AN IMPROVED MERCURY STILL BY L. H. DUSCHAK AND
s.
0. SPENCER'
The mercury still, shown in the accompanying diagram, embodies the principle of the familiar Weinhold apparatus, which by certain additions has been adapted to the distillation of mercury in the presence of a small partial pressure of oxygen as recommended by Hulett. Under this condition the common metals contaminating the mercury are oxidized and separated much more effectively than by ordinary distillation. The mercury after a preliminary cleansing by one of the usual methods, if necessary, is placed in the cup A with a reserve supply in the reservoir B by which the level in the cup is automatically maintained. By closing the stopcock this reservoir may be filled periodically without interrupting the distillation. The bore of this stopcock should be at least 5 mm so that air bubbles entering the bottom of the delivery tube will have no difficulty in passing. The horizontal branch of the three-way stopcock C is joined to a water aspirator, or other vacuum supply of moderate capacity. This stopcock may be also used for the rapid admission of air to the apparatus when it is desired to stop the distillation quickly. When the vacuum is applied, mercury rises in the annular space between the extended neck of flask D and the condenser tube within. This flask will ordinarily have a capacity of 1,000to 1,500 cc and may be of soda lime glass, although Pyrex or Jena glass is preferable. The flask is supported by an ample wooden clamp, not shown in the sketch, which grips the neck just below the gas burner. The elevation of the flask above the cup should be such that the difference in Published by permission of the Director, U. S. Bureau of Mines. Hulett: Phys. Rev., 33, 307 (1911);Hulett and'Minchin: Ibid., 21, 388 (1905).
312
L. H.Dusckak and S . 0. Spencer
mercury levels will be
20
to 30 mm less than the mean barometric pressure. A very small stream of air enters the apparatus through the capillary tip a t E. This is carefully adjusted so that several bubbles per second pass up through the mercury in D and a plug of cotton wool prevents dust from altering the size of the orifice. The rate of distillation is so regulated that the condensation of the mercury vapor in the upper part of tube F is practically completed IO to I5 cm above the mercury level in the cup A. This may be judged by noting the temperature of the a s c e n d i n g mercury column and also observing the formation of gas bubbles next the outer glass tube, which occurs only in the zone of condensation. With the apparatus in our laboratory the rate of distillation is about 750 g per hour. In starting the apparatus it is obviously necessary either to close the lower end of tube F, or to introduce enough pure mercury to establish a baro-
-
mercury remains in the reservoir at the bottom for this purpose.
Aw Improved
Mercury Still
313
From time to time it is necessary to remove the flask D in order to wash out the accumulated dirt with nitric acid. To do this it is only necessary to loosen the clamp referred to above, and slip the flask off the condenser tube F, using care to avoid breaking the tiny capillary above. As the still shown in the sketch is heated by gas a piece of nichrome gauze and an asbestos hood are used to insure symmetrical heating, but an electric heater as described by Hulettl can be equally well applied. With a dependable source of vacuum the apparatus is entirely 'automatic in operation. The supports for the cup A, reservoir B, and flask D and the clamp for holding the stopper in the bottom of cup A should be robust and designed with due regard to the weight of mercury to be supported. Bureau of Mines Experiment Station Berkeley, California 1
LOC.cit.
