M. J. D. Low and 1. Abromsl Rutgers, The State University New Brunswick, New Jersey
An Automatic Mercury Still
T h e automatic vacuum still described below is useful for the continuous distillation of mercury for use in diffusion pumps, McLeod gauges, and polarographic and other equipment. The dual virtues of the system are that i t is simple and consequently trouble-free, and that it is inexpensive and consequently easily procured.
ac coil.) The heater can easily be made from Nichrome wire, and should have a resistance of about 20 to 25 ohms. A second length of Nichrome (not shown) can be used to control the heater, so that a Variac is not required. After a few trials, by using a clip, the length of wire is determined that will give the proper distilling rate without bumping of the mercury in the stillpot.
Still Construction Details of the still are shown in Figure 1. The design is similar to those of Jelinek, Huber, and Castle2 and Cannom3 The stillpot is made from a 250-ml Pyrex Ehrlenmeyer flask. A bubble is blown on top of the flask neck and the flask shaped as indicated in Figure 1. The boiler section is covered with asbestos, but the bubble is left uncovered. Mercury vapor is condensed on the bubble walls and flows into the annular space between the bubble top and the boiler section of the stillpot. A '/,in. Kovar-Pyrex seal is fused to the top of the bubble or, as shown in the sketch, to a 10/30 standard taper. A '/le-in. steel rod was passed through the Kovar seal to extend to within an inch or so of the bottom of the stillpot, and was then soldered to the Kovar. If used, the standard taper can be sealed with de Khotinslcy cement. The entire still was mounted on a solid board fixed to a cabinet fitted with casters. If a permanent place is allocated to the device, a water-cooled condenser can be incorporated, with a resulting increase in distilling rate.
The simple electrical circuit is shown in Figure 2. Except for the solenoid valve, the parts can be purchased a t any local electrical supply store. The transformers are required because a low voltage is needed a t electrode A. The contact between A and the mercury surface in the stillpot is made in vacuum and a discharge will occur if the voltage is excessive. TI, T,,T3, and T, are 115v/2.5v filament transformers. R,,R2, R3, and R4 are 120v, 60-cycle relays. The pilot lights L are useful, but may be omitted. Any pushbutton will serve for P. The solenoid valve S is two-way, normally closed, stainless steel body with Teflon seat, without shave coil (S90D120, Crown Controls, Rutherford, N. J.). (Note: This is a dc valve fitted with an
A stipend for L. A. by means of a contract from Sun Oil Co. is
*JELI;EK,H.,
H&R,
EDUC. 26, 597 (1949).
SOLENOID
Figure 1.
Electrical Circuit
-matefullv acknowledeed.
2
C. F.,AND CASTLE,M. J., J. CAEM.
'CANNON, W. A., J. CHEM.EDUC. 28, 272 (1951).
SRII Operation Mercury is placed in the sump B and in the reservoir, and the outlet C is dipped into clean mercury. The system is then evacuated. Mercury will rise in both vertical tubes. To start the still, the normally-open push button P is momentarily depressed, thus activating relay R,. If there is insufficient mercury in the sump to permit contact between steel electrodes d and 9, relays Rzand R3 are not activated and current will flow to the solenoid S. The normally closed solenoid valve will open and mercury will flow from the reservoir to the sump until contact is made between electrodes B and S and between A and 2, when R2 and R3 are activated. The solenoid valve will then close and the heater H will then be energized, and mercury will distill. The mercury level in the sump will drop as the distillation proceeds until contact between electrodes d and 9 is broken. Relay R2 will then be deactivated, and the solenoid valve will open, permitting mercury Volume 42, Number 10, October 1965
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the operation of the still. Contiuuous operation of the vacuum pump is recommended, although not essential. Safety Faclors
Figure 2.
1 3
to flow into the sump. This cycling between solenoid valve and heater will continue throughout the distillation. The still has an output of about 2 lb/hour, and will run continuously. Essentially, the size of the mercury reservoir is the factor in determining the duration of
(1) If the reservoir is not refilled and empties completely, the distillation will continue until the contact between B and 3 is broken. The heater will then shut off, and the solenoid valve will remain open. (2) If vacuum is lost because of a crack in the system or other causes, the contact between A and B is broken. This will deactivate R3, and S will be activated. The mercury level in the sump will rise and contact will be made between 1 and 2, thus activating R4. This shuts off the current to R,,which therefore opens and thus completely shuts the system down. (3) An interruption in the 120 volt supply will deactivate RI, and cause a complete shut-down. (4) The T4, R4unit can also be used to monitor the mercury level in a receiver placed at outlet C. This is not shown in the figures. A &eel electrode connected to 3 is led to the bottom of a receiver. A second electrode connected to 1 is clamped so that when the mercury level in the receiver rises, contact is made between 1 and 5. This will activate Ra and cause a complete shut-down.
