Quantitative Application of Potassium Bromide Disk Technique in Infrared Spectroscopy J. J. KIRKLAND
E, 1. du Pont de Nemourr & Co., Inc., Wilmington, Del.
Grarrolli Chemicals Department, Experimental Station,
In a study of the potassium bromide disk technique for the quantitative infrared spectrophotometric analysis of solids, several prooedures for dispersingsolids in potassium bromide have been compared. A vibrator-grinding technique was found to be advantageous especially for the preparation of sample mixtures from difficultly ground materials. This simple procedure is rapid, requires no elaborate equipment, and is generally applicable to awidevariety of substances not soluble in useful infrared solvents. The reproduoibility of the various methods of sample preparation was determined, and the optimum procedure tested by analyzing model organic and inorganic systems. The precision of data obtained by the disk method compares favorably with conventional liquid-phase measurements. Although the techniques described have been applied to only a limited numher of quantitative studies, these experiments suggest the wide applicability of the disk method to the solution of complex analytical problems, which previously were not subject to infrared spectrophotometric treatment because of solubility limitations.
breakage. An evaeuablo, solid-casing die for preparing disks 12 mm. in diameter was found to be generally inferior to that of the divided-easing design for removing disks without fracture or breakage. A Perkin-Elmer Model 21 infrared wectrouhotometer was used .~~~~ , ~~
~
~~
~~~~
microcell adapter of the M o d e i i l instrument wzhout interfering with the sample heam. This bolder was designed to accommodate disks ranging from 0.5 to 2.0 mm. in thickness. A hydraulic press of 10-ton capacity was employed in the pressing operations.
S
TIMSON and O'Donnell (8) and Schiedt and Rheinwein
(6, 7) have demonstrated the advantages of preparing samples for solid phase infrared spectroscopy hy mixing the finely ground specimen with potassium bromide and pressing the mixtures into clear transparent wafers under high pressure. This method, known as the "potassium bromide disk" techinque, is particularly applicable to the study of minute quantities of insoluble solids. Anderson and Woodall ( 1 ) have obtained infrared absorption curves on as little as 10 y of sample. Infrared spectra taken by the disk technique show little or no scattered energy above 2 microns and are generally of better quality than those obtained by other methods of solid sampling. In addition, potassium bromide does not exhibit any absorption maxima u p to about 28 microns whereas mineral oil or other suspending media have hands which mask portions of the spectrum. Although investigators (8, 4 ) have used disk methods for the wlution of specific problems, they have not discussed a sample preparation procedure which is suited for general quantitative purposes. During the investigation reported, several methods of dispersing finely divided solids in potassium bromide were compared critically, and the reproducibility of these procedures w a determined. ~ The most reproducible method wms then tested hy analyzing model organic and inorganic systems. With this procedure, the precision of solid-phase measurements compared favorably m t h that of conventional liquid-phase methods, thus permitting the application of precise infrared spectrophotometry to a much wider variety of analytical problems. EQUIPMENT AND MATERIALS
A cylindrically ground, divided-casing die of the type described bv Sehiedt ( 6 ) was found to uroduce disks which could be used &tisfaetoril$ 'for both qualctative and quantitative purposes (Figure 1). This evacuable die w a s constructed of Carpenter Vega tool steel hardened t o Rockwell C-60 and was designed to press transparent disks 12 mm. in diameter in thicknesses varying from 0.5 to 2.0 mm., depending on the weight of potassium hromide mixtures used. Since the two halves of the die casing can be easily separated, the fused disks are readily removed without
Figure 2. A . Holder bod" B. Keeper c. Keeper L1ore"s D. Holder positioning
n;.r -. ..., F. Mounting p i n F
Disk holder
dots
".":I"
Left, disassembled view; right, assembled d e w
1537
ANALYTICAL CHEMISTRY
1538 Harshaw Chemical Co. optical grade potassium bromide was found to be the most suitable suspending material. Fused disks 1 mm. thick made from this highly purified material show a transmittance of 85% or better in the 2- t o 15-micron range. This pure salt exhibits none of the 7 - and 9-micron impurity bands sometimes seen in the spectra of disks prepared from reagent grade or chemically pure materials. These less expensive materials may be used satisiactorily for quantitative studies in cases where the bands chosen for analysis do not occur in wave-length regions in which these minor impurities interfere. The potassium bromide powder used t o prepare sample mixtures was obtained by first grinding the salt in a motor-driven mortar until it was thoroughly powdered. The material was then screened through a 230-mesh stainless steel cloth and dried a t 135” C. for 48 hours a t atmospheric pressure. Disks 1 mm. thick pressed from this powder show no bands except very weak peaks a t 2.9 and 6.1 microns, which cannot be conveniently removed by continued drying. The 2.9-micron band of a 1-mm. disk made from potassium bromide usually has a base-line intensity of less than 0.04 absorbance unit, whereas the 6.1-micron band appears as only a slight depression in the “background” line. It is most desirable to grind and screen a relatively large amount of salt (25 to 50 grams) and then t o separate the powder into 3- to 5-gram lots, which are dried separately just prior to use.
Table I.
Lambert’s Law Check
3-(p-chlorophenyl)-l,l-dimethylurea concn. -0.150% Disk Thickness, Ma. k” 9.19 p ‘k 11.99 p 0.50 ... 3.38 1.31 0.74 3.35 1.20 1.31 3.39 1.45 1.35 3.34 1.70 ... 1.32 1.96 3.38 1.35 AY. 1.33 3.37 A v . dev. 1.4% 0.9% baseline absorbance a k = weight per cent X disk thickness, mm.
EXPERIMENTAL
Disk Pressing Technique. During the present investigation, a11 disks were pressed by a standard procedure. First, the desired quantity of potassium bromide-sample mixture was placed in the partially assembled die (Figure 1). The mound of salt mixture was then smoothed to a uniform surface with a stirring rod, the end of which had been flattened. (All operations involving exposure of potassium bromide to the atmosphere were conducted as rapidly as possible to minimize the adsorption of water.) Xext the plunger was inserted and the assembled die placed in the hydraulic press. A slight pressure was exerted on the plunger to seal the gaskets to the casing, and evacuat,ion of the die was begun. After a minimum of about 2 minutes of evacuation at less than 15 mm. of mercury, the force on the plunger n s s l o d y increased to 8.5 tons and allowed to remain for 5 minutes (or 10 t,ons for 2 minutes) while the system was sMl under vacuum, The pressure was released slowly and air vented into the evacuation chamber. Next the die was disassembled, and the disk removed. For quantitative purposes, the disks were weighed or their thicknesses measured in order to relate the intensity of the absorption maxima t o the quantit,y of sample placed in the beam. Thickness measurements used in all of the quantitative studies reported were made with a S o . 522M dial micrometer with I/dinch contact point and anvil manufaclured by the B. C. Ames Co., Waltham 54, Mass. The surfaces of disks 1 mm. thick are essentially uniform and parallel when prepared by the procedure described. Measurements taken on various sectors of the disks have shown variations of no more than 0.05 mm., and usually are in the range of 0.01 to 0.02 mm. The average of several measurements made on the face of the disk was taken as the apparent disk thickness. The density of a potassium bromide disk prepared by t,he procedure described was measured as 2.65 grams per cc., as compared to 2.75 grams per cc. reported for crystalline potassium bromide. Lambert’s Law Check. Adherence t o Lambert’s l a F by the solid-phase mixtures in disks was tested using 3-(p-chlorophenyl)1,l-dimethylurea as the sample compound. The pure organic material was preground in a motor-driven mortar for 30 minutes. -1portion of this finely pulverized material was then ground with dry, powdered potassium bromide in this manner for 45 minutes, This procedure was used in order to prepare a sufficiently large sample (about 3.5 grams) so that several disks could be pressed from the same mixture.
The data in Table I show that the absorptivity of the analytical band was constant when the disk thickness was varied (constant sample concentration); therefore, it mukt be assumed that disk thickness is an accurate measure of the number of absorbing units in the sample beam when the disks are prepared in the manner described. Close adherence to Lambert’s law has also been found for all materials tested to date using the vibrator-grinding procedure. Study of Grinding Methods. T o obtain sharp, distinct infrared spectra of solids, energy losses resulting from scattering must be reduced t o a minimum. Scattering may be decreased by reducing the absorbing particles to dimensions significantly smaller than the wave lengths of energy xhich are being used in the spectrophotometric study. If the average sample particle size is maintained below about 0.1 micron, energy losses by scattering are usually negligible, even in the 2- to 3-micron region. Additional reductions are realized by fusing the finely ground sample with potassium bromide, thereby effecting an excellent matching of the refractive indices of absorbing materials with that of the suspending medium. The particle size effect is of particular importance in quantitative solid phase spectroscopy. Lejeune and Duyckaerts ( 5 )have shown that in the case of powdered calcite, the absorptivity of the maxima depends very largely on the particle size of the dispersed substance. Harp, Stone, and Otvos ( 3 )have reported that not only do particle size variations cause changes in the observed absorptivities but also that the dependence of apparent absorptivity with particle size is different for absorption bands of different intensity. These investigators concluded that sample particle sine must be controlled carefully when quantitative results are.desired. Since uniformly small particle size appeared to be a principal requirement in quantitative potassium bromide disk spectroscopy, a study of this important variable was undertaken. Organic materials which had been found particularly resistant to particle size reduction were selected for the investigation, as it was felt that if quantitative techniques could be developed with substances which were difficult to grind, they could also be applied to samples which could be ground more easily. 3-(pChloropheny1)-1,l-dimethylurea and related substituted ureas meet these requirements and were selected for study. These compounds give highly aggregated particles when ground in a mortar and develop a static charge which greatly increases handling difficulties. In addition, they are virtually insoluble in useful infrared solvents. Three modes of particle size reduction for solid-phase infrared sampling w r e studied: ball milling the sample with potassium bromide, grinding the material in a mortar with powdered potassium bromide, and grinding the substance with potassium bromide in a mechanical vibrator-grinder with the help of small steel balls. The use of a vibrator was suggested by Schiedt (6). The study of the efficiency of the vibrator-grinding technique was carried out using a Wig-L-Bug amalgamator, manufactured by the Crescent Dental Manufacturing Co., Chicago, Ill. This amalgamator mechanically vibrates a stainless steel cylinder 3 / 4 inch long and 3/g inch in diameter a t about 3200 cycles per minute in a back-and-forth motion. The most efficient’grinding of mixtures takes place when two l/g-inch steel balls are placed in the cylinder which is about one third filled with the potassium bromide-sample mixture. If larger quantities are loaded into the cylinder, less efficient particle size reduction tahes place. Approximately 0.8 to 1.5 mg. of sample, depending on the nature of the substance, and 0.35 + 0.03 gram of powdered potassium bromide are usually employed in the grinding-mixing process, bIost mixtures are easily removed from the cylinder if the potassium bromide has been properly dried. Since the mixture is homogeneous after the grinding operations, it is not necessary to recover it quantitatively. About 0.30 gram of the preparation is required for a 12-mm. diameter disk approximately 1 mm. in thickness.
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
1539
preground material. The variation in the band absorptivity data obtained using the two (Concn., 0.2'2 in RBr) starting materials is unexplained, but may give some indication of I r . Absorptivity (k") .Iv. Dev., r6 Max. D e i . , C; (Min. of 6 Disks) the expected long-term precision Grinding and .\fixing Technique 9.19 p 11.99 p 9.19 p 11.99 p 9.19 p 11.99 p of the over-all method on samA . Preground sample5 mixed with K B r by ples of varying starting particle hand-grinding for 5 minutes. 1 12 2.48 10.2 10.5 17.0 16 Y size, because the two sets of data B. Preground sample5 mixed Jvith KBr by motor-mortar grinding for 15 minutes. 1.27 2.81 5.1 5,4 10.2 7.5 were taken a week apart. Other methods for obtaining C . Same as B , except ground with KBr for 30 minutes. 1.45 3.33 1.8 2 3 4.1 5 4 reproducible particle size reducD. Large crystals of sample, mixed and tion were also attempted, but ground with KBr for 15 minutes by vibrator method described 1.57 3 , s 1.2 0.8 3.2 1 .j n-ere found to be generally less satisfactory or more tedious than k = where k = absorptivity the vibrator technique. PrepA s = absorbance, baseline aration of mixtures by ball C = concentration. weight Cic 1 = thickness of disk, mm. milling the sample with potaeb Ground in a motor-driven mortar for 30 minute.3 before dilution w i t h KBr. sium bromide in a small stainless steel cylinder with steel balls is not feasible, because of seiious caking of the mixture which takes place. Some To test the effectiveness of the vibrator technique as compared studies v-ere also carried out using liquids such a p acetone with other methods of particle size reduction, a series of mixtures and ethyl alcohol to assist in the mortar grinding. This necescontaining 3-(p-~hlorophenyl)-l,1-dimethylurea were prepared sitates a solvent removal step and results in only a slight by various grinding procedures. Disks were pressed from these mixtures and selected absorption bands scanned. The data obincrease in the efficiency of mortar grinding. The technique of dissolving the compound in an organic solvent and dropping or tained during this study are summarized in Table 11. spraying the solution on the finely pulverized salt, folloll-ed by Since the vibrator-grinding method produces disks shom ing the evaporation of the solvent under vacuum, has been used in this highest and most reproducible band absorptivities, it may be laboratory for obtaining qualitative spectra of extremely small concluded that this technique achieves the smallest average uniamounts of sample; however, this procedure is more difficult t o form particle sizes of any tested. use quantitatively than the vibrator method. Preparation of The advantages of the vibrator-grinding technique are most potassium bromide-sample mixtures by freeze-drying techniques pronounced with difficultly ground materials such as 3-(p-chlorois time-consuming and is not generally applicable, because of phenyl)-1,l-dimethylurea. Many easily ground compounds give solubility limitations. satisfactorily reproducible disks by simple hand grinding with The vibrator-grinding technique has a n additional important potassium bromide in a mortar. advantage over most of the other methods of sample preparation The grinding time required to disperse a sample for quantitain that contamination of the potassium bromide mixture by tive investigation should be determined for each material t o be water is maintained a t a low level. This is possible as the entire analyzed by the disk technique. This interval may vary with grinding and mixing process is carried out in an enclosed cylinder; the nature of the compound and may be determined empirically hence. the powdered dry potassium bromide in the resulting mixby observing the absorptivities of the bands to be used for the tures is exposed t o the air only during the weighing operations. quantitative measurement as a function of grinding time. The Serious interference by water bands was found in the spectra of results of a vibrator-grinding study using 3-(p-chlorophenyl)-lJldisks prepared by the hand- or motor-driven mortar grinding, dimethylurea are shorn-n in Table 111. methods even R hen the operations were carried out in an enclosed box through Lyhich dry nitrogen was flowing rapidly. Quantitative analyses in the 3- and 6-micron regions are difficult Table 111. Vibrator-Grinding Time Versus Absorptivity when these viater impurity bands are significantly intense. for 3-(p-Chlorophenyl)-l,l-dimethylurea I n order to evaluate the techniques described for the solution (Concn., 0.200% in KBr) of practical quantitative problems, model organic and inorganic k5 9 . 1 9 ~ 1Iinutes ka 9 . 1 9 ~ quantitative analyses were devised and tested. Large Crystals Preground Sampleb Vibrated Quantitative Applications to Organic Systems. ASALYSISFOR 1 1.80 1.71 1.65 3 1.78 MIXORCOYSTITUEUT.3-( p-Chlorophenv1)-1, 1-dimethylurea and 1.69 1.66 5 3-(3,4-dichlorophenyI)-l,I-dimethylurea were selected for demon1.59 1.65 10 16 1.70 1.57 strating a typical solid-phase quantitative analysis in which the 1.55 20 ... 1.61 25 ... determination of the minor constituent in a two-component sys= base-line absorbance tem is desired. Calibration mixtures nere prepared by weighing weight per cent X disk thickness, mm. the necessary quantities of the two pure organic materials into the b 30-minute grinding in a motor-driven mortar before dil,ition with IiBr. vibrator cylinder. Approximately 0.35 gram of pure, powdered potassium bromide was then accurately weighed into the same vessel so that the total organic content was 0.80001, of the mixThe apparent absorptivities of the 9.19-micron band in disks ture. The total weight was maintained at about 0.35 gram in prepared from both large crystals and preground material show a order to obtain maximum reproducibility in particle size reducslight decrease a t the start of the grinding process. This may tion. These preparations were then vibrator-ground for 15 have been caused by the slight shift in a base-line tangent angle, minutes as described. Approximately 0.30 gram mas then pressed which was observed and is presumably a function of sample parinto disks about 1 mni. in thickness, mounted in themetal holder ticle size. The apparent absorptivity of the 9.19-micron band and scanned. The calibration data plotted in Figure 3 shoF\appears to he fairly constant after 5-minute vibration of mixtures that Beer's lam- follows throughout the concentration range inprepared with l a ~ g ecrystals of sample, and after 3 minutes using vestigated. Several s\-nthetic mixtures containing known Table 11. Comparison of Grinding Techniques for 3-(pChlorophenyl)-l,ldimethylurea
~
~
Q
~
$
ANALYTICAL CHEMISTRY
1540
Table IV.
Determination of Minor Component 3-(p-Chlorophenyl)1,l-dimethylurea Added, % 2.5 8.0 9.6 14 9 16.9
Synthetic 1 2 3 4 5
3-(p-Chlorophenyl)1,l-dimethylurea Found, % 2.3 7.5 9.7 14.1 16 6
lu
__0
z
2 020
$ T
2
40
60
80
IO0
WEIGHT P E R C E N T 0.15
Figure 4. Calibration curves for 3-(p-chloropheny1)1,l-dimethylurea and 3-(3,4-dichlorophenyl)-l,l-dimethylurea mixtures
?
2t 2a
20
010
0.05
000
0
2
4
6
8
IO
12
14
16
18
20
WE1 G H T PERCENT 3-(p-CHLOROPHENYL)-l,I- D I M ETHYLU R E A
Figure 3 . Calibration curve for 3-(p-chlorophenyl)1,l-dimethylurea in 3-(3,4-dichlorophenyl)-l,l-dimethylurea amounts of pure 3-(p-~hlorophenyl)-l,1-dimethylurea in technical grade 3-(3,Pdichlorophenyl)-l,l-dirnethylurea were prepared and analyzed as a means of testing the accuracy of the method. The results of these analyses are shown in Table IT.
Table V. Analyses of Synthetic Two-Component Samples Added, % 51.7 pure 3-(p-chlorophenyl)-l,l-dimethylurea (I) 3-(3,4-dichlorophenyI)-l,l-di48.3 technical inethylurea (11)a 51.7 pure (11) 48.3 technical (IIa 28.9 pure (11) 71.1 technical (I)a a Technical samples about 9770 pure. b No corrections applied for known interfering impurities.
Found, % 52 44a 52 50 b 32 73 b
ANALYSIS OF Two-ColrPoxEvT ~ ~ I X T U R E S .Binary mixtures of 3-(p-chlorophenyl)-l,l-dimeth>lurea and 3-(3,4-dichlorophenyl)-1,l-dimethylurea in the concentration range 15 to 70% were prepared with potassium bromide by the vibrator technique. Absorption bands characteristic of earh component were chosen so that these peaks were in region of maximum transmittance for the other compound. Self-calibrating base-line intensity measurements were used t o obtain the calibration data shown in Figure 4. The base-line procedure used to obtain the data in Figure 4 was necessarily different from that in Figure 3 and also different from that employed in obtaining the more fundament81 data in Tables I1 and 111; therefore, no consistency in 9.19micron band absorptivity could be eupected. Adherence t o Beer’s law was found for both compounds in the concentration ranges shown; however, additional points in the 90 t o 100% range for 3-(p-chlorophen> 1)-1,l-dimethylurea indicated a noticeable downward trend in the curve. The apparent deviation from Beer’s l a x in this concentration range probably is a function of instrumental limitation since the intensity measurements were taken in the high absorbance region. The results of analysis of several synthetic samples containing the two test compounds are given in Table V. Quantitative Application to Inorganic Systems. Several of the
different crystalline forms of silica were used to demonstrate the applicability of the disk technique to the solution of quantitative inorganic problems. The study of silica seemed particularly interesting, because the usual methods of x-ray and electron diffraction, and differential thermal analysis do not possess sufficient sensitivity to permit the determination of small amounts of one form in the presence of another. Quartz and a-cristobalite display characteristic absorption bands a t 14.41 and 16.13 microns, respectively, vi hich are not shown by any of the other modifications of silica. These bands were utilized for the quantitative analysis of mixtures of theqe materials by the disk technique. Independent Beer’s l a x curves were constructed for each of these two substances by preparing a series of diqks containing known concentrations of the test substance using the general vibrator procedure desciibed. Both silica modifications were preground by hand in a mullite mortar and ground for 15 minutes n i t h potassium bromide by the vibrator technique. Band intensity measurements 15 ere again made by the base-line method. -4potassium bromide piiim was eniployed in the spectrophotometer in order that the 16 13-micron a-cristobalite band could be studied. Beer’s lan IS obe) ed for both quartz and a-cristobalite throughout the concentration ranges studied (Figure 5). % . slight deviation in the a-cristobalite curve was found when the concentration ,exceeds about 0.257& in potassium bromide. I n ordei to test the applicability of these curves for practical quantitative anal>-ses, several synthetic samples containing small amounts of acristobalite in quartz were prepared and analjzed Disks incorporating this mixture of silica modifications were made by the same technique employed for the Beer’s law curves except that the total silica content was maintained at 0 800%. The results of the analyses of the synthetic samples aIe given i n Tahle 1-1.
0 7 ,
06
lu
5 05
% 3
04
5
0.3
38 0 2
a
PT
0.1
nn 0
Figure 5.
0.2 0.4 0.6 0.8 WEIGHT PERCENT I N POTASSIUM BROMIDE
IO
Beer’s law plots for quartz and cristobalite
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
1541
a-Cristobalite also may be determined quantitatively in amorphous silica by the disk technique. The calibration data shown in Figure 6 were obtained from disks containing known mixtures of the two silica modifications using the same experimental technique described for the earlier silica studies. The infrared method appears to be capable of detecting about 1% a-cristobalite in amorphous silica. DISCUSSION
S o quantitative studies were attempted with gummy or tarry
materials, or with high-boiling liquids; however, several satisfactory qualitative spectra have been obtained on samples of this nature. Dispersion of materials of this type is probably the result of the coating of the sample fragments with potassium bromide, which prevents reaggregation and facilitates particle size reduction.
0600% S I L I C A IN K B r VALUES CORRECTED TO I O O M M DISCS
q 002
0
2
4
6
8
IO
12
WEIGHT PERCENT CRISTOBALITE
Figure 6.
Calibration curve for cristobalite in amorphous silica
Absorption bands to be used for a particular quantitative analysi- are selected in the same manner as for solution work. A study of band absorptivity as a function of grinding time usually indicates the feasibility of utilizing a particular peak for quantitative purposes. B further test is to prepare a series of replicate disks containing the sample and determine if the reproducibility of band absorptivities is within acceptable precision limits. To ensure maximum reproducibility in the preparation of disks by the vibrator method (and presumably by any other method employing a grinding process), the ratio of sample weight to potassium bromide and the total weight of potassium bromidesample mixture should be kept uniform. The weight of the mixture to be vibrator-ground should also be held at a minimum. -4n important advantage of the disk technique is that it permits the precise quantitative study of variables associated with the crystallinity of materials, which otherwise would be impossible by solution techniques. The solid-state analysis of two very similar species, such as diasterioisomers, can often be carried out on substances which show little or no spectral differences in the liquid phase. Ibsorption bands associated xith crystallinity of a suh-
Table VI. Sample No. 1
2 3
4
Analysis of a-Cristobalite in Quartz Cristobalite Added, % 2.4 4.7 10.0 15.2
Cristobalite Found, % 2.3 4.4
10.0 15.1
stance must be used with caution, however, since occasionally they give anomalous results. The absorptivities of these bands are sometimes more susceptible to particle size changes than is the case for those originating from functional group vibrations. I n several cases it was found that the wave length of the maximum and the shape of this type of absorption band were also dependent on the particle size of the sample. The illustrative quantitative examples given in this paper were all simple mechanical mixtures of solids. Caution must be used when attempting t o apply the disk technique to systems containing solid solutions or mixed crystals. Obviously, calibration data must be obtained on the same solid systems as are to be encountered in the desired analysis. I n instances TThere it is not known whether the samples to be analyzed are mechanical mixtures or solid solutions, mixed crystals, etc., it might be necessary to dissolve the calibration components in a common solvent (preferably the solvent from which the unknown sample is isolated), then evaporate the solvent before preparing the disk in the usual manner. XO detailed study of the effect of storage of disks on the absorption spectrum of an imbedded material has been made. Although the majority of disks prepared remain clear when kept in a desiccator, some do become turbid on standing; therefore, it is recommended that disks be scanned as soon as possible after preparation. N o significant changes have been noted in the spectra of the clear disks which have been rescanned in this laboratory. ACKKOW LEDGMENT
The author is indebted to Warren K. Lowen for his helpful suggestions and encouragement offered during this investigation and to George S. Ralser, m-ho obtained much of the data reported. LITERATURE CITED
(1) Anderson, D. H., and Woodall, S . B.,=Is%L. C H m f . , 25, 1906
(1953).
(2) French,' R. O., Wadsworth, 31. E., Cook, 11. -i.,and Cutler, J. B., J . Phys. Chem., 58, 805 (1954). (3) Harp, W. R., Jr., Stone, H., and Otvos, J. W., Pittsburgh Confer-
ence on dnalytical and .\pplied Spectroscopy, March 1954.
(4) Jensen, J. B., Acta Chem. Scand., 8, 393 (1954). (5) Lejeune, R., and Duyckaerts, G., S p e c t r o c h m . Acta, 6, 194
(1964).
(6) Schiedt, U., 2. 2\7uturforsch., 8b, 66 (1953). (7) Schiedt, U., and Rheinwein, H., Ibid., 7b, 270 (1052). (8) Stimson, 11. M., and O'Donnell, M. J., J . A m . C h e m . Soc., 74, 1805 (1952). RECEIVED for review February 7, 1955.
Accepted June 27, 1955.
