ANALYTICAL EDITION
February, 1944
starting and holding contacts] and finally through the relay and magnetic valve coils. In operation, when the control manometer contact closes, the magnetic valves and relay close, injecting liquid air into the still head, and slowly lifting the control contact. Although the starting contact is opened as soon as the piston starts upward, the holding relay contact in parallel with it is still closed, so that current continues to flow through the valve coils until the manometer contact opens again and releases the valves and relay. After this, the electrical circuit cannot be vompleted again until the piston reaches the down position, even though the manometer contact might have closed, because the circuit cannot be completed until the starting contact closes. The drive piston is connected by R lever and linkage to the tube (wrying the movable contact in such a way that a large movement of the piston produces a small Figure 3. Contact Drive movement of the contact. A 200Cylinder gram lead weight is carried on the Diston rod to return the Diston t o the how, position, and a stih spring connected to the top of the contact tube keeps the system of linkages i n tension and thus eliminates lost motion. The stop on the contact drive piston (not shown) which restricts its upward motion is set t o allow the movable contact to be lifted about 3 mm. The control contact consists of a platinum tip soldered t o a copper lead wire and sealed with de Khotinsky cement into the end of a glass tube 2.0 mm. in diameter to give stiffness to the contact wire. A fixed platinum contact is sealed through the manometer tube below the surfare of the mercury to complete the circuit through the mercury. Regulation of the speed of rise and descent of the control contact is accomplished by adjusting the screw clamps in the line from the air supply to the magnetic valve and in the line from the valve to the contact drive cylinder. The method of delaying injection of liquid air until the moving manometer contact has reached the down position was adopted to improve the stability of the system and t o prevent chattering of the controls.
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I n all other respects, the operation of this column is almost the same as that described by Booth and Bosarth. This apparatus is intended primarily for purification of a single gas, so that an automatic stopcock is not used to control the take-off from the column because it is unnecessary, With a mixture of gases, the automatic stopcock described by Booth and Bozarth should be used. Exhaustion of a fraction from the column was indicated by a slight rise in the time-temperature curve of the still-head thermocouple, indicating the presence of a higher boiling fraction in the still head. With the automatic stopcock this would be indicated by increasingly infrequent operation of the stopcock. LITERATURE CITED (1) Booth,
H. S., and Bozarth, 9.R.,IND. ENQ.CHEW.,29, 470
(1937).
W e i g h i n g Funnels MILTON S. SCHECHTER AND H. L. HALLER U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine, Beltsville, Md.
TW O
types of easily constructed weighing funnels have been used in this laboratory t o facilitate the accurate weighing and quantitative transfer of solids or liquids t o volumet,ric flaska or other containers for analytical determinations. I n the accompanying sketch funnel A is suspended from the balance hook during the weighing by a wire attached to the Sshaped stem by twisting, the solid or liquid sample is placed in the shoulder, S, of the funnel, and the exact weight of sample desired is adjusted. The funnel is then placed as shown at the left, and the sample is nTashed quantitatively into the flask by a stream of solvent from a wash bottle, The intermittent siphoning action facilitates the washing of solids into the receptacle.
OPERATION
OPERATION OF THB COLUMN. First, the gas to be purified is
condensed in the still pot with liquid air and, after transfer, any noncondensable gases in the system are pumped out. The Dewar of liquid air is removed and the heating coil is put in place under the still pot after the coil has been immersed in liquid air for a moment to avoid sudden warming of the still pot. Then an empty silvered Dewar flask is placed mound the still ot and the current is turned on in the heating circuit, While t i e pressure in the column is rising, the injector Dewar flask is filled with liquid air nhich is injected into the still head manually before the operating pressure is reached. This cools the still head and column mole quickly. After the operating pressure has been reached, the contacting manometer holds it constant. When the column ha- been operating for a short time, the contact drive mechanism is adjusted to give the best compensation for the response lag of the system. The rate of ascent and de. scent of the drive piston and the rate of liquid air injection are adjusted so that the total movement of the contact point is between l and 2 mm. The pause between the return of the drive piston to the down position and the start of the next injection is about a half second in duration. If the injection period, which is normally about 2 or 3 seconds, becomes too long and the control contact rises t o o far, the rate of liquid air injection should be cut down. When properly adjusted, this control system can hold the pressure within 1 or 2 m a . of the control point, even during change of components in the still head, when the pressure is subject to wide fluctuations with the usual method of control. This column will handle a low-boiling gas like tetrafluoromethane easily, while previous types have been so unstable that operation with such a compound was very difficult.
Funnel B is made with a flat bottom for placement on the balance pan. After the sample is weighed, the funnel is tilted and the sample washed into a flask with an appropriate solvent. Either funnel may be fitted with ground-glass caps if desired. The diameter of A which the authors have used is 2.5 cm. and the over-all length is 7 cm. The diameter of B is 2 cm. and the over-all length is 6 cm. The shape and dimensions may be modified to meet the requirements of the analyses t o he performed.
