has been adopted because the ammonium ion form of the resin is commonly used in cationexchange separations of metal ions using chelating agent eluants.10F13 The relatively low efficiency of alkali metal retention in the presence of ammonium ion (56 %) is in accord with the findings of others .l” Thus, though it could have been predicted on the basis of steric considerations that the use of tetramethylammonium ion instead of ammonium ion would result in an improvement in the selective retention of the alkali metals, the magnitude of the improvement is remarkable. Using tetraethylammonium ion instead of tetramethylammonium ion could be expected to improve still further the relative affinity of the resin for the alkali metals. The small improvement over * BecauseDowex A-l is a weak acid resin and is subject to severe hydrol~is, it was necessary to make the sample solution for the Dowex A-l experiment O-01M in phosphoric acid before adjusting to pH 7 with tetramethylammonium hydroxide. With the phosphate buffer the etlluent pH remained within 0.1 pH unit of 7; without the buffer the effluent pH rose to 10, and lithium and sodium leaked very early because of complex& by EDTA.ls
EUGENE D.
86
OLSEN
and HAROLD R.
!SOBEL
tetramethylammonium ion that is found (28 % compared to 26 %) is probably attributable to slower rates of diffusion of the tetraethylammonium ion.20 Slower column flow rates might appreciably improve the alkali retention when using tetraethylammonium ion, but this was not tried. Degree of resin cross linkage and solution Jaw rates: Table II summarises the effects of using a higher degree of divinylbenzene (DVB) crosslinkage, and also shows the flow rate dependence of breakthrough capacity. When 10-Oml of Dowex 5OWXl2 is substituted for 10-Oml of Dowex 5OW-X8, the fastest flow obtainable without TABLE IL-EFFECTS OF DEGREEOF RESIN CROSSLINKAGE AND SOLUTION
FLOW
RATES ON ALKALI METAL
BREAKTHROUGH
CAPACITY
Influent:
O.OlOOM Li+, Na+, K+; O.OSOMCae+; 2% in EDTA at pH 7.0 with (CHI)*NOH; Resin : Dowex SOW in (CH,),N+ form. Flow rate,
Breakthrough capacity (% of total capacity) 12% DVB 8% DVB
ml/min
0.30 1.0 2.5
TABLE
24 19
40 29 -
III.-EFFECT OF CONCENTRATION OF EDTA AND ALKALI METAL BREAKTHROUGH CAPACITY
Influent: O.OlOOM
Li+, Na+,
K+;
pH 7.0 with (CH,),NOH; Resin : Dowex 5OW-X 8 in (CH&N+ Flow rate: I.0 ml/min. Concentration of EDTA, %, w/u 2.0 2.0 3.0 4.5
CALCIUM
ON
Cal+ at concentration specified; EDTA
at
form;
Concentration
of Ca*+,
mole/lilre
0~0100 0.050 0.050 0.110
Breakthrough capacity ( % of total capacity) 26 24 18 9
modifying the apparatus used was 0.30 ml/min, in contrast to the rate of 1-Oml/min conveniently obtained for all other experiments. A significant improvement in breakthrough capacity is realised by using the higher degree of crosslinkage, in agreement with predictions based on steric effects and swelling pressures.21 Because 0.30ml/min is too slow to be convenient, flow rates were increased by attaching 60 cm of 2-mm (i.d.) capillary tubing below the columns to increase the total head of liquid. Thus, at I.0 ml/min, the resin with 12 % DVB gives only a small improvement (29 % compared to 24 %) over the resin with 8 % DVB. Increasing the flow rate with Dowex-X8 likewise decreases the breakthrough capacity, but the dependence does not seem quite so marked as with the 12% crosslinked resin. Dowex 5OW-X8, operated at a flow rate of I.0 ml/min, was chosen for all subsequent studies because of the simplicity of the column equipment and ease of obtaining the reproducible flow rate of 1.0 ml/min. Concentration of EDTA and concentration of chelated ions: The more EDTA contained in the sample solution, the greater is the concentration of multivalent metals that can be separated from the alkali metals. However, as can be seen from Table III,
Retention of alkali metals on cation-exchange resins
87
increasing the concentration of EDTA, and with it the concentration of tetramethylammonium ion, causes the efficiency of alkali metal uptake to decrease. This decrease can be qualitatively explained on the basis of the following equilibrium: RSO,N(CH$,
+ Na+ + RSO,Na
+ (CHJ,N+.
Thus, as the concentration of tetramethylammonium ion increases, the alkali metal capacity should decrease. Likewise, increasing the concentration of calcium ion tends to decrease the alkali metal breakthrough capacity, but the concentration dependence is much smaller than that of EDTA, because at pH 7 EDTA is predominantlyz2 in the form HY”, and little extra tetramethylammonium hydroxide need be added to neutralise the hydrogen ion released in the chelation reaction. The effect of concentration of multivalent ions on the extent of chelation is discussed in the next section. pH and concentration of multivalent ions: Because EDTA complexes with lithium above pH 8.3 and with sodium above 9.0, 22~23it was necessary to work at pH 8 or below. On the other hand, EDTA will not complex with many metal ions at low pH, and in addition, EDTA becomes rather insoluble below pH 4*5.* Therefore, the
potentially useable pH range for this separation was pH 5-8, with the optimum pH being the lowest pH at which multivalent metal ions could be complexed quantitatively into the anionic form. The lower this pH, the better, because then the tetramethylammonium ion concentration would be at a minimum, and presumably the efficiency of alkali retention would be at a maximum. Because barium forms the weakest EDTA complex of all the multivalent metals, 26it was chosen to determine the lower pH limit. An 0.050M barium ion solution was made 2 % in EDTA and adjusted to the required pH with tetramethylammonium hydroxide. At pH 5 it was found that virtually all the barium ion was retained by the resin. At pH 7 the breakthrough effluent analysis indicated that approximately 80 % of the barium was complexed into the anionic form, 20 % being retained on the resin. At pH 8 the breakthrough effluent analysis indicated quantitative complexing of barium. However, elution of the resin with 5M HClindicated that about 1-2 Y0of the total barium had been retained by the resin during the breakthrough run. Higher pH’s resulted in early breakthrough of lithium, and at pH 10 both lithium and sodium are complexed and reach influent concentration at around 2530 ml. Even at pH 10 a trace of barium is retained on the resin.* Because negligible lithium complexing occurs at pH 8, and only a small amount of barium remains uncomplexed at this pH, pH 8 was chosen as the optimum pH for the separation. ’ Tests with varying concentrations of barium revealed that the retention of barium (as revealed by a 5M HCI elution) drops to about 1% in samples containing 0.OIM barium, and if 0.005M barium is used, only a trace (co-5 %) can be detected. The concentration level of strontium, which forms the second weakest EDTA complex of all the multivalent metals,% also effects the quantitativeness of its separation from the * Attempts were made to increase the stability of the barium-EDTA complex by working in 50% ethanol-waterand 50% dioxan-water, but although the extent of complexing was increased, retention of traces of barium by the resin could not be prevented. Attempts were also made to complex barium quantitatively with DTPA (dietbylenetriaminepenta-acetic acid), which forms a complex with barium that is over lOO-fold stronger than the EDTA complex .*6 Although quantitative complexing of barium could be achieved at pH 10, lithium and sodium were likewise complexed at this pH, and so no further attempts were made.
EUGENE D.
88
OL~BN and HAROLD R. SOBEL
alkali metals at pH 8. Whereas O*OlM strontium appears quantitatively complexed, 0.05M strontium is retained by the resin to the extent of about 2 %. Thus, to keep the retention level of multivalent metal cations at 1 ‘A or below, the concentration of each of these ions, in general, should be O*OlM or below. Calcium, and metals forming more stable EDTA complexes than calcium, could probably be tolerated to the extent of about 0*05M, but the requirement of excess EDTA further restricts the total concentration of multivalent metals to about 0.05M when 2 % EDTA is used. Concentration of alkali metals: Table IV shows the effect of various concentration levels of alkali metals on the alkali metal breakthrough capacity. The sharp drop in breakthrough capacity at trace concentrations of the alkali metals is not surprising in view of the high ratio of tetramethylammonium ions to alkali metal ions at these TABLE IV.-EFFECT OF CONCENTRATIONOF ALKALI METALS ON ALKALI METAL BREAKTHROUGHCAPACITY Influent:
O.OlOOM Ca*+; Li+, Na+, K+ at concentration indicated; EDTA at pH 8.0 with (CH,),NOH; Resin: Dowex 5OW-X8 in (CH,),N+ form; Flow rate : 1 .O ml/min. Concentration of Li+, Na+, K+,
2%
Breakthrough capacity (% of total capacity)
molellitre 1.00 1.00 1.00 1.00
x x x x
10-i 10-e 10-a 10-d
49 20 3.2 0.34
concentrations. At 10-3M alkali metals the ratio of tetramethylammonium ions to alkali metal ions is approximately 100: 1, and at lo-*Malkali metals the ratio is about 1000: 1. These are rather startling competitive odds against the retention of the alkali metals, especially in view of the weakness with which the alkali metals are normally held. Nevertheless, the low breakthrough capacities at the 10-3M and 1O4M levels of alkali metals should not detract from the usefulness of this technique at low concentrations. It is still possible to achieveexcellent separations ofalkali metals from multivalent metals. For example, with a sample containing 10-4M alkali metals under the conditions given in Table IV, over 140 ml of sample solution can be passed through 10 ml of Dowex 5OW-X8 in the tetramethylammonium ion form before the first breakthrough of the alkali metals occurs. (Sodium breaks through at 144 ml, potassium at 188 ml and lithium at 278 ml.) After washing with 20 ml of water, 5M HCl elutes the retained alkali metals quantitatively in less than 20 ml, giving a peak concentration of each of the alkali metal ions of approximately 10-2M, representing a lOO-fold increase in the sensitivity with which they can be detected flame photometrically. Over the 20-ml fraction, the average concentration of each of the alkali metals is 10-3M, representing a lo-fold concentration factor over the original sample. Length of the resin bed: Although the majority of the separations were made with 10-Oml of resin, several runs were repeated with 5.0 and 15-Oml of resin to test the effect of length of the resin bed on the alkali metal breakthrough capacity. In all cases the relative breakthrough capacity (percentage of total exchange capacity) remained the same within the accuracy and reproducibility of measuring breakthrough volumes (about * 3 ml). Thus, the breakthrough capacity figures cited should be
Retention
of alkali metals on cation+xchange
resins
89
useful in estimating the amount of resin that should be used to isolate a given amount of alkali metals. CONCLUSIONS
Results presented in this paper emphasise the separation of the alkali metals from the alkaline earths. Because other multivalent metals form even stronger EDTA complexes than do the alkaline earths, 26 the method described should be useful to separate a great many of these metals from the alkali metals, provided that an excess of EDTA is maintained. The separation of cadmium from the alkali metals is given as an example. The separation and isolation of the alkali group metals is simple and rapid and serves as a way of concentrating dilute solutions of these metals. The method is only useful for a separation of the alkali metals as a group. Further separation of the individual elements was not attempted, but should be possible using any of a number of separation procedures that have been devised.3l13 In this procedure the original sample solution is completely freed of alkali metals. If, for example, one were determining the alkaline earths by flame photometry, the complete removal of the alkali metals should contribute greatly to the ease and accuracy of the analysis. At the same time, the presence of EDTA in the sample solution should have the advantage of minimising anion interferences in the flame photometric analysis.27 Acknowledgmenrs-Appreciation is expressed to Charles E. Forbes confirmat breakthrough experiments. The work was supported National Science Foundation.
for performing many of the in part by a grant from the
Zusammenfassnn8--Die Gruppe der Alkalimetalle kann von mehrwertigen Metallen schnell getrennt werden durch portionsweise Komplexbildung der mehrwertigen Metalle mit EDTA bei pH 8 und Passieren der Losung durch einen Kationenaustauscher in der Tetramethylammonium-Form. Nach Isolierung der Alkalimetalle auf dem Harz kann die game Gruppe schnell eluiert und konzentriert werden. Trennungen der Alkalimetalle von Erdalkalien und Cadmium werden gezeigt. Bedingungen, die den Wirkungsgrad der Alkalimetallaufnahme beeinflussen, wurden im einzelnen untersucht. R&n&-On peut &parer rapidement les metaux alcalins, en tant que groupe, des metaux polyvalents, en complexant au pr6alable l’ensemble de ceux-ci au moyen d’EDTA a pH 8, puis en passant la solution sur une r&sine cationique sous forme ion t6tramethylammonium. Apres isolement des metaux alcalins sur la r&sine, on peut les bluer rapidement en tant que groupe et les concentrer. On pmsente, a titre d’exemple, des separations des metaux alcalins d’alcalino-terreux et de cadmium. On a CtudiC en detail les variables affectant l’efficacitb de l’absorption des metaux alcalins. REFERENCES 1 J. A. Dean, Flame Photometry. McGraw-Hill Book Company, Inc., New York and London, 1960. * W. H. Foster, Jr. and D. N. Hume, Analyt. Chem., 1959, 31,2033. s K. A. Kraus, H. 0. Phillips, T. A. Carlson and J. S. Johnson, Second International Conference on Peaceful Uses of Atomic Energy, Geneva, 1958, paper 1832. 4 J. Van R. Smit, W. Robb and J. J. Jacobs, J. Inorp. Nuclear C/rem., 1959, 12. 104. 5 C. B. Amphlett, L. A. McDonald, J. S. Burgess a;d J. C. Maynard, ibid., 1959, 10,69. B H. L. Caron and T. T. Sueihara. Analvt. Chem.. 1962. 34. 1082. ’ C. B. Amphlett, P. Eaton,-L. A.‘McDonald and A. J: Miller, J. Inorg. Nucl. Chem., 1964,26,297. 8 H. Tsubota, Nippon Kagaku zasshi, 1960,81,927; Chem. Abs., 1961,55,226681. g E. Schumacher, Helv. Chim. Acta, 1956,39, 531. lo W. J. Blaedel, E. D. Olsen and R. F. Buchanan, Anulyt. Chem., 1960,32, 1866.
90
EUGENE
D.
OLSEN
and HAROLD R.
SOBEL
I1 0. Samuelson and E. Sjostriim, ibid., 1954, 26, 1908. i* 0. Samuelson, E. Sjijstrijm and S. Forsblom, Z. ana@. Chem., 1954, 144,323. I8 0. Samuelson, Zon Exchange Separations in Analytical Chemistry. John Wiley and Sons, New York and London, 1963. I4 Dow Chemical Company, Dowex Zon Exchange. Midland, Michigan, U.S.A. 1958. I6 E. S. Peracchio and V. W. Meloche, J. Amer. Chem. Sot., 1938, 60, 1770. I8 D. K. Hale, D. I. Packham and K. W. Pepper, J. Chem. Sot., 1953,844. Interscience I7 G. Schwarzenbach, Complexometric Titrations. Methuen and Co. Ltd., London; Publishers, Inc., New York, 1957. is A. F. Clifford, Inorganic Chemistry of Qualitative Analysis. Prentice-Hall, Inc., Englewood Cliffs, N.J., U.S.A. 1961. I8 H. R. Sobel and E. D. Olsen, unpublished results, 1963. *OT. R. E. Kressman, J. Phys. Chem., 1952,56, 118. 21F. Helfferich, Zon Exchange. McGraw-Hill Book Co., Inc., New York, 1962. pa A. E. Martell and M. Calvin, Chemistry of the Metal Chehzte Compounds. Prentice-Hall, New York, 1952. 23 F. Nelson, J. Amer. Chem. Sot., 1955, 77, 813. z4 H. Flaschka, EDTA Titrations. Pergamon Press, New York, London and Paris, 1959. *6 C. N. Reilley, R. W. Schmid and F. S. Sadek, J. Chem. Educ., 1959,36, 555. 26J. Bjerrum, G. Schwarzenbach and L. G. Sillen, Stability Constants, Part I: Organic Ligands. The Chemical Society, Burlington House, London, 1958. e7 A. C. West and W. D. Cooke, Andyt. Chem., 1960,32,1471.
SELECTIVE RETENTION OF ALKALI METALS ON CATION-EXCHANGE RESINS RAPID SEPARATION OF ALKALI
EUGENE D. Or_?&
METALS FROM OTHER METALS* and HAROLD R. SOBEL
Department of Chemistry, Franklin and Marshall College Lancaster, Pennsylvania, U.S.A. (Received 17 August 1964. Accepted 22 September 1964) Summary-The alkali metals, as a group, can be rapidly separated from multivalent metals by batchwise precomplexing of the multivalent metals with EDTA at pH 8 and passing the solution through a cation resin in the tetramethylammonium ion form. After isolation of the alkali metals on the resin, they can be rapidly eluted as a group and concentrated. Separations of the alkali metals from the alkaline earths and cadmium are illustrated. Variables affecting the efficiency of alkali metal uptake have been studied in detail. INTRODUCTION
THEREis a need for a simple, rapid method of separating the alkali metals, as a group, from other metal ions. In flame photometry, for example, many alkaline earths interfere with the analysis of the alkali metals,l and vice uer.sa.1~2 The high selectivity of various inorganic ion-exchange crystals for certain alkali metals in acidic solutions is becoming well known, and has been used very successfully for the separation of individual alkali metals.%’ A group separation of the alkali metals from the alkaline earths has been worked out on the ammonium form of zirconium phosphate, but the ammonium form has essentially no selectivity left for the alkali metals, and they are eluted as a group, ahead of the alkaline earths3 Likewise, in previously reported group separations of the alkali metals on organic ion-exchange resins, the heavier metals are preferentially held on the resin while the alkali metals are made to pass into the effluent. For example, Tsubota elutes sodium and potassium from a cation-exchange resin with dilute HCl or a formate buffer eluant, and then removed heavier metals with various formate buffer mixtures.* Schumacher preferentially eluted microgram amounts of potassium and rubidium from Dowex-50 ahead of the alkaline earths and rare earths .s Blaedel, Olsen and Buchanan held radiotracer amounts of most of the alkali metals on a cation resin while sequentially eluting groups of heavier metals, but the separation necessitated very low ionic strength eluants, and is applicable only to microgram amounts of metals10 Anion-exchange resins have been used to separate the alkali metals from certain other metal ions. Samuelson and SjiistrGm used Dowex-2 anion-exchange resin in a mixture of EDTA and acetate forms to absorb calcium and magnesium from solutions containing sodium and potassium .ll Samuelson, Sjiistriim and Forsblom separated the alkali metals from calcium, magnesium, vanadium(IV), iron(III), aluminium(III), l Abstracted in part from the Honors Dissertation of Harold R. Sobel, A. B., Franklin and Marshall College, l!X4. t Present address : Department of Chemistry, University of South Florida,Tampa, Florida, U.S.A.
81
82
EUGENE
D. OLSENand HAROLD R.
SOBEL
copper( nickel(II), cobalt(II), manganese(I1) and zinc(I1) by passing solutions of these ions through a column of Dowex-2 in the EDTA, acetate, and oxalate forms. The alkali metals pass into the effluent while the multivalent ions are retained on the resin.12 Although this is a satisfactory group separation, the alkali metals cannot be concentrated by this method. In contrast, the procedure described in this paper retains the alkali metals on a cation-exchange resin and allows the other metal cations to pass through. Thus, even though the alkali metals ordinarily possess the weakest exchange affinity of all the metal ions,13 and can readily be displaced by dilute hydrogen ion or ammonium ion,1° the selectivity can be partially reversed by batchwise precomplexing of the multivalent cations with EDTA and passing this solution through a cation resin in the tetramethylammonium ion form. Variables affecting the efficiency of alkali metal uptake have been studied in detail. EXPERIMENTAL
Apparatus and reagents The ion-exchange resin used in most of this work was Baker Analyzed Dowex SOW-X8, 100-200 mesh, having a total exchange capacity of 1.8 meq/ml in the water-swollen hydrogen ion form. Baker Analyzed Dowex 5OW-X12, 100-200 mesh, had a total exchange capacity of 2.2 meq/ml, and Dowex A-l resin, used in preliminary studies, was obtained from the Dow Chemical Company and had a total exchange capacity of 0.66 meq/ml. The breakthrough capacity of the resin (the number of meq of ions that can be. taken up quantitatively during column operations) varies with experimental conditions, and is expressed in this paper as the percentage of total exchange capacity. The resins were cleaned in large lots as described previously.1o Theglass columns used to hold the resin were 20 cm long by 12 mm inside diameter, closed at the lower end by a coarse sintered-glass disk upon which the resin bed rested, and fitted at the top with a bowl 8 cm in diameter to hold eluants. A resin volume of 10.0 ml (H-form) was used in most of this work, giving a bed height of about 9.5 cm in the hydrogen ion form, and 10.2 cm in the tetramethylammonium ion form. The column free volume is about 3.8 ml.” Five-ml siphon pipettes, used to collect 5- or lo-ml fractions of column effluent, were obtained from the Ace Glass Company, Vineland, New Jersey, U.S.A. and were recalibrated under the conditions of use. Reagent-grade EDTA (ethylenediaminetetra-acetic acid) was obtained in the acid form. Tetramethylammonium hydroxide was prepared according to the method of Peracchio and Meloche,ls using purified silver oxide (Fisher Scientific Co.) and highest purity tetramethylammonium chloride (Eastman Organic Chemical Co.). Highest purity tetraethylammonium hydroxide was obtained as a 10 % solution from Eastman Organic Chemical Co., but commercial tetramethylammonium hydroxide could not be used because the sodium ion content was over O.OSM. The nitrate or chloride salts of all metals were reagent-grade. Procedure Ten ml of Dowex 5OW-X8 in the hydrogen ion from was converted to the tetramethylammonium ion form by passing 05-0.8M tetramethylammonium hydroxide through the column at a flow rate of about 1 ml/min until the effluent was basic to litmus; then the column was washed with 20 ml of water. In preliminary experiments in which effluent pH was plotted against effluent volume, using an apparatus described previously, lo it was found that the column is completely converted with an equivalent amount of the reagent, indicating virtually 100’~ efficiency of conversion. When the conversion was attempted with l.OM tetramethylammonium chloride, it was found that even a ten-fold excess was insufficient for quantitative conversion, as indicated by a slow rise of efliluent pH. This finding is consistant with that of others.lB The sample solution, which may contain all the alkali metals at any concentration up to about O.lM, and heavier metals up to a total concentration of about O.O5M, was made 2% in EDTA (0.068M) and adjusted to pH 8.0 with tetramethylammonium hydroxide. The sample solution was then passed continuously through the column at a flow rate-of 1.0 ml/min and 5- or lO-ml fractions were collected until breakthrough equilibrium was achieved. After washing the column with 20 ml of water, the alkali metals were eluted with 5MHCI,2-ml fractions being collected in a graduated cylinder.
Retention of alkali metals on cation-exchange
a3
resins
Analysis A Beckman Model DU spectrophotometer with flame attachment was used for analysis of effluent fractions. An oxygen-hydrogen flame was used for barium, and an oxygen-acetylene flame was used for all other elements. At high concentrations, dilutions were performed to minim& selfabsorption. To enhance barium readings, 1: 1 dilutions with acetone were made. Standard solutions used for flame analysis contained the same concentration of EDTA as the effluent fraction analysed. Because the flame photometric analysis of magnesium and cadmium is not very sensitive,’ the behaviour of these elements was tested by running them individually, with only the alkali metals present, and effluent fractions were analysed by indirect titration of the excess EDTA with standard magnesium nitrate solution, using Eriochrome Black T indicator. I7 After elution of the resin with HCI, magnesium was qualitatively tested for with p-nitrobenzeneazoresorcinol,‘8 and cadmium was tested for polarographically. RESULTS
AND
DISCUSSION
Fig. 1 illustrates the separation that can be achieved when an ionic solution containing calcium, strontium, lithium, potassium, rubidium and caesium, each at a
"s *
0.60
f
040
Et = 0.20 z
0
20
40
60
60
Effluent volume,
FIG. l.-Breakthrough
100
I20
140
0
ml
of lo-‘M Li, Na, K, Rb, Cs, Ca and Sr on 10.0 ml of Dowex 5OW-X8, (CHJ)aN+ form.
concentration of 0.OlOM, is made 2 ‘A in EDTA at pH 8 with tetramethylammonium hydroxide and is passed continuously through 10 ml of Dowex 5OW-X8 in the tetramethylammonium form at a flow rate of 1.0 ml/min. Calcium and strontium breakthrough immediately, and rapidly rise to influent concentration, and no leakage of the alkali metals occurs until over 80 ml of solution have passed. Thus, the first 80 ml can be collected free from all alkali metals. Magnesium, barium and cadmium, tested individually, behave similarly to calcium and strontium. After washing the column with 20 ml of water, 5M HCl can be used to elute the alkali metals rapidly from the resin, as illustrated in Fig. 2. Only the lithium and caesium curves are shown, because the curve for sodium is similar to that of lithium, and the curve for rubidium is almost identical to that of caesium, with the potassium curve comming in between. No calcium or strontium can be detected in the HCl fraction. In samples containing magnesium and barium, 0.5 % of the total magnesium and about 1 o/0of the total barium appear in the HCl fraction. Cadmium, like calcium and strontium, is quantitatively complexed by the EDTA, and does not appear in the
84
EUGENE
D. OLSN and
HAROLD
R.
S~BEL
HCI fraction. The amount of magnesium and barium retained on the resin during the original passing of the sample solution is too small to give a measurable deer ease in the flame analysis of the EDTA effluent, and likewise the conta~nation of the 5M HCI fraction is too small to interfere in the ffame photometric analysis of the alkali metals. If it were desired to obtain the alkali metals completely free from magnesium and barium, this could be accomplished by elution of the column with @lM HCI instead of 5M HCl. Three hundred ml of 0-M HCl were found to be sufficient to elute all the alkalies quantitatively *, leaving magnesium and barium on the resin from which they
Effluent volume, mi
FOG.2.--Eiution
of alkali metals with SM HCl after passing 10~*I%?alkali metals through 10-O ml of Dowex 5OW-X8 until b~kthrough equiIib~um.
can be rapidly eluted with 5M HCI. No attempt was made to improve on these latter separation conditions. It can be calculated from Fig. 1 that a total of 4.74 meq of alkali metals are quantitatively retained on the resin up to breakthrough of each of the ions. This represents 26 % of the total exchange capacity of the resin. Since this breakthrough capacity is critically dependent upon a large number of variables, the next section is devoted to a study of these variables. Conditions afecting breakthrough capacity
In most of the following breakthrough studies, lithium, sodium and potassium were chosen to be representative of all the alkaiies, because rubidium and caesium can * Efutionof the alkali metals with either O*lOM HCI or @05M HCI results in mutuaily~veriapping elution curves, indicating that the various alkali metals are probably retained in a somewhat random distribution on the resin, rather than in sharply-defined bands. No separation of the individual alkali metals can be expected by this technique because of the excessive loading of the resin.
Retention of alkali metals on cation-exchange resins
85
be expected to show at least equal or greater uptake on the resin. Calcium was chosen to represent the alkaline earths because of its high sensitivity of detection with a flame phot0meter.l Type of resin: Previous studies in this laboratory indicated that a rather high degree of selective retention of the alkali metals in the presence of other metal ions could be accomplished on Dowex A-l, a chelating resin, when the resin is used in the
tetramethylammonium ion form and EDTA is used to complex the other metals into the anionic form.ls To compare the breakthrough capacities of Dowex SOW-X8 and Dowex A-l, breakthrough curves were determined for each kind of resin using an influent solution containing 0,OIM Li, Na, K and Ca, and 2% in EDTA at pH 7-O with tetramethylammonium hydroxide. * The breakthrough capacity on Dowex A-l was 8.0% of its total exchange capacity, whereas on Dowex SOW the breakthrough capacity was 26% of its total exchange capacity. It is interesting that in both runs the order of breakthrough is sodium, potassium, and then lithium. For Dower, A-l this order has been explained on the basis of chelation of lithium by the i~nodiacetic acid groups on the resin. Is With Dowex SOW this explanation would not be valid, but no satisfactory explanation has yet been found. Because of the superior breakthrough capacity of Dowex SOW, it is used in all the studies that follow. TABLE I.-EFFECT OF RESINCXXMTERION ON ALKALI METAL BREAKTHROUGH
CAPACITY
0.01~~ in Li+, Na+, KC, Ca*+; 2 % in EDTA at pH 7.0 with hydroxide of counter ion; Resin: Dowex SOW-X8, in counter ion form indicated; Flow rate: 1.0 ml/min.
Influent
:
Breakthrough capacity Counter ion
(% of total capacity)
g&N+ (‘GH,),N+
5-6 26 28
Type of counter ion: Table I summarises the effect of using Dowex SOW-X8 in the ammonium ion and te~aethylammonium ion forms as compared to the tetramethylammonium ion form. It was of considerable interest to compare the effect of substituting ammonium ion in all places in the procedure where tetramethylammonium ion has been adopted because the ammonium ion form of the resin is commonly used in cationexchange separations of metal ions using chelating agent eluants.10F13 The relatively low efficiency of alkali metal retention in the presence of ammonium ion (56 %) is in accord with the findings of others .l” Thus, though it could have been predicted on the basis of steric considerations that the use of tetramethylammonium ion instead of ammonium ion would result in an improvement in the selective retention of the alkali metals, the magnitude of the improvement is remarkable. Using tetraethylammonium ion instead of tetramethylammonium ion could be expected to improve still further the relative affinity of the resin for the alkali metals. The small improvement over * BecauseDowex A-l is a weak acid resin and is subject to severe hydrol~is, it was necessary to make the sample solution for the Dowex A-l experiment O-01M in phosphoric acid before adjusting to pH 7 with tetramethylammonium hydroxide. With the phosphate buffer the etlluent pH remained within 0.1 pH unit of 7; without the buffer the effluent pH rose to 10, and lithium and sodium leaked very early because of complex& by EDTA.ls
EUGENE D.
86
OLSEN
and HAROLD R.
!SOBEL
tetramethylammonium ion that is found (28 % compared to 26 %) is probably attributable to slower rates of diffusion of the tetraethylammonium ion.20 Slower column flow rates might appreciably improve the alkali retention when using tetraethylammonium ion, but this was not tried. Degree of resin cross linkage and solution Jaw rates: Table II summarises the effects of using a higher degree of divinylbenzene (DVB) crosslinkage, and also shows the flow rate dependence of breakthrough capacity. When 10-Oml of Dowex 5OWXl2 is substituted for 10-Oml of Dowex 5OW-X8, the fastest flow obtainable without TABLE IL-EFFECTS OF DEGREEOF RESIN CROSSLINKAGE AND SOLUTION
FLOW
RATES ON ALKALI METAL
BREAKTHROUGH
CAPACITY
Influent:
O.OlOOM Li+, Na+, K+; O.OSOMCae+; 2% in EDTA at pH 7.0 with (CHI)*NOH; Resin : Dowex SOW in (CH,),N+ form. Flow rate,
Breakthrough capacity (% of total capacity) 12% DVB 8% DVB
ml/min
0.30 1.0 2.5
TABLE
24 19
40 29 -
III.-EFFECT OF CONCENTRATION OF EDTA AND ALKALI METAL BREAKTHROUGH CAPACITY
Influent: O.OlOOM
Li+, Na+,
K+;
pH 7.0 with (CH,),NOH; Resin : Dowex 5OW-X 8 in (CH&N+ Flow rate: I.0 ml/min. Concentration of EDTA, %, w/u 2.0 2.0 3.0 4.5
CALCIUM
ON
Cal+ at concentration specified; EDTA
at
form;
Concentration
of Ca*+,
mole/lilre
0~0100 0.050 0.050 0.110
Breakthrough capacity ( % of total capacity) 26 24 18 9
modifying the apparatus used was 0.30 ml/min, in contrast to the rate of 1-Oml/min conveniently obtained for all other experiments. A significant improvement in breakthrough capacity is realised by using the higher degree of crosslinkage, in agreement with predictions based on steric effects and swelling pressures.21 Because 0.30ml/min is too slow to be convenient, flow rates were increased by attaching 60 cm of 2-mm (i.d.) capillary tubing below the columns to increase the total head of liquid. Thus, at I.0 ml/min, the resin with 12 % DVB gives only a small improvement (29 % compared to 24 %) over the resin with 8 % DVB. Increasing the flow rate with Dowex-X8 likewise decreases the breakthrough capacity, but the dependence does not seem quite so marked as with the 12% crosslinked resin. Dowex 5OW-X8, operated at a flow rate of I.0 ml/min, was chosen for all subsequent studies because of the simplicity of the column equipment and ease of obtaining the reproducible flow rate of 1.0 ml/min. Concentration of EDTA and concentration of chelated ions: The more EDTA contained in the sample solution, the greater is the concentration of multivalent metals that can be separated from the alkali metals. However, as can be seen from Table III,
Retention of alkali metals on cation-exchange resins
87
increasing the concentration of EDTA, and with it the concentration of tetramethylammonium ion, causes the efficiency of alkali metal uptake to decrease. This decrease can be qualitatively explained on the basis of the following equilibrium: RSO,N(CH$,
+ Na+ + RSO,Na
+ (CHJ,N+.
Thus, as the concentration of tetramethylammonium ion increases, the alkali metal capacity should decrease. Likewise, increasing the concentration of calcium ion tends to decrease the alkali metal breakthrough capacity, but the concentration dependence is much smaller than that of EDTA, because at pH 7 EDTA is predominantlyz2 in the form HY”, and little extra tetramethylammonium hydroxide need be added to neutralise the hydrogen ion released in the chelation reaction. The effect of concentration of multivalent ions on the extent of chelation is discussed in the next section. pH and concentration of multivalent ions: Because EDTA complexes with lithium above pH 8.3 and with sodium above 9.0, 22~23it was necessary to work at pH 8 or below. On the other hand, EDTA will not complex with many metal ions at low pH, and in addition, EDTA becomes rather insoluble below pH 4*5.* Therefore, the
potentially useable pH range for this separation was pH 5-8, with the optimum pH being the lowest pH at which multivalent metal ions could be complexed quantitatively into the anionic form. The lower this pH, the better, because then the tetramethylammonium ion concentration would be at a minimum, and presumably the efficiency of alkali retention would be at a maximum. Because barium forms the weakest EDTA complex of all the multivalent metals, 26it was chosen to determine the lower pH limit. An 0.050M barium ion solution was made 2 % in EDTA and adjusted to the required pH with tetramethylammonium hydroxide. At pH 5 it was found that virtually all the barium ion was retained by the resin. At pH 7 the breakthrough effluent analysis indicated that approximately 80 % of the barium was complexed into the anionic form, 20 % being retained on the resin. At pH 8 the breakthrough effluent analysis indicated quantitative complexing of barium. However, elution of the resin with 5M HClindicated that about 1-2 Y0of the total barium had been retained by the resin during the breakthrough run. Higher pH’s resulted in early breakthrough of lithium, and at pH 10 both lithium and sodium are complexed and reach influent concentration at around 2530 ml. Even at pH 10 a trace of barium is retained on the resin.* Because negligible lithium complexing occurs at pH 8, and only a small amount of barium remains uncomplexed at this pH, pH 8 was chosen as the optimum pH for the separation. ’ Tests with varying concentrations of barium revealed that the retention of barium (as revealed by a 5M HCI elution) drops to about 1% in samples containing 0.OIM barium, and if 0.005M barium is used, only a trace (co-5 %) can be detected. The concentration level of strontium, which forms the second weakest EDTA complex of all the multivalent metals,% also effects the quantitativeness of its separation from the * Attempts were made to increase the stability of the barium-EDTA complex by working in 50% ethanol-waterand 50% dioxan-water, but although the extent of complexing was increased, retention of traces of barium by the resin could not be prevented. Attempts were also made to complex barium quantitatively with DTPA (dietbylenetriaminepenta-acetic acid), which forms a complex with barium that is over lOO-fold stronger than the EDTA complex .*6 Although quantitative complexing of barium could be achieved at pH 10, lithium and sodium were likewise complexed at this pH, and so no further attempts were made.
EUGENE D.
88
OL~BN and HAROLD R. SOBEL
alkali metals at pH 8. Whereas O*OlM strontium appears quantitatively complexed, 0.05M strontium is retained by the resin to the extent of about 2 %. Thus, to keep the retention level of multivalent metal cations at 1 ‘A or below, the concentration of each of these ions, in general, should be O*OlM or below. Calcium, and metals forming more stable EDTA complexes than calcium, could probably be tolerated to the extent of about 0*05M, but the requirement of excess EDTA further restricts the total concentration of multivalent metals to about 0.05M when 2 % EDTA is used. Concentration of alkali metals: Table IV shows the effect of various concentration levels of alkali metals on the alkali metal breakthrough capacity. The sharp drop in breakthrough capacity at trace concentrations of the alkali metals is not surprising in view of the high ratio of tetramethylammonium ions to alkali metal ions at these TABLE IV.-EFFECT OF CONCENTRATIONOF ALKALI METALS ON ALKALI METAL BREAKTHROUGHCAPACITY Influent:
O.OlOOM Ca*+; Li+, Na+, K+ at concentration indicated; EDTA at pH 8.0 with (CH,),NOH; Resin: Dowex 5OW-X8 in (CH,),N+ form; Flow rate : 1 .O ml/min. Concentration of Li+, Na+, K+,
2%
Breakthrough capacity (% of total capacity)
molellitre 1.00 1.00 1.00 1.00
x x x x
10-i 10-e 10-a 10-d
49 20 3.2 0.34
concentrations. At 10-3M alkali metals the ratio of tetramethylammonium ions to alkali metal ions is approximately 100: 1, and at lo-*Malkali metals the ratio is about 1000: 1. These are rather startling competitive odds against the retention of the alkali metals, especially in view of the weakness with which the alkali metals are normally held. Nevertheless, the low breakthrough capacities at the 10-3M and 1O4M levels of alkali metals should not detract from the usefulness of this technique at low concentrations. It is still possible to achieveexcellent separations ofalkali metals from multivalent metals. For example, with a sample containing 10-4M alkali metals under the conditions given in Table IV, over 140 ml of sample solution can be passed through 10 ml of Dowex 5OW-X8 in the tetramethylammonium ion form before the first breakthrough of the alkali metals occurs. (Sodium breaks through at 144 ml, potassium at 188 ml and lithium at 278 ml.) After washing with 20 ml of water, 5M HCl elutes the retained alkali metals quantitatively in less than 20 ml, giving a peak concentration of each of the alkali metal ions of approximately 10-2M, representing a lOO-fold increase in the sensitivity with which they can be detected flame photometrically. Over the 20-ml fraction, the average concentration of each of the alkali metals is 10-3M, representing a lo-fold concentration factor over the original sample. Length of the resin bed: Although the majority of the separations were made with 10-Oml of resin, several runs were repeated with 5.0 and 15-Oml of resin to test the effect of length of the resin bed on the alkali metal breakthrough capacity. In all cases the relative breakthrough capacity (percentage of total exchange capacity) remained the same within the accuracy and reproducibility of measuring breakthrough volumes (about * 3 ml). Thus, the breakthrough capacity figures cited should be
Retention
of alkali metals on cation+xchange
resins
89
useful in estimating the amount of resin that should be used to isolate a given amount of alkali metals. CONCLUSIONS
Results presented in this paper emphasise the separation of the alkali metals from the alkaline earths. Because other multivalent metals form even stronger EDTA complexes than do the alkaline earths, 26 the method described should be useful to separate a great many of these metals from the alkali metals, provided that an excess of EDTA is maintained. The separation of cadmium from the alkali metals is given as an example. The separation and isolation of the alkali group metals is simple and rapid and serves as a way of concentrating dilute solutions of these metals. The method is only useful for a separation of the alkali metals as a group. Further separation of the individual elements was not attempted, but should be possible using any of a number of separation procedures that have been devised.3l13 In this procedure the original sample solution is completely freed of alkali metals. If, for example, one were determining the alkaline earths by flame photometry, the complete removal of the alkali metals should contribute greatly to the ease and accuracy of the analysis. At the same time, the presence of EDTA in the sample solution should have the advantage of minimising anion interferences in the flame photometric analysis.27 Acknowledgmenrs-Appreciation is expressed to Charles E. Forbes confirmat breakthrough experiments. The work was supported National Science Foundation.
for performing many of the in part by a grant from the
Zusammenfassnn8--Die Gruppe der Alkalimetalle kann von mehrwertigen Metallen schnell getrennt werden durch portionsweise Komplexbildung der mehrwertigen Metalle mit EDTA bei pH 8 und Passieren der Losung durch einen Kationenaustauscher in der Tetramethylammonium-Form. Nach Isolierung der Alkalimetalle auf dem Harz kann die game Gruppe schnell eluiert und konzentriert werden. Trennungen der Alkalimetalle von Erdalkalien und Cadmium werden gezeigt. Bedingungen, die den Wirkungsgrad der Alkalimetallaufnahme beeinflussen, wurden im einzelnen untersucht. R&n&-On peut &parer rapidement les metaux alcalins, en tant que groupe, des metaux polyvalents, en complexant au pr6alable l’ensemble de ceux-ci au moyen d’EDTA a pH 8, puis en passant la solution sur une r&sine cationique sous forme ion t6tramethylammonium. Apres isolement des metaux alcalins sur la r&sine, on peut les bluer rapidement en tant que groupe et les concentrer. On pmsente, a titre d’exemple, des separations des metaux alcalins d’alcalino-terreux et de cadmium. On a CtudiC en detail les variables affectant l’efficacitb de l’absorption des metaux alcalins. REFERENCES 1 J. A. Dean, Flame Photometry. McGraw-Hill Book Company, Inc., New York and London, 1960. * W. H. Foster, Jr. and D. N. Hume, Analyt. Chem., 1959, 31,2033. s K. A. Kraus, H. 0. Phillips, T. A. Carlson and J. S. Johnson, Second International Conference on Peaceful Uses of Atomic Energy, Geneva, 1958, paper 1832. 4 J. Van R. Smit, W. Robb and J. J. Jacobs, J. Inorp. Nuclear C/rem., 1959, 12. 104. 5 C. B. Amphlett, L. A. McDonald, J. S. Burgess a;d J. C. Maynard, ibid., 1959, 10,69. B H. L. Caron and T. T. Sueihara. Analvt. Chem.. 1962. 34. 1082. ’ C. B. Amphlett, P. Eaton,-L. A.‘McDonald and A. J: Miller, J. Inorg. Nucl. Chem., 1964,26,297. 8 H. Tsubota, Nippon Kagaku zasshi, 1960,81,927; Chem. Abs., 1961,55,226681. g E. Schumacher, Helv. Chim. Acta, 1956,39, 531. lo W. J. Blaedel, E. D. Olsen and R. F. Buchanan, Anulyt. Chem., 1960,32, 1866.
90
EUGENE
D.
OLSEN
and HAROLD R.
SOBEL
I1 0. Samuelson and E. Sjostriim, ibid., 1954, 26, 1908. i* 0. Samuelson, E. Sjijstrijm and S. Forsblom, Z. ana@. Chem., 1954, 144,323. I8 0. Samuelson, Zon Exchange Separations in Analytical Chemistry. John Wiley and Sons, New York and London, 1963. I4 Dow Chemical Company, Dowex Zon Exchange. Midland, Michigan, U.S.A. 1958. I6 E. S. Peracchio and V. W. Meloche, J. Amer. Chem. Sot., 1938, 60, 1770. I8 D. K. Hale, D. I. Packham and K. W. Pepper, J. Chem. Sot., 1953,844. Interscience I7 G. Schwarzenbach, Complexometric Titrations. Methuen and Co. Ltd., London; Publishers, Inc., New York, 1957. is A. F. Clifford, Inorganic Chemistry of Qualitative Analysis. Prentice-Hall, Inc., Englewood Cliffs, N.J., U.S.A. 1961. I8 H. R. Sobel and E. D. Olsen, unpublished results, 1963. *OT. R. E. Kressman, J. Phys. Chem., 1952,56, 118. 21F. Helfferich, Zon Exchange. McGraw-Hill Book Co., Inc., New York, 1962. pa A. E. Martell and M. Calvin, Chemistry of the Metal Chehzte Compounds. Prentice-Hall, New York, 1952. 23 F. Nelson, J. Amer. Chem. Sot., 1955, 77, 813. z4 H. Flaschka, EDTA Titrations. Pergamon Press, New York, London and Paris, 1959. *6 C. N. Reilley, R. W. Schmid and F. S. Sadek, J. Chem. Educ., 1959,36, 555. 26J. Bjerrum, G. Schwarzenbach and L. G. Sillen, Stability Constants, Part I: Organic Ligands. The Chemical Society, Burlington House, London, 1958. e7 A. C. West and W. D. Cooke, Andyt. Chem., 1960,32,1471.