Gene E. Kalbus
California State College at Long Beach and Lee H. Kalbus California State College at San Bernardino
Use of Infrared Spectrophotometry
I
in the Analysis of Limestone
Infrared spectrophotometry is finding increasing use as a teaching aid because of the ease with which it can demonstrate chemical and physical principles and phenomena. The importance of infrared spectrophotometry in chemistry is enhanced by the recent introduction of less expensive instruments. Consequently, it can be used more and more to stimulate students' interest. At the present time, infrared spectrophotometry is mainly employed in undergraduate organic and physical chemistry courses to demonstrate the value of infrared inorganic analysis and in the determination of physical constants. This paper reports an exploration and extension of the usefulness of infrared as a teaching aid in the field of inorganic analytical chemistry. It is effective in demonstrating the principles associated with the quantitative gravimetric analysis of limestone. Although there are a numher of reviews dealing with infrared in the inorganic field, the usefulness of infrared in this area has not been fully exploited (1-8). This specific analysis was selected for the following reasons. First, the majority of the compounds originally present in a limestone sample and those formed during the gravimetric analysis possess polyatomic anions and are therefore infrared active. Second, if students in an analysis laboratory are given a complex sample to analyze by successive determinations, often limestone is the choice since this analysis demonstrates a numher of important analytical principles and techniques. Third, the knowledge of the usefulness of infrared in providing a visual demonstration of reactions such as coprecipitation, postprecipitation, decomposition, solid state reactions, etc. occurring during the
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analysis of a relatively complex material such as limestone would permit the extension of its benefits to numerous other common analyses. Procedure
A limestone sample was analyzed according to the common gravimetric procedure for its proximate analysis (9, 10). The procedures for the five following successive gravimetric determinations were performed: percent loss on ignition, percent SO2, percent mixed oxides or percent R203(where R stands for all of the cations precipitated with ammonium hydroxide a t this point; Fe, Al, and Ti being most common), percent CaO, and percent MgO. At various stages of this analysis, small portions of the sample or product were taken and subjected to infrared examination. The resulting spectra are illustrated and their usefulness in demonstrating principles and in aiding the analyst in the understanding of the procedures will be discussed. The synthetic limestone sample used in this exploratory investigation had the following composition: 40.23% CaMg(COa)z (dolomite), 46.98% CaC03, 7.50% SiO?, 2.14% F&Oa, 1.99% ALOa, and 1.16% TiOz. A Baird model 4-55 double beam infrared recording spectrophotometer with a sodium chloride prism was employed. The potassium bromide pellet technique was used in preparing the samples (11). The samples were weighed out to the closest 0.01 mg on a Cahn microhalance and mixed for ten minutes with a weighed amount of Harshaw spectroscopic grade potassium bromide in a Wig-1-Bug amalgamator (Crescent Dental Mfg. Co., Chicago). A portion of this mixture was
then formed into a pellet which was weighed, thus permitting the calculation of the milligrams of sample present in the infrared radiation beam.
sorption spectrum will reveal its identity. However, if they occur together, it is impossible to determine which oxides are present. WAVE NUMBERS IN 40W
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Figure 1 . la1 MgCO? spectrum, with CoMglCO& b m d ond CaCOa bond indicated. lbl SiOl spectrum, with Fe20s,TiOl, and AlsOa bands indicoted.
Results
Figure 1illustrates the infrared spectra of some of the individual pure compounds that might comprise a limestone sample. They are presented for comparison purposes. The complete spectrum of MgCOa, shown in Figure la, is typical of almost all carbonates, with the characteristic intense absorption band a t approximately 1450 cm-' (6.90 p), the two medium size ahsorption bands a t longer wavelengths, and the smaller bands in the lower wavelength region due to overtones and combinations of the fundamental motions. All absorption bands, except the one a t about 3510 em-' (2.85 p) caused by the oxygen-hydrogen stretching motion of water, are due to the stretching and bending motions of the carbon-oxygen bonds in the carbonate anion. For a theoretical discussion of the various complex vibrations of the carbon and oxygen atoms which give rise to the absorption bands shown in this spectrum, the reader is referred to the book by Herzberg which illustrates the modes of vibration (directions and magnitudes of the movements of each atom) for the planar COT - group and gives the approximate wave number of absorption for each motion ( I $ ) . Previous work has indicated that, in general, as the atomic number of the cation increases, the absorption bands of the carbonate group exhibit a slight shift to smaller wave numbers (5,4). On a spectrum which is linear with respect to wavelength, the absorption band a t approximately 750 cm-I (13.35 p) is shifted most drastically with a change of cation and therefore its position is most useful in identifying the cation. Figure l a indicates the position of this band a t 730 cm-' (13.7 p) for dolomite and a t 712 em-' (14.0t5p) for calcium carbonate. Except for this one band the spectra of these three carbonates are essentially the same. Figure l b illustrates the infrared spectrum of SiOl and portions of the spectra of three other oxides commonly found in a limestone sample. Since FenOa,TiOz, and AlzOahave only one characteristic absorption hand a t longer wavelengths, only this portion of interest is indicated. Their shapes in this region are different enough so that if one occurs by itself the infrared ah-
'Ir
Figure 2. lo) Limestone smmple. ( b ) Limestone sample ignited for 1 hr at 7 0 0 ' C . (4 Limestone sample ignited for 4 hr at 950°C. Id1 Limestone sample ignited for 4 hr at 9 5 0 D C and allowed to rtond unprotected in atmosphere for 1 hr, with relevant portions of spectrum indicated ofter standing unprotected in atmosphere for 3 days.
The spectrum shown in Figure 2a was obtained from a potassium bromide pellet containing 3.60 mg of the original limestone sample. An unusually large sample was used in order to bring out some of the absorption bands of lesser intensity. By examining this spectrum, the analyst could make the following qualitative observations on the original sample. (1) The appearance of the large absorption band a t approximately 1430 em-' (7.0 p) and the smaller absorption band a t 877 em-' (11.4 p) indicates the presence of large amounts of carbonate in the sample. (2) The positions of the absorption bands at approximately 730 cm-' (13.7 p) and 712 em-' (14.05 p ) indicate the presence of CaMg(C03)2and CaCOa,while the absence of an absorption band a t 750 cm-' (13.35 @) indicates that 1L2gCOs is either completely absent or is present in such small amounts that it is not detectable by means of infrared. (3) The position of the broad absorption band a t abont 1100 cm-1 (9.1 p) and the small absorption band at abont 800 em-' (12.5 p) reveals the presence of SiOz. (4) The general lowering of the spectrum in the region from 833 em-' to 625 cm-' (12 p to 16 p) indicates the presence of small amounts of oxides such as FerOs TiOl, and AlzOa. (5) The absorption band a t approximately 3500 cm-1 (2.9 p), due to oxygen-hydrogen stretching, indicates the presence of a small amount of moisture in the pellet. The spectra obtained from the limestone sample a t Volume 43, Number 6, June 1966
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various stages of the conventional gravimetric analysis illustrate what can be accomplished using infrared as an
into the final product and that the residue should he reignited.
aid.
Loss on Ignition. Figure 2b shows the spectrum of the limestone sample after it had been ignited for 11/%hrs a t 700°C. The intensity of the 1430 em-' (7.0 p) absorption band indicates that a large quantity of carbonate is still uudecomposed. Examination of the 730 cm-'(13.7 p) absorption band due to dolomite and the 712 em-' (14.05 p) absorption band due to calcium carbonate reveals that the dolomite is practically all decomposed while the calcium carbonate is still abundant. The intense silica band a t 1100 cm-' (9.1 p) is altered and a number of new absorption hands appear in the region between 1200 cm-I to 900 cm-' (8.3 p to 11.1 r ) . These absorption bands are due to magnesium silicates and indicate that a solid-state reaction has occurred between the MgO (the product of the MgCO, decomposition) and the SiOz during the ignition period. Figure 2c shows the spectrum of the limestone sample after it had been ignited for four hours a t 950°C. Examination of this spectrum indicates that the carbonate is almost entirely decomposed. Only the most intense hand of carbonate is barely visible a t 1430 cm-I (7.0 p). Absorption bands between 1200 cm-' to 840 cm-' (8.3 p to 11.9 p) are due to calcium and magnesium silicates which have formed during the ignition period. The spectrum shown in Figure 2d was obtained by taking the pellet used in the preparation of Figure 2c, regrinding it, allowing it to stand unprotected for one hour in the atmosphere, reforming it into a pellet, and redetermining its spectrum. The large increase in the water bands a t approximately 3570 cm-' (2.8 p) and 1610 cm-' (6.2 p ) , due to oxygen-hydrogen stretching and oxygen-hydrogen bending respectively, indicates the absorption of water by the sample and the potassium bromide mixture. The appearance of the strong carbonate absorption band a t 1430 cm-' (7.0 p) reveals that the ignited sample has reacted with the carbon dioxide in the atmosphere to reform the carbonate. When the ignited limestone sample was allowed to stand unprotected in the atmosphere for several days, the carbonate and water bands increased in size as shown by the dotted spectral hands in Figure 2d. The small band making its appearance a t 712 cm-' (14.05 p) reveals that calcium carbonate and not magnesium carbonate or dolomite has been formed during this long period of contact mith the atmosphere. This spectrum emphasizes the importance of protecting the ignited limestone sample from the atmosphere during the period of cooling and weighing. Silica. The ignited sample was then analyzed for its silica content by dissolution in hydrochloric acid and subjection to a number of dehydrations which rendered the SiOl insoluble. Figure 3 was obtained from 0.70 mg of this resulting SiOI residue which had been ignited a t 1000°C for two hours. It appears to be similar to the spectrum of pure Si02 (Fig. lb) except for the slight band a t approximately 950 cm-I (10.5 p). Since previous work had indicated that this band, which is present in hydrated silica, disappears after prolonged high temperature ignition, the appearance of this band indicates that the two hours ignition a t 1000°C was not quite sufficient to convert the hydrated silica 316
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4000
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Figure 3.
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SiOl (from the limestone sample1 ignited for 2 hr a t 100O0C.
Combined Oxides (R203). The precipitate of hydrated mixed oxides (Fez03,TiOz, and AlzOa)was ohtained by the slow addition of ammonium hydroxide to the hot filtrate from the previous silica determination. Ignition a t 1100°C for 4 hrs served to remove the water of hydration. The solid line spectrum in Figure 4 shows 1.78 mg of the ignited &03precipitate which came from a solution which had undergone only one dehydration in the previous silica analysis. The absorption band a t approximately 1100 em-' (9.1 p) shows that this RzOais contaminated by a considerable amount of silica. The dotted line spectrum is of an ignited Rz03precipitate which was obtained from a solution which had undergone three dehydrations with intervening filtrations to remove the silica. The silica band a t 1100 em-' (9.1 p) is barely visible in this spectrum. Comparison of these two spectra indicates the importance of several dehydrations in removing the SiOzso that it does not later contaminate the RzOaprecipitate. Since the R203precipitate is gelatinous, and therefore numerous impurities are usually occluded, a reprecipitation of the RzOais always recommended. A spectrum of this reprecipitated R203was also obtained to determine if the benefits of reprecipitation would be evident upon examination of the infrared spectrum. Comparison of this spectrum with the dotted line spectrum of R203 (Fig. 4) did not reveal any visual benefits of this reprecipitation procedure. Therefore, in this case, the benefits, although they could be considerable, were either not of the magnitude required to show up on an infrared spectrum, or they involved substances which were infrared inactive. WAVE NUMBERS IN CM-' 900 600
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Figure 4. Solid line: RSOI ignited for 4 hr ot 1 100DC, from 0 limestone sample undergoing only one previous SiOz dehydrmtion. Dotkd line: R20a ignited for 4 hr a t I 100DC,from CI limestonerample undergoing three previous Si01 dehydrations.
Calcium Oxide. The calcium oxalate monohydrate precipitate was obtained by acidifying the solution from the RZ03determination, adding ammonium oxalate,
and then slowly neutralizing a t elevated temperatures with ammonium hydroxide. Figure 5a shows the spectrum of 0.50 mg of this precipitate which was collected after i t had remained in contact with the mother liquor for 1 hr and then heated a t 1 0 5 T for 2 hrs. No discrepancies were evident when this spectrum was compared with a reference spectrum of pure calcium oxalate monohydrate. WAVE
NUMBERS IN CM-'
and 1610 em-' (6.2 p), the ammonium bands a t approximately 3100 cm-I (3.2 p) and 1390 em-' (7.2 p), and the large characteristic band of phosphate in the vicinity of 1100 cm-I to 1000 cm-' (9.1 p to 10.0 p) are all evident. A reprecipitation of the hexahydrate was performed under more closely controlled conditions. Comparison of the spectrum of this reprecipitated NIgNH4POn~6H20 with Figure 6a revealed three differences. The absorption band at about 1280 cm-I (7.8 p) and the absorption band a t 940 cm-' (10.6 p) were missing and the large band between 1100 cm-I and 1000 cm-' (9.1 p and 10.0 p) did not exhibit the splitting shown in Figure 6a. WAVE NUMBERS IN CM-I 1800 1400 1100 900
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Figure 5. lo) CoCIO1.H1O (from the limertone ramplel collected after 1 hr of contact with the mother liquor, with the appearonce of the MgCaOl bond indicating portprecipitation occurring during 6 hr contact with the mother liquor. ( b ) Solid line: CoCzOr.H20 (from the limestone rompiel ignited for 2 hr at 500'C to produce CoC08. Dotted line: C o G 0 4 H x 0 (from the limestone sample) ignited far 2 hr ot 1100' C to produce CaO.
When this precipitate was collected, after remaining in contact with the mother liquor for 6 hrs, and then heated a t 105°C for 2 hrs, its spectrum exhibited a new absorption band a t approximately 826 em-' (12.1 p) as shown in Figure 5a. This band is due to the magnesium oxalate which has postprecipitated during this six hour period of digestion. This visually demonstrates the classical example of the postprecipitation of magnesium oxalate and emphasizes that the calcium oxalate precipitate must be separated from its mother liquor shortly after precipitation in order to minimize this type of contamination. Instead of heating the calcium oxalate monohydrate a t 105°C with the retention of its original formula, this precipitate is often ignited at 500°C to CaC08 or a t 1100" C to CaO. Figure 5b (solid line) shows the spectrum of the calcium oxalate monohydrate after two hours of heating a t 500°C. It is identical to a reference spectrum of pure CaC03. Since no oxalate bands can be seen on this spectrum i t can be assumed that this duration and temperature of heating were sufficient for complete decomposition of the oxalate to the carbonate. Figure 5b (dotted line) shows the spectrum of the calcium oxalate monohydrate after ignition for two hours a t 1100°C. No carbonate or oxalate hands can be seen, indicating decomposition of the oxalate to the oxide. Magnesium Oxide. After destroying the excess ammonium salts and oxalate by the evaporation to dryness of the decantate in the presence of nitric acid, the residue was dissolved and treated with a solution of diammonium hydrogen phosphate. Slow neutralization with ammonium hydroxide produced the MgNHr POa. 6Hz0 precipitate. The spectrum, illustrated in Figure 6a, was ohtained from 1.00 mg of this precipitate. The water hands a t approximately 3330 cm-I (3.0 p)
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Figure 6. 101 M g N H 8 O r 6 H p O (from the limertone sample1 with lines indicating contamination by NHIHzPOI ond MgrlPOlle.4Hs0. Ib) MgNH4POc6H20(from the limestone romple) rsprecipitoted, ignited for 3 hr at 1 100°C to produce MglPnOi.
By examining the infrared spectra of various ammonium phosphates and magnesium phosphates, it appears that the 1280 em-' (7.8 p) band is due to contamination by NH,H2PO4while the 940 cm-I (10.6 p) band and the slight splitting of the 1100 em-' to 1000 em-' (9.1 p to 10.0 p) band are characteristic of Mga(POa)9.4Hz0. Observation of these extra absorption bands would therefore reveal the benefits of reprecipitation in removing these two contaminations. Since the percent of magnesium oxide is more commonly determined by transforming the hexahydrate into magnesium pyrophosphate by ignition a t 1100" C for 3 hrs, Figure 6b, obtained from the reprecipitated hexahydrate which had undergone such an ignition, is presented. The transformation to another compound can he observed by noting the disappearance of the ammonium bands and the altered appearance of the intense PO4- - - band between 1100 em-' and 1000 cm-' ( 9 . 1 to ~ 10.0p). Conclusion
Benefits of theoretical interest ohtained from observing spectra, such as those presented in this paper, would include the following: (1) the demonstration (by observing the disappearance of absorption bands) that certain transformations or decompositions are occurring during ignitions and the demonstration of the completeness of these transformations as the temperatures and times of ignition are varied; (2) the observation that the conversion of silica to magnesium and calcium silicates is actually occurring during the ignition Volume 43, Number 6, June 1966
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of the original limestone sample; (3) the observation of the tendency of the ignited limestone sample to pick up moisture and carbon dioxide from the atmosphere; (4) the observation of the importance of the number of dehydrations in the separation of the silica; (5) the demonstration of the classical example of the postprecipitation of magnesium oxalate; and (6) the demonstration of the value of the technique of reprecipitation in minimizing contamination. Although this particular investigation was designed to determine the usefulness of infrared in the analysis of limestone, the knowledge gained during this investigation would be of value in extending its usefulness to many quantitative inorganic analyses such as the following: (1) It would he useful in the analysis of practically all rocks and minerals in demonstrating principles similar to those discussed in the limestone analysis. (2) Infrared would be beneficial in the gravimetric analysis of brass and other alloys. Although an original brass sample cannot be examined by infrared, many infrared active substances (PbSOa, CuSCN, ZnNHr PO4.6H,O, etc.) are formed during the gravimetric analysis, and it is possible that many contaminations would be infrared active. At stages of the analysis, a portion of the precipitate could be collected and examined by infrared for possible impurities and also for completeness of transformations. (3) Infrared could he used to determine whether a precipitate in a singular analysis is contaminated. For example, the barium sulfate precipitate obtained in the determination of sulfate is often contaminated by chlorate and by nitrate, and the nickel dimethylglyoxime precipitate obtained in the determination of nickel is often contaminated by the precipitating agent dimethylglyoxime because of its limited solubility in aqueous solution. These contaminations could be detected by infrared. (4) Infrared offersa convenient method of studying many solid state reactions. Examination of infrared spectra of such reactions would reveal if reactions have occurred, what products have been formed, and the times and temperatures required for such reactions. The time the student would devote to obtaining the infrared spectra would ordinarily not be significant when compared to the total time required for an ordinary gravimetric analysis. The loss of the amount of
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sample required for obtaining infrared spectra would usually he negligible in the gravimetric analysis, but if important, the sample could always be returned to the primary reaction vessel. If time and equipment will not permit each student to obtain infrared spectra of his own sample, i t might be possible to provide the entire class with a suitable demonstration, assign the procedure as a group project, or simply permit the students to examine pre-prepared spectra illustrating important points such as those presented in this paper. In any case the student is able to observe what is happening to the sample instead of merely being told what reactions occur and what errors would result from wrong and careless manipulations. Acknowledgment
The authors wish to acknowledge the financial support provided by the Long Beach State College Foundation. Literature Cited
(1) BELLAMY, L., "TheInfrared Spectra of Complex Molecules" 2nd ed., John Wiley & Sons, Inc., New York, 1958, pp. 343-349. J. R., J. CHEM.EDUC.,38,201 (1961). (2) FERRARO, (3) HUNT,J. M., WISHERD, M. P., A N D BONHM,L. C., Anal. Chem., 22,1478 (1950). ( 4 ) KALBUS, G. E., PhD Thesis, University of Wiseonsin 11957). ~.... ,. ( 5 ) LAWSON,K. E., "Infrared Absorption of inorganic Substances," Reinhold Publishing Corp., New York, 1961. (6) MILLER,F. A,, CARLSON,G. L., BENTLEY,F. F., A N D JONES,W. H., Spectrochim. Acta, 16,135 (1960). (7) MILLER,F. A,, AND WILKINS,C. H., Anal. Chem., 24, 1253 (1952). (8) NAKAMOTO, K., "Infrared Spectra of Inorganic and Coordination Compounds," Jahn Wiley & Sons, Inr., New York, 1963. (9) HILLEBRAND, W. F., LUNDELL,G. E. F., BRIGHT,H. A,, and HOFFMAN,J. I., "Applied Inorganic Analysis," 2nd ed., John Wiley & Sons, Inc., New York, 1955, pp. .. 958-980. (10) PIERCE,W. C., HAENISCH,E. L., AND SAWYER, D. T., Quantitative Analysis," 4th ed., Jahn Wiley & Sons, Ine.. New York. 1963. DD. 382-395. (11) STIMSON, M. M., AND O'DONNELL, M. J., J . Am. Chm. Soc., 74,1805 (1952). G., "Molecular Spectra. and Molecular Strue(12) HEREBERG, ture, 11: Infrsred and Rsman Spectra of Polyatomie Molecules," 1st ed., D. Van Nostrand Co., Inc., New York, 1945, pp. 177-179.
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