be rationalized in terms of the widely differing cerium(1V) :and osalate concentrations under consideration and any uncertainties in the assumptions made concerning the degrees of dissociation of bisulfate and bioxalate. Equation 12 also permits the calculation of the concentration of the intermediate for the reactant mixtures of Table 111. The corresponding absorbances are obtained by the extrapolation of the plots of Figure 3 to zero time. The absorbance-concentration relationship was found to be linear, thus justifying the use of Beer’s law. The role of sulfate or bisulfate in retarding the rate of the overall cerium (1V)-oxalate reaction, therefore, centers about the ability of‘ these anions to repress the formation of the reactive intermediate by competing with oxalate for the available cerium(1V). Other anions, such as phosphate, which also effectively complex cerium(IV), show a
similar retarding effect upon the rate of the reaction (10). Conversely, in perchlorate media-Le. under noncomplesing conditions-the cerium (1V)-osalate reaction proceeds too rapidly to be followed by conventional spectrophotometric techniques. Because of irreversible intermediate steps in the mechanism, the trend in reaction rates as a function of solvent anion does not necessarily parallel the trends in standard potentials of the cerium(1V)cerium(II1) couple which involve the relative affinity of the ohidized and reduced forms for the compleling anion. LITERATURE CITED
(1) Black, A. H., Dodson, Y., J . Chem. Educ. 33, 562 (1956). (2) Blatz, L. A , , J . Phys. Chem. 66, 160 (1962). (3) CsBnyi, L. G., Szake, R., 2. Anal. Chem. 175. 187 11960). (4) Dodson,’V., Black; A. H., J . Am. Chem. SOC.79, 3657 (1957).
(5) Frost, A . A , , Pearson, R. G., “Kinetics and Mechanism,” 2nd ed., p. 41, JViIey, New York-London, 1961. (6) Hamilton, L. F., Simpson, S. G., “Calculations of Analytical Chemistry,” 6th ed.. u. 315. hlcGra\v-Hill. 1960. (7) Hardidck, T: J., Robertson, E., Can. J . Chem. 29, 828 (1951). (8) Kolthoff, I. hl., Sandell, E. G., “Textbook of Quantitative Inorganic Analysis,” 3rd ea., p. 37, hlacm~llan, New York, 1959. ( 9 ) Pound, J. It., Chem. Eng. Mining IZeview 32, 418 (1940). (10) Rao, G . P., Mohan, P. G., Sastri, M. N . j Z. Anal. C‘hem. 152, 336 (1957). (11) Rechnitz, C:. A , , A N A L .CHEM.36, 453R (1964). (12) Rechnitz, G. A , , F:l-Tantawy, Y., Z. Anal. Chem. 188, 173 (1962): 193, 443 (1963). (13) Ross, S. I)., Swain, C. G., J . ilm. Chem. SOC.69, 1325 (1947). (14) Weybrew, J. A , , Mortone, J., Baxley, H. hl., h A I , . CHEM.20, 739 (1948).
RECEIVED for review February 12, 1964. Accepted May 6, 1964. Supported by Laboratory for Research an the Structure of Matter, University of Pennsylvania.
Separation, Identification, and Spectra of the An ionic Chromium(Ill)-T hiocya na te Co m plexes SHELDON KAUFMAN and LEWIS S. KEYES Department of Chemistry, Princeton University, Princeton, N. 1.
b The cellulose anion exchanger, Selectacel DEAE (diethylaminoethyl cellulose) has been used to separate the anionic chromium(lll)-thiocyanate complexes, including the geometric isomers Cr(NCS)4(H20)2-. These complexes are so strongly adsorbed by conventional anion exchange resins that they cannot b e eluted without decomposition. The low affinity of the cellulose ion exchangers for these ions allows their eiution under mild conditions.
T
HE
CHROMIUH(III) - THIOCYANATE
system of complexes has been studied by Bjerrum ( I ) . who demonstrated the existenctl of the complexes Cr(r\;CS)n(H20)6c_3in,n = 1 to 6, inclusive. At room temperature equilibrium between the complexes is established only slowly, allowing. them to be separated and characterized. Ion exchange has been used (2, 4 ) to separate the cationic complexes, including the geometric isomers of Cr(NCS)2(H20)4+. The anionic complexes*have been separated by chromatography, using an alumina column ( 3 ) . This method was unsatisfactory, because the wide variation in properties of different batches of alumina and the dependence of adsorptive properties on the previous history of a gi\ en sample made it difficult to obtain reproducible results. In
addition to this inconvenience, it became apparent that alumina catalyzed the aquation of Cr(XCS)6-3 while it was adsorbed. il search was therefore made for a more satisfactory ion exchange method of separation. EXPERIMENTAL
Reagents. Mixtures of the chromium(II1)-thiocyanate complexes were prepared by heating solutions of chrorq@m(III) perchlorate and 1\’H4SCX at about 95’ C. for several hours, or by similarly heating a solution of (KH4)aCr(iYCS)6.4H20. The latter compound was prepared by the method of Bjerrum ( 1 ) . Different proportions of the anionic compleses were obtained by varying the relative amounts of chromium and thiocyanate. After the solution had cooled, the cationic cornpleses were removed by diluting until the S H 4 +concentration was less than 0.lA11and passing through a Dowes5OW column. The effluent, containing neutral Cr(SCS)3 and the anions, was stored in a refrigerator until used. Tracer esperiments were carried out with Cr51(TI,2 = 27.8d), which was counted in a well-type scintillation counter (NaI-TI). The tracer was obtained from Oak Ridge Sational Laboratory, Oak Ridge, Tenn., as CrCI3 in HCI, and was converted to the perchlorate by fuming with HC10, until orange CrOs formed, then reducing with H202. Labeled complexes were prepared as above.
The anion exchange resins, Dowex-1, Dowex-2, Dowex-3, and Dowes-21 K were obtained from ,J. T. Baker Co., Phillipsburg, h’. J. They were used in either the thiocyanate or perchlorate form. The cellulose anion esrhacger, Selectacel DEAE, was obtained from Carl Schleicher and Schuell Co., Keene,
N.H.
Procedure. Ion exchange columns were 1.0 cm.2 cross-section and 30-35 cm. long. The flow rate was about 1 ml. per minute. The cellulose ion eschanger was shaken vigorously with 1M HClO, before use, to break up the material to a fine suspension and convert it to the perchlorate form. Chemical analyses were performed with a Beckman Model 11 spcctrophotometer. Chromium was determined as chromate ( 5 ) and thiocyanate as the ferric comples ( b ) . Each solution was analyzed directly for free thiocyanate, then treated with S a O H to decompose the compleses and analyzed again for total thiocyanate. The amount of complesed thiocyanate is obtained by difference. RESULTS
Polystyrene Resins. The following anion eschange resins of the polystyrene - divinylbenzene type were tested: Dowex-1 (strong base), I>on.ex-2 (strong base), Do\vex-3 (weak base), and Dowex-21 E; (strong base, high porosity). Similar behavior was shown VOL. 36, NO. 9, A U G U S T 1964
1777
1
o,lo
t
,,o
HC104
HC,04
+3.OM
20% ACETONE40%ACETONEHC104 t 3 . 0 M HC104
-1
I
I
1
I
I
I
D
R
G
\
1 400
VOLUME, rnl
Figure 1. Elution curve of a mixture of chromium(ll1)-thiocyanate complexes from Selectacel-DEAE anion exchanger. The bands are identified in Table I
by all of them. They adsorbed the anionic complexes quantitatively, giving an intensely colored band a t the top of the column. This band could not be eluted or displaced by any common eluting agent, including thiocyanate and perchlorate. Concentrated HCI converted the adsorbed complexes to chloro complexes and thus removed them. Elevated temperatures also caused decomposition of the complexes. Concentrated HC104 caused oxidation of the thiocyanate. KO differentiation among the species was possible. Celhlose Ion Exchanger. The strong adsorption of these complexes suggests using a material with a relatively low density of ionic sites and a low affinity for ions. The cellulose anion exchanger DEAE (diethylaminoethyl cellulose) is such a material, and has been previously used in separating biological materials. It is characterized by easy elution under mild conditions. Preliminary experiments indicated that the anions were adsorbed from slightly acid solutions, and that washing with 1M HC104 resulted in development of several distinct bands, some of which were eluted. The remaining bands were only slowly displaced by HC1O4 alone, but mere readily eluted by a mixture of acetone and HCIOa. The procedure finally
Table
I.
I1
- -
settled on was to elute successively with 0.1M HC104, 1.OM HC1O4, 20% acetone (by volume) in 3.0M HC104, and 40% acetone (by volume) in 3.0M HC104. The elution curve of a mixture of the neutral and anionic complexes is shown in Figure 1. The composition of each peak in the elution curve was determined by measuring the ratio of complexed thiocyanate to chromium. The results are presented in Table I . Bands B and C are the isomeric tetrathiocyanato-diaquochromium(II1) anions, and by analogy with similar isomers, the more easily eluted one is presumed to be the trans isomer. The species in bands D and E decomposed during and after the elution because of the presence of acetone. This effect of organic solvents was noted by Bjerrum ( 2 ) . Hence, only a lower limit of the complexed thiocyanate: chromium ratio could be obtained. The results are consistent with the iden-
rn
c
D
2 95 f 0 05 4 09 f 0 . 0 5 3 93 f 0 07 >4 7
E
>5 5
A
B
1778
ANALYTICAL CHEMISTRY
I
I
300 I ‘
‘ ‘400 ‘ ‘ ‘ ‘ ‘ r500 ‘ ‘ ‘ l600 - 70
-- Cr(NCS)3(H
Figure 2. Spectra of - -frans-Cr(NCS)r(H20)-~
Analyses of Successive Bands in Elution Curve
Eluant
I
W a v e length, m p
[NCS-1
Band
2 o:
Species
Cr( N C S ) S - ~
20)s
and
tifications in Table I. In addition, a solution of (NH4)3Cr(NCS)6 showed identical behavior to that of band E. The spectra of Cr (NCS) 3(H20)3 and trans-Cr (NCS),(HLO)z- complexes are given in Figure 2. The cis isomer spectrum is substantially the same. Comparison with the spectra of the isomeric Cr(NCS)2(H20)4+complexes (2) shows that the curves are quite similar, except for the greater absorbancy of the higher complexes. The successful elution of these complexes suggests the possibility of using cellulose ion exchangers to separate other inorganic ions which are strongly adsorbed on conventional ion exchange resins. It would be of interest to study such ions as the chloro complexes of transition metals, and such highly charged cations as the polymeric chromium species. LITERATURE CITED
( 1 ) Bjerrurn, N., 2. Anorg. Chem. 118, 131 (1921): 119, 39, 54, 179 (1921). ( 2 ) Houaen, J. T., Schua. K.. Kine, E. L.. J. Amy Chem. goc. 79; 519 (1957). ( 3 ) Kaufman, P., Zbid., 82, 2963 (1960). ( 4 ) King, E. L., Disniukes, E. B., Zbid., 74, 1674 (1952). ( 5 ) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, rl’. Y., 1959.
RECEIVED for review March 19, 1964. Accepted April 29, 1964. Work supported in part by the U. s. Atomic Energy Commission.
