A CONTINUOUS MERCURY STILL H. NORMAN JELINEK, C. F. HUBER, and MELVIN J. ASTLE Case Institute of Technology, Cleveland, Ohio
A SUPPLT of freshly distilled mercury is needed in many laboratories,.particularly those in which polarographic equipment 1s used. The mercury still described in this paper has been found to be ideally suited for the purpose of maintaining an adequate supply of distilled mercury. It operates continuously a t a distillation rate of about 800 g. per hour. A single evacuation hefore the distillation is started is sufficient for one day's operation and there is no necessity for protecting the vacuum pump from mercury vapors. Except for an occasional adjustment of the mercury levels the strill needs little or no attention. Construction of the Still. A section of U-shaped, 25-
SECTION A A REPRESENTS SPACE OCCUPIED B Y MERCURY MERCURY ANDVAPOR
1 11
I I
Uc 25 MM TUBINGL
0
TUBING
10130 ML 6 . 6 JOIN
2 M CAP.
1
ram. tubing is sealed onto the top of the stiU pot A (Figure 1). The section of the tubing B on the right side of the still serves as an air condenser. The length, 360 mm., was necessary in order to obtain adequate condensation. Tubing of 15 mm. diameter was 597
JOURNAL OF CHEMICAL EDUCATION
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found to be unsatisfactory because of insufficient condensation surface. The still pot is insulated with s/8 inch of asbestos except for a small vertical window. The two arms C and D were made of 2-mm. capillary t,uhing in order to keep the weight of the column of mercury to a minimum. These tubes are attached to the still with ground-glass joints to facilitate cleaning and storage. The supply reservoir is a 1000-ml. round-bottomed flask with the neck sealed off, a hole blown in the bottom and a side arm sealed into the neck (see Figure 2). The heater consists of 161/9ft. of Nichrome V ribbon on layers of asbestos wrapped around an iron core 1 inch in diameter. The resistance of the heater,is 22 ohms. The heater is fastened to an aluminum plate separattedfrom the heater unit by several thicknesses of asbestos paper. 9 good perspective of the still can he obtained from t,he accompanying photograph (Figure 2). Operation of the Still. Mercury is added to the reservoir and a supply of pure mercury is placed under arm D. The vacuum pump is then connected at stopcock E and the system evacuated as far as possible. The mercury is thus pulled up into the still pot and into arm D t,o a point. which is 10 t2015 em. below the groundglass joint. The level of mercury in the still pot is adjusted by raising or lowering the reservoir until the st,ill pot is about two-t,hirds full. The stopcock E is
then closed and the vacuum pump disconnected. One milliliter or so of mercury condenses above the stopcock E during the distillation and would get into the pump if the stopcock were opened while the pump is connected to the still. To avoid the necessity of disconnecting the pump each time the system is evacuated. a trap may be placed between the still and the pump. The heater is turned on and the distillation proceeds smoothly with only an occasional adjustment of the mercury levels. The heat to the column is controlled by means of a Variac. Table 1 shows the relationship between heat input and distillation rate. The pressure increases with increasing rate of take-off because of less efficient condensation of the mercury vapors.
Trolts
Watts
Distillatim rate. ml./min.
P~essure,
mm. of Hu
The optimum operating condition involves a potential of about 70 volts across the heater which corresponds to about 225 watts. At higher potentials appreciable humping occurred and the dist.illation was less smoot.h.
