1000
800
900
for his assistance in designing the equipment used in this study.
700
Figure 4. Infrared spectrum of 2,6dimethylaniline 3sulfonic acid in 1% solution of aluminum stearate in
-
cs2 Concn.: 10 rng. per ml.
I
03 ‘
.8
O 9
I
L IO I II 12 WP,VELENGTH (MICRONS)
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, b u t are generally stable for a t least 15 minutes.
I
I
13
14
-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
‘P
0.070’’DIA.
1/32 WIDE 1
Figure block
.
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 D T A 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
I
I
44 6
I
I
100
200
Figure 2.
J
I
300 T .‘C.
400
DTA of sulfur
5-mg. sample; heating rate, 15’ stafic nitrogen atmosphere
VOL. 35,
501
NO. 7, JUNE 1963
C./minute;
933
peak liosition. The inflection temperature of 119" C. agrees very well with the reported melting point of sulfur at llS.73" C. ( 6 ) . I-pon further heating. the orange liquid turned red during the 179" endotherm, and became increasingly darker up to t'he boiling mdotherm a t 446" C. Molten sulfur is believed to be an ecluilibrium mixture of three forms, A. ,u, and K (6). -1s the temperature increase?, both K - and p-forms increase. In the region from 160' t o 200' C.!the quantity of X form reaches a masimum, and the quantity of ,u-form continues to increase up to the boiling point. Seemingly, the 179' endotherm coincides with the point where the K-form is a t its masiniuni. The CY -+ c? transition was previouJy detected b!- DT-1 (1, 2 ) . However. the indication of an endothermic effect along with a color change for the A e p e T transition is of great interest. Tkual DT.1 is useful in the interpretation of thermograms, especially when reactions are involved, and unexpected product's are formed. The following example nil1 illustrate this point. Figure 3 slion-s thermograms of mixtures of p-aniinobenzoic acid (P-IB-I) and 1,3,5trinitrobenzene (TSBl in various ratios. Eithcr P.IB,I or TKB showed only a siiigle melting endotherm a t 185" or 123" C.: respectively. However, a mixof T S B and 99% ture containing of P.iH?I shon-ed an endotherm at 11S2 C. and an exotherm a t 119" C.. aeconilianid hy the development of
lcc
I PABA
1185
I%TNB
118
I50
I
185
A
A
20%TNB
I
119'
I
1
I
I
I
I18 A
I
40%TNB
119"
120 A
I
I TNB I
1
167
-
1241
I
I
1123 I
I
I
100
I50 T,
I
2 00
OC.
Figure 3. DTA of mixtures of p-aminobenzoic acid and 1,3,5-trinitrobenzene 1 0-mg. samples; heating rate, 15' C./minule; static nitrogen atmosphere
orange spots in the sample. The orange spots melted, showing an endotherm a t 150' C. Then the whole sample melted during the 185' endotherm. With increasing amounts of T X B in the mixture, the peak sizes of the 118" endot,herni and 119' exotherm increased. and so did the peak size of the 150" melting endotherm and the color of the orange spots. Accordingly! both the peak size and the peak temprrature of the melting
endotherm of P.113.1 decreased. For a mixture with 607, T S B , the melting endotherm of P.1B-\ as eliminated and the t n o endotherms a t 121' and 153" C. remained. With still higher ratios of TSH. the 153' endotherni also disappeared. -111 these can he explained by the formation of a new compound betn-een P.1B.I and T S B . which is orange or red in color and shows a melting endotherm a t 153' C. From the fact that almost no excess PIB.1 was left for a mixture ivith 60% T S B , the compound would be expected to have a coniposition of 1 :1 P.1B-1:TSR. The formation of such a compound b e b e e n P.II3.1 and T X B was previously reported (6). For very high ratios of T S l 3 such as 807,, the compound produced dissolved in the melt of TSB. and no longer .;bowed a aeiiarate melting endotherm. ACKNOWLEDGMENT
The author thanks J . S.Clark and R. -1.Pnrkinson for assirtance in this work. LITERATURE CITED
ill. L.. Sircar. A. I
