California Association of Chemistry Teachers
George B. Kauffmanl and L I O Y ~T. Takahashi Fresno State College Fresno, California R. C. Vickery
The Lighter Lanthanides A l a b o r a t o r y experiment in r a r e e a r t h chemistry
Malibu, California
Great advances are now being made in our knowledge of the chemistry of the rare earths, and compounds of these elements are no longer rare. These two essential facts have unfortunately not yet filtered down to many high school and undergraduate college chemistry instructors, the group primarily responsible for chemical education in this country. That an hiatus exists between the research frontier and the classroom is to he expected, but the unusual teaching value inherent in this largest natural group of metals, their relationship to the actinide series, and the student motivation derived from their "exotic" status, make it especially important that attempts be made to reduce this gap. Particularly needed are reproducible laboratory experiments specifically designed for undergraduate college or high school courses. As an example we offer the present experiment, which is schematically summarized in Table 1. It stresses Presented in part before the Division of Chemical Education a t the 140th Meeting of the American Chemical Society, Chicago, Illinois, September, 1961. Address for the academic year 1963-64, ,, AnorganisehChernisches Institut der Univemtiit Zuneh, Ranustrasse 76, Ziirich, Rehweiz. Table 1.
Separation of Lindsay Code 330 Rare Earth Oxide into Ce, Lo, Pr, ond Nd.
Dissolve the samnle in HNO. and heat.
1
Pmn'pitote: KM~O.
Ce(OHh -+ Ce(OH).,
Solution: Discard.
For Details See "The Experiment."
Save the solution containing Ce4+, Laat, NdS', and Pra+. Add NH4N08.
Precipitate: Dissolve (NH+).Ce(NO& in water. Add HzOt t o reduce to Cesf. Dllute wlth HIO, and heat to boiling. Add NapS04. Soluticn: Discard.
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basic principles encountered in traditional courses as well as phenomena peculiir to the rare earths. Our approach is an heuristic one; although the experiment can he performed exactly as described, we hope that it will also serve to stimulate original thinking about rare earths and give those educators whose primary interest lie outside these elements an inkling of what can be done in a school laboratory with simple equipment and a few inexpensive raw materials. I n this experiment, important separatiou methods such as precipitation, crystallization, redox reactions, and complex formation, which have been largely overshadowed by ion exchange, are illustrated (ref. ( I ) , pp. 65-86, ref. (2), pp. 40-48). Here many of the complex and tedious procedures characteristic of rare earth separations are simplified for student use although admittedly a t sacrifice of purity and yield. Our primary purpose is to illustrate, with a reasonable expenditure of time, the separation and properties of rare earths. If pure products are desired, the more time-consumiug procedures given in the original literature should be consulted.
Solution: Lax+, Nda+, PP+, Ce", Precipitate: La(NOs)s.2N&N( -see section 3. add CaC03 and Preci 'late: %(O%MNO~)~-., spot test - see section 4.
evaporate and add H.02.
4H20, run teak for Ce8+,NdJ+, and Pr3+ )issolve remaining precipitate in water, irOs, and evaporate. Dilute with water.
Solution: Nd*+. Pr'+, either sedarate b.y ion
Solution: La3+, add NH&Os and NH,, and heat. Solution: Precipitate: Nd(OHh, P$OHh. Las+, add (CH,)*C?O,. La>(C2O4h Either d~scard, wll pree~pltate. or dissolve in Ienite t o farm HNO. and add L&Oi -see secto Nd-Pr filtion 6 trate.
Volume 40, Number 8, August 1963
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433
The Lanthanide Contraction
Reference to primary sources is given in order to encourage students to consult the literature; major reference, however, is directed to the standard textsin this field (1, 2) so that students in institutions with modest libraries will not be handicapped.
This term, coined by the geochemist V. M. Goldschmidt (7), refers to the diminution in atomic and ionic radii which begins with lanthanum (Las+, 1.22A) and continues regularly throughou$ the rare earth series through lutecium (Lu3+, 0.99A). Because it is a natural consequence of an increase in nuclear charge without a compensating increase in distance from the nucleus of the additional electrons, the contraction is not anomaly, and it should be considered in general chemistry courses along with other trends in atomic and ionic size. A student who has observed the decreasing basicity and increasing amphotericity in Group 11-A from Ra2+ to be Be2+ should encounter no difficulty with the similar trend seen among the trivalent lanthanides. The small but important differences in properties between trivalent lanthanides which form the basis for most separations1 procedures (ref. (g), p. 58) may be ascribed to these size differences. For example, the lanthanide contraction is exploited in the fractional precipitation of hydroxides, which affords the basis for a number of separations of La from the other, less basic lanthanides (a), ref. (I), pp. 102, 157). As ionic radii decrease (e.g., Las+, 1.22A; Pr3+, 1.16A; Kda+, 1 1 5 ) the basicities and solubilities of trivalent lanthanide hydroxides decrease as shown by K., values (La3+, 1.0 X 10WL9;Pr3+, 2.7 X lo-="; Nd3+, 1.9 X (9, 10) and pH values for incipient precipitation (La3+, 8.03; Pr3+, 7.05; Nd3+ 7.02) (11, 12). In section 5, the Trombe method (IS, 14) as modified by Vickery ((15), ref. (I), p. 105) is used to precipitate Pra+ and Nd3+ as hydroxides, leaving most of the La3+ in solution. By bubbling air containing NH3 through a boiling, buffered solution, this method not only avoids the dilution produced by aqueous NH3 but also permits closer pH control and yields denser, more easily filterable precipitates. The consequences of the lanthanide contraction are not, however, limited to the lanthanides, but have profound effects on atomic volumes, ionization potentials, densities, and other properties of elements in widely separated areas of the periodic table (16). Thus, the marked similarities in size and properties between corresponding pre-lanthanide and postlanthanide eler~entssuch as Zr-Hf, Kb-Ta, and Mo-W are direct results of the lanthanide contraction (17).
Electronic Configurations
The rare earths, which consist of the lanthanides, lanthanum (at. no. 57) through lutecium (at. no. 71), plus scandium (at. no. 21) and yttrium (at. no. 39) in periodic group 111-B, are closely related, not only in the periodic table but also in nature. I n considering the implications of the periodic table, long a familiar part of every high school or freshman chemistry course, the instructor cannot avoid referring to this family, if only to dismiss them uneasily with the statement that they do not readily "fit" into the table. In the light of modem orbital theory, however, their physical and chemical properties are seen as logical consequences of their electronic configurations. The marked similarity in chemical properties among lanthanides is due to a close similarity in electronic structures, either 4f"5d6s2 or 4f"+' 69%. Although chemists and physicists differ slightly in their choice of configurations, those of the tripositive ions are the same on either basis. The regular filling of the if orbitals with n electrons from La(4fo) through Lu(4f1') is reflected in a corresponding periodicity in physical and chemical properties. A familiar example is the MainSmith color sequence ((S), ref. (I), p. 42, ref. (d), p. 20), which is shown in Table 2 for the ions in this experiment. A trivalent lanthanide ion having n electrons more than La3+ has almost the same color as an ion with 14-n electrons more, since both possess the same number of unpaired electrons according to Hund's rule of maximum multiplicity. Oxalic Acid, The Lanthanide "Group Reagent"
One property shared by all lanthanides is the insolubility of their oxalates in acid solution. Consequently, oxalate precipitation is often used for rapid separation of lanthanides from other elements. Oxalates are particularly attractive derivatives since on ignition they yield oxides, which can be dissolved in acids (sections 3 and 5) :1 LndGO,h Ln.Os 3C01 t 3C0 t I n section 5, La is precipitated by the long known but only recently exploited technique of homogeneous precipitation (4). Methyl oxalate, which hydrolyzes in boiling aqueous solution to generate oxalic acid in situ, produces a coarsely crystalline, easily filterable product (5, 6). No oxalate of cerium(1V) is known, since oxalic acid reduces Ce4+ ion (section 3): 2Ce4+ 4CzOF Ce%(C*O& I ZCO2 t
-
+
+
-
+
Fractional Crysfallization, Precipitation, and Complex Formation
Although most compounds vary regularly in solubility throughout the lanthanide series as a result of the lanthanide contraction, most of the fractional crystallizations and precipitations used for separation purposes involve double and complex salts rather than simple compounds. For example, in section 1, the
+
Section numbers refer to the experimental procedure.
Table 2.
434
Ion
Color
Laa+ Cea+ Pr'+ Nda*
Colorlees Colorless Yellow-green Red-violet
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lourncd of Chemical Education
Main-Smith Color Seauence
Confirmration -
Unpaired electrons
4f0 4f'
0 1 2 3
2;
Confixuration . 4f" 4.f'" 4flP
4f11
Color Colorlnss Colorless Pale green Rose
Ion Lua+ Yba+ TmX+ EraC
complex lanthanide mixture is separated by fractional crystallization of the isomorphous double ammonium nitrates (ref. (I), pp. 70,115), the method used by Baron Auer von Welsbach (18)for his classic separation of didymium into praseodymium and neodymium. Three fractions are obtained in order of increasing solubility (numbers in parentheses give the relative solubilities of the double ammonium nitrates a t 20°C): Ce4+ (
