The variation of k,,, with p H in Figure 2 is qualitatively that expected for an acid-base catalyzed reaction. For System I1 in acid media dehydration might be expected to follow a mechanism of the type
0 H ‘ H ‘04 H’ N ‘’
H
N’
H
There the first step is assumed to be in equilibrium (with equilibrium constant ICE+), and the rate determining step involves kE+. A similar mechanism can be suggested for base catalysis. Thus, k,,,, the rate constant measured polarographically in buffered solutions, should be a measure of the overall rate of reactions of Type 111, and as a result k,,, should be of the form
LITERATURE CITED
(1) Alberts, G. S.,Shain, I., ANAL.CHEW.
There k , is the self-dehydration rate constant in the absence of catalysis, and other constants already have been defined. The data of Figure 2 are in agreement with this concept, since the dashed line is a plot of Equation 35 with k H + K H + = 4.45 M-l-sec-l, k O ~ - K 0 n -= 4.49 X 107LM-1-sec.-1, and k , = 0.36 sec.-l The polarographic behavior of pnitrosophenol, therefore, is consistent with the overall mechanism of System I1 where the mechanism of the dehydration is that indicated by System I11 (and the inferred system for base catalysis). While these results serve to establish further the mechanism for reduction of p-nitrosophenol, certain features of the overall reaction remain uncertain. These include the adsorption phenomena reported above, effects of chemical reactions preceding the first charge transfer, and the heterogeneous rates of the individual electrode reactions. All of these processes should be amenable to study by application of modern electroanalytical techniques, such as stationary electrode polarography (619)’
35,1859 (1963). (2) Churyfiill, R. V., “Operational Mathematics, 2nd. ed., McGraw-Hill, New York. 1958. (3) Davis, P. J., “Hydbook of illathematical Functions, M. Abramowitz and I. Stegun, eds., p. 253, National Bureau of Standards, Washington, D. C., 1964. (4) Herman, H. B., Bard, A. J., J. Phys. Chem. 70,396 (1966). (5) MacDonald, R. J., J. A p p . Phys. 35, 3034 (1964). (6) Nicholson, R. S., ANAL.CHEW 37, 1351(1965). (7)lv~cholson,R. S.,Shain, I., Ibid., p. IIO.
(8) Zbid., p. 190. (9) Ibid., 36,706 (1964). (10) Sherwood, G. E. F., Taylor, A. E., “Calculus,” PrenticeHall, New York, 19.54. ~ . . ~ (11) Tachi, I., S,enda,
M.,“Advances in Polarography, I. Longmuir, ed., p. 454, Pergamon Press, Xew York, 1960. (12) Testa, A. C., Reininuth, W. H., ANAL.CHEM.33, 1320 (1961). RECEIVED for review December 2, 1965. Accepted February 11, 1966. Presented in part at the Division of Physical Chemistry, 151st Meeting ACS, Pittsburgh, Pa., March 1966. Work supported by funds received from the National ScienceFoundation, under Grant No. GP-3830. During the summer of 1965, J. bl. Wilson was an NSF .Undergraduate Research Participant a t Michigan State University.
Se pa ration of Lead(II) from Bismuth(Ill), T ha IIium(Ill), Ca d mium(II), M erc ury(II), Go Id(III), PIa ti num(IV), Palladium(Il), and Other Elements by Anion Excha nge Chroma tog ra p hy F. W. E. STRELOW and F. V O N S. TOERIEN National Chemical Reseorch laboratory, Council for Scientific and Industrial Research, Pretoria, South Africa
b Lead and other elements are absorbed from between 0.1 and 4.ON hydrobromic acid solution on a column of A G l - X 8 anion exchange resin in the bromide form. The following elements are eluted with 0.1N HBr: U(VI), Th(lV), Zr(lV), Hf(lV), Ti(IV), Sc(lll), Y(III), La(”, and the rare earths, AI(III), Ga(lll), In(lll), Fe(lll), Be(ll), Mg(ll), Ca(ll), Sr(ll), Ba(ll), Zn(ll), Mn(ll), Co(ll), Cu(ll), Ni(ll), Cr(lll), Sb(lll), Ge(lV), Li(l), Na(l), K(I), Pb(l), and Cs(l). Then lead i s eluted selectively with 0.30N H N 0 3 plus 0.025N HBr, and after evaporation of the acid it can be determined b y EDTA titration or b y the mass spectrometric isotope dilution method. Bi(lll), TI(IIl), Cd(ll), Hg(ll), Au(lll), Pt(lV), and Pd(ll) are retained b y the column quantitatively. No element which could interfere with the EDTA titration accompanies lead.
T
of lead from other elements by ion exchange chromatography has received some attention in recent years. Among the most selective methods which have been described are the cation exchange procedure of Fritz (2, 3) which uses hydrobromic acid as eluent, and the anion exchange procedure of Korkisch (5) in nitric acidtetrahydrofuran media. Unfortunately, in the first lead is eluted preferentially, while most of the common bulk elements remain absorbed. This limits the amount of material which can be handled and makes the method less suitable for the separation of small amounts of lead from large amounts of such elements as Al(III), Fe(III), Ca(II), Mg(II), Cu(II), and U(V1). I n the second, the rare earths which are main constituents of many radioactive ores are among the few elements that HE SEPARATION
accompany lead. Furthermore, some elements with tendencies t o the formation of nitrate complexes, such as Sr(II), Ba(II), Hg(II), and Au(III), have not been investigated by Korkisch. The fairly high distribution coefficient
Kd = amount of element in resin phase X amount of element in solution grams of dry resin ml . of solution for Ca(I1) suggests that Ba(I1) and Sr(I1) are also likely t o accompany lead. The classical anion exchange separation of lead in hydrochloric acid based on the work of Kraus (7, 8) is probably the most selective of all the methods available; but the low maximum value of the distribution coefficient of lead a t VOL. 38,
NO. 4,
APRIL 1966
545
Table 1. Stability Constants of Some Anionic Bromide Complexes ( 9 ) 1 Mean -log ON
N
AuBr4HgBr4-2 PtBrg-2 PtBr4-2 TlBr4RhBr4PdBr4-2 AgBr3-2 BiBr6-3 CdBr4-2 PbBr4-2
8.0 5.5
Very stable, no data 5.0 5.0 4.6 3.3 2.8 1.6 0.9 0.7
about 1.5N HC1 (Kd = 27) allows only limited amounts of solution to be percolated through a column before lead appears in the eluate. Andersen (1) has shown that lead is absorbed by anion exchange resins from bromide much more strongly than from chloride solutions, but he does not present elution data for lead. The maximum for the distribution coefficient is about 500. Andersen's distribution data suggest that some elements forming weak bromide complexes, such as In(III), Ga(III), Ge(IV), Sb(III), and Sn(IV), should be eluted together with those elements which do not tend to form bromide complexes, such as Al(III), U(IV), etc., by using dilute HBr as eluent, while lead together with Tl(III), Cd(II), and Hg(I1) should be retained by the column. Our own work revealed that among the elements not investigated by Andersen Bi(III), Au(III), Pt(IV), and Pd(II), which form very stable bromide complexes, also accompany lead. Korkisch (6) has investigated the anion exchange behavior of bromide complexes in partly organic media and found that, besides the elements accompanying lead in purely aqueous solution, some others, such as Zn(1I) and In(III), had similar tendencies. The presence of organic solvent therefore does not seem to offer any advantages in selectivity. Our study was undertaken, first, to establish good conditions for the quantitative separation of lead from all the elements less strongly absorbed from dilute hydrobromic acid solution, and, secondly, to develop a method for the separation of lead from those elements which are retained strongly and accompany lead. A systematic study of the anion exchange behavior of bromide complex-forming elements in mixed nitric acid-hydrobromic acid media indicated that it should be possible to separate lead from the latter elements by preferential elution with a mixed nitric acid-hydrobromic acid eluent, provided the hydrobromic acid concentration was selected in such a way that the more stable bromide complexes were stabilized while the lead complex, being the least 546
ANALYTICAL CHEMISTRY
ELUATE m l .
Figure 1 .
Elution curve for U(VI), Pb(ll), and Bi(1ll)
Column of 20 g r a m s A G l - X 8 resin In Br- form. 1.00N " 0 3 for Pb(llJ a n d Bi(llll. Flow r a t e 3.0
stable, was dissociated. From this a very selective method capable of separating lead from almost all other elements, including the rare earths and Tl(II1) and Bi(III), has been developed. EXPERIMENTAL
Reagents and Apparatus. Analytical grade chemicals were employed whenever possible. When only small amounts of lead were separated and determined, special purification procedures were employed, or, alternatively, blank determinations were carried out. Methyl thymol blue and pyrocatechol violet indicators were supplied by E. Merck A.G., Darmstadt, Germany, and the xylenol orange by E. Gurr, Ltd., London. Gold and platinum (99.99yo pure) metals were obtained from Johnson, Matthey and Co., Ltd., London. The resin was AG1-X8 strongly basic anion exchanger of 100- to 200-mesh particle size, supplied by the BIO-RAD Laboratories, Richmond, Calif. Borosilicate glass tubes 35 cm. in length and 2 cm. in diameter with fused-in glass sinters of No. 2 porosity and a buret tap a t the bottom were used as columns. Unless stated otherwise, the columns were loaded with 10.0 h 0.1 grams of resin to give a resin column 7.0 cm. long. The amounts of resin are given in weight of the chloride form on drying a t 105' C. This ensures that distribution coefficients and elution curves are reproducible and comparable with experimental results for other anion exchange systems, provided these are also referred to the same standard conditions. For the spectrophotometric work (elution curves), a Beckman DU model spectrophotometer was employed. Elution with HBr. Using a column of 10 grams of resin in the bromide form, and a flow rate of 3.0 & 0.2 ml. per minute, it was established that no lead appeared in the first 1000 ml.
Eluent 0.10N Her for U(VI),
f 0.2 ml. p e r minute
when 0.10N HBr was used as eluent or in the first 500 ml. when 4.ON HBr was used as eluent. In addition to the work of Andersen, it has been experimentally shown by Herber (4) that Ga(III), Cu(II), Co(II), and Ni(1I) can be eluted easily with dilute HBr (
