ACKNOWLEDGMENT
The authors gratefully acknowledge helpful discussions with M. C. R. Symons, A. W. Petrocelli, and M. M. Rauhut, and the assistance of W. H. Jura and L. B. D’Errico, who obtained the aqueous polarograms. LITERATURE CITED
(1) Bennett, J. E., Ingram, D. J. E.,
Symons, M . C. R., George, P., Griffith, J. S., Phil. Mag. 46, 443 (1955). (2) . , Coetzee, J. F., Kolthoff, I. M., J. Am. C h e k SOC.79, 6110 (1957).
(3) Galus, A., Lee, H. Y., Adams, R. N., J. Electroanab Chem. 5, 17 (1963). (4) Given, P. H., Peover, M. E., J . Chem. SOC.,1960, 385. (5) Kaitmazov, G. D., Prokhorov, A. M., Fiz. Tverd. Tela 5 , 347 (1963). (6) Knect, L. A., Kolthoff, I. M . , Inorg. Chem. I I, 195 (1962). (7) Kneu buhl, F. K., J. Chem. Phys. 33, 1074 (1960). ( 8 ) Kolthoff, I. M., Reddy, T. B., J. Blectrochem. SOC.108, 980 (1961). (9) Kroh, J., Green, B. C., Spinks, J. W. F., J. Am. Chem. SOC.83, 2201 (1961). (10) IhIcElroy, A. D., Hashman, J. S., Inorg. Chem. 3,1798 (1964). :; ANAL.&EM. 35, (11) Rlaricle, D. L 683 (1963). ’
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(12) Office of Naval Research and Advanced Research Projects Agency, Contract Nonr. 4200(00h ,, Tech. ReDt. 4 (July 1964). (13) Panfilov, V. N., Kinetika i Kataliz 5, 211 (1964). (14) Russell. G. A,. Chem. Ena. News 42, 49 (April 20, 1964). (15) Smith, R. C., Wyard, S. J., Nature 186, 226 (1960). (16) Vannenberg, N., “Progress in Inorganic Chemistry,” Vol. 4, pp. 125-97, Interscience, New York, 1962. ~
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RECEIVEDfor review June 18, 1965. Accepted August 25, 1965. Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
Preconcentration of Trace Elements by Precipitation Ion Exchange Using Organic-Hydrochloric Acid Systems R. R. RUCH,’ FOUAD TERA, and G. H. MORRISON Deparfrnenf o f Chemistry, Cornell University, Ifhaca, N. Y . The approach to preconcentration of trace elements by precipitation ion exchange is extended and improved by the use of organic-HCI eluent systems. The matrix and traces are first sorbed on a cation exchange column; the matrix is then precipitated by the organic-HCI eluent, followed by nonselective elution of the traces. The separation i s based on the insolubility of the matrix and the low selectivity of the traces in 7070 pdioxane-3070 concentrated hydrochloric acid (v./v.). As many as 40 trace elements may b e preconcentrated in greater than 90% yields, matrix-free or with only small amounts of matrix from the following matrices: Li, Na, K, Rb, M g , Ca, Sr, Bo, Sc, Y, and La. Using 81% dioxane-10% ethanol-9% concentrated HCI (by volume) as eluent, matrices such as Nil Cr, Mn, Pb, and AI demonstrate sufficient insolubility and delayed column breakthrough, as well as favorable trace recovery, to b e of value in preconcentration.
A
NONSELECTIVE and general approach to the simultaneous preconcentration of many trace elements from a variety of matrices has been accomplished through the combination of ion exchange and precipitation. The method, called “precipitation ion exchange” ( 2 ) , is based on the initial sorption of the sample on a n ion exchange column, followed by immobilization of the matrix on the column by precipitation, and nonselective elution of as many trace elements as possible before breakthrough of the matrix by gradual dissolution of the precipitate.
The previous study employing a cation exchange column and concentrated HCl as the precipitant-eluent permitted the possible preconcentration of more than 30 trace elements with greater than 90% recovery in a small volume from such matrix elements as Na, K, Sr, Ba, and Ag. The present study enlarges the scope of the general approach through the use of 70% pdioxane-30yo concentrated HCl (v./v.) as the precipitant-eluent. Resulting lower matrix solubilities coupled with low distribution coefficients for most trace transition elements permit the possible preconcentration of more than 40 elements in a small volume from the following matrices: Li, Na, K, Rb, Mg, Ca, Sr, Ba, Sc, Y, and La. Using 81% dioxane-lOyo ethanol9% concentrated HC1 (by volume) as eluent, matrices such as Xi, Cr, Mn, Pb, and A1 demonstrated sufficient insolubility and delayed column breakthrough, as well as favorable trace recovery, to be of value in preconcentration. EXPERIMENTAL
Reagents, Tracers, Apparatus. All reagents used were of reagent grade and were employed without further purification. Dowex 50W-X2, 100t o 200-mesh, was used almost exclusively. It was purified by passing large excesses of 6iM and 2-44 HC1 and washing thoroughly with distilled water until a negative acid test was obtained. Radioactive tracers were obtained from Oak Ridge National Laboratory or Isoserve, Inc., or produced in the Cornel1 University TRIGA Mark I1 Reactor. The apparatus for fraction collection and the equipment
used for counting radioactivity hav been described (2). General Procedure. All studies of trace behavior were performed using radioactive tracers. Matrix studies involved radioactive tracers, flame photometry, or gravimetry. The individual studies of various matrix and trace elements involved the sequence of sorption of the matrix and trace on t h e cation exchange resin, precipitation of the matrix on the column, and then elution of the trace elements. Procedure Using 70% Dioxane30% Concentrated H C I System.
SORPTION. Twelve milliequivalents of each matrix element studied (Li, Na, K , Mg, Sc, and Y) were sorbed at a flow rate of 0.5 t o 1 ml. per minute on a 1.1-cm.-diameter column containing 18 ml. of water-equilibrated Dowex 50W-X2 resin. Other elements studied (Rb, Ca, Sr, Ba, and La) were sorbed on a column containing 20 ml. of resin. The columns were then washed with about 5 ml. of water t o ensure complete sorption. The eluate from this operation was collected and combined with t h e eluate obtained in the elution step. The resins containing matrix elements Li, Na, K, Mg, Sc, and Y were columnwashed with 30 ml. of dioxane and suction-dried for l/* hour. The resins containing matrix elements Rb, Ca, and La were column-washed with 15 ml. of a solution consisting of 7 parts of dioxane and 3 parts of water by volume. The resins containing Sr and Ba were treated as directed below. PRECIPITATION. The dried resins containing matrix elements Li, Na, K, Mg, Sc, and Y were then transferred to a 1.1-cm.-diameter column containing Dowex 50W-X2 equilibrated with a 1 Present address, National Bureau of Standards, Washington 25, D. C.
VOL. 37, NO. 12, NOVEMBER 1965
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Figure 2. Comparison of NaCl matrix breakthrough using concentrated HCI and 70% dioxane-30% concentrated HCI (v./v.) eluents Amount of matrix. 12 meq. (0.702 9.) NoCl Flow rate. 0.2-0.3 ml./min.
solution of 7001, dioxane30% concentrated HC1 (v./v.). The volume of equilibrated resin was 5 ml. for Na, K, SC, and y, and 10 ml, for Li and Mg. The level of solution above the equilibrated resin was 5 ml. for Ka, K, SC,and Y, and 10 ml. for Li and ;LTg. ~~~i~~ the transfer the matrix-contaming resin was extensively stirred to avoid coagulation of the system. Care was taken not to disturb the lower column, This procedure is referred to as the twocolumn procedure. In the one-column procedure employed for Rb, Ca, Sr, Ba, and La, excess wash liquid was drained from the column, followed by gradual column elution (0.2 to 0.3 mi. per minute) using dioxane-HC1 solution to effect precipitation- After about lo ml* Of had passed, the upper portion
washed with dioxane, and suctiondried as described above. The dried resin was then transferred to l*l-cm*diameter containing ml* Of with 50w-x8 resin the dioxane-ethanol-HC1 solution, the level above the equilibrated resin being 10 ml. The system was allowed to settle and subsequently eluted at a flow rate Of L Oa3 ml- per minute. RESULTS AND DISCUSSION
As demonstrated in the previous study (a), the applicability of precipitation ion exchange is governed by two main conditions: The matrix must be insoluble or only sparingly soluble in the eluent to prevent early breakthrough, and the trace elements must have a very low distribution (D,< 1) in the eluting medium to permit a high yield of recovery in a small volume. To increase the scope of the method to include additional matrices, an eluent system resulting in decreased solubility of these major elements was required. The use of organic solvents together with high concentrations of HCl greatly decreased the solubility of many metal chlorides as compared with concentrated HCI alone, while still maintaining low distribution coefficients for most trace
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taken not to disturb the lower part. ELUTION.The systems for all matrix elements were allowed to settle about 5 minutes before elution. Twenty milliliters of eluate was usually sufficient to recover trace elements in 90% yields O r better. The eluate from the sorPtion step was combined with the above
e l u ~ ~ ~ c e d uusing re 81% Dioxanelo% Ethanol-()% concentrated HCI System. ~~~l~~ milliequivalents of matrices Ni+2, Cr+3, bln+2, pb+2, and Al+3 were sorbed on 18 ml. of resin, 1566
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ANALYTICAL CHEMISTRY
elements. p-Dioxane was chosen for its low dielectric constant, inert character, fair miscibility, and excellent viscosity, The various precipitant-eluent systems available for investigation were dictated by immiscibility of concentrated hydrochloric acid and pdioxane a t concentrations higher than 70% by volume of the latter. The addition of ethyl alcohol a t higher dioxane concentrations with HCl eliminated immiscibility and extended the range of applicability to include matrices of Ni, Cr, Mn, Al, and Pb. Under extreme conditions involving dioxane saturated with hydrogen chloride gas, the solubilities of many metal chlorides were greatly decreased; however, trace recovery behavior was generally unfavorable. Each of these systems was investigated in detail to ascertain optimum conditions for delayed matrix column breakthrough and good trace recovery. Matrix Column Behavior. Using an eluent system consisting of 70% dioxane-30% concentrated HCl, the following elements demonstrated sufficiently delayed breakthrough as well as diminished levels upon breakthrough to permit their applicability to preconcentration: Li, Na, K, Rb,
Mg, Ca, Sr, Ba, Sc, Y, and La (Figure 1). The matrix column behavior is governed by matrix solubility and D, in the particular eluent system. In general, the amount of expected saturated breakthrough corresponded to the solubility. I n the case of Mg, the beneficial combination of intermediate D, and slight solubility in the above eluent system permitted its applicability, whereas using just concentrated HCl, the combined higher solubility, and low D, of this matrix element resulted in prohibitive breakthrough. Breakthrough behavior of Na, K, Rb, Sr, and Ba was greatly improved by using the mixed solvent system as compared with the results obtained in the previous study using concentrated HC1. Figure 2 compares the breakthrough behavior of NaCl using these two systems. The Do’s are < I and 6, and the solubilities are -1 and -0.2 mg. per ml., respectively, for concentrated HCl and 70% dioxane-30% concentrated HC1. Using 81% dioxane-lO~c ethanol9% concentrated HC1, a matrix breakthrough of
