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'I
ANALYTICAL CHEMISTRY
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Table 11. Determination of Acetone Acetone Present,
4.
{
Acetone Found,
3.8
Substances Present
PH
Y
0 2.4 4.7 6.3
3.25 2.77 2.56 2.47
0 2.4 4.7 6.3
Ben Ien e
0 4 7 7.9
3.25 2.06 1.96 3.25 2.26 2.02
0 19.7 27.6 0 11.1 23.7
Dioxane
Y
0 4.7 9.5
%-Amyl alcohol
SUM.MARY
A rapid, accurate method for the determination of acetone present in concentrations of about 25 niicrograms per nil. or less depends upon the calibration of a standard curve, a plot of known quantities of acetone zs. pH, for each set of samples. Acctone may then be directly determined by pH measurement. The range of sensitivity of the curves may be extended by varying the concentration of hydroxylamine hydrochloride used. Dcterminations arc not perceptibly affected by variations of room temperature from the norm (20' to 30' C.). Kormal laboratory atmospheres.apparently do not influence the accuracy of the results. ACKNOWLEDGMENT
The author wishes to express appreciation to Fredcrick R. Dukc: for his guidance and assistance.
Figure 2 other carbonyl compounds, will interfere with this determination. .ketone may not be determined by this method in solvents from which it cannot be completely extractcd with water.
LITERATURE CITED
(1) Marasco, M., Znd. Eng. Chem., 18, 701 (1926). RECEIVED October 31, 1947.
Modification of Hershberg Melting Point Apparatus for Internal Heating and Silicone Fluid FREDERIC C. XIERRI.AM' Haroard C'nicersity, Cambridge, Mass.
URINC; the 12 years that have elapsed since Hershberg ( 7 ) D described his precision melting po'nt apparatus design, silicone fluids for high temperature heat transfer have been discovered. These fluids, which are now commercially available, are superior to sulfuric acid in that they are noncorrosive and are cwellent melting point bath fluids a t temperatures far above the boiling point of either sulfuric acid or mineral oil (6, 8). Like mineral oil they are nonconductors of electricity and permit direct heating with a coiled resistance wire immersed directly in the fluid without excessive discoloration or oxidation of the fluid. The apparatus of earlier design (7), using mineral oil, denionstrated the advantages of internal heating, but had to be put aside because of the shortcomings of mineral oil in this application. White and Hartwell (6, 8) have recently used equipment of similar design for tests on fluids. At temperatures up to 150" C., Present address, Department of Chemistry, Boston University, Boston, Mass. I
Graft' (6)employed infrared light for internal heating, but its use is necessarily restricted. The apparatus described below employs a nev heater design, with suitable provision for the greater expansion of the silicone fluids on heating. With its rapid response it is possible to select quickly and to maintain a series of increasing bath temperaturcs. This is necessary in order to determine the instantaneous decomposition points of unstable compounds (1). Dow-Corning silicone fluid Type 550-112 centistoke viscosity grade has been used successfully in this laboratory for temperatures up to 360" C., and the 500-50 centistoke grade is satisfactory for temperatures up to 300 O C. APPARATUS
The construction of the apparatus is illustrated in Figure 1. The tube and sleeves were constructed of larger tubing than is ordinarily used for a Hershberg apparatus, so as to have more space for the heater and melting point capillaries. An expansion
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V O L U M E 2 0 , NO. 1 2 , D E C E M B E R 1 9 4 8 bulb was provided to accommodate the rather large increase in volume of the silicone fluid a t elevated temperatures. The heater was constructed from 255 cm. (8.5 feet) of No. 28 B. and S. gage Chromel wire (4.10 ohms per foot). This was tightly wound on a 2.34 mm. (3/32 inch) rod, then removed and stretched over three glass hooks on the heater sleeve. The coil was arranged so that it was 1 mm. or more from the sleeve a t all points to provide good contact between the fluid and heater \Tithout the development of local hot spots. The ends of the heater coil were looped through two holes a t the upper end of the sleeve, leaving 1 cm. of wire free a t each end. The free ends \?-eresilversoldered to No. 22 B. and s. gage copper wire after it had been threaded through the side arms. This copper wire was used to avoid dissipation of heat in the side arms. After the heater was inserted, the leads were anchored to the stirrer neck either with stainless steel straps ( 7 ) or by soldering them to the twisted ends o f Yo. 18B. and S. gage copper wire loops. The thermometer arm and cap were made as short as possible to obtain good immersion and complete visibility of short total immersion Anschutz thermometers. Both caps were fitted by grinding in place with 120-mesh Carborunduni porn-der to obtain
,H
Figure 1.
Diagram of -4pparatus
Pyrex tubing 30-mm. 0 . d . X 27-mm. i.d. 190 mm. (7.5 inches) C. 96 mm. (3.8 inches) D. 22-mm. 0.d. E. Two side arms of 2-mm. i.d. X 7 - m m . 0.d. capillary tubing for heater leads F. Knobs to center sleeves G. Heater sleeve, 22-mm. 0.d. a n d 60 m m . long with three hooks m a d e from 2-mm. Pyrex rod a n d two holes punched w i t h 2mm. (0.08-inch) tungsten wire H . Insulating sleeve 22-mm. 0 . d . a n d 90 mm. long. Loops of No. 24 B. a n d S. gage Chromel wire I . Stirrer c a p J . Stirrer, 5-mm. 0.d. tubing m o u n t e d i n ground ball bearings with 8 - m m . hole and 23-mm. o.d., enclosed in Pyrex t u b e 26mm. 0.d. X 23.5-mm. i.d. a n d 100 mm. long K . Thermometer c a p with center hole 7 m m . i . d . a n d t w o 2 - m m . i.d. holes for capillaries
A. E.
a firm seat, free from wobble. The loops for centering the thermometer and capillaries Jvere made from two pieces of No. 24 B. and S. gage Chromel n-ire silver-soldered a t the points of contact for durability. The ends were bent perpendicular to the plane of the three loops in such a fashion that the whole "spider" could be slipped over the sleeve and held there by the tension in its legs. This spider was withdrawn from the sleeve with forceps when thermometers were calibrated in the apparatus.
The stirrer bearings were mounted in a glass tube with a onehole stopper and constriction a t the bottom for protection from dust and fumes. The annular spnce betvieen the bearings and tube was filled by winding cellulose tape on each bearing until it would just slide in the tube with a slight pressure. The stirrer motor was of the unmounted type suggested by Fieser (4). It was mounted on a 1.25 em. (0.5-inch) steel tube with an L-shaped aluminum panel. The horizontal portion of the panel was large enough to cover the motor and protect it from dust, while the vertical portion carried the motor control rheostat, a 1000-ohm 25-n-att potentiometer. A toggle switch on the same panel was wired to turn on both the stirrer motor and a showcase lamp which provided illumination. OPER4TION
The heater in ihe apparatus was controlled by an autotransformer capable of delivering from 0 to 135 volts. At 135 volts the heater dissipated slightly over 500 watts, so that it was possible to heat the fluid from 25 O to 300' C. in 7 minutes. This high rate of heating, combined with the small lag in temperature response when the voltage was changed, made it possible to take precise melting points in a few minutes for compounds whose approximate melting points were known. As the apparatus was not insulated, it cooled fairly rapidly when the heater and lamp were turned off. -2 40-watt showcase lamp warmed the apparatus a t a sufficient rate for the determination of melting points a few degrees above room temperature. In a room relatively free from drafts of air it took 3 minutes for the apparatus to cool from 300" to 200" C., 12 minutes to cool from 200" t o 100" C., and 18 minutes to cool from 100' to 50 O C.
For calibration of the equipment to obtain the proper heating rate at all temperatures, the method of Domard and Russo ( 3 ) is the most useful. If the line voltage in the laboratory is well regulated, autotransformer tap settings (voltage) can be used instead of ammeter readings. E. B. Hershberg suggested that it is convenient t o mount calibration data on a substitute dial on the autotransformer that controls the heater. The sensitive control of the apparatus made it possible to determine instantaneous melting or decomposition points in as little as 15 minutes. Although this did not compare with the short time required to use a Dennis bar (Z), it was much faster than was possible with an externally heated apparatus, and the Dennis bar usually required more material. The procedure (1) consisted in observing the time required for decomposition of a compound a t various temperatures above the decomposition point observed with slow heating. The temperature of the bath n a s held constant at each level while the time for decomposition rvas measured after a aample n a s inserted in the fluid. From the data obtained a t several temperatures a chart was draivn showing time of melting or decomposition as a function of temperature. The lomest temperature a t which a sample melted in the minimum time for heat transfer through the capillary (usually 2 or 3 seconds) corresponded to the instantaneous melting point observed with a Dennis bar. ACKNOWLEDGMENT
The author wishes to thank E. B. Hershberg and K. AI. McLamore for their helpful suggestions. LITERATURE CITED
(1) Bruce, TI'.
F., "Organic Syntheses," Coll. Vol. 11, p. 14, New York, John TTiley & Sons, 1943. (2) Dennis, L. M.,and Shelton, R. S., J . Am. Chem. Soc., 52, 3128
(1930). (3) Dowaard, Edwin, and Russo, M., IND.EKG.CHEM.,ANAL.ED., 15, 219 (1943). (4) Fieser, L. F., "Experiments in Organic Chemistry," 2nd ed., p. 327, Boston, D. C. Heath & Co., 1941. ( 5 ) Graff, M. M., ISD. ENG.CHEM.,ASAL. ED.,15, 638 (1943). (6) Hartwell, J. L., Ihid., 20, 374 (1948). (7) Hershberg, E. B., Ihid., 8, 312 (1936). (8) White, L. M., Ibid., 19,432 (1947). RECEIVE January D 5 , 1918. Apparatus shown in apparatus exhibit a t Tenth National Organic Chemistry Symposium, AMERICAN CHEJIICAL SOCIETY, Boston, Mass.
