Infrared Spectra and Characteristic Frequencies of ...

1 Infrared Spectra and Characteristic Frequencies of Inorganic ions Their Use in Qualitative Analysis FOIL A. MILLER AND CHARLES H. WILKINS. Department of Research in Chemical Physics, Mellon Institute, Pittsburgh 13, Pa. polyatomic ions exhibit Characteristic Infrared spec- A brief classification of the various types of vibrations in tra. Although such Spectra are potentially useful, crystals may be appropriate. Ionic solids are considered fiist. there is very little reference to them in the recent In a crystal composed solely of monatomic ions , such as sodium literature. In particular, the literature contains no chloride, potassium bromide, and calcium fluoride, the only extensive collection of Infrared Spectra of pure in- vibrations are lattice vibrations, in which the individual ions organic salts obtained with a modern spectrometer.

2 Undergo translatory oscillations. The resulting spectral bands are In order to investigate the possible utility of such broad and are responsible for the long wave-length cutoff in data, the Infrared Spectra of 159 pure inorganic transmission. I n a crystal containing polyatomic ions , such as compounds (principally salts of polyatomic ions ) calcium carbonate or ammonium chloride, the lattice vibrations have been obtained and are presented here in both also include rotatory oscillations. Of greater interest in this graphical and tabular form. A table of character- case, however, is the existence of internal vibrations. These istic Frequencies for 33 polyatomic ions is given. are essentially the distortions of molecules whose centers of mass These Characteristic Frequencies are shown to be and principal axes of rotation are a t rest.

3 The internal vibrations useful in the qualitative analysis of inorganic un- are Characteristic of each particular kind of ion. knowns. Still more fruitful is a combination of I n molecular solids, such as benzene, phosphorus, and ice, the emission analysis, Infrared examination, and x-ray units are uncharged molecules held in the lattice by weak forces diffraction, in that order. Several actual examples of the van der Waals type, andoften also by hydrogen bonds. The are given. It is evident that a number of problems same classification into internal and lattice modes can be made. involving inorganic salts containing polyatomic ions A few examples of such solids are represented in this paper (boric will benefit by Infrared study. The chief limitation acid, and possibly the oxides of arsenic and antimony).

4 At present is the practical necessity of working with Finally there are the covalent solids, such as diamond and powders, which makes it difficult to put the Spectra quartz, in which the entire lattice is held together by covalent on a quantitative basis. bonds. Here the distinction between lattice and internal vibra- tions disappears. One might a t first expect an ill-defined and featureless spectrum, but such is not the case. Actually there A LTHOUGH there has been a vast amount of work on the Raman Spectra of inorganic salts ( 2 , 4 ) , the study of them in the Infrared has been relatively neglected. Schaefer and Mat- are bands that are very Characteristic . The situation is in some ways analogous to that in a polymer, which in spite of its ossi (10) have reviewed work done up to 1930, most of which deals size and complexity possesses a remarkably discrete spectrum.

5 With reflection Spectra . The most extensive surveys of Infrared Silica gel is the only representative of this type included here. absorption Spectra have been made by Lecomte and his coiTorkers EXPERIMENTAL. (6, 7 ) , but unfortunately many of their data are somewhat out of date and are not always presented in the most useful form. Origin and Preparation of Samples. Practically all the samples References to studies on a few ions are given in the books by Wu were commercial products of or analytical reagent grade. (12) and by Herzberg ( 3 ) . There has recently been renewed The samples were gound to a fine powder to minimize the scatter- interest in the detailed study of the Infrared Spectra of selected ing of light, and were examined as Sujol mulls.

6 When there were salts, as exemplified by the papers of Halford ( 8 ) , Hornig ( I I ) , spectral features that were obscured by the Sujol bands, the and their coworkers. The well known Colthup chart ( 1 ) con- samples were either run as a dry powder or mulled in fluorolube (a tains Characteristic Frequencies for nitrate, sulfate, carbonate, mixture of completely fluorinated hydrocarbons. Fluorolube is phosphate, and ammonium ions . An excellent recent paper by a product of the Hooker Electrochemical Co., perfluoro lube oil Hunt, Kisherd, and Bonham ( 5 ) contains the Spectra of 64 of E. I. du Pont de Semours & Co.). Some compounds, such naturally occurring minerals and related inorganic compounds. as ferric nitrate nonahydrate (No. 49) and calcium permanganate Aside from sixteen Spectra in this latter paper, there is in the tetrahydrate ( S o.)

7 L50),seemed to mull up in their own water of literature no compilation of Infrared Spectra of inorganic salts hydration. When the fine powder x a s rubbed between salt obtained with a modern spectrometer. I t therefore seemed worth plates, it acquired the appearance and feel of a typical mull, but while to make a fairly extensive survey to seek answers to the fol- no appreciable fogging of the salt plates resulted. For other com- lowing questions: Is it generally possible to obtain good Spectra ? pounds, such as potassium carbonate, breathing on the sample Do the ions possess Frequencies which are sufficiently characteris- achieved the same result. This is not recommended, however, tic to be useful for analytical purposes? What is the effect on for it varies the water content unnecessarily, and with potas- the vibrational Frequencies of varying the positive ion?

8 Is sium carbonate some of the bands are shifted. Infrared spectroscopy useful in the analysis of salts? Although these techniques are satisfactory for qualitative This paper presents the Spectra from 2 to 16 microns of 159 examination, it may be of interest t o list some other methods pure inorganic compounds, most of which are salts containing which have been mentioned in the literature for handling inor- polyatomic ions . A chart of Characteristic Frequencies for 33 such ganic solids. Lecomte, who introduced most of them, has pointed ions is given. The use of these data for the qualitative analysis out that a finely ground dry powder scatters very little radiation of inorganic mixtures is demonstrated. Finally, a number of of wave length greater than 6 microns and consequently it may interesting or puzzling features of the Spectra are described.

9 Be used directly in that region (6, 7 ) . He also suggests coating 1253. 1254 ANALYTICAL CHEMISTRY. Table I. Index to Infrared Curves and Tables of Data Formula No. Type Formula No. Boron Sulfur (Contd.). hletaborate 1 Sulfate 2. 3. Tetraborate 4. 5. 6. Perborate 7. Misc. 8 Bisulfate 9. Carbon Carbonate L1&03 10 Thiosulfate NazCOz 11. KzCOa 12. 3hfgCOs hlg(OH) 13. CaCOa 14. BaCOa 15. coco3 16 Metabisulfite 103. PbCOa 17 104. Bicarbonate SHiHCOa 18 Persulfate 105. NaHCOa 19 106. KHCOa 20 Selenium Selenite NazSeOa 107. Cyanide NaCN 21 CuSeOa, 2H20 108. KCY 22. Selenate (NHhSeOi 109. Cyanate KOCX 23 NazSeOi. lOHzO 110. AgOCN 24 KzSeOh 111. CuSeOl. 5 HzO 112. Thiocyanate NHISCN. NaSCN Chlorate NaC108 113. KSCN KClOa 114. Ba(SCN)z. 2Hz0 Ba(Cl0s) 115.)

10 Hg(SCS)z Pb(SCN)z Perchlorate NHiClOi 116. Silicon SaClOa, H20 117. Metasilicate 31 KClOa 118. 32 Mg(CIO*)z 119. Bromine Silicofluoride NazSiFs 33 Bromate NaBrOa 120. Silica gel SiOr , XHZO 34 KBr03 121. AgBrOa 122. Sitrogen Iodine Nitrite Nah-02 36 Iodate 123. KNO? 36 124. AgNOz 37 125. Ba(SOz!z Hz0 38. Periodate KIOi 126. Nitrate 39. 40 Vanadium 41 Metavanadate 127. 42 128. 43 Chromium 44 Chromate (NHl)zCrOa 129. 45 NazCrOi 130. 46 KzCrOa 131. 47 132. 48 BaCrOb 133. 49 ZnCrOa, 7 HzO 134. PbCrOa 135. Subnitrate 50 Alr(CrOd8 136. Phosphorus Dichromate (NH4)zCrzOi 137. Phosphate, tribasic 51 NazCrtO~.2H20 138. 52 KzCrrOr 139. 53 140. 54 CuCrzOr ,2Hz0 141. 55 Molybdenum 56 Molybdate NazMoOi, 2 HzO 142. 67 Kd~ 143. 58. 59 Heptamolybdate (~ 144.)))