I
John M. Haschke University of Michigan Arin'Arbor, 48104
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In recent years the numher of new chemical compounds has increased at a rapid rate; and in an almost simultaneous and perhaps synergistic way, the numher of exotic chemicals which are commercially available in high purity has also increased. The situation with speciality chemicals seems to have much in common with both the interesting article in this Journal on nonexistent compounds ( 1 ) and with the continuing series on common textbook errors. Although most chemicals are probably of high quality, a disproportionate numher of speciality inorganic products are not. Data Collection In an effort to inform chemists of the chemical identity problem with which they are faced, catalog information1 and experimental results for several inorganic compounds are presented in the table. Samples of several of these materials have been obtained and characterized by X-ray diffraction procedures employing a 114.6mm Haegg-type Guinier camera. Manipulations of air sensitive compounds were performed in an inert atmosphere glove box (2). Results and Discussion Evaluation of the catalog data and the experimental results indicates that all entries in the table fall into one of two categories, nonexistent (N.E.) compounds or mislabeled (M.L.) compounds. Nonexistent Compounds The author is surprised and somewhat appalled to discover that several commercially available compounds cannot he prepared in the laboratory. A well documented example is silicon monoxide which is stable between 1150°C and 1300"C, hut spontaneously disproportionates into silicon plus silic,ondioxide at lower temperatures (31. 'Since the purpose of this article is to acquaint chemists with a potential problem which could alter the quality of their teaching and research, individual companies and catalog sources have not been cited.
Speciality Chemicals An identity problem Additional examples of N.E. compounds are found among the large number of lanthanide materials which have recently become available. Contrary to the standard textbook description, the lanthanides are a diverse series of elements. In addition to the trivalent ions, Ln(III), Ln(I1) (Ln = Sm, Eu, Yh), and Ln(IV) (Ln = Ce, PI, T b ) states are well known and give rise to a variety of chemical and physical properties. In their chemical behavior, Eu and Yb are verv similar to the alkaline earths and their CaH1-type dihidrides are readily obtained (4); however. all attemots to orenare their trihvdrides even at high . Hz pressures have been ~nsuccessful(5j. Similar efforts to prepare anhydrous EuI3 by reacting the diiodide with iodine at high pressure (6) have also been unsuccessful. The estimated thermodynamic functions for the europium iodides (7) indicate that EuI3 should spontaneously disproportionate into EuIz and Iz a t or above 25°C. The free energies of disproportionation of EuBra and YbI3 are similar to that of EuIs, but these trihalides can be prepared under carefully controlled conditions (2, 61, and they are not classified as N.E. compounds along with EuH3, EuI3, and YbHs. Mislabeled Compounds The second category includes those compounds which are known and adequately characterized. The difficulty here is simply that the material in the container does not correspond with that indicated by the label. In only one case, that of EuBrs, was a sample purchased for the purpose of investigating its identity. All other M.L. samples in the table were purchased from commercial suppliers by other investigators and submitted to the author for characterization by X-ray diffraction. Commercial EuBra, with the alleged properties listed in the table was found to be EuBr3 .6Hz0. The sample, which was received in a glass bottle with a screw cap, plastic gasket, and taped closure, was a colorless solid composed of macroscopic crystals. Anhydrous EuBra is a rust-brown solid (2, 8) which probably derives its intense color from the existence of a B r to Eu(III) charge trans-
Erarnnles of lnoraanic Chemicals Available from Commercial Sources
CoWOl
99.96
., .
crystal grade
M.L.
crystalline powder pawder
M.L. N.E. N.E. M.L.
crystal grade
M.L.
(1% 11)
powder nowder
N.E. N.E.
(3) ( 4 . 5)
...
NiWO,
99.96
SiO
99.9 999
YhH.
... >I702 ..
.
(9-11)
annealing a t 1000°C gave CoWOI + WO. mixture actually EuBr., 6H30 possibly EuH. possibly EuL or EuIa.6H?O mixture of MaOlC12 and other ohaseki ,. amorphous to X-rays; annealing a t 1000°C gave NiWOl + NiO mixture probably Si SiOl mixture possibly YbH? or YhH3.r;
+
"In cases where fine mesh sizes were listed, only "powder" is indicated. ".E. indicates that the catalog entry is a nonexistent (unknown) compound; M.L. indicates that the commercial sample was found to he mislabeled. Volume 52, Number 3, March 1975 / 157
fer reaction. The X-ray diffraction pattern contained only reflections assignable to a NdC13. 6Hz0-ty e lattice with a = 10.026 0.003& b = 6.753 0.0021, c = 8.162 0.003A, and 0 = 93.45 f 0.02". Although it is impossible to ascertain whether or not the sample hydrated during shipment, its high crystallinity suggests that it did not; hydrolysis of the anhydrous EuBra normally produces finely divided hexahydrate. It is interesting to note that the physical properties listed in the table for EuBra are completely irrational. Since the pressure of Brz (g) in equilibrium with EuBra (s) and EuBrz (s) is 1 atm at approximately 400°C (8), a normal melting point (702°C) is rather difficult to understand and appears to be the hypothetical value estimated from trends established by other lanthanide tribromides (7).
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Conclusions
Since their inception, the chemistry-related sciences have been plagued by the problems of sample identity and purity; however two important points seem to characterize the present situation: (1) Chemists are apparently so assured by quoted purity levels and the liberal use of such words as "ultrapure" and "quality control" that they seldom raise questions concerning the integrity of chemicals. The consequences in our teaching and research lahoratories may he more severe than one would like to imagine and (2) The attitude held by some speciality chemical suppliers seems to be one of cauaet emptor, let the purchaser beware. These companies either offer no guarantee or disclaim essentially all responsibility regarding their products. Sweeping generalizations obviously cannot be made on either point, but careful reading of chemical catalogs frequently reveals such suhleties as listing the purity level for metals with respect to total metal content only. Ionic impurities and dissolved gases, which may comprise
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Journal of Chemical Education
several mole percent of the sample, are not included. Unfortunately it is often unclear how percentages are defined. What action should users of speciality chemicals take? Two approaches are obvious: (1) adequate characterization of all commercial samples or (2) preparation of ones own samples. The first option is the minimum response. A serious problem arising from the lack of characterization is the introduction of incorrect information into the literature. Even after errors have been recognized, they are frequently difficult to correct. Another consideration is safety. The second option, preparation of ones own samples, may sound somewhat quaint and regressive, but in a time of dwindling financial resources it may become the most reasonable solution. The author was amazed to discover that a sample, which he recently prepared from inexpensive equipment and starting materials costing less than $45, has a retail price of greater than $2000. Literature Cited (I! Daren,. W. E.. J . CHEM. EDUC.. 10, 110 119631; "Nonexistent Compounds." MsrcelDokker, Inc.. New York. 1966. (2) Haschke,.l.M., andEick. H.A.. J. lnarg. N u c l Chsm., 32.2153 119701. 13) Rochow, E. C.,"Comprehensive Inorganic Chemisrry." Vol I. (Editors' Bailer. J. C., Emeleua. H. J., Nyholm. R.. and Tmtman~Dickenlon.A. F . ! Compendium Puhlishem. Elmsford. N.Y.. 1973.o. 135%. . 9.12(19561. 14) K& w . i . and W . ~ J . C . , A ~ Ldlystaiiogr.. 15) Hardcastle, K . I., and WarL J . C.. Inon Cham., 5,1728 flY66i. L. B.. Keenan, T. K . , andKruw. F.H.. h o r g Chem .3.1137 119641. 16) APPTOY, (7) Wicks, C. E.. and Binek, F . E.. "Thormodynamies Proprtieiof 65 Elements-Their Oxides, Halides, Carbides and Nitrides," U.S. Bureau 01 Mines Bulletin 605 ~ . ~ . ~ ~ ~ ~ washingon. ~ ~ ~D.C..~ 1963. ~ ~ ~ ~ i ~ t (8)Hsschk%.). M . J . Cham. Thermadynnmicr, 5.28311973). (91 Bmch. E . Z . Phyr Chrm.. A b f A , . 1.409(1929). (10) U.S. National Bureau of Standards Monograph 25. U S . Government Printing Office. Washington, D.C.. 1962. (11) "X-Ray Powder Diffraction File," ASTM Special Technical Publieation G ~ JAmer. icanSociety foiTesting Materials. Philadelnhia, 1960. (12) Banaclouph. C G . . and Stals. J.,Ausf. J Chrm., 19,741 f19ffi). (13) Afovmyan. L. 0.. Aliev. 2. G.. and Tsrakanov. 8. M.. Zh. Strukl. Khim.. 9. 1W7 lL968l.
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