ANALYTICAL CHEMISTRY
1416
Table I.
Apparent Parathion C o n t e n t of Benzene
Grade of Benzene
Manufacturer
C.P. thiophene-free
1-lb. bottles 5-gal. drums 1-lb. bottles Purified (99-100%), 5-gal. drums Technical, 6-gal. drums 6
This sample was turbid.
1 2
3
2
1
y ’200 m l .
-4pparent Parathion after Distillation y/lOO ml.
11.0 93.0 6 9 68.8 13. 3a
0 0 0 0 2 ‘4a
ripparent Parathion
Actual color was very faint.
Readings were taken 10 minutes after addition of the coupling agent, as in routine parathion determinations. The color, however, continued to deepen and did not become stable for several hours. Because all samples of benzene developed some color, distillation was tried to remove the impurities. The results obtained after a simple distillation, in which 10% of the benzene was discarded as forerun and tailings, are given in Table I. The impurities were not appreciably volatile on the steam bath. Further tests indicated that the impurity is probably a nitro aromatic compound, possibly nitrobenzene formed during the acid wash to remove thiophene. For these tests the sample of
benzene from manufacturer 2 was chosen, as it had shown the highest percentage of impurity. When the reduction with zinc and hydrochloric acid was omitted, about 20% of the color waa developed, an indication that the impurity was primarily a nitro compound with small amounts of an amine. Kitrobenzene, m-nitrophenol, p-nitrotoluene, arid o-nitrotoluene were accordingly tested, and in each case a magenta color was developed which to the eye &-as identical with the true parathion color. Comparative transmittance-wave-length curves were then plotted for a sample of benzene containing the impurity and for pure nitrobenzene. These data were obtained with a Beckman spectrophotometer, and are shown in Figure 1. Absorption maxima in both cases fall about 555 mp, which corresponds to the parathion maximum. Although the results set forth in this paper are not important where small quantities of benzene are involved, the analysis of spray residues requires the use of 200 ml. or more of benzene, and in such cases the error introduced may be appreciable. It would therefore seem that C.P. benzene should be distilled and wherever possible a control analysis be used as recommended ( 1 ) . LITERATURE CITED
(1) Arerell, P. R., and Norris, 51. V., A S A L .CHEZI..20,753 (1948).
RECEIVED Decemher 9, 1948.
Device for Control of Still-Head Pressures during Isothermal Distillation HAROLD SIhIMOKS BOOTH AND ROGER L. JARRY Western Reserce Unicersity, Cleveland 6, Ohio
0 DECREASE pressure surges due to interniittant applicaTtion of cooling to the still head during distillation, Booth and McKabney ( 2 ) developed the “anticipator,” a complex mechanoelectrical device, which, as its name suggests, anticipates pressure changes and thereby administers the cooling in a manner that eliminates the surge characteristic. By means of this device distillations utilizing liquid nitrogen as the still-head coolant were accomplished with very small pressure variants. McNabney (5)has outlined a proposed modification to this anticipator for use especially with dry ice-acetone still-head cooling. B s the modification had not previously been tried out, it was made in this laboratory and tested. This article describes the construction of this modified anticipator and its operation. The modified anticipator is made entirely of soda-lime soft glass. All parts are fused together, and the entire piece is mounted on a wooden plank, after which it is fused to the stillhead line of the distillation apparatus. Tube A is made of 7-mm glass tubing, B and D of 20 mm. tubing, and C of 5-mm. tubing. Stopcock E, used as a throttling valve, is a 2-mm. bore capillary stopcock. Reservoir F is connected below D by means of an ordinary stopcock. Length J is just over 760 mm., while length T is slightly greater, so that the bottom of D is a t least 760 mm. from the center of B. *4t H a platinum wire is sealed in and the electrical contact between it and the adjustable probe, G , made of 26-gage nickel wire, is made through a 6-volt relay, to energize a small pump which supplies the coolant to the still head. In operation, the mercury is set by means of the leveling bulb F , so that the desired pressure head will result, the level in B being near the center. As shown in Figure 1, the mercury level in the three columns is set for atmospheric pressure distillation; for distillation a t pressures less than atmospheric the level in the three columns has to he adjusted so that when a vacuum is attained, the level in A will he in the middle of B and the difference in height between that level and those in C and D gives the pressure a t which distillation is to be carried out. As the still-head pressure increases the mercury will he forced down in A and up in C and D,contact will he made with the adjustable probe, and the still-head cooling agent will be injected into the still head. As the still head cools, the mercury will rise in A , owing to dropping pres-
sure, and fall in C and D,thereby breaking the 6-volt circuit and stopping the cooling pump. At this equilibrium point, D which has fallen less rapidly than C, forces the mercury in that column up and thereby starts it slightly in advance of the building pressure in A . By this means future contacts will be in advance of the full pressure-Le., anticipate full pressure. In a similar manner D,rising more slowly than C, xi11 tend to deplete that column and make it fall sooner, anticipating the pressure drop. The rate a t which the mercury flows into D ill necessarily control the sensi-
QWILlY
Figure 1. Modified Anticipator
V O L U M E 2 1 , NO. 11, NOVEMBER 1 9 4 9 tivity of the device; this rate is controlled by manual setting of stopcock E.
1417 gave control within + 2 mm., and the resulting fractions proved pure.
This type of regulator has the advantage of easy operation with LITERATURE CITED
a positive force opposing the pressure of the still head. Elimination of the automatic stopcock of systems such as those described by Booth and Bozarth ( 1 ) greatly simplifies the adjustments necessary during distillation. .4lthough the greatest accuracy of pressure control with this type of regulator is in the region around atmospheric pressures, distillations a t as low as 250 mm. are accomplished easily. The control a t atmospheric pressure for distillations lasting as long as 8 hours, for 50 to 75 ml. of sample,
(1)
Booth, H. S., and Bozarth, A. R., I n d . Eng. Chem., 29, 470
(2)
Booth, H. S., and McNabney, R., IND.ENG.CHEM.,-4x.4~.ED.,
(1937); J . A m . Chem. SOC.,61, 2927 (1939). 16,131 (1944). (3) NcNabney, R., private communication. RECEIVED November 2 4 , 1948. Based upon work done i n connection with a research project sponsored b y the Office of Naval Research.
An Improved Tensimeter-Still WILLIAM P. RATCHFORD AND C. E. REHBEKG Eastern Regional Research Laboratory, Philadelphia 18, Pa. STUDIES of high-boiling derivatives of lactic acid, need INarose for a device which would serve as a simple low-pressure
still and also make possible accurate vapor pressure-i.e., boiling point-measurements in the low-pressure region-via., 0.02 to 10 mm. of mercury,
At pressures below 0.01 to 0.03 mm., noncondensahle gases diffuse against the distillate vapor stream and vitiate boiling point measurements ( 7 ) . Hence it is futile to attempt such measurements a t extremely low pressures, even though some substances can be distilled a t such pressures in the tensimeter-still. If a t approximately 0.03 mm. the boiling point of the distilland is too high to permit distillation without decomposition, it is advisable to use a molecular still, where permanent gases are pumped away efficiently and distillate molecules have a short path to the condenser. The tensimeter-still is similar in appearance to commercial equipment that has been described recently (2); the latter, however, is recommended for use in the range 0.001 to 0.1 mm., and is stated to have a separation efficiency of less than one theoretical plate. The present still gains its usefulness from vigorous agitation of the distilland, obtained by rocking the entire still to and fro. The agitation minimizes or eliminates superheating and inhomogeneities in the distilland, and thus facilitates accurate boiling point measurements and equilibrium distillation. The separation efficiency is at least one theoretical plate. APPARATUS AND METHOD
Figure 1. Tensimeter-Still A . Boiler, 500- or 1000-ml. flask with
thermometer side arm 45/50 ground-glass joint Thermometers 12-mm. connection to vacuum gage 12-mm. connection to trap and pump F. Alembic, formed from 40-mm. tubing 6. Connection to pump
B. C. D. E.
Several types of stills m r e tried before the design described in this paper was evolved. The improved still has been in use for more than 2 years and has proved reliable and simple to operate over the desired range. It has been used to obtain vapor pressure information on a wide variety of lactic acid derivatives (10). Various means have been used to overcome difficulties in obtaining accurate boiling point measurements a t low pressures. The “automanometer still” of Hickman and Weyerts (8) exposes the distilland to a relatively high pressure drop between still pot and condenser, for it is precisely by measuring the pressure drop across one or more orifices that this still gains its usefulness. It is operable doivn to a condenser pressure of perhaps 0.5 mm. of mercury. On the other hand, the tensimeter-hypsometer of Hickman, Hecker, and Embree ( 7 ) offers negligible pressure drop but operates on total reflux, so that distillation is impossible. This device, moreover, in the present authors’ hands failed to operate satisfactorily above 2 mm., because of excessive bumping. It is stated to be operable down to 0.03 mm. of mercury.
The tensimeter-still (Figure 1) consists of a 500- or 1000-ml. round-bottomed short-necked flask fitted with a thermometer side arm and a 45/50 ground-glass joint. The joint is attached to a wide upright condenser with an alembic take-off located so that the vapor path is as short as possible. The condenser bears a thermometer and connections for a vacuum gage and pump. T o maintain vapor-liquid equilibrium and avoid bumping, the still is suspended from the side tubes near the top of the condenser and rocked to and fro by a crank powered by a stirrer motor. The crank is attached a t the ground joint after the flask is connected and the joint is well wrapped with asbestos tape. The joint is lubricated with silicone-type grease to eliminate the sticking sometimes encountered with ordinary grease. I n operation as a tensimeter, the rocker is started, and heat is applied with a mantle heater, while vacuum is maintained. When boiling begins, the connection to the pump is throttled down, and the pressure is adjusted to the desired value by means of a slow leak. Heating is continued until the sample is distilling gently. The heat input must be adjusted so that the difference between liquid and vapor temperatures is a minimum, indicating minimum superheating and pressure drop between flask and condenser. At several millimeters this difference is usually 1’ C. or less, but a t low pressures may become several degrees. To assist in condensing the vapors, a strip of moist cloth is sometimes wrapped around the condenser. The stopcock makes it possible to operate on total reflux (usually necessary on small samples). The boiling point is taken as the vapor temperature a t the prevailing pressure. T o make certain that the system is in equilibrium, the boiling is continued until the vapor temperature remains constant. The pressure is then adjusted to another value, and the process repeated. It is possible to cover the range 0.03 to 10 mm. in a few hours. The data thus obtained are adequate for the preparation of the usual log P US. 1/T charts.
