1070 psi to 2000 psi at 20 psi/min. The sample size was 2.5 pl. Detection was carried out at 217 nm. This large sample size was necessary because alkylbromides have a low extinction coefficient in the UV, even at a wavelength as low as 217 nm. Some overloading of the column may be the result. Figure 8 shows a chromatogram of a synthetic mixture of ferrocene (dicyclopentadienyl iron), cyclopentadienyl Mn(CO)a, and dicyclopentadienyl Tic&. These metalloorganic compounds are not very stable at elevated temperatures. Dicyclopentadienyl TiClz (as well as other cyclopentadienyl compounds) is reported to be a 1 :1 electrolyte according to conductivity measurements in various ethers (13). The
separation shown was carried out on the same column described previously. The temperature was maintained at 33 “C. Detection was carried out at 250 nm. The pressure was held isobarically at 1000 psi for 30 minutes to resolve the iron and manganese compounds and then rapidly increased in pressure to 3000 psi at 130 psi/minute to elute the titanium compound. This particular program type is especially beneficial for the separation of similar substances in the presence of a compound with a very much longer elution time.
(13) W. Strohmeier, H. Landsfield, and F. Gernert, Z . Electrochem., 66, 823 (1962).
RECEIVED for review September 13, 1971. Accepted December 7,1971.
Synergistic Effects in Ion Exchange in Mixed Solvents-C hloride A. P. Rao and S. P. Dubey Chemistry Department, Indian Institute of Technology, Delhi, New Delhi-29, India
The anion-exchange behavior of Co(ll), Cu(ll), and UOz(ll) in ketone-alcohol-HCI mixtures was studied using Dowex L X 8 . The synergistic enhancement of Kd values while using mixtures of two solvents has been attributed to increased formation of the species undergoing exchange equilibria in the mixture as compared to the individual solvents. From the absorption spectral measurements, the species taking part in the exchange equilibria are shown to be CoCI3-, CuCI3-, and U02C12in the solution phase. They are present as CoCI2-, CuCI42-, and as a mixture of UOzClzand UO2CI3on the resin.
Solvents. Reagent grade methanol, ethanol, n-propanol, acetone, methyl ethyl ketone, and diethyl ketone were purified by standard methods ( 4 ) and used. Determination of the Elements. Co(I1) and UO,(II) were determined spectrophotometrically ( 5 ) while Cu(I1) was determined using micro EDTA titrations (6). The distribution coefficients ( K d )were calculated from; mg of the element/g of the resin K d = mg of the element/ml of solution
KORKISCH (I) HAS REPORTED that the adsorption of uranium onto the anion-exchange resin Dowex 1-X8 from methanol or ethanol solutions, increases when part of the alcohol is replaced by dibutyl-carbinol or butoxyethanol. The distribution values are higher in the mixture than in individual solvents. Subramanyam and Sastri ( 2 ) made similar observations during their studies on the adsorption of cobalt from alcohols and acetone. Recently we (3) have also reported similar enhanced distribution values for some elements in acetone-methanol-HC1 mixtures, using the anion exchange resinDowex 1-X8. This work was undertaken to investigate the mechanism for this synergistic enhancement in Co(II), Cu(II), and UO2(II) in ketone-alcohol-HCl mixtures.
The distribution coefficients were determined by the batch equilibrium method (batch method). Each equilibrium was performed with 50. ml of a mixture containing sufficient HCl to give the required overall acid normality, and 5 mg of the element. To this mixture, 1 gram of the resin was added and the solution was agitated for 12 hours at room temperature (30 f 1 “C). The resin was then separated by filtration and the element was determined in the filtrate, after evaporating the organic solvent and the excess acid. The experimental eiror for the determination of distribution coefficients was =k5 for K d values below 1000 and f10 for Kd values above 1000. Spectrophotometric Studies. The absorption spectra were taken on Unicam SP 500 and SP 700 spectrophotometers using 1-cm cells. For taking the spectra of the metal complexes adsorbed on the resin, a procedure similar to that of Ryan (7) was employed using 2-mm cells.
EXPERIMENTAL
RESULTS AND DISCUSSION
Materials. The strongly basic anion exchanger Dowex 1-X8 (chloride form, 20-50 mesh), dried at 30 OC was used for the experiments. Standard Solutions. Exactly weighed amounts of Co(II), Cu(II), and U02(II) as chlorides were dissolved in 0.01M HCl to give solutions containing 5 mg of the element per ml.
The variation of Kd values of Co(II), Cu(II), and UOz(I1) with the composition of the solvent mixtures at various HC1 concentrations is given in Figures 1, 2, and 3 . It can be seen from the Figures that the Kd values pass through a maximum. On the other hand, other ions which were studied previously (3) [e.g., Zn(II), Cd(II), Ni(II), and Th(1V)I did not
(1) J. Korkisch, Progress Report to IAEA and U.S. Atomic Energy Commission under Contract No. At (30-1),2623, Oct.
(4) A. Weissberger, “Technique of Organic Chemistry, Organic Solvents,” Vol. VII, Interscience, New York, N.Y., 1955. ( 5 ) G. Charlot, “Colorimetric Determination of Elements,”
1964.
(2) J. Subramanyam and M. N. Sastri, J. Inorg. Nucl. Chem., 31,
199 (1969). (3) A. P. Rao and S. P. Dubey, hid. J . Chem., 7 , 396 (1969). 686
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
z
z
Elsevier, Amsterdam, 1964. (6) H. Flaschka and H. Abdine, Chemist-Analyst, 45, 2 (1956). (7) J. L. Ryan, Inorg. Chem., 2, 348 (1963).
/o'-
&
€4
40
50
0
t:
Figure 1. Distribution coefficients of Co(I1) in 80% organic solvent (acetone-methanol mixtures)-20 % HCI, (1) OSN, (2) LON, (3) lSN, (4) 2.ONoverall
I
I
Bo
1
60
I
1
1
40
W
0
% &&me
Figure 3. Distribution coefficients of UO,(II) in 80 organic solvent (acetone-methanol mixtures)-20 % HCI, (1) OJN, (2) 0.25N, (3) OSN, (4) LONoverall
Figure 2. Distribution coefficients of Cu(I1) in 80% organic solvents (acetone-methanol rnixtures)-20 HCI, (1) OSN, (2) l.ON, (3) l S N , (4) 2.ONoverall give any maxima. There may be many reasons for observing a maxima in Kd values. One of the factors that need to be considered is changes in swelling of the resin as the composition of the solvent is varied. If it is just a change in swelling that is responsible for giving a maximum in K d , such a maximum should be given for all the metal ions at the same composition. However, Zn(II), Cd(II),
lv' X &e&-
Figure 4. Distribution coefficients of Co(II) in 90 % organic solvent (acetone-alcohol mixtures)-10 %, 10N HCI, (1) acetone-methanol, (2) acetone-ethanol,(3) acetone-propanol ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
687
J 1
TO
XI
I
so
I
e
o
2( K C h
Figure 5. Distribution coefficients of Co(I1) in 90% organic solvent (ketone-methanol mixtures)-10 %, 10N HCI, (1) acetone-methanol, (2) methyl ethyl ketone-methanol, (3) diethyl ketone-methanol
Figure 7. Spectra of Co(I1) in anhydrous acetonemethanol mixtures with a Co2+ to C1- ratio of 1:45, (1) pure acetone, (2) 85% acetone-lsz methanol, (3) pure methanol Co(I1). In order to see whether this synergistic behavior is restricted to acetone-methanol mixtures or is a common behavior, exhibited in mixtures of other alcohols and ketones, distribution studies were made using 90 % organic solvent10 % H20 containing sufficient HC1 to give an overall acidity of 1N . The mixtures employed in these studies are acetonemethanol, acetone-ethanol, acetone-n-propanol, methyl ethyl ketone-methanol, and diethyl ketone-methanol. Co(I1) exhibits maxima in all the mixtures studied (Figures 4 and 5 ) . Co(I1) is well known to be taken up by anion-exchange resins from aqueous chloride solutions as tetrahedral anionic chloro complexes. Obviously the composition of the solvent mixtures must be affecting the formation of these chloro complexes. The octahedral-tetrahedral transformation of Co(1I) with and without the addition of chloride was investigated by several workers (8,9). The various equilibria that can be present are:
5X I 0
lo
xd
LI
+ 4 C1- $ COc14'- + 6 ROH + ROH $ CoC13(ROH)- + C1-
Co(ROH)G2+ COc14'-
CoCls(R0H)- f ROH CoC&(ROH)2
d Figure 6. Distribution coefficients of Co(I1) in 90% organic solvent (acetone-methanol mixtures)-10 %, 8NHBr Ni(II), and Th(1V) do not show any maxima. On the other hand, Co(I1) exhibits a maximum in a mixture of 60 acetone20% methanol, Cu(I1) in 40% acetone-40Z methanol, and UOz(I1) in 30% acetone-50Z methanol in experiments with 1N HC1. Obviously factors other than swelling greatly influence the exchange equilibria. These no doubt are the various complex equilibria in these mixtures, and hence these will be discussed individually. 688
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
$ CoC12(ROH)2
f C1-
+ 4ROH $ Co(ROH)e2+ + 2C1-
where ROH can be HzO,alcohol, or any other solvent with solvating properties. A high C1- concentration and a solvent of low dielectric constant, E, with weak donor properties will favor the formation of CoC14'-, while a solvent with high E and solvating power will favor the replacement of C1- in C0c14~-by the solvent molecules. From the above observations, it can be concluded that formation of C0C142- will be maximum in solvent mixtures containing diethyl ketone (E = 15.10) and least in methanol ( E = 32.6). If formation of CoC14'- is essential for the adsorption of cobalt by the resin, the Kd values in diethyl ketone should be higher than in acetone (E = 21.0). But the Kd values in acetone are higher than in diethyl ketone. Further, since (8) Y . Libus and I. Uruska, Znorg. Chem., 5, 56 (1966). (9) L. I. Katzin, J. Chem. Phys., 36, 3034 (1962).
Table I. Kd Values of Co(II), Cu(II), and U02(II) in Mixtures of Anhydrous Acetone and Methanol Containing LiCl M"+-CI Acetone-methanol ratio, Ion Mm+ ratio 100:O 90:lO 80:20 70:30 60:40 50:50 40:60 30:60 20:80 10:90 0:100 Co(I1) 1 :6 26 14575 14886 8320 7262 ... ... 157 346 134 56 1 :45 5.5 ... 49146 ... 78232 ... 22666 .,. 8083 317 100 Cu(I1) 1 :45 69 100 231 ... 474 ... 300 ... 210 174 146 UO*(II) 1:100 1 ... 100 ... 530 900 1650 ... 975 . .. 600
addition of a solvent of high E and solvating power decreases the formation of C O C ~ , ~the - , Kd values should decrease as we replace acetone by methanol, but the Kd values are passing through a maximum. The above observations indicate that a species other than C0C142- is taking part in the exchange equilibrium. Katzin and Sullivan (10) have pointed out that ion exchange in mixed solvents involves not only ion exchange, but also distribution of low charged and neutral species between the resin phase and the solution. Then formation of CoC13(ROH)- and CoC12(ROH)* (which are taking part in exchange equilibrium) in solvent mixtures can make the Kd values pass through a maximum. The stabilities of halo complexes of Co(I1) decrease in the order C1- > Br- > I- (11). If a cobalt halo complex, in which some of the halides are replaced by the solvent molecules, is taking part in the exchange equilibrium, then addition of a smaller percentage of methanol to acetone should produce a species adsorbed by the resin in the case of bromide as compared to chloride. This means we should observe a maximum in Kd in a mixture containing a smaller percentage of methanol in bromide media when compared with chloride media. An experiment in 90% organic solvent-10% 8M HBr, with acetone-methanol mixtures was also performed (Figure 6). As expected, maximum Kd was observed in a mixture containing only 20 % methanol (in chloride media maximum K d was observed in a mixture containing 40% methanol). Also as an increased concentration of C1- favors the formation of CoC142-,a greater percentage of methanol will be required to displace the chloride by methanol to produce the species taking part in the exchange equilibrium, as the concentration of chloride is increased. The results obtained in 80% organic solvent (acetone-methanol mixtures) containing various concentrations of HC1 (Figure 1) show that as the concentration of HCl increases, maximum Kd is observed in mixtures containing an increasing percentage of methanol. It is easier to interpret the data obtained in anhydrous solvents as compared to the solvents containing water, so distribution studies were made in anhydrous acetone-methanol mixtures with added chloride (LiCl), using the anion-exchange resin Dowex 1-X8 (containing 25% water). [Marcus and Eyal (12) have pointed out that the small admixture of water caused by nonperfect drying of the solvents and the resin, and introduced with the small concentrations of metal chlorides, is of no consequence.] Two experiments were performed, one with a Co2+-C1- ratio of 1 : 6 and another with a ratio of 1 : 45. The results obtained are given in Table I. In the first case, maximum K d was observed in a mixture of 85% acetone 15 % methanol while in the other in a mixture of 60% acetone 40% methanol. This again points to the fact that a species other than c0c142-istaking part in the exchange equilibrium.
+
+
(10)L. I. Katzin and J. C. Sullivan, U.S. Patent 2,840,451,June 24, 1958. (11)D.A . Fine,J. Amer. Chem. SOC.,84, 1139 (1962). (12) Y.Marcus and E. Eyal, .I Znorg. . Nucl. Chem., 32,2045 (1970).
Table 11. Molar Absorptivity Values of the Species Formed emax.
Species CoCla(CH30H)coc142-
a
Sh
=
Wavelength, nm 590 -630 (Sh)' 688 -610 (Sh) 625 -640 (Sh) 667 697
Observed 220 230 400 240 340 285 565 610
-
Fine (11) 237 150 455 248 353 287 557 612
shoulder,
To see the nature of the species responsible for giving a maximum in K d , the spectra of Co(I1) in anhydrous acetonemethanol mixtures (with a Co-C1 ratio of 1 :45) were recorded (Figure 7). The spectrum in pure acetone has peaks at 625, 667,and 697 nm and shoulders at 610 and 640 nm. So Co(I1) exists as C0C142- in acetone (11). The spectrum in 60% acetone-4Ox methanol has peaks at 590 and 688 nm, showing that Co(I1) is present in this mixture as CoC13(CH30H)- (11). The spectrum in methanol has a peak at 545 nm indicating that Co(I1) is present in methanol as C O ( C H ~ O H ) ~ ~The +. molar absorptivity values calculated assuming that Co(1I) is present as CoC142- in pure acetone and as CoCl3(CH30H)-in the mixture agree with those of Fine (11) (Table 11). In order to see the behavior in mixed solvents, the spectra of cobalt chloride were recorded in 90% organic solvent-10 %, 10N HCI. The spectrum in 90% acetone-10% 10N HC1 has peaks at 625,660,and 695 nm and shoulders at 610 and 640 nm and is very similar to the CoC142- spectrum. In addition it also shows a small shoulder at 590 nm, indicative of a small percentage of CoCl3-. In the mixture where maximum K d iS observed, there is a pronounced shoulder at 590 nm with increased absorption, and the peaks at 660 and 695 nm merge to give a single peak at 685 nm showing the formation of COc13(solvent).- In 90% methanol-lO%, 10N HC1 the spectrum has a peak at 545 nm showing that Co(I1) is present as Co2+ in this mixture. The fact that K d values in the mixture are very high, where the formation of CoC1,- is also high shows that CoC13(CH3OH)- is the species taking part in the exchange equilibrium. In order to verify the nature of the species on the resin, the spectra of the resin after adsorbing Co(I1) on it from these mixtures were recorded. These spectra show the species on the resin to be CoC1,- as only a single peak is observed at 690 nm . Cu(I1). The results obtained in acetone-methanol mixtures (80 % organic solvent-20 % water containing HCI) are given in Figure 2. As in the case of Co(II), as the concentration of HC1 increases, maximum Kd is observed in mixtures containing an increasing percentage of methanol. Distribution studies were also made in anhydrous acetonemethanol mixtures with a Cu2+ to CI- ratio of 1:45. Here ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
689
,i\ I
O7I
425
Figure 8. Spectra of Cu(I1) in anhydrous acetone-methanol mixtures with a Cu2+-Clratio of 1:45, and acetone-methanol, (1) 100:0, (2) 90:10, (3) 80:20, (4) 60:40, (5) 40:60, (6) 20:80, (7) 10:90, (8) 0:lOO
050
an
w
nbmee@-(m)
WwgtS[nm)
Figure 10. Spectra of U02(II) in anhydrous acetonemethanol mixtures with a U-Cl ratio of 1:100, (1) 90% acetone-10 % methanol, (2) 40 Z acetone-60 % methanol, (3) pure methanol (13). Replacement of a part of acetone by methanol de-
&na?qq/h-(nm)
Figure 9. Spectra of Cu(I1) loaded on Dowex 1-X8 from 80% organic solvent (acetone-methanol mixtures)-20 % 5N HCl, (1) 80 acetone, (2) 40 % acetone40 % methanol, (3) 80 % methanol maximum K d was observed in a mixture of 60 % acetone-40 % methanol (Table I). In order to have an understanding about the nature of the species taking part in the exchange equilibrium, the absorption spectra of Cu(I1) in these mixtures with a Cu2+to C1- ratio of 1 :45 were recorded and are given in Figure 8. The spectrum in acetone has a peak at 475 nm and a shoulder at 410 nm showing that Cu(I1) is present as CuCI3- in these solutions 690
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
creases the intensity of the 475-nm peak and a new peak starts appearing at 430 nrn. Manahan and Iwamoto (13) have shown that CuCL2- absorbs at 406 nm while CuC12 and CuCl+ absorb below 400 nm. The shift in absorption maximum to 430 nm cannot be due to the formation of CuC1d2-, as we are adding a solvent of high solvating power to acetone. Obviously this can be due only to the occupation of the fourth coordination position of CuC13- by the alcohol which has strong donor properties. Similar changes were observed by Katzin (9) when dimethyl formamide was added to acetone solution of Cu(1I) with added chloride. So the absorption at 430 nm can be attributed to the formation of CuC13(CHaOH)-. On further addition of methanol, the peak at 430 nm also disappears showing that CuCl3(CH30H)-is dissociating. During swelling resin prefers methanol and water to acetone (14). Then the resin is expected to prefer a low charged species and among the low charged ones, those containing methanol in their coordination positions. That is, the resin will prefer CuC13(CH30H)- to CuC142- or CuC13(acetone)-. This is the reason for observing an increase in Kd values on addition of methanol to anhydrous acetone up to a certain percentage. Above this, further replacements of acetone by methanol will break the CuC13- complex and thereby give lower Kd values. The spectra of CuClz were also recorded in 80% organic solvent (acetone methanol in various proportions)-20 5N HC1. The spectrum in 80% acetone has a peak at 405 nm, showing that Cu2+is present as CuC142-. With increasing replacements of acetone by methanol, the peak intensity at 405 nm decreases, showing that CuC142- is disappearing. In the mixture where maximum Kd is observed, there is a shoulder at 415 nm (flat portion in absorption extending from 410-420 nm). This is due to the formation of CuC13(CH30H)- or
+
(13) S. E. Manahan and R. T. Iwamoto, Inorg. Chem., 4, 1409
(1965). (14) C . W. Davies and B. D. R. Owen, J. Chem. SOC.,1956, 1646, 1681.
Table 111. Molar Absorptivity Values of the Species Formed emax.
Vdovenko et al Species
Wavelength, nm
Observed
(13
uo*c12
400 -415 (Shp 425 442 460 411 49 5 400 -418 (Sh) 425 442 415 490
13 22 29.8 26 16.9 13.5 1.4 9.1 14.2 28.5 17.4 20.8 13.6
12 22 30 26 17 13.5 7.5 9 14 28 17.5 21 14
UO2C13-
Q
Sh
=
shoulder.
CuCl3(H20)-. We have proved in anhydrous acetone-methanol mixtures that CuC13(CH30H)- absorbs at 430 nm. From these observations, it is evident that CuCI3(CH30H)or CuC1,(HZO)- is the species taking part in the exchange equilibrium. The CuCI3- that has diffused into the resin can undergo regular exchange with the counter ions or may associate with the resin functional groups to give C U C ~ ~ ~ - . In order to verify whether Cu(I1) exists on the resin as CuC13- or C U C ~ ~the ~ -absorption , spectra of Cu(I1) adsorbed on the anion-exchanger from acetone-methanol mixtures were recorded and are given in Figure 9. The spectrum has a peak at 400 nm showing that Cu(I1) is present on the exchanger as C U C ~ ~ ~ - . UOz(II). The results obtained with U02(II) in acetonemethanol-HC1 mixtures are given in Figure 3. Here also, as the concentration of HC1 is increased, maximum Kd is observed in mixtures containing an increasing per cent of methanol. Distribution studies were also made in anhydrous acetonemethanol mixtures with a U-C1 ratio of 1 :100. The results are given in Table I. In these mixtures, UOz(II) gave maximum Kd in a mixture of 40% acetone-6Ox methanol. The spectra of uranyl chloride in these solvents were recorded. Figure 10 gives the spectrum in pure methanol, in the mixture where maximum K d is observed and in 90% acetone-10% methanol. The spectrum in pure acetone could not be recorded as there is precipitation of uranyl chloride in acetone solution containing LiC1.
The spectrum in 90% acetone-10% methanol has peaks at 400,425,442,455,475, and 490 nm and a shoulder at 418 nm, showing that UOz(I1) is present in this mixture as UOZCl3(15). The spectrum in 40% acetone-6OZ methanol (where maximum K d is observed) has peaks at 425,442,460,477, and 495 nm and a shoulder at 415 nm. In this spectrum the ratio of absorbance at 475 to 460 nm, is different from that in 90% acetone-10 % methanol, showing UOz(I1) to be present in this mixture as UO2ClZ. The spectrum in pure methanol has peaks at 420,460, and 475 nm and shoulders at 395,410, and 430 nm showing it to be a mixture of UOzClzand U02C1+. The molar absorptivity values calculated assuming that U 0 2 (11) is present as U0zCl3- in 90% acetone-10% methanol and as UOzClz in 40 % acetone-60 % methanol agree with those of Vdovenko et al. (15) (Table 111). From the above observations, it is clear that in the mixture where Kd is maximum, formation of UOzCl2is also maximum, showing UOzClz to be the species taking part in the exchange equilibrium. The spectra in 80 % organic solvent-20 % 5 N HC1 were also recorded. The spectrum in 80% acetone is close to the spectrum of Uo&-; in the mixture where maximum Kd is observed, it is close to the spectrum of UOZCl2; and in 80% methanol, it appears as a mixture of UOzCl2 and UOZCl+. These results show that in hydrous solvents also UOzClzis the species taking part in the exchange equilibrium. The UOzClz that has diffused into the resin phase can remain as U02Clz or can associate with the resin functional groups to give uoZcl3-. In order to see the actual species on the resin, the absorption spectra of UOz(II),adsorbed on the resin from these solutions (80% organic solvent-20% 5N HC1) were recorded. In all the cases, the resin spectrum resembles the spectrum of UOz(II) in concentrated LiCl and also the UOzC12and UOZCl3-spectra. This proves the resin phise species to be a mixture of UOzClz and Uo&-. These observation agree with those of Marcus (16) but do not agree with the observations of Ryan (7). The disagreement with Ryan’s result is most probably due to the differences in cross-linking of the resins used.
RECEIVED for review August 31, 1971. Accepted December 14,1971. (15) V. M. Vdovenko, A. A. Lipovskii, and S. A. Nikitina, Zhur. Neorg. Khim., 4, 391 (1959). (16) Y. Marcus, Coord. Chem. Rea., 2, 257 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
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