Martin N. A c k e r m a n n Oberlin College Oberlin, Ohio 44074
Infrared Spectrometry Of Inorganic S a b A general chemistry experiment
At the present time in most undergraduate curricula n student first encounters infrared spectrometry in the organic chemistry course. We believe, however, that this topic can be successfully and profitably introduced in the general chemistry course. Accordingly, we have developed an experiment on inorganic qualitative analysis by infrared spectrometry for our general chemistry program. This experiment may stand alone in a course which includes essentially no qualitative analysis, or it can serve to introduce an important instrumental method into qualitative analysis. The purpose of this paper is to describe this experiment and to offer some observations on the use of inorganic iufrared spectra as instructional aids. Experimenl Each student receives an unknown binary salt chosen from a list of thirty-two. He makes s. preliminary identification of his unknown by wet chemistry of his choosing and then confirms
Table 1.
Anhydrous Binary Salts f o r lnfrored Analysis
Anion
Salts
Acetate Bicarbonate Carbonate
NH,C,H,O,, Ba(CzHzOd3, KCsHaOa KHC03, N a F m BaCOs, CaCi N%Ci KIOI, N NHINO. NaNOa, Sr(NO& K N 0 2 NaNOz NH,C~O,,LiCIO,, KCIO' (NH,)aS04,KBO,, NanSO., SrSG NsnSOJ NHBCN, KSCN, NaSCN
Iodate Nitrate Nitrite Perchlorate Sulfate Sulfite Thiocyanate
tion have been described (I),but i t is the relatively simple MiniPress met,hod of pellet preparation which makes this experiment feasible for the large numbers of students in the general chemistry laboratory. To prepare B pellet about 70 mg of infrared grade potassium bromide and 1-2 mg of sample are ground thoroughly with an acate or mullite mortar and ~ e s t l efor a t least 4 min. T h i step i* thc rnr~it inllmrtat,t i!. 1hc pellet-mnkisg pnwe.s. .\ rh.lrp, wll-rcwlverl ,pccrrum re-iolt*mly i f the \ n m p l ~pnrtirlc s i x IS T C ~ I I I . P10~ ~mdlcr.tlra!. 2 p etrd the minlurc of p o . ~ i ~ i u m Iworn:tlc nml i m k ~ n m19 ~h ~m w p ~ ~ eI I ~. w IJuriug gri~vling0 8 e h l l l d itup periodirnlly lo wmpr 1l.c s n m p l ~tt,~erhcrfor lwrtrr I I I ~ X I I I1E.0. f m n thc ~cllertlw mixtore ulaved it%tothp rrws and the two bolts areturned against one another as tightiy as possible with hand-held wrenches. Pressure from the wrenches is maintained for 10-15 see, then the press is set aside for a t least 2 min. When the bolts are removed, an opalescent to transparent pellet remains in the bolt hole in the metallic block. Even cracked or somewhat opalescent pellets yield surprisingly good spectra. In the Mini-Press technique the sample remains in the press while the infrared spectrum is recorded. The diameter of t,he bolt hole is such that a significant redoction in the light iniensity occurs in the sample beam, and some attenuation of the reference beam intensity is desirable. A suitable attenuator has been described (2). By providing several of the inexpensive Mini-Presses far alaboratory, we have been able to have students prepare their own pellets without causing delays in the use of the spectrometer. In this way i t has been possible to obtain fifteen to twenty spectra from a single spectrometer in a three hour laboratory session. Each student is provided with information similar to that in Table 2 which gives the frequency range in which the normal modes of the structured ions absorb in typical salts. The less symmetric acetate and bicarbonate ions are omitted from Table 2 because of the greater number of normal modes of these ions of lower symmetry. With this information t,he student is encouraged to try to identify the structured ions in his salt. He then consults a collection of infrared spect,ra recorded on the same instrument he used and makes an identification by compa~isonof t,he unknown and known spectra. His attention is directed to comparing absorption frequencies, peak shapes, and relative peak intensitie~.~The cation identification (except ammonium) generally depends upon the flame test. If the wet and infrared analyses differ, the student is asked to perform additional tests to reconcile them. Discusrion
The anions selected represent a range of solution chemistry, and most are common to quditative analysis programs. Except for the ammonium ion the cations are identifiable by flame tests. This restriction simplifies the wet chemical analysis, but s wider range of cations with varied chemistry can be included. Since numerous examples of stmctured anions are available, while the ammonium ion is the only familiar s t , ~ ~ ~ c t ucation, red the salts have been selected to emphasize the infrared spectra of the anions. For the infrared analysis each student prepares a potassium bromide pellet of his unknown using the Mini-Press' method and then records the spect,mm. Several methods of sample preparaWilks Scientific Corporation, South Norwalk, Connecticut. An extensive compill~tionof the infrared spectra of inorganic salts has been oublished for the 500&625 em-' (3) and 700-300
' ~ h k u s i a approximation l that symmetry coordinstes may be used to represent normal modesis made.
Inorganic salts offer some features which make them attractive for a first encounter with molecular vibrations and infrared spectrometry. The inorganic ions in this experiment contain only a few atoms and have a high symmetry. Therefore, it is possible to go beyond the qualitative identification approach in an introduction to infrared spectrometry and to give a more complete and quant,itative discussion of molecular vibrations. Such discussions have appeared in introductory chemistry texts (5-7). The normal modes for the ions in Table 2 are fairly easy to represent (8, 9), and a student can consider the motion of all of the atoms without diffic u l t ~ . Consequently, ~ one need not start with a group frequency argument as is done in the organic course. Instead, this treatment forms a background for making Volume 47, Number 1 , January
1970
/
69
Table 2.
6
Frequencies of the Normal Modes of Some Inorganic Ions"
Ion
No. of Atom
SCN-
3
Linear
NOS-
3
Bent
COP
4
NOs-
4
Soas-
4
Equileteral Triangle Equilateral Triangle Pyramid
lo3-
4
Pyramid
NHl+
5
Tetrahedron
SOP
5
Tetrahedron
C104c
5
Tetrahedron
Shape
Symmetric Stretch 2030-2150 IR,R 1325-1380 IR,R 1025-1090 R 1025-1070 R 960-1220 IR,R 75fP790 IR,R -3040 R 980-1020 R -935 R
Frequencies of Vibrations (cm-')bC Symmetric Asymmetric Bend Stretch 450-500 (2) IR,R 810-850 IR,R 820-890 IR 810-890 IR 620-655 IR,R 350-390 IR,R -1680 (2) R 450465 (2) R -460 (2) R
710-760 IR,R 1230-1275 IR,R 1420-1480 (2) IR,R 1340-1430 (2) IR,R 905-975 (2) IR,R -825 (2) IR,R 3120-3150 (3) IR,R 1070-1200 (3) IR,R 1050-1170 (3) IR,R
hqymmetric Bend
680-750 (2) IR,R 700-730 (2) IR,R 450-550 (2) IR,R 300-330 (2) IR,R 1390-1410 (3) IR,R 580470 (3) IR,R 610-635 (3) IR,R
Taken from reference (8). The number in arentheses following a frequenoy indicate the degeneracy of that vibration. The symbols b e i w a frequency indicate whether a vibration absorbs in the infrared (IR) or causes a Ramm (R) effect.
the group frequency approximation. We include a presentation of some pertinent theory in our experimental write-up. Among the topics discussed are normal modes, quantization of vibrational energy, degeneracy, and the criterion for infrared activity of a normal mode. Carbon dioxide is used to illustrate these concepts. In addition some of the relationships exemplified by the ions in Table 2 and the relevance of the spectra to the ionic model of bonding are discussed. Table 2 is a useful summary of many of the relationships that can be examined in a study of molecular vibrations. The collection of ions illustrates the dependence of the number and frequency of the normal modes upon the number of atoms in the ion, the shape of the ion, and the masses of the atoms. Thus, the sulfate and perchlorate ions are similar in all respects and so are their normal mode frequencies. The sulfite and iodate ions, which have the same shape, demonstrate the effect of mass upon frequency, and the sulfite and nitrate ions the effect of geometry (with allowance for a small mass effect). A study of the infrared spectra of inorganic salts also supports a discussion of the ionic model of bonding. The fact that the fundamental frequencies for an ion are transferrable from one salt to another strongly suggests that its motions are largely independent of the counter ion and supports the idea that it exists as an independent entity in the crystal. Thus, spectra of potassium nitrate and sodium nitrate in the 4000-650 cm-I region are practically indistinguishable with a moderate resolution spectrometer, even though their crystal structures differ (10). In some cases the effect of the counter ion on the vibrational motions is sufficient to alter the shape and multiplicity of an absorption band, hut there is little effect upon the frequency. This is probably the result of a partial removal of the degeneracy of several modes through site symmetry effects. In favorable cases it is possible to make a complete identification of a salt from its infrared spectrum alone, even if one ion is unstructured. This is illustrated with some sulfate salts in the figure. Even the symmetric stretch, which would be inactive in the isolated ion, is observable s,t 980 em-', in the potassium and strontium sulfates as a result of crystaleffects. 70
/
Journal o f Chemical Education
Infrared spectra of some sulfate ralh: solid line, K1S04; broken line, Na250r; a n d broken and dotted line, SrSOc Spectra were run on o Perkin-Elmer 700 Infrared Spectrometer "ring KBr pelleh. The feature at 1640 cm-I in each spectrum is a water band. The salts differ in crystol rtrusture (1 01.
Despite some of the advantages described above, inorganic salts generally receive little attention in an introduction to infrared spectrometry. This may be due in part to the belief that high quality spectra are not obtainable from these compounds. This experiment is one mechanism for introducing this topic into the undergraduate curriculum; many variations on the experiment are conceivable. For example, a possibility is the analysis of mixtures of salts, although complications in the interpretation of the spectra can be a problem (3). A copy of the experimental write-up distributed to students and of the spectra of the salts listed in Table 1 will be provided to interested persons. Literature Cited (1) POTTB. w. J., "Chemical Infrared Speotros~op~;'John W i l w 61 Sons. Inc.. New York. 1963, Vol. I. (2) Crcx0nz.R. 8.. AND VAN ATTA.R. E., J. CHEM. E~u0..45.271 (19681. (3) M L L L EF. ~ ,A,. A N D W ~ m m aC , . H., And. Cham.,24, 1253 (19521. (4) MILLER, F. A , , C A R L ~ O G N ., L.,BENTLET. F. F., AND JONES. W. H.. Speclrochim. A d a . 16,135 (IQGOI. (5) B n ~ s c mF.. , Anems, J.,M ~ r s b ~ cIt., x , m n Tuns,A,, "Fundamentals of Chemistry," Academic Press, Ino., New York. 1966. P.597. (GI GUY. IT. B.. A N D IIAIDXT, G . P.,JR.,''Uaaic Principles of Chemistry." W.A. Benjamin, Inc., New York, 1967, p. 196. (7) G A R R E T T .B.. ~ . LIPPINCOTT, W. T., A N D VBRIIO&EC. F. H.."Chemistw." Blhiadell Publishing Co., Waltham. Massachusetts. 1968, P.252. (81 N ~ n ~ v oK.. ~ o"Infrared , Speotra of Inorganic and Coordination Corn pounds." John Wilev 61 Sona. Ino.. New York. 1963. (9) H ~ n z s r ~G o .,. "Infrared and Raman Spectra of Polystomic Molecules." D. Van Noatrsnd Co.. New York. 1945. (10) WEL'B, A. F., ' ' S t r u ~ t ~ rInorganio d Chemistry."(3rd d l , Clarendoo Presa. Oxford, 1962.
