I Improved Suspension Technique for Quantitative Infrared Analysis of Insoluble Solids Meyer Dolinsky and J. A. Wenninger, Division of Calor and Cosmetics, Food and Drug Administration, U. S. Department of Health, Education, and Welfare, Washington 25, D. C.
W. E. Schmidt, Department of Chemistry, George Washington University, Washington, D. C.
Q sohds . .
analysis Of . difficult unless the sa.mple can be dissolved in a suitable solvent. Since only a few doupolar solvents are sufficiently transparent in the 2- to 16mieron region, those solids which cannot be dissolved must be analyzed by other methods. In some cases, quantitative analyses may be carried out in water solution (7), and other specialized techniques have been described (4,61, but most attempts a t quantitative infrared analyses of insoluble solids have been undertaken using the well-established Nujol mull (d) or, especially, the KBr-disk technique (9, l l ) ,both of which are applicable to a wide variety of Solids. Many workers have reported excellent results on various types of compounds using these methods; other workers have encountered a number of difficulties. These difficulties are due to degradation of samples during preparation; cocrystallization or interaction betn-een components of mixtures; control .of particle size so as to reduce light scattering and Christiausen filter effect; nonuniform particle distribution; crystal orientation; bonding, mixed crystal formation, or other interaction with the KBr matrix; polymorphism; and control or measurement of thickness of films. These problems have been reviewed by several workers in the field of infrared analysis ( 1 , 5, S, 10). UANTITATIVE INFRARED 1s
Because of these difficulties, accura.te ana.lysis is possible only under rigorously controlled and standardized conditions of sample preparation. An alternative technique for quantitative infrared analysis of insoluble solids was presented Some years ago (31, in which suspensions of the finely divided insoluble material were analyzed. These suspensions vere prepared by shaking the material with glass beads in a flask containing a 1% soiution of aiuminum stearatein carbon disulfide, This technique could produce fluid, stable suspensions which could be examined in standard infrared liquid cells and in which the particle size was sufficiently small for acc.nrate quantitative measurements. There r e r e several disadvautages to the method as originally used-it required a special tgpe of shaker, a rela.tively large sample,and a prolonged time. However, the method as modified by the use of a %-L-Bug shaker gives satisfactory suspensions in approximately 4 minutes when using 1 t o 2 ml. of CSt and approximately 5 to 50 mg. of sample. The method is not limited to cs,; equally suspensions can be obtained in CC4, CHCIB,tetrachloroethylene, ete. The suspension procedure as Illodified may, in some cases, offer advantages over the mull or disk methods for quantitative work. I t is rapid, requires no expensive- apparatus, and give-s excellent
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EXPERIMENTAL
with Apparatus. a
Stainless steel capsules
of rill, mere used. These measured approximately 41 mm. x 13 nlm. in dialneter and were fitted with a friction cap and polyethylene gasket. I n order t o adapt the capsules t o the Wig-LBug (Crescent Dental Manufacturing Co., Chicago, Ill.), a metal spacer was inserted into the base of the prongs of the Wig-LBug, and a clamp nras used to hold the capsule during shaking pignre 1). The stainless steel grinding halls were 3 nlm, in diameter. Qualitative infrared spectra m.ere obtained on a Perkin-Elmer Infracord Model 137, and quantitative measurements were made on a PerkinElmer Model 21 infrared spectrophotometer. Cell thickness wa? 0.5 mm. in
grade, algzc;nts. stearate, csI, AERO FS
Aluminum commercial, ( ~ Cyanamid ~ ~ or ~ comparable). Commercial aluminum stearates vary greatly as to their ability to form sui& able suspensions for this work. The material used must he comP1etely soluble in CS,, show no strong ahsorption between 5 and 6 microns, and have moderately-high gelling power. The 1% so~utions in cs, are prepared by adding 1 gram of mate. rial to 100 ml. of CS2in aglass-stoppered flask, heating to boiling, then stoppering
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: =:.20 4
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SO .60 .70
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WAVELENGTH lUlCRONSl
~i~~~~ 1, capsule clamped into modi. fled Wig-1-Bug holder
Figure 2.
Infrared spectrum of 1 yo solution of aluminum stearate in CCI,
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Cell length: 0.5 mm. VOL. 35, NO. 7, JUNE 1963
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and allowing to cool for 15 to 20 minutes. A clear, slightly opalescent solution, having a gel-like consistency should
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Concn. (mg./ml. 1 12.2 16.3 21.5
(mg./ml.) 8.35 10.0 13.2
several times by hand before opening. The suspension is transferred to a sandwich-type infrared cell by means of a hypodermic syringe, being careful that no bubbles are present, and the
11.35 p 0.040 0.036 0.039 0.041
9.47 p 0.056 0.054 0.058
12.0 p 0.037 0.037 0.038
12.60 p 0.038 0.040 0.035 0.037
13.25 p 0.046 0 043 0 046 0 047
9.87~ 0.048 0.050 0.052
12.07~ 0.016 0.017 0.016
14.45~ 0.034 0.036 0.035
Sodium bicarbonate Absorbance/mg./ml.
D. Sodium carbonate Bbsorbance/mg./ml.
Concn.
(mg./ml.) 7.3 10.1 12.6
11.40 p 0.044 0,045 0.042
Table II.
Components’
Analysis of Mixtures Mixture KO. 1 Mixture No. 2
Added, Found; Added, Found, % % % % 40.6 59.4
2,6-Dimethylaniline-3-sulfonicacid 2,6-Dimethylaniline4-sulfonicacid Components
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ANALYTICAL CHEMISTRY
40.5 57.0
21.3 78.7
20.1 76.5
Mixture No. 1 Mixture No. 2 Mixture No. 3 Added, Found, Added, Found, Added, Found,
% Sodium carbonate 42.4 Sodium bicarbonate 57.6 .;Based on measurements at 12.07 p and 11.35 p * Based on measurements 3t 11.4 p and 12.0
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B. 2,6-Dimethylaniline-3-sulfonicacid Absorbance/mg./ml.
C.
Concn.
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Infrared spectrum of 1? ’ & aluminum stearate in CSs vs. air
Figure 3.
11.13 p 0.036 0.032 0.034 0 034
9.00 p 0.027 0.026 0.026
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A . Potassium dichromate Absorbance/mg. /ml. 10.60 p 0.073 0.064 0.067 0 065
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Table I. Applicability o f Beer’s Law
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result. The solution should be sufficiently fluid, however, to be measured with a pipet or syringe. Fresh solutions must be prepared daily, because on long standing the solutions lose their suspending power. Procedure. A weighed sample of the material t o be analyzed is placed in the steel capsule, 1 or 2 ml. of t h e aluminum stearate solution is added by means of a hypodermic syringe or wide-tip pipet, 15 t o 20 steel balls are then added, and the sealed capsule is clamped into t h e shaker. The capsule is shaken for 4 minutes with a 10- t o 15-second pause after each minute of shaking. It was advantageous to cool the capsule with ice during each pause t o prevent slight loss of carbon disulfide. It is not necessary t o remove the capsule from the shaker while cooling. After shaking, the capsule is removed from the shaker, cooled, and shaken
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43.5 54.3
28.6 71.4
26.5 70. 1
69.2 30.8
68.0 32.6
spectrum is obtained, using the 1% aluminum stearate in CSz solution as reference. Before refilling the cell with another suspension, it is thoroughly flushed with several portions of carbon disulfide and sucked dry. Care must be taken that all of the suspension is completely flushed from the cell before drying, otherwise insoluble material will deposit inside of the cell. After all measurements are made, the cells are dismounted and cleaned. I n this work, 0.5-mm. stainless-steel spacers were used. The use of metal spacers ensures uniform thickness of the cell even after disassembling and repolishing of the rock-salt plates. The suspensions are sufficiently viscous so that no gaskets or amalgamation is necessary to prevent leakage. DISCUSSION
The infrared spectrum of aluminum stearate in CC1, and CSt is shown in Figures 2 and 3. hlthough an obvious disadvantage of the suspension technique is the added absorbance of the aluminum stearate plus solvent, a considerable portion of the 2- to 15-micron region is sufficiently transparent for analytical measurements, using cell thicknesses up to 0.5 mm. typical spectrum obtained by this technique is shown in Figure 4. ilbsorbance values obtained at several different concentrations for a number of compounds are shown in Table I. In no case did any appreciable scattering of light occur above 7 microns, and no background corrections have been made. A wide variety of materials including sulfonic acids, sugars, inorganic salts, etc., have been examined by this technique. Beer’s law has been found to be applicable in all cases where the materials can be pulverized and are completely insoluble in carbon disulfide. I n cases where solubilization occurs, deviations from Beer’s law are to be expected since a combination of solution and solid phase spectra is obtained. Practically all crystalline substances can be satisfactorily suspended; fibrous and waxy materials generally do not give satisfactory suspensions.
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for his assistance in designing the equipment used in this study.
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Figure 4. Infrared spectrum of 2,6dimethylaniline 3sulfonic acid in 1% solution of aluminum stearate in
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cs2 Concn.: 10 rng. per ml.
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Suspensions of organic compounds, prepared as described, are generally stable (no change in absorbance on successive scans) for at least 30 minUtes-generally for much longer. Inorganic salts tend to Fettle out somewhat faster, but are generally stable for a t least 15 minutes.
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-4nalytical results obtained in the analysis of two mechanical mixtures are shown in Table 11. ACKNOWLEDGMENT
The authors thank Paul J. Vollmer, Division of Color and Cosmetics,
LITERATURE CITED (1) Baker, A. W., J . Phys. Chetn. 6 1 , 450
(1957). (2) Barnes, R. B., Gore, R. C., Williams, E. F., Linsley, S. G., Petersen, E. AI., ANAL.CHEM.19, 620 (1947). (3),Dolinsky, iM.,J . Assoc. O j i c . A g r . Chemists 34, 748 (1951). (4) Dolinsky, M., Stein, C., AKAL.CHEM. 34, 127 (1962). (5) Duyckaerts, G., Analyst 84, 201 (1959).
’ foorsch. 7b, 270 (1952). (10) Simmons, I. L., “The Spex Speaker,” 5’01. 5 , No. 3, (1960), Spex Industries,
P. 0. Box 98, Scotch Plains, K.J.
(11) Stimson, M. M., O’Donnell, XI. J., J . Am. Chem. SOC.74, 1805 (1952). Taken in part from a thesis submitted by
J. A. W. to the George Washington Gniversity in partial fulfillment of requirements for Master of -1rts degree in Chemietry.
Visual Observaiion in Differential Thermal Analysis Jen Chiu, Plastics Department, E.
I. du Pont d e Nemours & Co., Wilmington, Del.
obtained by S differentialthermograms thermal analysis (DT.4) INCE THE
are often complex, the interpretation of the various thermal effects usually needs the aid of other analytical techniques such as thermogravimetry, infrared spectrometry, mass spectrometry, x-ray measurements, etc. However, a visual observation during differential thermal analysis should simpLify greatly the interpretation, especially when the thermal effects involve changes in physical state or color. Hogrtn and Gordon (4) achieved visual observation by enclosing borosilicate glass sE.mple tubes in a
0.75“ DIP. I/16”SLOT
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0.070’’DIA.
1/32 WIDE 1
Figure block
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Top view of visual DTA cell
A and B, sample and wference thermocouple holes C, temperature controlling thermocouple hole D, borosilicate glass sleeve
transparent quartz tube, and by using radiant heat from a n electric Bunsen heater. This design provides good visibility of the samples, but does not give adequate protection from drafts and convection currents. Uniform heating of the sample tubes cannot be obtained for lack of a heat reservoir. I n this work, visual analysis is made possible by a simple modification of the DTA cell block previously described (3). The top view of the modified block is shown in Figure 1. The sample and reference cavities, A and B, are moved to the edge of the aluminum block ( 5 1 8 inch in diameter X 1.5 inches) and cut open to give a slot about 1 31 inch wide and inch deep. The block is then enclosed tightly by a borosilicate glass sleeve to prevent outbide disturbances, and to reduce heat losses The sleeve has a slot about 1/16 inch nide against the programming thermocouple to allow difference in thermal e\lJansion betneeri the sleeve and the block. The sample is vierzed directly through a stereoscopic microscope outside the bell-jar. il fluorescent lamp is attached to the microscope to illuminate the sample tube. Ordinary tungsten lamps give spurious thermal effects and should not be used. This arrangement enables the analyst to observe clearly physical transitions and cheniical reactions taking place in the sample without sacrificing the other features associated with the unmodified block. The noise level is below 0.2 p v . or 0.006’ C.
Sulfur (sublimed, ,llinckrodt, S e w York) was analyzed by the visual apparatus, and showed four well-defined endotherms with peak maxima at 113’. 124’, 179”, and 446’ C., respectively (Figure 2). X o apparent change was visually observed during the 113’ endotherm. Thi> thermal effect is ascribed to the enantiotropic change from the cy- to the p-form. The inflection point at 108’ C. is higher than the reported transition temperature of 95.5’ C. ( 6 ) , presumably because of the thermal history of the sample. Melting was eiident when the 124’ cndotherm started t o deviate from the base line n-a, fastebt during the inflection of the peak, and ended immediately after the
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DTA of sulfur
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