Anal. Chem. w a 4 , 56,1053-1056
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Distribution Coefficients and Ion Exchange Behavior of 46 Elements with a Macroreticular Cation Exchange Resin in Hydrochloric Acid F. W. E. Strelow National Chemical Research Laboratory, P.O. Box 395, Pretoria 0001, Republic of South Africa Systematic information on distribution coefficients forms the fundamental base from which ion exchange separations can be planned. For the gel-type microporous cation exchangers such systematic information, including large numbers (3), of elements, is available for aqueous HC1 (1,2), "03 H2S04(3),HBr (4,5),HC104 (2,6),and HCl-HC104 mixtures (7) as well as for HCl-ethanol@), HC1-acetone (9),(10, I I ) , HBr-acetone (4,12),and various other systems. The so-called macroreticular resins, which have a rigid wide-open macroporous structure, were already introduced in 1962 by Kunin and his co-workers (13) but have found relatively little attention in the field of inorganic analytical chemistry. Because of their rigid structure volume changes with changes in eluent concentration or kind are very much less than those with the gel-type resins. This makes the macroreticular resins much more suitable for applications in high-pressure liquid chromatography procedures for the separation of inorganic ions. Most of the work published in this field is due to the investigations of J. S. Fritz, K. Kawazu, and their co-workers (14-16). Though many of their publications contain sporadic information about distribution coefficients, this relates almost entirely to a few elements with chloride complex forming tendencies in Hcl-organic solvent mixtures (mostly acetone). A systematic study of the distribution coefficients of a large number of elements, including those with negligible tendencies toward chloride complex formation, does not seem to be available for the hydrochloric acid system. A study exists for the nitric acid system (17),but the results are presented as percentages of absorption in the form of small graphs without experimental points. This allows only an approximate estimation of the actual values of the coefficients and is not very satisfactory for estimating optimum conditions for critical separations. Because cation exchange distribution coefficients are influenced quite strongly by the degree of cross linkage of the resin and this influence varies considerably depending on the size of the hydration shell of the cation in question, the rigid macroreticular resin with its very high effective cross linkage (about 25%) in the exchange region should show some marked differences in the distribution coefficients of some elements as compared with a 8% cross-linked gel-type resin. It has been shown (18) that the distribution coefficient of magnesium when plotted against percentage of cross linkage shows a maximum a t about 12% cross linkage and then slightly decreases again, while that for calcium shows an increase up to 25% cross linkage. As a result, the separation factor between Mg and Ca in 1 M aqueous HC1 increases from a value of about 2 with an 8% cross-linked gel-type resin to about 8 for the AG MP-50 macroreticular resin. It seems reasonable to assume that some other spectacular increases in separation factors are also possible. A systematic study of distribution coefficients in aqueous HCl using the macroreticular resin AG MP-50 was therefore undertaken, and the results are here presented and discussed. EXPERIMENTAL SECTION Reagents. The AG MP-50 macroreticular sulfonate cation exchanger on polystyrene basis marketed by BIO-RAD Laboratories of Richmond, CA, was used for this study. Resin of 100-200 mesh particle size was employed for distribution coefficient de0003-2700/84/0356-1053$01.50/0
terminations and resin of 200-400 mesh for column work. The resin as received was found to contain appreciable amounts of calcium. For purification it was packed into large columns ( 3 cm inner diameter) and the calcium was eluted with 5 M HNO, until the eluate was free from calcium. Water was distilled and, for further purification, passed through an Elgastat deionizer. Only reagents of A.R. quality were used. Standard solutions were in most cases prepared from the chlorides of the elements. In a few cases the nitrates were used [Mg(II), Hg(I1) in HNO,, Tl(I), Pb(I1) and U(VI)]. In the case of scandium and the lanthanides the oxides were dissolved and converted to dry chlorides. The metals were dissolved in the case of gold and the platinum metals, using a pressure vessel lined with Teflon for Rh(II1) and Ir(III/IV). Most standard solutions were in deionized water, but those of some multivalent elements contained 0.5 or 1.0 M HCl to avoid hydrolysis, and those of Ti(IV), V(V), Mo(V1) and W(VI) also contained hydrogen peroxide for stablization. Apparatus. Borosilicate glass tubes of 21 mm diameter and about 450 mm in length were used as columns, fitted with a No. 2 porosity glass sinter and a buret tap at the bottom and a B19 joint at the top. The columns were filled with a slurry of AG MP-50 resin (200-400mesh) in the hydrogen form until the settled resin reached the 54-mL mark. The columns were purified by passing through about 500 mL of 5 M "OB followed by 100 mL of deionized water, shaking with water, and allowing the resin to resettle again. Atomic absorption measurements were carried out with a Varian-TechtronAA-5 instrument using the air-acetylene or the nitrous oxide-acetylene flame. A Zeiss PMQII was used for spectrophotometric measurements. Distribution Coefficients. The resin was dried at 60 "C in a Gallenkamp vacuum pistol with magnesium perchlorate (anhydrone) as drying agent and kept in a desiccator over the same drying agent. Residual water was determined by drying at 120 "C and the weights of resin were corrected accordingly. Distribution coefficients were determined by equilibrating 2.500 g of dry resin in the hydrogen form with 250 mL of solution by shaking for 24 h in a mechanical shaker at 20 "C. The concentrations of HCl were those shown in the table. In a few cases coefficients were determined in HNO:, either because the chlorides were insoluble, Ag and Hg(I), or for because we wished to compare results, Hg(I1). Usually 2 exchange mequiv of the cations were used for equilibration, but in some cases, Pb(I1) and Tl(I), smaller amounts were used because of the limited solubilities of the chlorides of these cations. Where the resin could be ignited without loss of the particular element, it was separated by using a filter paper and ashed at low temperature. The ash was dissolved for determination by a suitable method or weighed directly. Otherwise the resin was transferred to a short large-diameter column (21 mm in diameter) and the element to be determined eluted by a minumum amount of a suitable reagent. The amounts of the elements in both the aqueous and the resin phase were then determined by appropriate analytical methods. From the results equilibrium distribution coefficients ( I ) were calculated and are presented in Table I. Elution Curves. a. Cd-Zn-Ga-Yb-Sc. A solution containing about 0.5 mmol of each Cd, Zn, and Ga and 0.33 mmol of each Yb and Sc in about 50 mL of 0.1 M hydrochloric acid was passed through a column containing 54 mL (20 g) of clean AG MP-50 resin (200-400 mesh) in the hydrogen form. The cations were washed onto the column with a few small portions of 0.1 M hydrochloric acid. Cadmium was then eluted with 350 mL of 0.70 M hydrochloric acid, followed by zinc with 300 mL of 1.50 M hydrochloric acid, gallium with 300 mL of 2.50 M hydrochloric 1984 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 0, MAY 1984
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Table I. Cation Exchange Distribution Coefficients with AG MP-50 Macroporous Resin in HCl molarity HC1 element Th Zr La sc Hg(Ua Ba Y Yb Hg( 111a Sr Ca Tl(1) Aga Ga Pb(II)c A1 Fe( 111) U(V1) cs Rb Mn( 11) K Fe( 11) Co( 11) Cu(11) Ni(I1) Mg Zn Be Na V(IV) Ti(IV) Li Cd In Bi(111) Rh(111) Te(1V) Sn(1V) Ir(III/IV) Pd(11) Au(111) Pt( IV)
vwe
W(V1)d MO(VI)~
As(II1) MV) )"
Sb(II1)
0.2 M >io5
>l o s
>los 105 > i o 4 > i o 4 > i o 4
>io4 > i o 4
9300 5500 1480 581 > i o 4 > i o 4
6300 6600 970 255 2 26 1130 160 760 730 740 6 70 469 690 330 61 238 128 23.6 171 286
f
8.6 15.5
f
3.9 2.3 2.2 0.8 0.6 0.5 0.4 0.2 C0.5 105
>los
> 104 > 104 6200 4400 9500 6700 2600 1320 850 414 233 880 1270 780 790 20 3 110 97 161 69 113 109 117 108 89 99 55 26.0 43.0 37.8 9.8 15.1 9.6 7.4 6.1 5.7 5.0 3.1 2.1 1.3 0.7 0.7 0.6 0.4 0.3 < 0.5 < 1.0 g
1.0 M
> 104 > 104 4320 2270 1580 1140 1040 870 435 3 20 214 122 111 110 102 89 87 68 59 50
43.6 34.9 30.0 27.7 27.7 27.4 24.8 19.7 15.9 13.6 11.5 10.7 5.1 2.1 1.4 1.3 3.9 2.4 1.4 2.0 1.8 1.1 0.8 0.5 0.5
0.4 0.2
