mined. The kinetic formulation could be explained by means of a plausible mechanism. C 0 NCLUSIO NS
The instrument described has given very good service over one year. Intended as a straightforward extrapolation of the well-known radio frequency titration lrocedures, its utility has greatlJ- exceeded expectations. \Then low cost is not of primary importance, considerably more accurate and sensitive instruments can be constructed according to the same principles. -1 possible refinement, a t the exliense of simplicity, is the frequency sta1)ilization of the klystron referred to above. Both heights of transmission curve and tuning of the cavity can then he measured. -1more elegant and simpler approach is to use a varactor microwave source, n.hich generates microwave energy by multil)lying, by means of voltagevariable capacitance diodes (8),the frequency of the output of a crystal-
controlled r.f. transistor-oscillator by an exact factor in the order of 200. These rugged all-solid-state devices are capable of furnishing several milliwatts of microwave power a t very constant frequency. Surprisingly accurate and sensitive measurements can be carried out by means of these generators. Using the hlicrouave Associates MA1-8262 B varactor source n i t h an output of 12 mw. a t 9000 f 0.01 hlc. per second it was found that, in a balanced microwave bridge, the d.c. detector output changes by as much as 10 mv. when dry nitrogen is subitituted for air inside a 9-mm. i.d. quartz tube in the axis of the microwave cavity. X more detailed description of the latter initrument and its possibilities will be published in the near future. LITERATURE CITED
(1) Adema, E. H., Rec. Trav. Chim., in press. (2) Adema, E. H., Bartelink, H. J. M., Smidt, J., Zbid., 80, 173 (1961). (3) Bakker, &I. J. A., Smidt, J., A p p l . Sci. Res. B9, 199 (1961).
(4) Jacober, J., Kraus, C. A., J . Am. Chem. SOC.71, 2409 (1949). (5) Klages, F., Meuresch, H., Steppich, W., Ann. Chem. 592, 81 (1955). (6) Luther, H., Mootz, D., Radwitz, F., J . Prakt. Chem. 5, [4] 256 (1958). ( 7 ) Montgomery, C. G., IXcke. R. H., Purcell. E. >I.. “Princides of hlicrowave Circuits,’; Vol. 8, hl I. T. Radiation Laboratory Series, p. 239, RIcCrawHill, New York York, 1948. 1948. 9) Muller, R. H., ANAL. CHEM. 36, No. 8, 97A (: (1964). W., Z . Physik. Chem. B16, 9 ) Nespital, I% 153 (1932). 10) Thorn&, C. A,, “Anhydrous Aluminum Chloride in Organic Chemistry,” p. 52, Reinhold, New York, 1941. 11) Ulich, H., Hertel, E., Nespital, W., Z. Physik. Chem. B17, 21 (1932). 12) Ulich, H., Kespital, W., Angew. Chem. 44, 750 (1931). 13) Van Dyke, R . E., J . Am. Chem. SOC. 73. 398 (19511. (14) ’Van hyke,’ R. E., Crawford, H. E., Zbid., 72, 2831 (1950). (15) Weissberger, A., Proskauer, E. S., Riddick, J. A , , TOCJPS. E. E.. Jr.. “Technique of Organic Chemistry,’’ Vol. VII, Interscience, ru’ew York, 1955. ’
RECEIVEDfor review August 3, 1964. Accepted October 26, 1964.
Cation Exchange Separations Using Ammonium Thio cya na te-0 rga nic So Ivent-Wa te r EIua nts DONALD J. PIETRZYK and DONALD L. KISER’ Department of Chemistry, State University o f Iowa, Iowa City, Iowa
b The
distribution coefficients of several metals have been measured on Dowex 50-X8 with systems containing various ratios of NHISCN, organic solvent, and water. The concentration of organic solvent in a system greatly affects the distribution values. The nature of the organic solvent also has some effect. Most metals have decreasing distribution coefficients with increasing amounts of NH4SCN in methanol. Several separations were performed using eluting agents suggested by the distribution data.
A
of inorganic anions have been used as eluting agents for the separation of metal ions on ion exchange resins. Of these the thiocyanate ion, although utilized to some extent in both anion and cation exchange, has not been systematically studied. Coleman et al. ( 3 ) separated rare earths from Am by first removing the metal ions from a cation column with WIDE VARIETY
* Present address, Grain Processing,
Muscatine, Iowa.
5M SH,SCN and passing the effluent into an anion column. Surls and Choppin ( 1 4 ) studied the adsorption of thiocyanate complexes of actinides and lanthanides on anion and cation resins. Using aqueous thiocyanate solutions and anion resin, Teicher and Gordon (15) separated Fe+3 and A1 and Turner, Philp, and D a y (16) separated Cr-3 from Sc and Ni and Co from Fe+3. Hamaguchi and workers (6-9) have used aqueous NH4SCK-HC1 solutions to separate Sc from a number of nietals on anion or cation resins and to study the thiocyanato-chloro complexes of some other metals. Korkisch and Hecht (10) separated G a from Fe+3 on anion resin. These latter two methods depend, in part, on elutions with HC1. One problem which has limited the use of thiocyanate solutions as an eluting agent is the fact that very large concentrations of thiocyanate salt solutions are often needed to remove many cations froin a cation resin or to cause the metal ions to be retained by anion resins. The concentrated solutions are more viscous and difficult to work with. If recovery or analysis of the separated metal is desired, one is faced
with removing or analyzing for the metal ion in presence of large concentrations of thiocyanate salts. In part, this problem has been overcome by employing NHISCN-HC1 mixtures as eluting agent (6-9). Another approach to the problem, which is considered in this report’, is the use of an organic solvent mixed with the aqueous thiocyanate salt solution. Previous work by Fritz and Pietrzyk ( 4 ) and others (5, 1I , 17) with inorganic anions as eluting agents has shown that the presence of water-miscible organic solvent greatly affects the retention properties of many metals on anion and cat’ion resins through enhanced complex formation. Thus considerably lower concentrations of the complexing agent are required for the separations. I t would be expected that thiocyanate complex formation in lii’esence of a water-miscible organic solvent ~ o u l dbe enhanced and consequently would have a pronounced effect on exchange seiiaration of metal ions. The present work describes a systematic study of the sorption of metal ions on strongly acidic cation exchange resins in presence of waterVOL. 37, NO. 2, FEBRUARY 1965
* 233
ammonium thiocyanate-organic solvent mixtures. Separation of several synthetic metal ion mixtures as a result of these studies is also described. EXPERIMENTAL
Table I. Distribution Coefficients of Zn and Cd in Various Solvents, 0.1M NHISCN on Dowex 50-X8, NHl+ Form, 100- to 200-Mesh Resin
Solvent
Zn
Cd
40% 807, 2-Propanol
110 0.5 59 150
260 20 110
Resin and Reagents. Dowex 50X8 (100 to 200 mesh for batch measurements and 200 to 400 mesh for column separations) was washed with methanol, then charged into the ammonium form by passing a large excess of aqueous 2 to 3M N H 4 S C S through a column of resin. Resin was rinsed first with water and then with methanol and air dried. A.C.S. Reagent grade solvents and NH4SCN were used without further treatment. Stock solutions of metals were prepared by dissolving metal salts (chlorides or nitrates) in organic or water-organic solvents. The solutions contained about 1 meq. of metal in 50 ml. of solvent mixture which depended on the measurement being made. Ammonium thiocyanate stock solutions for column elutions were prepared by adding measured volumes of water and organic solvent to weighed amounts of NH4SCN. Volume contractions were disergarded. Distribution Coefficients. hliquots of N H , S C S in organic solvent, metal ion solution, organic solvent, and water totaling 25 ml. were added to 1 gram of air-dried, 100- to 200-mesh resin in a 125-mI. glass-stoppered flask. All final metal ion concentrations were corrected for volume contractions due to mixing of solvents and were in the range 0.100 ==I 0.010 meq./25 ml. For samples that were 0.8.11 XH4SCN or higher, weighed amounts of NH4SCN were added to the metal ion-solvent mixture. Flasks were mechanically shaken for 16 hours or longer. Twenty-milliliter aliquots were then analyzed by titration with disodium dihydrogen (ethylenedinitril0)tetraacetate dihydrate using established methods (1, 4 ) . Distribution coefficients, K,, were calculated on a dry weight basis using:
Kd
=
meq. metal on resin/g. dry resin meq. metal in soln./ml. s o h .
60 7% 40
cx
0 0
25
290
5 2 126
300
Usually a first fraction of 30 ml. was collected and analyzed. Thereafter 5 to 10 ml. were titrated until the effluent contained none of the metal being eluted. The eluant was then changed to elute another component. A flow rate of 0.3 to 0.6 ml. per minute was maintained throughout the sample addition and elution. RESULTS A N D DISCUSSION
Distribution Coefficients. A metal ion can be removed from a cation resin by mass action replacement with another cation or by tying the metal ion up as a complex. Generally, the presence of a n organic solvent will enhance a n exchange reaction for a given pair of cations. A favorable enhancement would therefore result in a lower K d . However, large concentrations of the eluting cation are still needed for elution and the separation is still based on differences of cation charge. Of more importance to metal ion removal is complex formation. I n the case of thiocyanate ion the complexation reaction, Reaction 1, would be as follows:
-
The dry weight of the resin was calculated from the air-dried weight and the moisture content of the resin. The moisture content, 13.0%, was determined by vacuum drying of a 2-gram sample for 24 hours a t room temperature. Separations. -\ slurry of swollen, 200- to 400-mesh resin in methanol or 807, methanol was added to a glass column equipped with a coarse frit and a n i.d. of 1.0 cm. T h e resin bed was adjusted to about 5 to 8 em. and rinsed with solvent of composition of the stock solution (less the metal). Aliyuots of metal solutions were added and the liquid level wab allowed to approach the surface of the resin, then more iolvent was added. h n eluant chosen on the basis of the Kd data was added to the column.
234
60 %
40% 80yoAcetone
ANALYTICAL CHEMISTRY
The formation of a complex with lower positive, neutral, or negative charge would remove M+"and consequently, reduce sorption to a cation resin (lower K J . I t is, therefore, not necessary to produce negatively charged complexes to effectively rcmove AI+" from the resin. Indeed, the concentration of negatively charged metal-thiocyanate complexes in most cases in aqueous solution is small or negligible ( g ) , Upon addition of an organic solvent M + n would be even lower in concentration because of enhanced complex formation and therefore would have a lower K d than it would in the absence of the organic solvent. To facilitate a choice of eluting conditions, K d data for a wide range of eluting agent concentrations are
measured. When a water-miscible organic solvent is to be utilized, several variables must be considered. Distribution coefficients in thiocyanate-organic solvent-water systems can be altered by change in concentration of thiocyanate ion at fixed water-organic solvent ratios, by change in ratio of organic solvent to water at fixed thiocyanate concentration, or by change to another organic solvent. The effect of the nature of the organic solvent is seen in Tables I and 11. The general trend is that the K d data are lower in solvents of low dielectric constant than they are in solvents of high dielectric constant. However, this is not always the case. The K d data for l l g are higher in 0.4 and 0.8M ?;H,SCS in 2-propanol than they are in methanol solutions of the same NH4SCK concentration. In Table 11, the K d values are affected to a greater extent by increasing X H S C X concentration in the more polar organic solvents than they are in the solvents of low polarity. Increasing NH,SCS concentration has little effect on the distribution of l I g or La in 2-propanol. The effect of changing the organic solvent-water ratio is indicated in Tables I, 11, and 111. Decreasing the amount of water decreases the K d . The effect of increasing amounts of KH4SCN is seen in Tables I1 and IV. I n general, the distribution coefficients decrease with an increase in ?;H,SCS concentration. The values of Co, Zn, Cd, and Fe+3 decrease to a minimum, then increase with increasing SH,SCN concentration. Aill the data reported here are for NH4+ form resin and SH4SCN. S o noticeable changes in K , values are observed when NaSCN or KSCK salts are used and the resin is charged in the respective form. The effedveness of NH,SC?J in methanol as an eluting agent is illustrated by comparing data in Table IV with K d data for aqueous HCl (23) and acetone-HC1 ( 5 ) systems. (Qualitative comparisons only can be made because of the lack of uniform experimental conditions.) Only slight differences are noticeable between the niethanol-SH4SCS and acetone-HC1 systems. A large difference is noticed, however, when compared to aqueous HC1. Distribution coefficients are lowered by a factor of l o 4 for T h and Zr and lo3 for Fef3 and A1 by changing from 0.2.11 aqueous HC1 to 0.231 S H 4 SCN in methanol. Reduction of K d is also observed for the other metals measured but to a lesser degree. The T h value is particularly interesting, 0.7 in 1.6.11 XH4SCN in methanol as compared to 67 in aqueous 4.11 HC1 (Th mas not measured in acetoneHC1). Quantitative recovery of T h
Table II. Distribution Coefficients of Ca, M g , and La in Various Organic Solvents and 90% Acetone with Different Concentrations of NH4SCN on Dowex 50-X8, NH4+ Form, 100- to 200-Mesh Resin
Table Ill. Distribution Coefficients in Various Concentrations of Methanol, 0.1 M NHbSCN, with Dowex 50-X8, NH4+Form, 100- to 200-Mesh Resin
Methanol, 70 80 90 750 500 1100 68 34
c
'Ti System AIethanol
XHaSCX 1 6 0 8
0 4
Ethanol
0 2 1, 6
0.8 0 4
0 2
Ca 43 6 204 1130 3660 45.0 115 650
0 1
Acetone
90'7' Acetone
1.6 0.8 0.4 0.2 16 0 8
0 4 0 2
17 0 34.7 57 8 132 19 3 83 8 306 372
Metal A1 Cr + 3 Ga WIn + 2
60 5000 1400 800 400 COZ+~ 130 co 114 S i 95 Fe'3 74 Cd 70 In 29 5'0 1.2 18 zn 23
La 19 7 135 970 4000 9.97 23.8
M g
0
0 83
3 68 16 6 0
0.65 15.6 16.6 45
8 84
15.7 33.8 47.9 200 875
1 64
9 31
43 9 193 10 8 15 6 16 4 21.1
1860
70
100 10.2 330 0.4 300.0 5 s 1 2 1 8 0 2 0 6 0 6
85
6 4
40 20 22 14 13 17 2 9 0.5
19
3 1 14 1 2
0.1
_. 8 0 % M e t h a n o l
10,000 9 30 11 9 11 3 14.9
-0.1
M NYSCN, 80% Methanol
c. 0
from a cation resin is difficult because of its tetravalency. Strelow (fa)resorted to ashing the resin for quantitative recovery. The advantage of the organic solvent is further illustrat'ed by the fact that the K , of T h in 1.6JI aqueous NH4SCX is 164. The K , of T h decreases as the T h concentration is increased. The values ranged from 250 at 0.02iO meq. of added T h to 19.7 a t 0.404 meq. of added T h in 0.4Jf SH4SCN in methanol and fixed resin weight. llagnesium K d , on the other hand, is independent of M g concentration under similar conditions. The increase in K d was most pronounced a t resin loadings of less t'han 1%. The highly polar resin has a strong preference for water or more polar solvent if a mixed solvent is used. The possibility of partitioning (or salting in) taking place between the strongly bound water (13y0 in this case) and external solvent mixture must always be considered when employing organic solvents or materorganic solvent mixtures. This may account for the increase in K , for T h a t low resin loading. Separations. T o separate ta.0 metal ions the elution condit,ions are chosen so that the ratio, K , slow moving to K d fast moving, for the two metal ions is as large as possible. At the same time the value for the fast moving ion should be Jmall, Ireferably one, so that only small volumes of eluting agent are needed. Distribution data as presented in Tables I to I\' perinit the prediction of suitable eluting agent mixtures of organic solvent-n-ater-SH4SCS-H4SCX for the se1)aration of metal mistures. The results of the separation of several
E r
0 0 -
5 0 -
1
synthetic metai mixtures are shown in Table V. A typical elution curve is shown in Figure 1. Most of the separations studied were about 1: 1 ratios of the metal ions in the mixtures. The method, however, is not restricted to this ratio as evidenced by separation of 10:1 and 1:10 ratios of Zn to Cd and 9 : l ratio of >In+* to M g (Table V). Also setlarations in which
solvents other than methanol are used are possible. For example, the K , data in Table I for Zn and Cd in acetone suggest a very favorable separation factor. This separation is illustrated in Table V. Separations depending on solvent change from 90 to 100% methanol were attempted. Tailing of varying degrees was present during separations per-
Distribution Coefficients of Several Metals a t Various Concentrations of NH4SCN in Methanol on Dowex 50-X8, NH4+ Form, 100- to 200-Mesh Resin M NH4SCN LIetal 0-01 0 05 0.1 0 2 0 4 0 8 1 6 Zn 1000 1 4 0 07 0 5 2 4 n_ f_i Cd 1 4 1 5 co 62 2 2 1 2 8 8 3 5 Xl 41 3 8 1 8 1 2 Fe ++ 33 3 8 1 7 0 1 AIn"2 3 .00
Table IV.
1'02 + 2 Zr M g
Cr + 3 Pb Th La Ca A1 Ga In
5.8 6.4
0.8
16.6 2980
14
10 0 4 0 6
187 4000 3660 2.8 0 2 0 6
0.4
3.7 39 49 80 970 1130 1.2 0 2 0
0 0.8
12 26 7.9 135 204
0 8 0
2 0 0 7 l9,7 43 6
0 0
VOL. 37, NO. 2, FEBRUARY 1965
235
formed with eluants that contained no water, but recoveries for Co-Mn+Z and Cd-Mn+2 mixtures were quantitative. The elution of Co is satisfactory with an eluant of 0.1M NH,SCN in 90% methanol. However, if 0.1M NH4SCN is used in methanol containing no water, elution is not quantitative. llnder the latter condition, Co separates into two easily seen blue bands; an immobile band that remains tenaciously a t the top of the column and a mobile band that migrates down the column in a normal manner. A slight bleeding occurs from the immobile band as eluting agent is passed through. If the NH,SCN concentration is 0.5M in methanol the relative amount of Co remaining in the immobile band increases. Iron cannot be eluted satisfactorily. Even 0.1M NH4SCN in 90% methanol
Separations on Dowex 50-X8, NH4f Form, 200- to 400-Mesh Resin in 1 .O-cm. Columns Using NHdSCN-Methanol-Water Eluants
Table V.
Metals separated Zn-Cd Zn Cd Zn Cd Zn Cd Zn Cd Zn Cd Zn-Co Zn co Zn-Ni Zn Ni Zn Xi Cd-Mn + 2 Cd Mn + 2 Co-PvIn + 2 co Mn + 2 Ni-Mn+2 Ni Mn + 2 TO +2-Mn + 2 VO + 2 Mn+2 T O +2
Mn + 2 Zn-Cd-Mn Zn Cd
Mn+* Mn +2-Mg hln + l alg
Mg VO2 +'-Th u02 +%
a
Th 7cAcetone.
236
will not remove Fe+l (about 15% removed). The immobile band of Fe+3, like Co, also becomes greater in concentration as the NH4SCN is increased. Elution of Zn on the other hand with 0.5M S€14SC?; in methanol is quantitative . K d values for several metals reach a minimum a t 0.1-U KH4SCNin methanol and then increases with increahing N H 4 S C S (Table IV). One might expect a slower elution of the metal ion:, at higher NH4SCS. However, the magnitude of the K d compared to the tenacity of the band suggests borne other sorption mechanism. Possibly, a Co or Fe + 3 thocyanate specieq is trapped inside the pores of the only slightly swollen resin or partitioning is occurring. This property is being investigated further.
-
Eluting conditions M 70 Methanol XHiSCS
Bed depth, cm.
Loaded, mg.
Recovered, mg. 1.29 2.08 3.28 5.13 0.660 6.50 0.95 0.849 1.13
5 0
0.1
1.29 2.06 3.23 5.17 0,646 5.17 6.46 1.03 0.835 Ll7
80 90
0.1 0.1
2.61 0.176
2.61 0.176
5 4
70 90 80 90
0.1 0.1 0.1 0.1
0.673 0,800 2.61 0.400
0.667 0.806 2.62 0,395
6 2
90 100
0.1 0.2
1.11 0.616
1.10 0.626
5 7
90 100
0.1 0.2
1.05 1.23
1.04 1.25
5 5
90 100
0.1 0.2
1.60 0.270
1.61 0,274
5 0
80 100 80 100
0.2 0.1 0.2
0.1
0.523 0.616 1.57 0.616
0.327 0.630 1.55 0.628
5 8
80
0.1 0.1 0.2
1.35 1.11 0.616
1.33 1.10 0.633
6 0
100 100 100 100
0.1 0.5
1.90 0,247 0.628 1.43
7 1
0.1 0.5
1,85 0.237 0,616 1.42
5 9
100 100
0.1 1.6
5.66 2.55
5.46 1.85
6 8
80
90 80 90 80
90 80 90 60a 9oa
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
5 5
8 0
5 5
5.00
T O
5 5
9 2
+2
90 100
ANALYTICAL CHEMISTRY
For separation based on changes of the methanol-water ratio at constant NH,SCS concentration, it is e.sential that a wash solution of the came coniposition as the first eluant. iiiinus the thiocyanate, be used before the first eluant is added. For esample, when thr separation of Zn and S i ivas attempted, omitting the mised solvrlnt wash, poor results were obtained. \ 10% Recovery for Zn was a l n a > ~about high, whereas Xi was always 10TGlow. -1pparcntly such a procedurc produces a lclading edge on the eluant that is enough greatpr than 807,, mcthanol to remove some of thc Xi n-ith the Zn. By reverting to the suggestetl wash quantitative recoveries were made for both Zn and Si. I-ranyl ion was separated from T h by use of 0.1.11' SH,SCX in methanol. but T h was not quantitativcly rrnioved from the column nith 1.6.11 SH,SCN in methanol, even though the K,,is favorable (Table 5'). The distriliution data suggest several different eluting conditions for the separation of 11g and Ca. ITnfortunately, quantitative results for the seliaration were never obtained for the several conditions tried. LITERATURE CITED
(1) Barnard, A . J., Broad, W. C., Flaschka, H., "The EIITA Titration: Sature and 1Iethoda of End Point Detection," J. T. Baker Chemical Co., Phillipsburg, S . J., 1957. (2) Bjerrum, J., Schwarzenbach, G., Sillen, L. G., "Stability Conrtants of l\Iet,al-Ion Complexes," Part 11, The Cheniical Society, London, 1957. (3) Coleman, J. S., Penneman, R. z4.j Keenan, T. K., LalIar, L. E., Armstrong, 1). E., Asprey, L. B., J . Znorg. Sucl. L'hem. 3 , 327 (1956). (4) Fritz, J. S., Pietrq-k, 11. J., Talanta 8, 143 (1961). (5) Fritz, J. S., Rettig, T. .I.,ASAI,. CHEM.34, 1562 (1962). (6) Hamaguchi, H., Kawabuchi, K., Kuroda, R., Ibid., 36, 1654 (1964). ( 7 ) Hamaguchi, H., Kuroda, R., Aoki, K., Sugistita, R., Onuma, S . , Talanta 10, 153 11963). (8)-Han;aguchi, H., Kuroda, R., Onuma, 1 ., Ibid., p. 120. (9) Haniaguchi, H., Onuma, S . , Kishi, JI.j Kuroda, R., Ibid., 11, 495 (1964). (10) Korkisch, J., Hecht, F.. Microchim. dcta 1956. 1230. (11) Korkis;:h, J., Janauer, G. E., 7'alanta 9,957 (1962). (12) Strelow, F. W. E., A x . 4 ~ .CHEM.31, 1201 (1959). (13) Strelow, F. W. E., I b i d . , 32, 1185 (1960). (14) Surls, J. P. Jr., Choppin, G. R., J . Inorg. .Yucl. Chem. 4, 62 (1957). (15) Teicher. H.. Gordon., L.., AZI.LL. CHEM. 23,8:30 (1951). (16) Turner, J. B., Philp, R. H., Day, R. A . , Anal. f'him. z-Ic.tn26, 94 (1962). (17) \Vilkens, 11. H., Smith, 11. E., Talanta 8, 138 (1961). RECEIVED for review August 24, 1964. Accepted Soveniber 23, 1964. Financial assistance to one of the authors (DLK) in the forni of a LhPont Fellowship is gratefully acknowledged.
