Collecting and Using the Rare Earths Sally ~ o l o m o nand l Alan ~ e e ' Drexel University, Philadelphia, PA 19104 Thirteen of the lanthanides, with the exception of promethium (61Pm),which has no stable isotopes, are added to a collection of elements described in "Collecting and Using the Chemical Elements" (1). Discussed here are cerium (&el to lutetium (71Lu), including prices, sources, handling tips, and specific suggestions about how to use the rare earth, or lanthanide, elements in the classroom and in the laboratory. Charles R. Hammond, who described sources and techniques for collecting the rare earths (along with other elements,, predicted in 1964 that tho mow abundant rare earth metals may eventually cost fmm $5 to $20 per pound (2). In fact, this has nearly happened. A pound lump of 95% neodymium, now wsts (in 1964 currency) about $30 compared to $225 in 1964. The discussion below is limited to the study of properties that can be observed using this collection and a few simple reagents. Reference books and inorganic textbooks (3-5) can be used to learn about discoveries, names, uses, electronic confieurations. a n d other features of t h e .. lanthanides. ~ a m m o n dprovides ' an excellent current oveniew ofall ol'the rhemical elements in *The Klements" section of the Handbook of Chemistry and Physics (6). Forms of Elements The rare earths are available as powder, chips, ingot, foil, and for some of them, wire. In general, all powdered substances are much more reactive than bulk materials and are hazardous to store and breathe. Powders of the extremely reactive rare earths must be ampouled under argon and none have been included in this collection. Chips, which look like turnings, are more expensive than ingots, but are the most convenient forms to use for many of the experiments described here. Chips are prepared commercially by scraping ingots with a special tool. For each element Table 1lists the most useful form, the source, and the approximate cost for a convenient quantity. The number of chips supplied in the recommended quantities is intended to guide collectors who may want to know how many chemical reactions can be done without reordering, or who may wish to create more than one display Display and Storage Each element is placed in its own transparent 20-mL screw-cap glass scintillation vial accompanied by caps with cone-shaped plastic liners to provide reliable ~ e a l i n gThe .~ original packaging tray with compartments for 100 vials serves as a portable storage case with unused spaces available to house small tools and reagents. Combining two packaging trays allows room for displaying the elements as they appear in the periodic table including space for the elements following lanthanum, numbered 58 (cerium) to 71 (lutetium). Circular labels placed on top of each vial cap identify the elements by symbol and atomic number. In the original collection, color coding indicated general handling techniques (1).In general, the rare earths are not thought
' Author to whom correspondence should be addressed. Suppolted by an Academy of Applied Science REAP grant.
Fisher Scientific, 711 Forbes Ave., Pittsburgh, PA 15219; catalog number 03-337-7.
Table 1. Information for Obtaining Samples of the Rare Earths Element
Source
Cost a
Mass
Number of
Cerium Alfa (Ce)Chips turnings) (in oil) Dysprosium Aldrich 400 (Dy) Chips Erbium Aldrich 30&400 (Er) Chips 1 Europium Alfa (Eu) lngot 2 Gadolinium Aldrich (Gd) lngot Chips Aldrich 700 Holmium Aldrich 1 (Ho) lngot Lanthanum Aldrich 3105 (La) lngot (in oil) Lutetium Aidrich 1 (Lu) lngot (Nz) Neodymium Alfa 800 (Nd)Chips Prase Alfa 300 odymium (Pr) Chips lngot (in oil) Aldrich 1 Samarium Alfa 1500 (Sm)Chips 20 Terbium Alfa (Tb)Chips Thulium Alfa 100 (Tm) Chips Ytterbium Alfa 25 (Yb)Chips 'CoSt in U S dollars. 'Johnson MaffheyiAlfa Products, P.O. 60x8247,WardHill, MA01835-0747. to be very toxic, but they should be handled with care because they are highly reactive and their toxicities are not entirely understood. Yellow labels, meaning "handle carefully with gloves", have been chosen here for all the rare earths. The discussion below is divided into outstanding features of the lanthanide elements, including their physical, chemical, and spectral properties. Details on how to collect, cut, store, and handle the rare earth elements are accompanied by specific instructions for demonstrating their interesting properties. Physical Properties The silvery luster of the lanthanide metals can be observed when they are freshly cut or scraped. Their hardVolume 71
Number 3 March 1994
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ness generally increases with atomic number and melting point. Our collection includes ingots of gadolinium, praseodymium, europium, holmium, and lutetium. Praseodymium (59P~, mp 931 'C) is so easy to cut that a knife can be used to form chips by scraping turnings from the ingot. Europium ingot ( 6 3 E ~mp , 822 O C )can be cut, but with much greater difficulty than the praseodymium. The others, gadolinium ( M G ~mp , 1313 W , holmium &Ho, mp 1470 'C), and lutetium (TILU,mp 1663 'C) are so hard that scraping succeeds only in removing small protruding pieces of metal.
-
OA
--
a3
--
az -al --
.,
Maanetic Pmnerties Many of the heavier rare earth metals exhibit states of magnetic order (7). For each one there is a transition temperature below which magnetic structures are observed. For gadolinium the transition which takes place at 16 'C, is easily noticed by woling the gadolinium in a n ice-salt bath and noting that it is attracted readilv to a mametic stir bar., comoared to a comparatively weak attraction a t room temperature. The five elements that follow eadolinium. elements 65 through 69, Tb, Dy, Ho, Er, and &I exhibit similar pmperties. but a t lower temperatures. Terbium has a transition temperature a t 4 3 -6 (230 K) and is readily magnetized when cooled in a dry ice acetone bath. Dysprosium, with a transition temperature of'-94 'C ,179 K);hecomes magnetic at the temperature of liquid nitrogen, but the effect is noticeable a t the temperature of acetone and dry ice.4 To observe the magnetization of holmium and erbium that have transition temperatures of -141 'C (132 K) and -188 'C (85 K) respectively, cooling to the temperature of liquid nitrogen is necessary. Thulium has a transition temperature (58 K) too low for magnetization to be observed at liquid nitrogen temperature.
-
05
0.
.
Figure 1. a. Spectrum of 0.14 M NdCb(aq)taken with a Spectronic20; path length 1 cm. b. Spectrum of 0.14 ErCi$aq) taken with a Spectrono-20;path length 1 cm.
Chemical Properties
The rare earth elements are highly electropositive, with reactivities similar to alkaline earth elements. All readilv form trivalent lanthanide(II1) ions. However, there are significant differences amone individual metals. Some resemble calcium, others magGesium. Comparisons of the elements and their reactions with air, water, and dilute acids follow. A discussion of trend; in prbpcrties of the lanthanides is thorouahlv treated in 'l'herilld Moellrr's Pcriodicity and the an than ides and Actinides (8). Reaction with Air
All the rare earths form oxides in contact with air. Cerium sparks spontaneously when scraped with a knife and is used in lighter flints as described in Demonstration of Lunthanide Reactivity (9). Reaction with Water
Lanthanides react with water to produce oxides, Lnz03, and hydrogen gas. Europium is the most reactive, liberating hydrogen upon contact with cold water just as metallic calcium would do. When this reaction is performed by adding a very small piece of the europium metal to water, fizzing can be heard as the europium reacts vigorously to produce light-colored europium oxide and hydrogen gas. Cerium is the next most reactive and after a day or two in water produces generous amounts of gray-green oxide. Cerium and europium, like sodium, must be stored under mineral oil to protect them from moisture and air. The other elements eventually liberate bubbles of hydrogen, creating small discolored areas of pitted metal surface.
248
Journal of Chemical Education
Reaction with Acids The lanthanide metals all react immediatelv with dilute acids. For example, the reaction of praseodymium metal with dilute HC1 or HNOa results in brieht ereen ( ~ r a s i oin s Neo: Greek means green) solkions of ~ r ~ l i a nPT(NO~)~. dymium in acid produces violet solutions and samarium, yellow. The distinctive wlors of the lanthanide(II1) ions are discussed below in the section on spectral properties Spectral Properties The lanthanide ions have interesting visible spectra that The arise o' m electronic transitions among f orbitals (10). 4f orbitals involved are deep in the atoms and so are effectively shielded from their chemical environments by outer 5s and 5p electrons. As a result the spectra arising fmm f-f electronic transitions are extremely sharp with line character similar to that of atomic spectra, and are unaffected by surrounding ligands. The spectra of NdC13 a n d Nd(N0313, for instance, are virtually identical. The wlors of the lanthanide(II1) ions made by preparing chlorides from the addition of lanthanide metals to dilute HC1 are listed in Table 2. Colors of ions with the same number of unpaired f electrons in their electronic cohfigurations happen, by accident, to be similar, but are not the same (4). Praseodvrnium(1II)solutions are a brieht ereen while thulium(11i) soluti'ons are pale yellow-green.-~eodvmium(II1)solutions are a distinct violet color wmoared to the brown-violet hue of erbium(II1).The visible spectra
We have noticed that dysprosium woled to liquid nitrogen often retains its magnetism at higher temperatures, even at room ternperature.
OJ
-
a2
--
Abarbaee
a1
-.
om
o
au
aos M t r
Figure 2. Beers law plot for NdC13(aq);path length 1 cm.
Table 2. Colors of ~ n ~ + ( a ~ ) '
4f Configuration
Ion
Color
4f Config- Ion uration
Color
4f0
La3+
Colorless
4f14
Lu3+ Colorless
41'
Ce3+
Colorless
4f13
yb3+ Colorless
4f
P?+
Green
4f12
~ m Pale ~ +yellowgreen E?+ Pale brownviolet
4f3
~ d ~ Violet '
4f1'
4f4
pm3+
(Pink, yellow)
4f10
H O ~ + Yellow
4f5
sm3+
Yellow
41'
~ y ~~ o ~ + or~ess
4f=
EU&
Pale yellow
4f8
~ b colorlessb ~ +
~ d ~ Colorless ' Exceptfar Pm(lll),whichis not in thiscollectian,ellthscolomarsof chloride solutions made by reacting ead metal with dilute HCI. '~hesemay be pale yellow or pink. 4f7
a
of NdCls(aq) and ErCldaq) are different as shown in Figures l a and lb.5 The study of spectra of various concentrations of NdC13 is a n experiment suitable for laboratory classes in general chemistry. Solutions of NdC13 follow Beers law very well a t concentrations less than 0.13 M (see Fig. 2). A stock solution can be in made by dissolving about 12 g NdC13. 6H206 water to make 250-mL solution. The violet solution has a maximum absorbance of roughly 0.3 a t 580 nm. Alternatively the compound can be made diredly from Nd metal by putting 0.1 to 0.2 g (about 5 to 10 large chips) of neodymium in 10 mL of 6M HC1. This produces a residue-free solution that also has a maximum absorbance of about 0.3. Making the stock solution from the commercial compound costs about $3 for every 100 mL compared to about $5 for the same volume of solution prepared from the metal. Students are impressed by the presence of structure consisting of several distinct peaks (See Fig. l a ) in contrast to the single broad hand that appears in the spectrum of cobalt chloride, a compound oRen chosen for this type ~.of experiment (11). Acknowledgment We wish to acknowledge Charles R. Hammond who gave us invaluable advice and encouraged us to collect as many elements as possible. Also we want to thank Amar ~ a t h and Louis Pytlewski for helpful discussions. Literature Cited 1. S O I O ~ s.; O ~~.%te=, , D.J.
chew. E ~ W I S. S I , ~ , W I . ~C. R. J. them. ~ E~ZC. ~1964,41,403. ~ ~ d , 3. Emsley. J. The E k m f s ; M o r d P r e s s : New York, 1989. 4. cottan. FA.; willdnson. ~ . ~ d ~ ~ ~c h ecm i~s ~ ~d ; $l 1N~ ~ W ~ymk. ~ :~ 1980; ~ ~ r Chap 23, pp 981-1004. 5. Dleenwood, N. N.; Earnshaw, A. Chemistv of the Ekm n t a : Pergammon Ress: New Ymk, 1986; Chap 30, pp 1423-1449. 6. CRC Handbook ofchemistry and Physic*;CRC Ress, 70th ed.. 198%19W, B.5. 7. Crangle, J. The MognoticPmp~rtiiiifSsIidid;Arnold: London, 1977:pp8-9. 8. M O ~ U ~ TT, J. ckm. E ~ Z 1. ~ 0 , 4 7 , 4 1 7 . 9. Schimelpfenig, C. W. J. Chem. Edue. 1974,51, 196. ~ 10. ~h~ R~~ ~ ~ r t hspdding, s. F H.:D ~ ~ ~H., ~ E,~ sA.wiley: ;. N ~ W YOFL,1961; ~595. 11. WentwolU1,R.Exprimnf9 in GenrmlChomkfry; Houghton Mlmin: Boston,1981: ~95. 2. ~
'Many of the solutions are residue free;others such as ErCI, may have to be filtered. Aldrich Chemical Company, Inc., 1001 West Saint Paul Avenue, Milwaukee. WI 53233; the catalog number is 28,918-3.
Volume 71 Number 3 March 1994
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