would be suitable. Because the slowing down medium for the proton will not be the same, K will change in a way related to the change in electron density. Using the identical irradiation, separation, and counting system described, we have evaluated K for small samples of ethanol of normal 0 ’ 8 abundance and find a value of K about 0.0263, which is somewhat lower than in the aqueous determination. This value is in agreement with the first approximation one might make, that K will be inversely proportional t o the electron density of the medium.
ACKNOWLEDGMENT
Cooperation of the staff of the Xuclear Reactor Facility of The Pennsylvania State University and financial support by the Department of the Army of the graduate study under which this work was carried out are gratefully acknowledged. LITERATURE CITED
(1) Aumann, D. C., Born, H. J., ~Vaturwiss. 51, 159 (1964).
(2) Ballczo, H., Schiffner, H., Z. A n d . Chem. 152, 3 (1956). (3) Biefeld, L. P., Howe, D. E., IND. EKG.CHEM.,ANAL.ED.11, 251 (1939).
(4) Blanchard, C. H., Westinghouse Atomic Power Division, Rept. WAPDAlW(P)-51, December 1955. (5) Bowen, H. J. &I., Intern. J . A p p l . Radiation Isotopes 4: 214 (1959). (6) Villars, D. S., Statistical Design and Analysis of Experiments for Development Research,” p. 170-72, Wm. Brown Co., Dubuque, 1951. LARRYH. HUNT’ WARREN W. MILLER Department of Chemistry The Pennsylvania State University University Park, Pa. Present address: Department of Chemistry, United States Military Academy, West Point, N. Y.
Cation Exchange Separation of Molybdenum, Tungsten, Niobium, and Tantalum from Other Metal Ions SIR: Hydrogen peroxide forms stable complexes in acidic aqueous solution with only a few metal ions. Fritz and Abbink ( 7 ) used a dilute solution of hydrogen peroxide to elute vanadium from a cation exchange column and thus to separate it from a number of other metal ions. Strelow (22) used hydrogen peroxide and sulfuric acid to separate titanium from more than 20 cations by cation exchange. Various authors have used solutions of hydrogen peroxide to elute molybdenum(VI), tungsten(VI), niobium(V), and tantalum(V) from cation exchange columns (1, 2, 8, 10, I I ) , but a very limited number of separations have been reported and the conditions for elution have varied considerably. Strelow (12) indicated a successful elution of molybdenum(V1) and niobium(V) from a cation exchange column with acidic hydrogen peroxide but stated that tungsten(V1) and tantalum (V) showed a tendency to hydrolyze. The purpose of the present work was to study the cation exchange separation of molybdenum(VI), tungsten(VI), niobium(V), and tantalum(V) as a group from other metal ions. EXPERIMENTAL
Apparatus. Conventional 1.2-cm. i.d. ion exchange columns with coarse glass frits were used. A slurry of resin and eluting solution was added t o t h e column until t h e bed had a height of 12 cm. Sample solutions and eluting solution were added dropwise from a separatory funnel inserted in the top of t h e column through a one-holed rubber stopper. Resin. Dowex 5OW-X8 cation exchange resin, 100 to 200 mesh, was used in the hydrogen Form. Reagents. Except for t h e metal ion solutions listed, all metal ion stock solutions were 0.05M solutions 1272
ANALYTICAL CHEMISTRY
of the nitrate or perchlorate salt in dilute nitric or perchloric acid. Chromium(II1) was a 0.05-11 solution of chromium chloride in dilute nitric acid. Zirconium(1V) was a 0.05V solution of zirconyl chloride in 0.3J1 hydrochloric acid. The zirconium(1V) salt was dissolved in concentrated hydrochloric acid and diluted to volume. Titanium(1V) was a 0.05J1 solution of titanium tetrachloride in 0.2.11 sulfuric acid and 0.3% hydrogen peroxide. Tin(1V) was a 0.0511 solution of tin tetrachloride in 0.3X hydrochloric acid. Molybdenum(V1) was a 0.0551 solution of molybdic acid which was made slightly basic (pH 8.3) with sodium hydroxide. Tungsten(V1) was a 0.05M solution of potassium tungstate in distilled water (pH 8.5). The potassium tungstate was prepared and purified in Ames Laboratory. I t was analyzed for tungsten by hydrogen reduction and was found to be pure. The niobium(V) and tantalum(V) 0.0556 stock solutions were prepared as follows: weighed amounts of the high purity metal (99.97%) were dissolved in hydrofluoric and nitric acids in plastic beakers provided with plastic covers. Dissolution was complete in about 5 minutes a t room temperature. The resulting solution was evaporated to about 5-10 ml. in the plastic beaker and next was transferred to a platinum evaporating dish. After addition of 10 ml. of concentrated sulfuric acid, the solution was further evaporated to fumes of sulfur trioxide to remove traces of fluoride. The vessel was cooled and the solution was diluted with approximately equal quantities of concentrated sulfuric acid and 30% hydrogen peroxide. The solution was transferred to a 500-ml. flask, a total of 30-55 ml. of concentrated sulfuric acid and 5C-65 ml. of 30y0 hydrogen peroxide were added, and the solution was diluted to volume with distilled water. I n all instances these stock
solutions mere stable for at least a month. I n the fuming step it was found that if heating was continued much beyond the first appearance of sulfur trioxide fumes, i t was difficult to dissolve the Xb2O5and Ta2O6which formed. The eluting solution consisted of 0.25M sulfuric, perchloric, or nitric acid containing 1% hydrogen peroxide. Separation Procedure. Synthetic sample mixtures for separation were prepared b y mixing known quantities of molybdenum (-0.25 mmole), tungsten, niobium, or tantalum (-0.5 mmole for each of the last three) with approximately a n equal quantity of a second metal ion. T h e mixture of niobium or tantalum and other metal ion was already 0.5-1-11 in sulfuric acid and 1.5-2% in hydrogen peroxide (from stock solutions), so t h a t further addition of sulfuric acid and hydrogen peroxide was unnecessary. The sample was added to the ion exchange column, and molybdenum was eluted with 125 ml. of 0.2531 sulfuric or perchloric acid containing 1% hydrogen peroxide. About 80 ml. of a 0.2531 nitric acid, 1% hydrogen peroxide solution was used to elute tungsten, niobium, and tantalum. The use of nitric acid for the last three metals is desirable because recoveries in the gravimetric analytical method are reported to be slightly low (at least for tungsten), when much sulfuric acid is preaent in the precipitation medium ( 5 ) . I n each case, after elution the column was washed with about 25 ml. of distilled water. Then the other metal ion was stripped from the resin with the amount of eluting solution indicated in Table I. The maximum flow rate obtainable was used in all of the elutions. Analysis of Column EfRuents. T h e column effluents containing t h e other metal ion were evaporated almost t o dryness and then diluted t o 100 ml. with distilled water for titration.
T h e metal ions were titrated with 0.0551 EDTA b y standard methods. Uranium was determined b y t h e standard oxidation-reduction method of titrating with cerium(1V) after passing the uranium(V1) solution through a lead reductor. Chromium was determined by oxidation-reduction titration with iron(I1). Molybdenum in column effluents was determined gravimetrically as the oxinate according to the method of Ualanescu ( 3 ) . I n order to destroy the molybdenum-peroxy complex, sulfur dioxide gas was bubbled through the solution, which was then gently boiled for about 15 minutes. Tungsten, niobium and tantalum were determined by the homogeneous precipitation technique of Dams and Hoste (5, 6). The temperatures used for ignition of the precipitates were 550" C. for \ivo3 and 1000° C. for Nb205 and TazOs. It is important that WOS not be heated above 700" C., because Carey, Raby, and Banks ( 4 ) found that above thls temperature volatilization becomes a n important consideration. RESULTS AND DISCUSSION
I n the separation of niolybdenuni(V1) from other metal ions, the other metal ion is usually retained in a tight band a t the top of t'he column, while molybdenum is eluted from the column as an anionic peroxy complex. Xpproximately 125 ml. of 1%)hydrogen peroxide in 0.25-11 sulfuric acid elutes molybdenum quantitatively from a 1.2-X 12-cm. column. Quantitatiie data for ion exchange separation of molybdenum(V1) from other metals are summarized in Table 11. Using the color of tungsten blue as a qualitative test', it was found t h a t tungsten(V1) is completely eluted from a 1.2-X 12-cm. cation exchange column by 80 to 90 ml. of 1% hydrogen peroxide in 0.2531 sulfuric or nitric acid. Further evidence for complete elution was provided by ashing some of the resin after elution and analyzing for tungsten by emission spectrometry. S o tungsten was detected in the resin. The recovery of tungsten eluted from the resin was also checked by gravimetric analysis and was found to be quantitative. The homogeneous precipitation method (5) of analysis for tungsten was compared to the standard hydrogen reduction method. The results of four determinations by the homogeneous precipitation inethod agree well with those obtained by Raby (9) on five determinations by hj'drogen reduction: 56.33 .03y0 (gravirnetric-WO3), com.OS% (hydrogen pared , t o 56.34 reduction-W). The analyses were done on the same sample of high purity potassium tungstate. Separations of approximately 0.1 gram of \ivo3from 0.5 niniole of another metal ion were carried out for each of the following: aluminuni(III), cobalt
*
*
(11), chromium(III), copper (11), iron (111), nianganese(II), nickel(TI), lead (11), and zinc(I1). Quantitative recoveries were obtained in every case, both for tungsten and for the other metal ion. The relative standard deviation (18 determinations) for W03 was f0.15yo; the relative standard deviation for the other metal ion(l8 determinations) was =k0.12970. As was the case with the tungsten separations, the resins used in the niobium and tantalum separations were submitted for spectrographic analysis after ashing. The reports consistently showed no evidence of niobium or tantalum. Quantitative recoveries of niobium and tantalum were obtained, as shown by gravimetric analysis of column effluents. About 80-90 ml. of 0.25M nitric acid, 1% hydrogen peroxide solution was used as the eluent. Quantitative separations were obtained for samples containing approximately 0.1 gram of NbpOj and 0.5 minole of another metal; also for samples containing approximately 0.1 gram of T a 2 0 5and 0.5 mmole of another metal ion. Separations of niobium and tantalum from each of the following metal ions were carried out: aluminum (111), chromium(III), cobalt(II), copper (11), iron(II1) , manganese (11) , nickel(II), titanium(1V) (0.1 niniole), zirconium(1V) (0.1 mmole). Analysis of 36 samples gave quantitative recoveries for both sample components with a relative standard deviation of +0,25% for niobium or tantalum oxide and a relative standard deviation of
Table 1.
Eluting Solutions Used for Metal Ions
Metal ions Y(II1) U(Y1) Zn(II), Mn(II), Co(II), Ni(II), Cu(II), Fe(III), Cr(III), Ti(IV), Sn(1S') Pb(I1) AI( I1I ) Zr(IV)
Eluting solution 300 ml. of 4&@HCl 300 ml. of 2.1.1 HC1
150 ml. of 3M HC1
250 ml. of 3M HNOS 300 ml. of 3 M HC1 300 ml. of 4M HCl
Ion Exchange Separation of Molybdenum(V1) from Other Metal Ions
Table 11.
Results are av. of Diff., 70 Found 0.1172 -0.1 0.1174 +0.1 -0.1 0.1172 10.0 0.1173 0.1173 zko.0 t0.7 0.1187
Mo, mg.
Taken 0.1173 0.1173 0.1173 0.1173 0.1173 0,1179 a
j=0.26% for the other metal ion. Only niobium-tin, and presumably tantalumtin, failed to give a satisfactory separation. Strelow (22) found that tantalum was not completely eluted due to its very pronounced tendency to hydrolyze. However, in the present work this difficulty has been eliminated perhaps because of the higher concentration of hydrogen peroxide and sulfuric acid in the stock solution aliquot which was introduced to the column. Perhaps some fluoride remained attached to the metal following evaporation with sulfuric acid. The use of a shorter column may have also helped to decrease hydrolytic tendencies. The results of the analyses of several alloys are given in Table 111. TazOs required the longest time to go into solution. For practical analyses it is recommended that evaporation be carried only to the point where all or most of the oxide has precipitated. By doing
two or three determinations
Other metal CU(II) Fe(II1) Xn(I1) Ki(I.1) U(Y1) Y(II1)
Diff .,
Millilitersa Taken Found 4.78 4.79 4.22 4.225 4.02 4.03 3.97 3.99 16.7Sb 16.78* 4.93 4.93
% t0.2 +0.1 t0.2 +0.4 fO.0 A0.0
Results expressed in milliliters of O.05.W EDTA required for titration. Milliliters of 0.05M cerium(1V) required for titration.
Table 111.
Alloy Xb-Ni Ta-Fe w-cu
Analysis of Alloys
Results are av. of four determinations Per cent Metal detd. Taken Found 40.16 40.15 59.84 59.72 72.10 71.93 27.90 27.91 40.09 40.02 59.91 59.91
Difference -0.01 -0.12 -0.17 +0.01 -0.07 fO.OO
VOL. 37, NO. 10, SEPTEMBER 1965
1273
this with several trial tantalum and niobium solutions, it was found that the oxide dissolved within about 15 minutes. Since small amounts of zirconium (IV), titanium(IV), and tin(1V) are present in niobium and tantalum minerals, a study was made to determine the extent of interference of these metal ions in the gravimetric method used for niobium and tantalum. Precipitations were carried out in which equimolar amounts of niobium and other metal ion were present. Results for niobium were high in all trials. I n the case of zirconium results ranged from about O.3Y0 to 1.7% high when 0.1mmole and 0.5-mmole amounts of zirconium were present, respectively. The degree of interference increased with titanium, results being from 2.5% to 10% high, while for tin the range was from 21% to 10070 high. For column separations of zirconium or titanium from niobium, the total volume of solution added to the column was increased to 40 ml. to decrease the sul-
fate ion concentration (from 1M to about 0.25.V), so that titanium and zirconium would not be eluted with niobium. I n order to compensate for dilution the solution was made about 2% in hydrogen peroxide and 0.3 11.I in nitric acid by addition of appropriate amounts of each. A total of about 150 ml. of eluting solution was used. Quantitative separations for both 0.5-mmole and 0.1-mmole amounts were achieved only for titanium. The separation of tin from niobium failed, but niobium can be separated from 0.1mmole amounts of zirconium. LITERATURE CITED
(1) Alimarin, I. P., Medvedeva, A. M., Khromatog., ee Teoriya i Primenen., Akad. Nank. S.S.S.R., Trudy Vsesoyuz. Soveshchaniya, Moscov 1958, 379; C.A. 55, 17360d(1961). (2) Alimarin, I. P., hfedvedeva, A. M.,
Trudy Komissii Anal. Khim., Akad. Nank, S.S.S.R., Inst. Geokham. i Anal. Khim. 6 , 351-64 (1955).
(3) Balanescu, G., Z. Anal. Chem. 83, 470
(1931). (4) Carey, M. A., Raby, B. A., Banks, C. V. ANAL.CHEM.36, 1166 (1964). ( 5 ) Dams, R., Hoste, J., Talanta 8, 664 (1961). ( 6 ) Dams, R., Hoste, J., Ibid., 9, 86 (1962). (7) Fritz, J. S., Abbink, J. E., ANAL. CHEM.34, l0S0 (1962). (8) Pavlenko, L. I., Zhur. Anal. Khim. 15, 716 (1960). (9) lZaby, B. A., Ames Laboratory, Ames, Iowa, private communication, 1963. (10) Ryabchikov, D. I., Bukhtiarov, V. E., Zhur. Anal. Khim. 15, 242 (1960); C.A. 54, 19261s (1960). (11) Sano, H., Shiomi, It., J. Inorg. Nucl. Chem. 5, 251 (1958). (12) Strelow, F. W. E., ANAL.CHEM.35, 1279 (1963). JAMES S. FRITZ LIONELH. DAHMER Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Iowa
WORKperformed in Ames Laboratory of U. S. Atomic Energy Commission.
Cation Exchange Separation of Small Amounts of Metal Ions from Cadrnium(ll), Zinc(ll), and Iron(lll) SIR: Little has been reported to date on ion exchange separations of a small quantity of one element (or group of elements) from a very large excess of another element. Many studies have been made concerning anion exchange separations of metallic elements, where elements that form chloride complexes (or form complexes a t lower hydrochloric acid concentration) are retained by the column and the others pass through. It should be possible to use this method to separate traces of elemepts retained by a small anion exchange column from large amounts of elements that are not retained. If a trace constituent must be retained by an ion exchange column and a major constituent not be retained, a cation exchange procedure based on complex formation would supplement the above mentioned anion eschange method. Cation exchange separations based on the formation of metal-chloride complexes are limited to a very few elements, because low hydrochloric acid concentrations must be used to avoid unselective elution of metal ions by the mass action effect of hydrogen ions. However, Fritz and Rettig (2) have shown that many metal ions can be separated from each other on a hydrogenform cation exchange column using dilute hydrochloric acid in aqueousacetone solutions. The presence of acetone greatly strengthens metal1274
ANALYTICAL CHEMISTRY
chloride complexes and makes it possible to work at low hydrochloric acid concentrations so that that elution of metals by hydrogen ions is not a problem. Other authors have used acetone or other organic solvents in various separations based on this same principle. (3, 4,5 ) I n this paper is demonstrated the separation of two elements present in mole ratios of 1000 to 1 or 10,000 to 1 using a small cation exchange column and dilute hydrochloric acid in aqueousacetone solvent as the liquid phase. After the separation, the minor constituent can be stripped from the column and determined by a semimicro titration with EDTA or by standard spectrophotometric procedures. EXPERIMENTAL
Purified Dowex 50W-X8, 100 to 200 mesh, was used in the hydrogen form. It was placed in a large column and backwashed with distilled water to remove fine particles, then washed with 3 liters of 10% ammonium citrate, 3 liters of 3J9 hydrochloric acid, and finally distilled water until a negative test was obtained with silver nitrate. Excess water was removed by suction filtration, and the resin was allowed to air-dry. X 0.0139 solution of EDTA was prepared from analyzed reagent grade material. It was standardized at pH 6 against a solution prepared from pure zinc metal, using XXS as indicator. X 1.OM cadmium ion solution was made b y dissolving 99.99970 pure
metal in enough hydrochloric acid to give a concentration of 0.5X upon dilution. For high concentrations, a solution of iron(II1) was made 3.0M by dissolving 99.9470 metal in hydrochloric acid. A% 5.0M solution of ZnClz in 0.5JI hydrochloric acid was used. For low concentrations, copper and manganese were 0.01Ji solutions of 99.999yo pure metal in 0 . 5 X hydrochloric acid. Kickel and cobalt were 0.01X solutions of the chloride salt in 0.531 hydrochloric acid. Iron(II1) was a 0.005.19 solution of 99.94y0 pure metal in 0.5M hydrochloric acid. Eluents were prepared as volume percentages, while the concentration of hydrochloric acid is expressed in molarity. Volume changes due to mixing were disregarded. Ion eschange columns with 1.2-cm. i.d. and stopcocks of Teflon were used. The resin was equilibrated by passing acetone (20 ml.) through a t maximum flow rate. Samples were prepared in the following manner: Appropriate amounts of metal ion solutions were pipetted into small beakers; the solutions were evaporated to 1 or 2 ml., and a few drops of concentrated HXOI were added when necessary to prevent hydrolysis. The cooled samples were diluted with 20 ml. of the acetone eluent. The prepared samples were transferred to a 125-m1. cylindrical separatory funnel which was attached to the top of the ion exchange column. ‘I he beaker was rinsed several times with a total of 10 ml. of the acetone eluent, which was then added to the sample.
