Quantitative Separation of Barium from Strontium and Other Elements by Cation Exchange Chromatography in Hydrochloric Acid-Organic Solvent Eluents F. W . E . Strelow National Chemical Research Laboratory, South African Council for Scientific and Industrial Research, Pretoria, South Africa Cation exchange distribution coefficients for Ba and Sr in mixtures of HCI with the organic solvents methanol, ethanol, acetone, and dioxane are presented with AG50W-X8 and AG50W-Xl2 resins. A method for the quantitative separation of Ba from Sr and other elements is described. Sr is eluted from a column of 60 ml AG50W-X8 resin of 200- to 400-mesh particle size by 3.OM HCI containing 20% ethanol at a flow rate of 2.5 f 0.3 ml per minute. Ba is retained by the column and can be eluted with 3M HN03. AI, Ca, Mg, Fe(lll), Ti(lV), Mn(ll), Cu(ll), Co(ll), Ni(ll), Zn, Cd, U(VI), Ga, In, Pb(ll), and most other elements accompany Sr quantitatively. Zr, Hf, and Th accompany Ba, but are retained by the column when Ba is eluted with 2 M HN03. The rare earths interfere and have to be removed by an extra separation step. An elution curve for the favored eluent concentration and results on quantitative separations of synthetic mixtures are included.
THESATISFACTORY SEPARATION of Ba from Sr is still one of the more difficult problems in inorganic analytical chemistry when small amounts of one of these elements are present together with larger amounts of the other. Various organic complexing reagents such as lactate (1, 2), citrate (3), OLhydroxy isobutyrate (4-6), and malonate (7) have been employed as eluents for cation or anion exchange procedures. The separation factors
(=5,
where K
=
millimoles of element in resin X millimoles of element in aqueous phase volume of aqueous phase in ml weight of dry resin in grams are between 2.5 and 3.3 for all these eluting agents. Factors of about 7 can be obtained with the ammonium salts of the complexones EDTA (8, 9) and DCyTA (IO), but the distribution coefficients in this case are very strongly pH-dependent which, to some extent, neutralizes the advantage. Furthermore, the destruction of all the above reagents with the exception of ammonium malonate is time-consuming and
tedious. Generally, one would prefer a volatile inorganic acid such as HCI or " 0 3 as an eluent, because it considerably simplifies further work on the eluates. Recently, Tanaka (11) has separated Sr from Ba by elution with 1.2N HCl and L. Mazza, Sardo, and FrachC (12, 13) have employed 0.9N and 2.0N HCI. The distribution coefficients for Sr at the lower acid concentrations are rather high, with values of 45 and 75, respectively, for the equivalent AG50W-X8 resin (14). Asymmetrical elution curves and tendencies to tailing therefore can be expected. Furthermore, the separation factor at 2N HCl arBa = 2.0 is rather small. Quantitative separations therefore are possibly only for limited amounts of Ba and Sr. A study of distribution coefficients undertaken in this laboratory showed that with a resin of 1 2 z DVB crosslinkage, the separation factor agrBa, could be increased to 2.6. A further increase to 2.9 could be obtained with AG50W-Xl6 resin. Yet at the highest cross-linkage, exchange rates were slowed down considerably, and band spreading and tailing occurred. The addition of water-miscible nonaqueous solvents has been used successfully by Kember, MacDonald, and Wells (15) to promote metal-halide complex formation for selective elution of metal ions from cation exchange resins. Fritz and Rettig (16) have determined distribution coefficients for 14 elements in hydrochloric acid-acetone mixtures with Dowex 50W-X8 resin. Because the presence of the organic solvent causes changes in the hydration shells of the ions, one would expect that elements with negligible or no tendencies to halide complex formation nevertheless could show changes in distribution coefficients which could vary in degree appreciably and therefore lead to considerably increased separation factors. Thus cation exchange separation procedures have been developed for the alkali metals in HCl-methanol(17), HCl-ethanol (18), and HC1-methanolphenol (19) mixtures. Tanaka (11) has investigated the cation exchange behavior of alkaline earth elements in HCImethanol mixtures, but suggests 1.2M and 1.6M HC1 without the addition of methanol as eluents for Sr and Ba, respectively. A systematic survey of distribution coefficients in HCIethanol mixtures carried out in this laboratory suggested that
~
(1) M. Lerner and W. Rieman, ANAL.CHEM., 26, 610 (1954). (2) F. H. Pollard, G. Nickles, and D. Spincer, J . Chromatog., 10, 215 (1963). (3) F. Nelson and K. A Kraus, J. Am. Chem. SOC.,77, 801 (1955). (4) A. P. Baerg and R. M. Bartholomew, Can. J. Chem., 35, 980 (1957). (5) L. Wish, ANAL.CHEM., 33, 53 (1961). (6) F. H. Pollard, G. Nickles, and D. Spincer, J Chromutog., 13, 224 (1964). (7) F. W. E. Strelow, C. R. van Zyl, and C. R. Nolte, Anal. Chim. Acta, 40, 145 (1968). (8) R. Bory and G. Duykaertes, Anal. Chim. Acta, 11, 134 (1954). (9) J. J. Bouquiaux and J. H. C. Gillard, Anal. Chim. Acta, 30, 273 (1964). (10) Z . Sulcek, P. Povondra, and R. Stangl, Talanra, 9,647 (1962).
928
ANALYTICAL CHEMISTRY
(11) M. Tanaka, J . Chem. SOC.Japan, Pure Chem. Sect., 85, 117 (1964). (12) L. Mazza, L. Sardo, and R. Frache', Gazz. Chim. Ita/., 95, 599 (1965). (13) L. Mazza, L. Sardo, and R. Frache', ibid., p. 610 (14) F. W. E. Strelow, ANAL.CHEM., 32, 1185 (1960). (15) N. F. Kember, P. J. MacDonald, and R. A. Wells, J. Chem. SOC.,2273 (1955). (16) J. S. Fritz and T. A. Rettig, ANAL. CHEM.,34, 1562 ' (1962). (17) H. Okuma, M. Honda, and T. Ishimori, Japan Analyst, 2, 428 (1958). (18) V. Nevoral, Z . Anal. Chem., 195,332 (1963). (19) I. Mazzei, C. Gualandi, G. Burana and V. Venturello, Ann. Chim. (Rome), 53, 368 (1963); Chem. Abstr., 59, 8103c (1963).
Table I. Ba
Results of Quantitative Separations. Found, mg
Taken, mg Other element
Sr 138.2 Sr 276.4 Sr 0.276 AI 138.2 Fe(II1) 138.2 Ti(1V) 138.2 Ca 138.2 138.2 Mg Mn(I1) 138.2 Cu(I1) 138.2 138.2 Co(I1) Ni(I1) 138.2 Zn 138.2 138.2 Cd 138.2 U(W Ga 138.2 In 138.2 Pb(I1) 138.2 The results are means of triplicate determinations.
89.0 0.223 267.0 27.14 56.16 49.13 40.28 24.52 54.60 64.12 58.65 58.96 65.10 112.6 238.4 69.6 114.3 206.5
improved separation factors for the Sr-Ba pair and excellent separations could be obtained by using a considerably higher acid concentration (about 3M) and about 20 to 30% ethanol as eluent. Furthermore most other elements including Ca, AI, and Fe could be separated from Ba by the same method. EXPERIMENTAL
Reagents and Apparatus. Analytical reagent grade chemicals were used throughout with the exception of ethanol, which was purified by distillation. The AR grade hydrochloric acid was also further purified by redistillation. The resins were the AG50W-X8 or AG50W-X12 sulfonated polystyrenes supplied by the Bio-Rad Laboratories, Richmond, Calif. Resin of 100- to 200-mesh particle size was used for equilibrium and a 200- to 400-mesh for column experiments. Borosilicate glass tubes of 20-mm or 10-mm diameter, fitted with a fused-in-glass sinter of No. 2 porosity and a buret tap at the bottom and a B 19 ground-glass joint at the top, were used as columns. A Perkin-Elmer 303 instrument was used for determinations by atomic absorption spectrometry. Distribution Coefficients, Distribution coefficients were determined with 2.500 grams (dry weight at 105" C) of the resin in 250-ml solution, 5 milliequivalents (exchange equivalents = moles X valency) of the element, and a shaking time of 24 hours at 20" C. The experimental procedure has been described previously (14,ZO). Elution Curves. Experimental elution curves were determined with mixtures of Ba and Sr, or Ba and one other element, with the most promising eluent concentrations. The elements were absorbed from 0.5M HCI containing 20% ethanol and eluted with the indicated eluent taking 25-ml fractions for analysis. Quantitative Separations. From the foregoing, a method was elaborated and applied to the analysis of synthetic mixtures containing known amounts of Ba and one other element. The elements were absorbed from 0.5M HCI containing 20% ethanol on a column of 60 ml(20 gram dry weight) AG5OWX8 resin of 200- to 400-mesh particle size. Pb(I1) was absorbed from 3M HCI in 2 0 z ethanol. The resin column was 18 cm long and 2.0 cm in diameter. Sr and Ca were eluted with 500 ml of 3.00M HCl containing 2 0 z ethanol. Only (20) F. W. E. Strelow, Ruthild Rethemeyer, and C. J. C. Bothma, ANAL.CHEM., 37, 106 (1965).
Ba
Other element
138.2 f 0.1 276.5 f 0.2 0.274 f 0.005 138.1 f 0.1 138.2 f 0 . 2 138.3 f 0.2 138.2 f 0.1 138.2 f 0 . 1 138.3 i 0.1 138.1 f 0 . 2 138.2 f 0.1 138.3 =t 0 . 1 138.2 =t 0.1 138.2 f 0.2 138.2 f 0.1 138.2 f 0.1 138.1 i 0.1 138.2 i 0.1
89.1 f 0.1 0.224 f 0.003 266.8 f 0.3 27.16 i 0.05 56.13 i 0.06 49.15 f 0.06 40.30 f 0.04 24.53 f 0.03 54.62 f 0.05 64.09 f 0.06 58.61 f 0.08 58.97 f 0.06 65.12 f 0.06 112.5 f 0 . 2 238.3 f 0 . 2 69.6 f 0.1 114.2 f 0.2 206.5 f 0.2
Table 11. Distribution Coefficients in HCI and " 0 3 AG50W-Xl2 Resin HCl "03 Normality Sr Ba Sr Ba 2940 6300 0.1 3040 8900 0.2 2960 968 2450 960 550 221 489 0.5 208 69 175 66 127 1.0 22.0 58 16.2 28.6 2.0 12.0 29.8 7.1 3.0 7.5 4.0 8.1 19.9 4.2 3.8
300 ml of the same eluent were used for A1 and the other elements. The eluates were taken from the beginning of the absorption step. The ethanol was washed from the column with 50 ml of 0.1M HC1 and then the Ba eluted with 250 ml of 3.00M "03. A flow rate of 2.5 f 0.3 ml per minute was maintained throughout. After the excess of acid had been removed by evaporation, the elements were determined by conventional analytical methods. Small amounts of Sr were determined by atomic absorptip spectrometry with an air acetylene flame and the 4607 A line, while a nitrous oxide acetylene flame and the 5536 A line were employed for Ba which was measured as the chloride in 80% methanol after the nitrate had been removed by evaporation with HC1. The results are presented in Table I. RESULTS AND DISCUSSION
Distribution Coefficients. Table 11 shows coefficients for Sr and Ba in HC1 and " 0 3 with 12% cross-linked resin. The coefficients in the relevant range of acid concentration are higher than those with 8z cross-linked resin (14, 20). The separation factor in 2.OM HCl is increased from 2.0 to 2.6. Coefficients for Sr and Ba with AG50W-X8 resin in HClethanol mixtures of various concentrations are presented in Table 111, and Table IV shows coefficients for these two elements and the same resin with various concentrations of methanol, acetone, and dioxane. Coefficients in HClethanol mixtures with the AG50W-Xl2 resin are shown in Table V. The percentages organic solvent refer to volume VOL. 40, NO. 6, MAY 1968
929
IO
-
8-
6-
4-
2-
il,
per cent before mixing. Thus 40% ethanol means 100 ml ethanol plus 150 ml aqueous HCl. The coefficients for Ba increase much more strongly with increasing percentage of organic solvent than the coefficients of Sr. This leads to much improved separation factors. The four organic solvents show only negligible differences in their effects on the distribution coefficients. Elution Curves. Figure 1 shows an elution curve for the Ba-Sr pair (1 mmole each). The resin bed contained 60 ml (20 grams) of AG50W-X8 resin of 200- to 400-mesh particle size and was 19 cm long and 2.0 cm in diameter. Elution was carried out with 3.00M HCI in 2 0 x ethanol at a flow rate of 2.5 ml per minute. The separation factor is 3.5 under these conditions. It increases to 4.3 when the ethanol concentration is increased to 30%, but the rate of exchange, especially for Ba, is slowed down considerably. Even higher
Ethanol 0 10 20 30 40
60
z
Organic solvent 10 20 30
40 60
930
0
Sr 17.8 18.8 22.3 31 .O 49.3 192
separation factors can be obtained with AGSOW-Xl2 resin (Table V), yet the exchange rates are even slower. Quantitative separations. The described method provides a useful means for the separation of Ba from Sr. Separations are quantitative for weight ratios from 1000 :1 to 1 :1000 and likely also for higher ones. AI, Ca, Mg, Fe(III), Ti(IV), Mn(II), Cu(II), Co(II), Ni(II), Zn, Cd, U(VI), Ga, In, and Pb(I1) are eluted together with or before Sr and can be separated from Ba quantitatively. Separations of Hg(II), Au(III), Pd(II), Pt(IV), Ir(IV), Rh(III), V(V), Mo(VI), Bi(III), Ge(IV), Se(IV), Te(IV), As(III), Sb(III), Be, Li, Na, K, Rb, and Cs from Ba have not been carried out but should be possible under the same conditions according to the distribution coefficients of these elements. Z r , Hf, and Th are retained by the column together with Ba. When Ba is eluted with 400 ml of 2.00M H N 0 3 instead of 250 ml of 3.00M "08,
Table 111. Distribution Coefficients in Ethanol AG50W-X8 Resin 2.00N HCl 3.00N HC1 Ba Sr Ba 36 50 74 116 197 95 1
10.0 10.9 13.1 18.7 29.8 152
18.5 26.9 42.6 75 143
prec.
4.00N HCl
Sr 7.5 7.8 9.3 13.4 23.4
...
Table IV. Distribution Coefficients in 3.00N HCI with Various Organic Solvents AG50W-X8 Resin Methanol Acetone Sr Ba Sr Ba Sr 11.4 14.6 18.6 27.6 93.3
ANALYTICAL CHEMISTRY
26.7 42.0 71 121
prec.
11.2 13.0 18.9 30.3 151
26.3 40.0
73 130
prec.
Ba 11.9 17.8 31.3 52 89 prec.
11.0 12.7 18.5 30.0 132
Dioxane Ba 26.0 36.7 68 119
prec.
these elements are not eluted and can be separated from Ba. The heaviest rare earths are eluted almost together with Sr when 3.00M HC1 in 20% ethanol is used as eluent, while La appears almost together with Ba. The rare earths therefore interfere with the method and, when present, have to be separated. This can be done by cation exchange chromatography using a 75-ml (25-gram) column of AG50W-X8 resin of 200- to 400-mesh particle size. Ba and Sr can be eluted with 500 ml of 2.00M H N 0 3 while the rare earths (up to 2 mmole) are retained quantitatively. The separation factor of asrBa= 3.5 in 3.00M HC1 20z ethanol with the AG50W-X8 resin is slightly higher than those obtained with organic complexing agents except EDTA and DCyTA, and the method has the important advantage that the eluting agent can very easily be removed by evaporation. Exchange rates in the above eluent concentration are reasonably fast and flow rates of 2 to 3 ml per minute (linear flow rate 0.6 to 0.9 ml per minute per cmz) can be employed. Increasing the ethanol concentration gives higher separation factors but slower exchange rates. The same applies to using resin of higher cross-linkage. Larger amounts of Ba precipitate as the chloride from 3M HCl in 50% ethanol or 4 M HCl in 40% ethanol. Disturbing effects in the column operation appear already at somewhat lower concentrations. This and the fact that the distribution coefficients of Sr increase more sharply at higher concentrations of ethanol, limit the favorable conditions for separation to a fairly narrow range. The most convenient separations for most purposes are obtained with an 8% cross-linked resin of 200to 400- mesh particle size and 3.00M HCl in 20% ethanol. Smaller columns can be used when only several milligrams or less of Ba plus Sr are present. Ba (0.1 mmole) could be
+
FIuoride MicroanaIysis by Linear
Table V. Distribution Coefficients in Ethanol AG50W-Xl2 Resin 3.00N HC1 4.00N HC1 Ethanol Sr Ba Sr Ba
z
0 10 20 30 40 60
12.0 15.5 21.5 32.3 52.8 207
29.8 47.8 82 133 235
8.1 11.6 14.7 24.2 41.5
...
prec.
19.9 35.2 64 120
prec. prec.
separated from 0.1 mmole Sr on a column of 15 ml (5 g dry weight) of AG50W-X8 resin of 200- to 400-mesh particle size. The resin bed was 13.5 cm in length and 1.1 cm in diameter, and the Sr was eluted quantitatively with 140 ml of 3.00M HC1 in 20% ethanol at a flow rate of 0.8 f 0.1 ml per minute. When very small amounts of Sr have to be separated from very large amounts of Ba or vice versa, 3.00M HC1 in 30% ethanol is a better eluting agent because of the higher separation factor of 4.3. Resin of 12% cross-linkage does not offer advantages with ethanolic HCl eluents. Separations are less satisfactory when resins of 100- or 200-mesh particle size and high flow rates are used. The distribution is a much more effeccoefficients in Table I show that " 0 3 tive eluting agent than HC1 for Ba from a 12% cross-linked resin. This is in accordance with results for an 8% crosslinked resin obtained previously ( I 4 , 2 0 ) . RECEIVED for review October 24, 1967. Accepted February 12, 1968.
Nu II-Point Potentiometry
Richard A. Durst Division of Analytical Chemistry, Institute for Materials Research, National Bureau of Standards, Washington, D . C . 20234 The modified fluoride activity electrode is used as the sample electrode in the technique of linear null-point potentiometry for the determination of fluoride at subnanogram levels in sample volumes of 10 , I. The emf VS. titrant concentration data are plotted semilogarithmically, and the equivalence point is obtained by a linear interpolation to the null-point potential. Data analysis is also accomplished by computer techniques whereby the equivalence point is obtained from the intercept of a linear least squares fit of the data. Fluoride solutions 10-3 to 2 X 10-'jM (containing 190 to 0.38 nanograms of fluoride in 10 @I)were determined with an accuracy of approximately 1% over the entire range and a relative standard deviation of the mean of about 0.5% for 5 determinations. At the lowest concentration level, 2 x lO-'jM fluoride, the error in determining 380 picograms of fluoride was approximately 2 picograms.
THE DETERMINATION of fluoride at subnanogram levels in sample volumes of 10 p1 has been achieved for the first time using the modified fluoride activity electrode ( I ) in combina-
tion with the concentration cell technique of linear nullpoint potentiometry [LNPP] (2). The fluoride sample in the inverted fluoride electrode microcell is titrated by the addition of a standard fluoride solution to a second electrochemical half cell connected to the former by a salt bridge. The emf U S . titrant concentration data are plotted semilogarithmically, and the equivalence point is obtained by a linear interpolation to the null-point potential. The concentration cell employed is LaFalF-(CA), KN03(0.1WI IKNO@lM)I 10 pl
salt bndge
I I
KN03(0.lM), F-(CT) LaF3 100 ml.
where LaF3 is the fluoride-specific membrane of the fluoride activity electrode, C, is the concentration of the fluoride solution being analyzed (z'.e., analate solution), and CT is the concentration of fluoride in the titrant half cell. The, emf of this cell is given by the Nernst equation for a concentration cell : ~~~~
(1) R. A. Durst and J. K. Taylor, ANAL.CHEM.,39, 1483 (1967).
(2) R. A. Durst and J. K. Taylor, ANAL.CHEM., 39,1374 (1967). VOL 40, NO. 6, MAY 1968
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