V O L U M E 2 8 , N O , 2, F E B R U A R Y 1 9 5 6 the walls hug the needle and obviate the necessity for a packing gland around the needle stem. This principle is also used t o provide a positive seal in the bore of the plug when the needle valve is fully closed [the dimension A (Figure 1) being slightly greater than the bore of the plug]. Teflon has the disadvantage that it is difficult t o get a completely leak-proof joint, between the plug and the barrel of the stopcock. Usually this is not a serious matter, and the ordinary spring and nut holding device is sufficient t o prevent excessive leakage. Where extreme tightness is essential, the plug should be carefully chosen to match the barrel, or the plug may be ni:ic~hinedfrom Monel metal, or some other material xhich may be ground into the stopcock barrel. Where the plug is made of a material other than Teflon, the design in Figure 2 has the advantage that there is more room to construct a packing gland around the needle stem. The ease of adjusting the flow rate depends upon the pitch of the threads and the diameter of the adjusting head (Figure 1,B). B should be small enough to pass through the barrel of the stopcock (for convenience in removal of the plug assembly, although this is not an essential feature). The design shown in Figure 2 permits the use of a big adjusting head, but it is not as convenient t o adjust as is the design shonn in Figure 1. If fine threads are cut in Teflon, care must be used not to screw the needle too tightly or the threads may be stripped. One of the most useful applications for this device is in the slow addition of reagent by means of a dropping funnel. It is possible to set the addition rate very accurately, and t o reproduce this rate time after time without altering the needle setting, as the flow can be stopped or started by turning the entire plug. Also if polids are present in the solution being metered, as in the :&lition of ethereal aluminum lithium hydride (AIIiIIa) t o a rewtion, plugging of the stopcock can be overcome by rotating the plug about its axis 180”. The flow through the bore is reversed and the plugging material is washed away, thus restoring the original flow rate. Anot,her useful applicat,ion is the control of flow in chromatograph and ion exchange columns. In liazardous reactions, such as the addition of sulfuric acid t o water, this device could present the accidental discharge of a lxrge, uncontrolled amount of the reagent. The arrangement illustrated in Figure 3 is designed to permit altetnat,e large and small flow rates-as, for example, in routine t i t ~ ~ t i o n s This . is fabricated from a three-way stopcock such as Corning S o . i 3 8 0 (Corning Glass Works, Corning, N. I-,),I n this arrangement, however, it is not’ possible to ov~i’conieplugging of the bore by rotating the plug.
287 Electrical contact is provided by silvering sectors of the plate
as shown by B. Each sector is approximately one fifth the circumference of the plate.
Clamps made of spring brass, which
fit over the edge of the glass, provide electrical contact with the silvered surfaces. Electrical leads through the desiccator wall are provided by drilling tlTo l/ls-inch holes, C, through opposite sides of the desiccator approximately 0.25 inch below the bottom surface of the heating plate. These leads are soldered t o the brass clamps. A third hole, D,is drilled about 1 inch above the upper surface of the plate t o accommodate a thermocouple for measuring the temperature of either the platr or thf? suhstance being dried.
Vacuum-tight seals for these \\iics can be made \$ith a porcelain cement. The temperature of the plate is controlled by adjusting the applied voltage Jvith a variable transformer. Temperatures as high as 80” C. may be maintained lor piolongetl periods R ithout heating the desiccator e\crssivrl\
Anhydrous Hydrogen Chloride Generator Harry Taniguchi and George J. Jam, Department of Chemistry, Rensselaer Polytechnic Institute, Tray, N. Y.
of investigations on electrode potent i d s in nonIsaqueowcourse media, an apparatus to meet a need for pure anhyTHE
drous hydrogen chloride vas designed in this laboratory. Some of the elements of labor and hazard associated with the conventional generator? (l\laxsori? R. N., “Inorganic Synthesis,” Vol. I, p. 14i>XlcGraw-Hill, X ~ York, K 1939) are effectively eliminated in the operation of this generator. I n addition, an all-glass construction avoids contact of the hydrogrri rhloride with rubber. The arrangement of the apparatus is shonn schematically in Figure 1. The required amount of sodium chloride is charged through side arm D t o the reaction chamber, F’. The glass beads in F’ have been helpful in preventing caking of the chloride charge
Heated Vacuum Desiccator F. T.
Wadsworth, American Oil Co., Texas City, Tex.
occasions arise in which it is desirable to d17- or F remove trace quantities of a solvent from a product in a relatively short time. Although vacuum ovens can be employed. HEQUENTLY,
space limitations and availability of such equipment are frequent obstacles. Drying pistols can be used for small samples but are not convenient for many applications. Many of these obstacle. can be overcome with a standard vacuum desiccator equipped with a low wattage, electric heating plate. The essential features of this equipment are shown in the diagram. The porcelain plate supplied with the desiccatoi is replaced by a circular plate of Corning heating glass (Corning Gluss Korks, Corning, S.P., Bull. B-89),rated at 500 matts pel squaie foot at 110 volts. This plate can be easily cut to fit xith LI glass saw and should be of such size that a minimum of 0 25-inch cslearance exists betneen the plate and the ivall of the desiccator The heating plate is supported by a glass franie, A , equipped with t l i i r e equally spaced supporting points to minimize heat transfer 10 the desiccator.
Figure 1.
Anhydrous hydrogen chloride generator
