Michael Cefola
Fordham University N e w York, New York
A Hot Stage for Evaporations and Melting Point Determinations
Evaporation to dryness of aqueous radioactive solutions poses no problems. On the other hand, it has been our experience that evaporation of solutions such as benzene which are often employed in solvent extraction work behave quite erratically in that the droplet will tend to spread and frequently may even creep under the disc or planchet. This will not only result in an incorrect value of the activity hut might also lead to a serious situation where contamination of the counting system might result. The design of the hot stage here described eliminates these difficulties. Although originally intended for radioactivity measurements, it was subsequently shown that the same heating block, with minor modifications, could also be employed for the determination of micro melting points under the microscope.
block. A tight fit between the block and the thermometer bulb may be assured by wrapping the latter with aluminum foil. Melting Point Determinations
In the use of the block for melting point determinations, an aluminum insert, as shown in the figure, is employed. This decreases the thermal gradient between the block and the sample and should enhance the reproducibility of the measurements. Because of the presence of a thermal gradient and other heat losses, the melting points obtained are, in general, lower than
Design of the Heating Block
The cylindrical aluminum block is heated from the periphery. The heat travels toward the cooler center of the block, thereby establishing a temperature gradient. Under these conditions, a droplet of benzene or other similar liquid will tend to accumulate in the center of the disc or planchet and will evaporate without spreading. For better control of this factor, the opening of the block has been made large enough to accommodate a tapered metal or glass tube from which a jet of cold air from a cylinder or some other coolant could be made to impinge against the planchet. Details for the construction of the hot stage are shown in the figure. Electrical insulation from the block is obtained by the use of two mica sheets, 0.04in. thick (weight 0.123) available from Mica Insulator Co., Schenectady, N. Y. This is followed by 16 turns (24 turns per in.) of Nichrome V wire, 0.0149 in. in diameter, and '/?-in. layer of Alundum RA-1162 cement. After setting, the cement is wrapped with one layer of asbestos tape which is covered with one coat of water glass. This last coating is allowed to harden before the unit is put to use. Temperature of the block may be obtained by drilling a thermometer well or thermocouple well into the
21 8 / Journal of Chemical Education
the literature values. Consequently, a calibration of the hot stage is required in order to correct the meltiug point observed. Typical calibration curves of observed and corrected values are linear. These are obtained by the use of standard substances whose melting points are known. Once the calibration curve is accurately drawn, the correct melting points may be rapidly obtained from it, provided that the procedure followed is the same as that used in the calibration. When a large number of determinations are performed daily and eye fatigue from extensive microscopic observations becomes a problem, the use of micro projection equipment such as Bausch and Lomb's Tri-Simplex Micro Projector may be readily employed. The author is grateful to Miss Priscilla Costich and Mr. John Casazza for their aid in testing the block for melting point determinations. This research was made possible, in part, by an Atomic Energy Commission Grant AT(30-1)-906.
