Vol. 13, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
204
setscrew as shown, for adjustments in height. All soldered connections are made with silver. The carbon disk resistors in the middle portion of the mortar provide for the production of less heat energy than that from the Nichrome rings at either end, which brings about an equal heat distribution not attainable if all heatin elements are either graphite or Nichrome. The resistance the whole mortar is diminished or increased in proportion to the tension applied by adjustment of the screw cap. The "make" and "break" of B may be adjusted by altering the position of the adjusting knob to higher or lower temperatures. The Variac control is powered by the regular 110-volt laboratory power source. It is used over the voltage range 5 to 15 volts and supplies the third heat control. The thermometer is an Anschdtz type having a range of 150" to 220' C. Temperatures at the thermometer well in the middle of the heating mortar can be readily adjusted at any point between 150" and 220" C. and maintained constant within 1' to 2". variation from center to extreme ends can be kept belowT h5 t temperature gradient.
07
ll
Carbon D k k
FIGURE 2. DETAILS OF CONSTRUCTION The thermometer tube is 18 mm. long, 1 cm. in outside diameter, and of 6-mm. bore. The tripod upright is 55 mm. long, and of 6-mm. diameter to provide a slip joint with 55 mm. of the same tubing as that used for the insertion of the thermometer. The heating mortar is supported in contact with it by means of a
The heating mortar described has been used for 6 months in the authors' microchemical laboratory a t the University of Illinois with perfectly satisfactory results and its life span should be practically endless, without need for further adjustments. The temperature at which it is operated does not alter the carbon-to-Xichrome resistance contact in a measurable degree. The heating mortar has been operated continuously for 30 days with no change required in its three temperature-adjusting variables; during this period there was no change in temperature that could not be attributed t o changes in room temperature.
Literature Cited (1) Fisher Scientific Co., Laboratory, 10, KO.4, 68 (1939). (2) Schneider and Van Mater, IXD. EXG.CHEM.,-4nal. Ed., 9, 295 (1937).
Volumetric Flasks for Microanalysis EARLE R. CALEY Frick Chemical Laboratory, Princeton University, Princeton, N. J .
W
HEN solutions are mixed in small volumetric flasks of the
usual design, the error caused by the trapping of pure solvent or partly mixed solution in the ground glass at the stoppered end of the flask is proportionately much greater than that caused by the same source of error in large flasks. Moreover, thorough mixing of solutions in conventional flasks of 5- or 10-ml. capacity is difficult to accomplish rapidly. Both this source of error and this lack of convenience in mixing are avoided by the use of flasks of the design shown in the figure. I n such a flask the upper part has a capacity about five times that of the calibrated part, so that after the solution is made u p to the mark i t may be passed into the upper part by tipping the flask on its side. Then by righting the flask and at the same time giving i t a slight circular motion the liquid is caused to swirl as i t passes back into the lower part. By repeating this about three times the solution is thoroughly mixed. The sides of the calibrated lower part should slope more than in flasks of the usual design, in order that the liquid may be entirely, or almost entirely, transferred to the upper part without coming into contact with the upper neck. The neck of the calibrated part may be made slightly smaller in diameter than the neck of conventional flasks, with a corresponding gain in accuracy of measurement. For use with aqueous
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This disadvantage is of no significance if such a flask is habitu-
