Automatic Microscopic Me,thod for Determination of Melting A. K. Kolb, C. L. Lee, and R. M. Trail Dow Corning Corp., Midland, Mich.
THE DETERMINATION OF MELTING POINT has conventionally been done by the capillary method. The need for improvement of this method has recently been evidenced by the introduction of the Mettler FP-1 automatic melting point apparatus ( I ) . This device, however, does not offer the advantage of direct observation of the material under study, nor does it provide the accuracy readily obtainable with the microscopic melting point method. An additional limitation is that in a mixture where two or more melting points occur, the first melt could be missed because of insufficient change in light intensity; or conversely, the major component would melt and automatically shut off the instrument, thus failing to measure the second component. McCrone (2) described various hot stage devices designed for microscopic measurement of the Kofler micromelting point, which depend on the disappearance of birefringence. More recently, Hock and Arbogast (3) have described a procedure for measurement of micro melting points of polymers using a specially-constructed hot stage and extensive control apparatus, and have demonstrated the advantages and utility of the microscopic procedure for measurement of micro melting point and rates of crystallization. In this paper we emphasize the fact that much more information can be obtained using the microscopic method. Changes which may take place during heating of the specimen, such as solid-solid transformations, loss of water of hydration, sublimation, decomposition, and evidence of impurities all can readily be seen; and point of formation, extent, and character of products formed may be recognized. Observation of melting points of mixtures of compounds with successively melting eutectics is also readily done. For example, the automatic microscopic method has been used to classify waxes in terms of their chemical type and degree of refinement, and also to detect contamination of butter by oleomargarine. Furthermore the behavior and melting point of individual particles of the specimen may be observed and their micromelting point recorded. EXPERIMENTAL
Apparatus. The arrangement of instruments is shown schematically in Figure 1. The components consist of a Leitz heating-stage microscope, Model 350, equipped with a quartz long-working-distance objective of 10 x magnification, crossed polarizers, and a first-order red compensator. Objective lenses of various magnifications may be selected appropriate t o the size of the specimen particles available or field of view desired. A Zeiss attachment camera basic body II and its integral photocell was coupled to the microscope. The photoamplifier of this exposure meter device was fitted with a closed circuit jack to allow convenient connection t o the x-y recorder (Moseley 7035A Hewlett-Packard), with exclusion of the galvanometer of the amplifier from the circuitry. The sample temperature is measured with a po-
(1) Mettler Instrument Corp., “Instrument for Automatic Determination of Melting and Boiling Points, FP-l.” (2) W. C. McCrone, “Fusion Methods in Chemical Microscopy,” Interscience, New York, 1957, p. 54. (3) C. W. Hock and J. F. Arbogast, ANAL.CHEM., 33,462 (1961).
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CAMERA I
FOCUSING TELESCOPE
EXPOSURE METER
X Y RECORDER
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SAMPLE
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HEATING STAGE
Figure 1. Schematic diagram of instrument assembly
tentiometer (Leeds and Northrup No. 8662) using a copperconstantan thermocouple placed in the thermometer well of the heating stage. A similar thermocouple is also used t o monitor the temperature at the specimen and thereby control the heat input to the stage from the power-proportioning temperature programmer ( F and M Model 240). Thermocouples to the stage are fitted through a short section of tubing o r collar approximately 10 mm in length. The thermocouple junctions, extending approximately 6 mm beyond the col!ar, are inserted into the thermometer well, with the collar serving to eliminate air currents and temperature fluctuations a t the point of measurement. The instrumentation is turned o n for 0.5 hour prior to use to stabilize all units. Best results were obtained with the Hewlett-Packard Autograf chart paper No. 9270-1006 supplied with the recorder (or other similar hard-finish paper); heating and cooling curves were differentiated by selection of different recorder pen and ink. The regulating transformer supplied with the Leitz heating stage is replaced with a Variac autotransformer (General Radio Co.). The 120-V ac output of the programmer is thus fed directly to the Variac, which is modified with a limit stop t o provide a maximum output of 24 V to the heater coil of the stage. Rates of heating may be selected from a minimum of 0.5” C/minute to a maximum of 30” C / minute. The temperature of the heating stage is recorded o n the x-axis of a n x-y recorder. The temperature may be calibrated with the potentiometer during the heating process without interrupting the experiment. The change in intensity of birefringence of the sample, detected by the photocell, is recorded on the y-axis of the x-y recorder and a continuous melting curve is thereby obtained. Any changes in the specimen during the heating process can be observed through the focusing telescope; photomicrographs can be taken in 3 5 m m o r larger format t o correlate the changing appearance to the specimen with the data recorded on the melting point curve. Sample Preparation. In the conventional preparation of samples for photometric melting points, the sample is either melted between cover slips or mounted in a silicone oil (Dow Corning 710 fluid) and covered with a cover slip. The oil cuts down light refraction and permits improved birefringence intensity to be measured by the meter. Where the possibility exists that heat used in preparation of the specimen slide may alter o r destroy some of the crystalline phases present, other methods of preparation must be used. Recrystallization from solvent has proven a n excellent method of preparation for hydrates, sublimable materials,
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100 120 140 TEMPERATURE, #C
160
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Figure 2. Melting curve of phenacetin
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decomposable compounds, and some polymers (4,5 , 6). In this preparation method a drop of solvent is placed on a slide. Small crystals of the sample are introduced into the solvent and as they dissolve, more crystals are added until a n optimum intensity of birefringence is obtained upon recrystallization. The sample is allowed to stand uncovered long enough to assure complete solvent evaporation. This preparation gives a sample which is thick in the center and thin on the edges and needs to be leveled. A cover slip with a hanging drop of solvent is lowered over the sample; this causes the sample to dissolve slightly in the center and to spread out, making it uniform in thickness as seen by examination in a polarizing microscope. Leveling helps to ensure that the sample will stay in approximately the same area in the microscope field when melted and will recrystallize in a position accessible to the photoelectric cell for a second melting cycle. The sample is allowed to stand to complete evaporation of solvent. For crystalline or partially crystalline polymers (7, 8), the sample is often difficult to prepare. For such samples, we developed an “embedding technique” as described below. A small amount of high viscosity polydimethylsiloxane polymer, Silastic 400 gum (Dow Corning Corp.), is spread over a microscope slide and pressed to give a film with a thickness of less than 0.5 mm. This is conveniently done by means of a simple metallurgical specimen hand press (Adolph Buehler and Sons, Ltd.). Several small holes (cells) approximately 0.5 mm in diameter are punched out of the film with a glass capillary. The gum is transparent to light and shows no crystallinity, melt, or decomposition in the temperature range in which we use it. The crystals and gum are covered with a cover slip and melted in the cells thus prepared. The embedding technique is also useful with crystals which sublime at low temperature. These stay in the cells when melted and recrystallize well, giving reproducible intensity measurement. It has been observed that crystallization of the sample can be influenced or controlled by the nature of the substrate. Crystals which are plate-like in habit often orient in such a manner that necessary birefringence is not achieved. Through the influence of the substrate of Silastic 400 gum, one can achieve crystal habit and birefringence intensity which is (4) R. A. Raff and K. W. Doak, “Crystalline Olefin Polymers,” Part I, Interscience, New York, 1965. ( 5 ) F. Bueche, “Physical Properties of Polymers,” Interscience,
New York, 1962. (6) P. H. Geil, “Polymer Single Crystals,” Interscience, New York, 1963. (7) J. W. S . Hearle, New Sci., 423, 835 (1964). (8) J. D. Hoffman, SPE Trans., 4(4), 315 (1964).
70 90 TEMPERATURE, ‘C
50
110
Figure 3. Melting curve of hexamethyIcy clotrisiloxane
suitable for the photometric intensity measurement of the sample melting point. A pertinent and particularly lucid discussion of nucleation and its effect on polymer crystallization is given by Walton (9); a discussion of polymer morphology is given by Geil (10). Procedure. The sample preparation is placed into the heating stage in its maximum birefringence intensity position. The stage cover plate is attached, and the thermocouples are placed into the thermometer well of the stage. The photocell and camera assembly are connected to the microscope, and the programmer is set to the desired rate of heating. With the digital temperature indicator at 20’ C, the programmer is run on the “off-at-limit” setting to approximately 26” C , switched to “isothermal,” and held at this point until start of the heating sequence. Synchronization is first established between the potentiometer, the programmer digital readout, and the pen position on the x-axis of the recorder. This calibration is made at the start and periodically during the experiment. The range setting on the x-axis is adjusted to expected maximum temperature, and the y-axis range selected so that the photocell intensity at start is approximately 80% of full scale. To begin the micro melting-point measurement, the programmer is switched to “off-at-limit” and the recorder pen is engaged. The behavior of the sample is observed through the observation telescope. Changes in appearance, such as phase transitions, loss of water of hydration, sublimation, decomposition, and formation of volatiles can be observed and noted on the recorded melting curve to characterize any or all of the phenomena which take place, A photographic record is conveniently made whenever desired by lifting the pen, exposing the film, and replacing the pen. After the final melt is reached, the programmer is switched to “hold,” pen and ink may be changed, and the y-axis zero intensity repositioned to avoid overlap of cooling and heating curve. The sample may be rapidly cooled t o room temperature with air purge connected to the cooling coil of the stage, or else it may be allowed to drift slowly without forced cooling. The rate of crystallization can be measured from the cooling curve by switching the programmer to “isothermal” at a desired temperature below the melting point. The time(9) A. Walton, Intern. Sci. Technol. (a) 28 ,(1966). (10) P. H. Geil, Chem. Eng. News, 43, 72 (1965). VOL. 3 9 , NO. 10, AUGUST 1967
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meaningful appraisal of what takes place prior to and during the melting process. The ability of the operator to draw upon his visual experience in noting changes in the behavior and composition of the sample is a valuable feature of the method described here. The ease of assembly of the component instrumentation permits general use of the equipment for other laboratory tasks when not needed for melting point work. A photographic record is easily obtained for future examination and comparison with other preparations at any point in the heating and cooling sequence without interruption of the experiment. The automatic micro melting-point method permits unattended collection of data through the entire temperature range of interest and measurement of melting points of multicomponent mixtures, together with simultaneous inspection and photographic recording of the features of the specimen.
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ACKNOWLEDGMENT I
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The authors express their appreciation to J. D. Helmer for modifying the exposure meter device to permit its use with the x-y recorder.
Figure 4. Melting curve of polypropylene
intensity plot is recorded by connecting a time-based strip chart recorder to the photocell, or in series with the x-y recorder.
RECEIVED for review January 23, 1967. Accepted May 3, 1967.
RESULTS AND DISCUSSION
A typical melting curve for pure crystals having a sharp melting point, represented by phenacetin, is shown in Figure 2. The melting point determined from the sharp break was 135" f 0.2" C in agreement with the reported value (11). To demonstrate the unique application of the embedding technique, hexamethylcyclotrisiloxane, [(CH3)&0I3, was chosen. This compound sublimes readily a t room temperature and also orients in a n extinction position so that a good sample slide could be made successfully only by the embedding technique. A satisfactory heating curve was obtained as shown in Figure 3. The melting point thus obtained was 65" + 0.5" C , in agreement with the reported value of 64.5" C (12). The melting behavior of high molecular weight crystalline polymers (3, 10) represented by polypropylene (Hercules T-20), is illustrated in Figure 4. As one would expect for crystalline polymers, the break in the melting curve was not as sharp as with the simple organic compounds. The melting point (175" i 0.5" C), however, was easily determined from the curve as demonstrated in the figure.
Corrections Kinetics of Osmium-Catalyzed Reaction Between Cerium(lV) and Arsenic(ll1) in Sulfuric Acid Medium I n this article by Robert L. Habig, Harry L. Pardue, and James B. Worthington [ANAL.CHEM.,39,600 (1967)], o n page 602, Figure 3, there are errors in the exponents of the osmium concentrations. These should read:
10s (vIII)]T
=
3.16
x
and
[OS (VIII)]T
1.58 X
Also, it was not stated that the [Os (vIII)]T for Figures 4 and 6 was 1.05 X lO-'M.
SUMMARY
The microscopic melting point method exemplifies the general usefulness of the microscope in providing a more
Spectrophotometric Determination of Primary Aromatic Amines with Thiotrithiazyl Chloride
(11) W. C. McCrone, "Fusion Methods in Chemical Microscopy," Interscience, New York, 1957, p. 248. (12) M. J. Hunter, J. F. Hyde, E. L. Warrick, and H. J. Fletcher, J . A m . Cliem. SOC.,68, 667 (1946).
I n this article by Vaughn Levin, B. W. Nippoldt, and R. L. Rebertus [ANAL. CHEM.39, 581 (1967)j o n page 584, column 1, reference (8) should read: K. Marcali, ANAL. CHEM.,29, 552 (1957).
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