Paper Chromatographic Separation of Metal 2-Thenoyltrifluoroacetone Chelates E U G E N E W. BERG and RUSSELL T. M C I N T Y R E Coates Chemical Laboratories, Louisiana State University, Baton Rouge, La,
and Nauman (9) have used the equation fi - w0 = qa in lightscattering work. The C. and po are the refractive indices of the solution and solvent, respectively, q is the number of particles per cubic centimeter of solvent, and a is the polarizability of the solute molecule. Anomalous behavior is ohserved where the refractive index measurements are made a t a wave length within a region of absorption. A measure of the relative dielectric constants of dilute equimolar solutions of similar solutes in an ideal solvent indicates the relative polarization of the solute molecules. The determination of relative dielectric constants usually involves the measurement of the capacitance of a cell filled first with solution and then with the solvent. The capacitance of the cell is measured at a frequency of approximately IO6 cycles per second, because the time of relaxation of the molecules is short in comparison to the period of alternation. Because interactions occur between solute and solvent, the polarization measured with the high frequency oscillometer is only an approximation of the total polarization.
The efficient paper chromatographic separation of iron(111), copper(II), nickel(II), cobalt(II), and manganese(I1) 2-thenoyltrifluoroacetone chelates was studied thoroughly to find factors influencing the separability of the various chelates. The relationship among R , values, different adsorbers, chelate solubilities, total polarizations, and polarizabilities was determined. No single factor nas found to predominate in determining the sequence of adsorption of the metal chelates on paper strips. .iplausible explanation for the adsorption sequence was obtained by considering the relative solubilities, polarizations, and polarizabilities of the various chelates.
E
ARLIER work by the authors ( 1 ) showed that various mixtures of the iron(III), cobalt(II), copper(II), nickel(II), and manganese(I1) 2-thenoyltrifluoroacetone chelates were resolved on paper by an ascending chromatographic technique employing a mixed solvent of benzene, methanol, and glacial acetic acid. Some difficulties experienced in finding a suitable solvent system for the separation of the components was thought to be due to the expected gross similarity of the structures of the various chelates. Each of the cobalt(II), nickel(II), manganese(11),and copper(I1) ions coordinates n i t h 2 moles of the chelating agent, whereas the iron(II1) ion coordinates with 3 moles of the chelating agent. The molecular weights of the iron(III), copper(II), nickel( 11), cobalt(II), and manganese(I1) 2-thenoyltrifluoroacetone chelates are, respectively, 718.8, 505.5, 500.7, 500.9, and 496.9. Except for iron, the differences in weight are very small. The molecule of each chelate is predominantly organic and their structures are probably similar. A study of this SJ stem was undertaken in order to obtain some information concerning the factors influencing the separability or Rf values of the various metal chelates. Chromatograms of the chelate mixtures were obtained on plain paper and paper impregnated with common adsorbera whose relative adsorptive abilities generally were recognized. This approach was employed for two reasons: It was hoped that by using different adsorbers the separation of the chelate mixtures could be improved, and any observed deviation or regularity in the adsorption sequence, or R/ values, of the metal chelate. might suggest the type of adsorption mechanism involved. .4ssuming the solubility of the solute in the developing solvent t o be the sole driving force moving the chelates up the paper, an attempt was made to find whether a relationship existed between the observed R f values and solubilities of the metal chelates i n various solvent compositions used. S o direct relationship was observed between the solubilities and Rf values. This suggested that resistive forces kere a t least partly responsible for the observed adsorption sequence. The resistive forces must be due to attractive forces between the solute molecules and the chromatographic medium. This suggested that the adsorption sequence might be explained if a relative measure of the total polarization and induced polarization or polarizability of each chelate were known. A first-order approximation of the polarizability of solute molecules may be obtained from the refractive index difference between a solution and its solvent, if the measurements are made a t a wave length removed from a region of absorption. Debye
APPARATUS
Beckman llodel D U spectrophotometer with matched 1.000cm. quartz cells. B-S differential refractometer (Phoenix Precision Instrument Co.) with a two-compartment optical cell and a sensitivity of 3 units in the sixth decimal place for measurements of refractive index difference. Bausch and Lomb grating monochromator with tungsten lamp. Sargent Model V chemical oscillometer with oscillometric cell compensator and cell holder, Type A. Oscillometer frequency i w s approximately 5 X lo6cycles per second. REAGENTS
Absolute ethyl alcohol. Bulk ben7ene redistilled over calcium oxide. Glacial acetic acid. Ethvl acetate, C.P. 2-Thenoyltrifluoroacetone (99% pure, molecular weight 222) was obtained from the University of California. A lo%, bv weight, solution of the 2-thenoyltrifluoroacetone in 95% ethyl alcohol was used as the chelating agent. Iron(III), copper(II), cobalt(II), manganese(II), and nickel(I1) nitrates (reagent grade) uere used as the source of metal ione without further purification. Aqueous solutions were made up to I%, by weight, of the metal ion. Whatman No. 1 filter paper was used as the chromatographic medium. Sodium chloride-impregnated paper was prepared by dipping strips of filter paper into a 10% aqueous sodium chloride solution and drying the paper in air. Alumina-impregnated paper was prepared by the method of Datta et al. ( 2 ) . Silicic acid-impregnated paper was prepared by the method described by Kirchner and Keller ( 4 ) . Starch-impregnated paper was prepared by dipping the paper into a suspension of starch (40 grams of soluble starch in 1 liter of distilled water) and then drying in air. Before use, the filter paper strips, both plain and impregnated, were stored in a vacuum desiccator over calcium chloride. Sodium sulfide, c.P.,was prepared as a 1% aqueous solution to be used as a spray reagent for the detection of cobalt. Dimethylglyoxime was dissolved in 95% ethyl alcohol to make a 1% solution of the reagent for the detection of nickel. Benzidine reagent for the detection of manganese was prepared by dissolving 0.05 gram of benzidine hvdrochloride in 10 ml. of glacial acetic acid, diluting to 100 ml. with distilled water, and filtering.
.
195
ANALYTICAL CHEMISTRY
196
Table I.
R J Values of RIetal 2-Theno~ltri~iioroacetone Chelates in \-arious Solvents
Benzene-Methanol.4cetic Acid (Vol. Ratio) Solvent 98:0:2
93:5:2
88:10:2
7 8 : 2 0 :2
Type of Paper
Plain KaCI Starch Silicicacid .41?0z Plain NaCl Starch Silicicacid AlzOs Plain h-aCI Starch Silicicacid Ah03 Plain SaCl Starch Silicicarid .41?0z
6 8 : 30: 2
Plain KaCI Starch Silicicacid AI?O1
R j Values Cu 0.95 0.93 0.98 0.94 0.99 0.99 0.90 0.88 0.91 0.58 0.92 0.92 0.94 0.94 0.93 0.93 0.91 0.91 0.91 0.89 0 97 0 96 0 95 0 92 0.94 0.93 0.90 0.92 0.93 0.92 0.97 0.96 0.95 0.94 1.0 1.0 0.93 0.92 0.93 0.92 O.9ti 0.96 0 96 0.94 0.99 1.0 0.97 0.97 0.90 0.94
Fe
of 3Ietal Chelates Ni Co hln ~~
0.00
0:08 0.01
0.00 0.00 0.00 0.00 0.00
0.89 0.02 0.93 0.04 0.00
0.45 0.27
0.14 0.04
0:iI;
0.00
0.00 0.00
0 91
0 52 0 21;
0 32 0 08
0:13 0.00
0.00 0.00
0.00 0.50 0.00 0.00
0 5.5
0.98 0.89 0.00 0.92 0.88 0.94 0.93 0.49
0.95 0.90 0.93 0.97 0.70
0.23 0.05
0.05
0.63 0.39 0.32 0 00 0 5H 0.54
O:;? 0.09
0 . ii
0.38 0.30
0.22 0.21 0 00
pipetted into a previously weighed vial. The solvent was tvaporated in vacuo and the vial and residue were weighed. The solubility data were checked twice a t 28" C. Solutions of each chelate in absolute ethyl alcohol-0.01000, 0.02000, 0.03000, and 0.04000JT-were prepared by weighing the calculated amount of the pure dry chelate into a glass vi:il. The weighed chelate was dissolved in a small amount of absolutt. ethyl alcohol, quantitatively transferred t o a volumetric flask, and diluted t o volume with absolute ethyl alcohol. I n a similar manner 0.01000M solutions of each of the chelates were prepared in C . P . ethyl acetate. The standard solutions thus prepared were used in measuring the refractive index difference and the relative dielectric-constant. RESULTS
Certain ratios of benzene, methanol, and acetic acid were found to be effective in resolving a mixture of the metal chelates. Table I gives the relationship between the R, values of the metal chelates, on plain and impregnated papers, in solvent systems in which the ratio of benzene and methanol was varied. The R, values given are the averages of three or more determinations.
0.53 0.49 0.36 0.41 0.00
Mn
PROCEDURE
Pure metal chelates were prepared by adding an excess of 1yo aqueous solutions of the metal ions t o a hot, aqueous, sodium acetate-buffered (pH 7 . 5 ) solution of 2-thenoyltrifluoroacetone. The precipitated metal chelate was filtered, washed with hot water, dried in air, and then dissolved in a minimum of ethyl alcohol. The alcoholic solution was centrifuged and the supernatant liquid decanted slowly into hot water with constant stirring t o reprecipitate the chelate. The fresh precipitate was filtered, washed with hot water, and dried in vacuo over calcium chloride. The iron chelate had a tendency t o form a colloidal suspension when added t o hot water, unless the water contained sodium acetate. Even after recrystallization the iron chelate contained a small quantity of hydrous oxides. T o remove these impurities the chelate was dissolved in ethyl alcohol and centrifuged. The alcoholic solution of the pure iron chelate was then evaporated t o dryneqs in vacuo; the residue was stored in vacuo over calcium chloride. The chelates prepared in the above manner were assumed to be pure compounds. Strips of Whatman KO. 1 filter paper, plain and :tdsorbcntimpregnated, were spotted successively with 1-pl. portions of methyl isopropyl ketone solutions of each chelate to form a mi\ture. The diameter of the final spot was approximately 5 mni. The spotted paper strips were then hung in a closed glass chaml)rr containing the solvent miyture t o be used in the development of the chromatogram. The walls of the chamber were lined 171th filter paper soaked a i t h the solvent t o saturate the enclosure with vapor rapidly. One hour was permitted for equilibrium to be established between the vapor and liquid phases and then the strips were placed in contact with the developing solvent. Development was generally complete in 2 hours, when the solvent front had moved approximately 25 cm. The positions of the manganese(II), cobalt(II), and nirkel(IT1 chelates were determined successivelv as follow3?,000
31,021 An , qtin ., 30,948 30,830 30,767
__ __
Table 111. Relative Capacitance of Oscillometer Cell Cmntaining 0.0100.M Solutions of Metal Chelates in Ethyl .icetate at 25" C.
6'oo Xletal Chelate Xl n Fe co Si
CU
Ethyl acetate
0.02
0 00 MOLAR
0.04
CONCENTRATION
Figure 3. Refractive index difference between absolute ethyl alcohol and metal 2-thenoyltrifluoroacetone chelates in absolute ethyl alcohol
The difference in refractive index between ahsolute ethyl alcohol and various concentrations of the metal 2-thenoyltrifluoroacetone chelates in absolute ethyl alcohol was determined on the differential refractometer a t a temperature of 28" C. The results of these measurements are plotted graphically in Figure 3. The relative dielectric constants for the absolute ethyl alcohol and equimolar solutions of the various metal chelates in absolute ethyl alcohol were determined a t 25" C. with the high frequency oscillometer. The relative capacitanre of the oscillometer cell
Relative Capacitance of Oscillometer Cell 13,981 13,969 13.945 13,941 13,937 13,796
Thtx rc1:ttivr tot:tl polarizations measured for the metal chelates in absolute ethyl alcohol were: iron > manganese > nickel > cobalt > copper, and in ethyl acetate were: manganese > iron > cobalt > nickel > copper. These two sequences were arrived a t by assuming that the relative dielectric constants, and thus the relative polarizations of the solute molecules, of the chelate solutions were directly proportional to the measured relative capacities of the cell. Comparison of the polarizahilities and total polarization of the metal chelates to their RI values indicated that no single factor was responsible for the observed adsorption sequence. IToR-rver, if due consideration was given to the relative values of polarizabilities, solubilities, and total polarizations of the metal chelates the adsorption sequence could be explained in a qualitative manner. For example, the R, value of the manganese(I1) chelate was small b u t its relative solubility was large. In order to account for this apparent discrepancy, it was necessary to assume that eonie factor was responsible for retarding the migration of the manganese(I1) chelate through the adsorption medium. In general, the greater the polarizability or polarization of a particle the more strongly it is adsorbed. The observed relative polarizability and polarization of the manganese are large. This could possibly account for the small R f value for the manganese(I1) chelate.
ANALYTICAL CHEMISTRY
198 Similar comparisons of the relative polarizabilities, solubilities, and polarizations of the various metal chelates will account for the observed adsorption sequence in every instance except one. The relative Rj value of the iron(II1) chelate is large, although this chelate has the greatest polarizability and one of the greatest polarizations of the group. The solubility of the iron(II1) chelate is, however, five- to sixfold greater than that of any other chelate. The relatively large sollJbility would probably be sufficient to overcome the differenccs in adsorbability of the various metal chelates due to the relatively small differences in polarizabilities and polarization
