Research Laboratory of Organic Chemistry, Massachusetts Institute of Technology, Cambridge, Mass

U

ing vacuum distillation. This condition has been largely due to the fact that no simple satisfactory device was available for general laboratory use. Under these circum­

stances there has been a tendency to avoid distillation under reduced pressure wherever possible or to adopt temporary hand-regulated makeshifts where its use became imperative.

Fi g u r e 1 . Pr e s s u r e- Re g u l a t i n g Un i t

Having occasion to prepare numerous compounds requir­

ing vacuum distillation and finding no suitable control de­

vices at hand, the authors have constructed a simple portable regulator capable of maintaining to =*=0.15 mm. any| pres­

sure within the range of 2 to 30 mm., which has been’ used in these laboratories with great convenience and satisfaction.

Before designing the apparatus here described a survey of the literature since 1840 showed that more than thirty published descriptions could be classified as follows:

A. The pump operates continuously.

1. A valve is interposed between the pump and thé system.

Mercury valve (5, 16, SI).

Mechanical valve (35).

2. A valve bleeds in air at atmospheric pressure.

Mercury valve (4, S, 15, 19, 35, 33, 34, 37).

Mechanical valve (1, S, 6, 9, 11, IS, 17, 20, 21, 23, 24, 26, 27, 28, 29, 32, 36, 40).

B. The pump operates intermittently (7, 10, 13, 38, 39).

In the present device (Figures 1 and

2

Once the desired pressure has been attained, the pump is switched off by the action of a thermionic relay system

(Figures

6

ties and the nature of the pump motor. That is, with an alternating current pump motor the relay system also operates on the same alternating current line; with a direct current, motor the relay system operates on the same direct current, line. Thus, for a given motor the entire apparatus is con­

trolled by a single switch, and in actual use is simply brought up to ahd attached to the distillation system, plugged in to the power line, and allowed to maintain whatever pressure is desired according to the setting.

Associated with the pump and its thermionic control are:

the usual necessities of vacuum distillation, such as an ab­

sorption train (Figures 2 and 5) to protect the pump from

Fi g u r e 2 . Pr e s s u r e- Re g u l a t i n g Un i t, Sh o w i n g Ab s o r p t i o n Tr a i n

a n d Re s e r v o i r

vapors, a manometer for pressure measurement, and a mano- stat to effect operation of the thermionic relay. The entire ensemble is mounted on a light four-wheeled carriage, afford­

ing ready mobility from one place to another.

Th e Ma n o s t a t

This is essentially a U-manometer capable of being fastened in an inclined position. A fixed tungsten electrode contacts with the mercury, completing the relay circuit which con­

trols the pump motor. Thus by tilting the m anostat th e 144

March 15, 1933 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 145

Fi g u r e 4 . Ma n o m e t e r

internal diameter so as to be suitable for the measurement of/pressures between 0 and 30 mm. of mercury. As shown injFigure 4, the open end terminates in a tee, one side of which connects with the manostat and the other through a short soda lim e-activated carbon absorption tube to a three-way stopcock. Constriction of the manometer tub­

ing is unnecessary, but the use of a lubricant, such as butyl ^{Fi g u r e 6 . Re l a y A}s s e m b l y

A small U-manometer 13 cm. high, with arms 4 cm. be­

tween centers, was constructed of Pyrex tubing of 7 mm.

(30). The right-hand ring is split and fastened to the slid­

ing scale, the lower edge being set at the zero index. The left-hand ring is slipped over the closed arm and moves freely, with a friction spring holding it against the glass. To facili­

tate using the instrument at night, the manometer is illumi­

nated from behind by a 25-watt lamp mounted on the back of the panel and shining through a double ground-glass window which protects from any heat. Both the manome­

ter and manostat are protected from sudden access of air by means of a

10

a greater or lesser force, or vacuum, to bring it to that level at which it will make contact with the fixed electrode.

The manostat is of Pyrex glass, 14 cm. high with arms 2.5 cm. between centers and 7 mm. internal diameter. One

t u n g s t e n contact is sealed through the concave side of the U-bend a n d t h e o t h e r through the top of the arm w h i c h c o n n e c t s with the system. The m a n o s t a t is held near its upper end in a rotating clamp provided with a graduated r a d i o dial for observation of the s e t t i n g and a thumbscrew for clamp­

in g in a f i x e d p o s i t i o n . Since the manostat is inde­

p e n d e n t of v a r i a t i o n s of barometric pressure, a small graph relating the dial setting and pressure may be attached permanently to the panel for

Fi g u r e 3. Ma n o s t a t convenient reference.

Constriction of the mano­

stat tubing is unnecessary, for in operation the apparatus panel is clamped against the laboratory desk and oscillation of mercury is negligible. Lubrication of the mercury in the open arm is infeasible, since the organic lubricant is slowly decom­

posed by the small thermionic current. The slight lag in­

duced by the cohesion of the mercury and the tungsten is not serious, but may be avoided by the use of iron or nickel.

Th e Ma n o m e t e r

benzyl phthalate or di-n-butyl phthalate saturated with diphenyl mercury or di-p-tolyl mercury (18), is recommended for increased sensitivity. A reading scale for the manometer is mounted on a square brass tube sliding on a square brass rod with a phosphor bronze friction spring. Parallax errors are avoided by observing the meniscus across brass rings blackened with ammonium sulfide and warmed over a flame

Fi g u r e 5 . Ab s o r p t i o n Tr a i n a n d Re s e r v o i r

Vol. 5, No. for the removal of all condensable vapors from the gas stream.

After considerable experimentation with various types and lengths of absorbents, the best combination was found to be low-freezing heat-transfer liquid. For the latter the authors prefer the use of a mixture of equal weights of chloroform and carbon tetrachloride (22), which avoids all fire hazard and upon whose surface the solid carbon dioxide floats. To facilitate cleaning out condensate from the glass bulb trap between runs without dismantling the train, a glass tube joined to the bottom of the trap is bent so as to extend out of the top of the bath as indicated. B y blowing back through the usual connection to the distillation system the trap may readily be emptied.

Since the volume of the distillation system with which this regulator is ordinarily employed is relatively small as com­

pared with the capacity of the pump it has been found de­

sirable to include a reservoir in the system. For this pur­

pose an ordinary 3-liter ring-neck Pyrex flask has served conveniently.

Th e Re l a y

The relay assembly shown in Figure

6

Application of a negative grid bias to the normally un­ parts, including vacuum tube, condenser, relays, incandes­

Pu m p a n d Ca r r i a g e

The manostat, manometer, absorption train, reservoir, stopcock, and thermionic relay are all mounted upon a vertical wood panel as shown in Figures 1 and

2

trifuge Corporation. The pump itself is mounted upon a wooden base, which rests in turn upon vibration insulating cushions of s p o n g e r u b b e r glued to the floating

Grateful acknowledgment is made for assistance and ideas to the members of the Research Laboratory of Organic Chem­

istry, especially to G. S. Crandall, E. A . Averill, and A. A.