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INSTRUMENTAL ANALYSIS AND 'PHYSICAL MEASUREMENTS1 R. C. VOTER E. I. d u Pont d e Nemours & Co., Inc., Polychemicals Department, Wilmington, Delaware
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SPECTROPHOTOMETRIC METHODS
' Presented as part of the Symposium on An Analysis Group in an Industrial Research Organization before the Divisions of Chemical Education and Analytical Chemistry a t the 130th Meeting of the American Chemical Society, Atlantic City, S e p tember, 1956.
The principal research, development, and service work performed in this area is associated with molecular structure studies, quantitative determination of low levels of contaminants and additives in a great variety of materials, qualitative identifications, and detection of unsuspected and often undesired functional groups in polymers and their intermediates. These latter may occur as the result of process conditions or because of degradation phenomena. Determinations of traces of aromatic compounds
two preceding papers in this series presented applications of infrared spectrometry and X-ray diffractometry. This paper is concerned with other instrumental techniques which are used extensively by the analysis group. Work dealing with physical measurements also is discussed briefly.
VOLUME 35, NO. 2, FEBRUARY, 1958
such as those used as polymer additives, initiators, stabilizers. and antioxidants, are usually made by methods based on ultraviolet absorption. I n many cases, a spectrum recorded directly from a pressed film of the polymer is employed for analyses. Extraction with organic solvents followed by concentration is often needed where levels are very low or where the polymer itself absorbs radiation at the wave length used for the analysis. Conventional colorimetric analyses, many of which depend on complexes formed with either organic or inorganic reagents, are performed on many compounds and ionic species. Analyses by this technique are important, particularly where low concentrations are involved or where only small amounts of sample are available. I n order to reduce analysis time, efforts are often directed toward methods which mill permit simultaneous determination of the components of interest in a single solution. As an example, a quantitative method was worked out by Kitson (1)for cobalt, copper, and iron employing complexes developed in the thiocyanate-acetone system. Spectrophotometric measurements a t three analytical wave lengths, 380, 480, and 625 miHimicrons, are converted into quantitative results. This relatively simple, rapid procedure is used for analyses of a wide variety of samples containing 50 micrograms or more of cobalt and/or copper and more than 5 micrograms of iron. The method gives results with accuracy and precision of =tloyo. The Komarowsky reaction provides a convenient means for the colorimetric determination of certain glycols, alcohols, and carbonyl compounds (2). Color developed from reaction with p-hydroxybenzaldehyde in sulfuric acid solution is measured spectrophotometrically. The procedure has been used for determining small amounts of 1,2-propylene glycol in ethylene glycol and aldehydes in the presence of organic acids. Also, cyclohexanol and cyclohexanone canbedetermined simultaneously from measurements a t 535 and 625 millimicrons. Only the reaction product of cyclohexanol absorbs at the latter wave length. An indirect colorimetric method is used for determining parts per million levels and less of hydroquinone in methyl methacrylate. I n this procedure, some of the ferric iron, which is added to the sample, is reduced by the hydroquinone. The ferrous ion concentration, which is then determined colorirnetrically with 1,lO-phenanthroline, is equivalent to the hydroquinone originally present in the sample. Turbidimetric procedures using a spectrophotometer for the analytical measurement are employed for low levels of sulfur and bromine, chlorine and iodine in organic compounds. Barium and silver ions are used as the precipitating agents for the sulfate and halogen ions, respectively. With proper sample degradation and careful control of the turbidity producing steps, great sensitivity and reasonably good precision and accuracy are obtained. In the case of halogens, the spectrophotometric measurement is made within five minutes after the silver nitrate is added, while with sulfate determinations a waiting period of thirty minutes was found to be advantageous. Temperature control other than having the solutions a t room temperature was not found to he important. Applications include the determination of traces of halogen- or
sulfur-containing impurities in starting materials or in reaction products including polymers. Sample preparation methods play an important part in absorptimetric analyses. Frequently, interferences are removed by prior treatment employing techniques such as extraction, chromatography, ion exchange, and electrodeposition. Complexing agents are used extensively. Trace levels of inorganic residues in polymer and other organic samples are determined quantitatively by colorimetric and turbidimetric methods after the organic material has been burned off. The Parr oxygen bomb is commonly used to prevent loss of volatile compounds that would occur during ashing and frequently to eliminate the need for lengthy digestions. Demand for ultramicro colorimetric procedures have led to the development of a long path, low total volun~e microahsorption cell (Figure 1) of improved design
STAINLES~ STEEL CAP
SOD; OF TEFLON
S T A I N ~ E SSTEEL S JACKET
OUARTZ WINDOW
.
Figure 1. Microabeo.ption C1 .1
that can be easily filled and positioned in a spertrophotometer (5). This cell is a modification of one made of Teflonmby Kirk, Rosenfels, and Hanahan (4). The stainless steel jacket and caps provide rigidity and permit tightening to seal the windows against the plastic inner body without distorting the critical alignment. The diameter of the sample-containing cavity is about four millimeters and the path length is seven centimeters. Photometric measurements can be made on solutions having a volume of oile milliliter or less. Together with sensitive organic analytical reagents and scaled down operations, quantitative analyses are made on samples containing as little as 20 millimicrograms of iron and 50 millimicrograms of copper. Fluorimetry is a useful analytical tool in determinations of, for example, alkyl ammonium salts in organic liquids. The intensity of the fluorescence produced upon the addition of eosin provides the basis of one very sensitive method of this type. MASS SPECTROMETRY The mass spectrometer plays a very important part. in qualitative and quantitative analyses of a wide range of hydrocarbons and fluorocarbons,and of oxygen-. (5), nitrogen-, and sulfur-containing compounds. Both gases and liquids are analyzed readily by this technique; many complex mixtures are completely characterized. The latter may require utilization of a set of^ simultaneous equations and possibly a computer (6). Mass spectrometry is another method for which^ only very small amounts of sample are required. With liquids a small fraction of a milliliter is usually needed, and for gases samples of a milliliter (STP) and even smaller are handled easily. A modification that is. employed in our laboratories involves sealing off theballast volume of the instrument's inlet system, which JOURNAL OF CHEMICAL EDUCATION:
normally is sufficiently large to maintain a nearly constant pressure during the mass peak scanning. With the inlet system thereby reduced to a small fraction of its original volume, quantitative analyses can be obtained on gaseous samples where as little as one or two microliters (STP) are available. However, the drop in pressure during the mass peak scanning and the occurrence of fractionation a t the orifice a t these low pressures make it necessary to use compensating corrections based on the diffusion equation and the molecular weights of the gases in the sample. Separation methods are often used on samples in order to concentrate a component of a mixture that is of special interest or to separate components that make solving of the mass spectral patterns unduly difficult. One technique involves the use of a high vacuum system and manipulations such as selective freezing out and transferring of gases by an automatic Toepler pump to a calibrated gas buret where it can be withsample $,,be, hi^ drawn into the mass spectrometer novel arrangement is shown in Figure 2. Gas chromatographic separations also are used with great success. The materials corresponding to observed peaks are condensed and then introduced into the mass spectrometer for analysis. A gas mixing manifold, shown in Figure 3, is employed for preparing known mixtures of gases for calibration purposes and for checking the accuracy of mass spectrometer analyses where mutual interferences are possible. The partial pressures of the successively added components are measured with the manometer. The paddle of Teflon tetrafluoroethylene resin is rotated in a manner similar to that used in a magnetic stirrer. It serves to circulate the gases through the system to insure complete mixing before the sample is withdrawn.
+-GAS
BURET
COMMON CONT
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oassepalation
KNOWN VOLUME O L A T I O N CHAMBERS
PUMP Figure 3. Cia* Mixing Manifold
The mass spectrometer is often used for the characterization of products from the vacuum pyrolysis of polymers. The compositions of these pyrolysates are more or less characteristics for given pyrolysis conditions of each polymeric material (7). For example, ~ 0 l ~ m e t h methacrylate yl pyrolysate is essentially all monomer. Studies are made of the mechanism of the decomposition of polymers and of the behavior of plastic materials under processing conditions where, for example, too high a temperature might be employed. EMISSION SPECTROSCOPY
The emission spectrograph plays a major role because of the speed with which analyses can be performed, because of the usually good sensitivity, and because only a small amount of sample is normally required. Most polymeric materials, unless highly plasticized, can be examined by arcing directly without prior ashing. Most spectrographic analyses are performed on a semiquantitative basis (8); that is, the elements are reported in percentage ranges which f magnitude of the concentration comparison with a collection of Quantitative spectrographic techniques, once worked out, provide comparatively rapid and reasonably accurate analyses for trace impurities in inorganic systems. This quantitative technique is very useful for determining low levels of metallic elements such as copper, iron, nickel, and barium in cobalt oxide (9). The flame photometer makes a natural laboratory partner for the emission spectrograph since it often can be used for reasonably accurate quantitative analytical results after the spectrograph has established the presence of metallic ions. Analyses can be performed with great speed and exhibit extreme sensitivity in many cases, in particular for the alkali metals. 85
X-ray emission spectrography enables us to perform, among other analyses, nondestructive determinations of trace metal impurities and residues in polymeric materials. MICROSCOPY
Electron Microscopy. Data obtained on samples from electron micrographs with their great magnification and resolution have provided the groundwork for the solution of difficult research problems. A good example of such a program is the study of the state of dispersion of fillers in plastics such as carbon black in polyethylene. Either the resin is dissolved and separated from the insoluble filler or ultrathim sections are examined directly in the microscope. Other studies have included examination of the nature of dispersion particles formed during an emulsion polymerization. Recent research has disclosed the nature of crystals of linear polyethylene recovered by crystallization from very dilute solutions (10). Stacked layers of thin plates about 100 A. in thickness resulted from polyethylene of molecular weight = 150,000 crystallized from xylene. The crystalline nature of these structures was verified by electron diiraction techniques. Single crystals were found on crystallization of a fraction of molecular weight = 850. These crystals grew by a dislocation mechanism evidenced by the clearly defined screw dislocation visible in electron micrographs. Visible Microscopy. Work in the field of visible microscopy includes phase contrast studies, chemical microscopy, and examination of specimens by reflected, transmitted, and polarized light. A microscope with a hot stage is employed for making melting-point determinations and for studies of crystallization phenomena and phase transitions. Photomicrographs are taken for studies of filler distribution in polymers, determination of particle sizes, and for examining the surface characteristics of polymers. Visible microscopy is employed in identication of organic compounds via derivatives. This technique was applied by Mitchell and Ryland (11) for semiquantitative analyses of binary mixtures of the p-bromoanilides of acetic and propionic acids and the 2,4-dinitrophenylhydrazones of ace& and propionaldehyde. OTHER INSTRUMENTAL METHODS
Nuclear magnetic resonance spectroscopy is important in two general scientific areas. High resolution NMR has taken its place with infrared and ultraviolet spectroscopy as a means of identifying and determining molecular structures. I n many instances information on molecular arrangements that is not obtainable by other means is provided relatively quickly. Broad line NMR studies yielded information on the extramolecular structure such as the crystalline-amorphous ratio of polymeric materials. Ranmn spectroscopy has proved very useful in molecular structure determinations of organic compounds, often in conjunction with infrared techniques. Information is obtained, for example, on the nature of geometric isomers of olefins and position isomers in aromatic compounds. Raman techniques are particularly valuable for compounds where critical vibrational
frequencies are inactive in the infrared and where overtones in the infrared interfere with primary absorptions. Aqueous solutions are more readily analyzed by Raman spectroscopy. Magnetic susceptibility data have been employed in studies of metallo-organic complexes. Para magnetic resonance spectroscopy is used for the examination of materials containing free radicals. COMBINED ATTACK ON PROBLEMS
Many instrumental techniques and the accompanying know-how of the staff are often brought to bear on difficult problems that must be solved quickly and which do not lend themselves to solution by a single technique. One example of a tough analytical undertaking solved by cooperative effort involved identification of an undesired, faint white streak in an article of Lucitem acrylic resin molded by a customer. The white streak represented only a small fraction by weight of the molded piece, but it was sufficient to cause a strange and undesirable appearance. Emission spectrographic analyses of a selected portion of the article and of a clear piece for a control indicated that the streak was not due to inorganic material. Streaked and clear sections were selected and separately heated under controlled conditions until depolymerization was complete. The condensed pyrolysis off-gases n.ere analyzed by mass spectrometry and ultraviolet spectroscopy. A ibw intensity mass peak characteristic of styrene was observed in the pattern of the condensate from the material with the white streak. This evidence indicated that possibly a trace amount of polystyrene had inadvertently been mixed with the Lucite prior to molding. The ultraviolet examination served to confirm these results, inasmuch as absorptions attributable to styrene were noticed, although another ultraviolet absorber present curtailed the sensitivity of detection. Neither method alone could give an answer without considerable doubt, but the multiple instrument attack provided a sound solution. The mass spectrometer, in effect, gave information on the molecular weight of the material not common to the two condensates, while the ultraviolet examination provided information about its structural makeup. PHYSICAL MEASUREMENTS
Another important phase of the work of our analysis group deals with physical measurements. Physical constants are important not only as reference information but often are used for qualitative determinations, checks on the purity of starting materials, and rapid methods for determining the composition of binary mixtures. Data from these measurements are f r e quently used in conjunction with information from instrumental or chemical analyses. Also, physical measurements provide quick survey or pilot analyses. For example, the failure of a material to exhibit an expected value for a physical constant will signal the need for further analysis in order to establish the source of contamination. Typical of the variety of physical measurements are refractive index, viscosity (including solution viscosity), density, boiling points, melting points, freezing points, vapor pressure, heats of combustion, solubility, equilibrium constants, surface area, and optical rotaJOURNAL OF CHEMICAL EDUCATION
tion. Many of these measurements are made WATER JACKET on the micro as well as macro scale. Molecular weight determinations aremade by several methods which involve a colligativeproperty. These measurements are of frequent interest and are necessary for materials that range LIQUID WITH DENSITY GRADIENT in molecular weight from below one hundred to CALIBRATED FLOAT those encountered in polymeric substances. Cryoscopy, ebulliometry (12, IS) osmometry and isopiestic methods are of particular value when polymers are the subject of study. The determination of densities of polymer specimens is of particular interest. Besides the familiar displace ment methods, the density gradient tube is used with a high degree of success (14). I n this technique a vertical liquid column with a continuous density gradient is employed. The gradient is established by partially mixing layers of liquids of different densities which have been carefully added to the column (Figure 4). Desired density ranges are obtained by selecting appropriate liquid combinations. For example, for the density range of 0.79 to 1.00 g./ml. a column can be prepared from methanol and water. Liquids selected, of course, must not swell or dissolve the samples and must be inert chemically. Glass floats of known density are placed in the column and are used to calibrate the gradient in terms of position versus density. A polymer sample placed in the top of the column sinks to an equilibrium position where the liquid density is exactly equal to its own. Gradient
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columns prepared in this manner are stable for months although periodic recalibration of density versus float position is advisable. For best results, constant temperature water (*O.l°C.) is circulated through the water jacket. Sample densities may be determined with an accuracy of better than 0.1%. Glass floats are available from Scientific Glass Apparatus Company, Bloomfield, New Jersey. Satisfactory floats can be prepared quickly and easily from Pyrex tubing by sealing both ends of short lengths (15-20 mm.) with a fine tipped oxygen-gas torch. Excessive accumulation of glass a t the seal is avoided. Different sizes of tubing give floats in various density ranges. For example, regular ten-millimeter Pyrex tubing will result in floats in the 0.89 to 1.03 range. Floats are calibrated by the usual displacement method for densities. LITERATURE CITED (1) KITSON, R. E., Anal. Chem., 22, 664 (1950). (2) DALNOGARE, S., AND J. MITCHELL, JR., Anal. Chem., 25, 1376 (1953). (3) MOORE, G. E., AND H. D. DEVERAUX, ,paper presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 6, 1953. AND D. J. H A N ~ AAnal. N, (4) KIRK,P. L., R. S. ROSENFELS, Chem., 19, 355 (1947). (5) KELLEY, H. M., Anal. Chem., 23, 1081 (1951). H. J., AND R. E. KITSON in "Encyclopedis of Chem(6) FREY, ical Technology," Vol. 8, Interscience Publishers, Inc., New York, 1952, p. 812. S., AND S. L. MADOESKY, J. Research Nat. BUT. (7) STRAUS, Standards, 50, 165 (1953). (81 . . WARING,C. L.. AND C. S. ANNELL,Anal. Chem., 25, 1174 (1953). (9) MCCLURE, J. H., AND R. E. KITSON, Anal. Chem., 25, 867 (1963). \ ~ ~
-
,
~
(10) (11)
TILL,P. H., J . Polymer Sci., 24, 301 (1957). MITCHELL, J., JR., AND A. L. RYLAND, Mikroehim. Acta,
(12)
KITSON, R. E., A N D J. MITCHELL, JR.,Anal. Chem., 21, 401
(13)
K~TSON, R. E., A. N. OEMLER, AND J. MITCHELL, JR.,Anal.
1-6, 422 (1956). (1949).
Chem., 21,404 (1949). (141 LAW.B. W.. AND F. M. RICHARDS. J. Am. Chem. Sac.. 7 4 ,
