Determination of Boiling Range of Methyl Bromide DKIGHT W I L L I i M S AlVD C . C . MEEkEH 7echnical D e p a r t m e n t , U'estraro Chemical Corporation, South Charleston, lc'. V a . The boiling range of niethj 1 bromide is determinecl b? distillation u n d e r a reflux condenser; the distillation flasl. and receker are enclosed in a dr? ice hrrr to permit measurement of the \apor temperature. The heater consists of a coil of resistance wire wound around a €'>rex finger aealed to the bottom of the flask. RJ use of a thermometer di\ided into 0.1" C. di\ isions, distillation temperatiires ma) he measured with a precision of =+=0.Oq5'C. during the first 9576 of the clistillation. during which time snperheating is easentiall? ahsent. Distillation temperatures ma? be quantitati\el? cwrrelated with the concentration of hnow n impurities that do not form r.oiictant-hoilitig impiirities.
T
HE boiling range is coiiimonly used as an inde\ of thc punt> of liquids. The determination of the boiling iangr of methyl bromide offers unique problems as a result of the fact that Its boiling point is belolT room temperature. When conventional distillation equipment is used, a thermometer in the vapor indicates a temperature clow to that of the lahoratorv One mc,thod
i V
___c_ CORK STOPPERS GLASS ROD TO STRENGTH EN CONDENSER BRACKET INSULATION FIEERGLAS THERMOMETER
iiicawring t h r , dirjtillatioii ~ t ~ n i p ~ i ' a rin u r cappiicatioiis ui this sort is to imnit~rsc:the thernioiiit~tei,bulh in the liquid. \Then this procedure was used, meth;vl bromide invariably superheated, the amount of suporheating being as much as 6 O C. Another method involves carrying out the distillation in a Hask, the neck of which is jacketed. -4refrigerant is circulated through or simp17 introduced into the jacket and the vapor temperature is measured. By this procedure wide and erratic boiling ranges were obtained. The method that was finally chosen involved distillation in 8 refrigerated cahinet which rrprotluced on a small scale the conditions that obtain during tlir distillation of higher boiling liquids. An adaptation of the boiling-point Hask described b y Quiggle, Tongberg, and Fenskc ( 2 ) provided a convenient and efficient, hmter for carrying out the distillation inside the cabinet. Sormally the methyl bromide uscd for determination of the boiling range is measured at a temperature much below its boiling point and thc temperature of the sample before distillation may be substantially different from that of the distillate. Because any rhaiige in temperature, Kill result in a cbhange of volume, and tho ttwpera.turr' of the distillatca, and hence its volume, may be g e a t e r than that of the distilland. it is inipractical t o take a 100nil. samplr for (listillation. Accordingly, a %ml. sample is taken pcr ccbnt ( i f tlistillatr, calculated fro]); 01
.-\lthouyh this method has hem appliul only to i h e determination of the hoiling range of nieth! 1 bromide, it should be apdicahle to any liquid that has a hoiling point bctn-ec'n room tc.ni]>i"'atiirc' anti tlit, suhliiiiatiori poiiit of d ~ , icc. y \IilERIALS
(-'oninitwial nit.thy1 broinidt~was used ~iiiierjaotherwise sprcificmd. Thc methyl bromide riactl for testing the feasibility of tlw nict,hod and apparatuh was pu~,ifiedhy scrubbing with sulfuric acid, followed by rcctificut ion. Tho scrubbing \vas carried out by passing vaporizrd methyl hromidr through a 180-em. (&foot) towvrr packed with 6-nim. glass ~,ings and filled with Y5V0 sulfuric acid, Boiling range data indicatrd thal thip meshod of purification renioved most of tht7 high boiling inipurities but was iiieiPect,ive in removing low boiling inipuritieh and t,hat the subsequent rectification t'hrough a 6-foot column packed with 0.125-inch 10.3em.) helixes removed t entiall>- all t h r low boiling impurities. The rectified product w uwtl in thr. t w t s tk-crihcd helon- n-hvre a purifitd prodiict i c intlicatcd. ~ P P W 4n;s
The cliqtillation apparatus
t-
31 CM.
>
i.q
illustratrd in Figure 1.
The distillation flask i. matk bj. inodifying a 100-ml. Eiigler dist,illation Haak. The condwser-flask assembly is supported by H suitable clamp attached to tht. cirndenser. .4 tight fit is made a t the standard-t,aper joint at the t o p of the flask by means of two small springs. The dry ictb h o s is divided inti) two conipartments (froiit and rearj with a substantial nire partition between. One conipart-
734
ANALYTICAL CHEMISTRY
propriat,eholes are drilled. One large hole over the receiver is covMeasure the temperature of the met,hyl bromide sample t o ered with an auxiliary piece of Plexiglas, so that access is promake sure that it is below -50" C. When this condition is met, vided to the stopcock on t'he take-off tube of the distillat,ion flask. measure 95 ml. of sample into t,he precooled cylinder, transfer to The receiver consists of a 100-ml. graduated cylinder, provided the distillation flask through the top of the condenser, and imwith a side arm t,hrough which the distillat,e enters. A 29/42 mediately replace t'he cork in the condenser. Plug the flask heatstandard-taper joint is sealed to the top of tmhecylinder and is ing coil into a Variac and adjust the voltage to give a distillation used t o at,tach a narrow Pyrex tube which extends through the rate of 5 to 6 nil. pcr minute (about 60 volts). Carefully loww cover of t,he cabinet. *\ glass float rides on the surface of the the thermometer through op of the condenser until the botmcthyl bromide and is provided with a long stem that extends tom of t,he mercury capilla opposite the bottom of the take-off into this tube. The tube is calibrated to permit easy reading of tube. Secure it in t,his position by wedging the corks at the top of the volume of distillate. This calibration is accomplished by inthe condenser against, the Fiberglas sleeving to which the thertroducing methyl bromide into the cylinder, noting the volumc, mometer is at,t,ached. then putting the float and tube in place and marking thr height , Observe the distillation tt'mperature at 1-minute intervals. on the stem of the float on the t,ube. Three or four points are obWhen two sucoessive readings agree within 0.01' C., record the tained by direct, measurement and other points by interpolation. temperature as the st,arting teniperat,ure. S o w open the stopDuring the expei~iniental work volume measurements werc, cock in the take-off tube and record the time. Read the dist,illamatle by means of a mirror located at the level of t,he liquid in thc' lion tempcxraturr when the volume reaches 80, 85, and 88 ml. and receiver and t,ilted 45' from vertical. This permitted measureevery milliliter thereafter until t,he dry point is reached. ment, of t,he volume at any point during the distillation by movThe t,emperature readings must be taken every 10 t o 12 seconds ing the mirror upward as the distillation progressed but is l t ~ during the latter part, of t,he distillation. Obviously, t,lie analyst convenient for routine use than the float. must be extremely alert in order t.o make readings with this The heating coil consists of 9 feet of Yo. 30 varnished chrome1 rapidity. If one reading is missed, proceed to the next without' deC !!-ire, resistance 6.9 ohms per foot. The \Tire is closely wound lay. The dry point cannot be determined in the usual manner around a finger which is sealed t'o t'he bott'om of the distillation because of lack of visibility. Aft,er the liquid level disappears inflask. The insulation is stribped from the ends of the wire and side the finger, carefully watch the intermittent flow of liquid they are soldered to short lengths of No. 20 copper wire which arc through the take-off tube. When this flow becomes inapprecitwisted around two glass knobs, one sealed to t'he bottom of the able, the dry point has been reached. Immediately turn off the finger and the ot,her to the bott,om side of the bulb. These heavirr current to prevent burning out the heating coil. Record the volcopper wires serve as convenient terminals for the ends of the ume of distillate, the dist,illation temperature, and the time a t windings and for the permanent electrical connection which plugs this point. Immediately remove the thermometer to prevent into a Variac. Several glass prongs are sealed to the bottom of the hreakage due to freezing t,hc mercury. Restopper the condenser. finger and three or four turns of the resistance wire are held in When the flask is removed, stopper the bottom of t'he condenser place by these prongs to ensure adequate heating at, the t,ip of and replace the Plexiglas cover on the dry ice box. the finger and complete volatilization of the sample. The heating Divide the t,ot,alvolume of dis'illate by the time to obtain the coil is covered with several turns of asbestos wicking which serves rate of distillation, which should be 5 to 6 ml. per minute. Read as heat insulation. the barometer and the temperature alongside during the course of A t,hermometer guide placed inside the neck of the flask above the distillation. Correct the pressure t o 0" C. as follows: the t,ake-off tube keeps the thermomet.er in the center of t8heneck. Po = Pt - ( t x 0.122) This guide consists of a short length of 9-mm. Pyrex tubing, held in place in the center of the neck by three glass rods inclined downwhere Poand Pt are the barometric pressure in millimeters of merward and sealed to the flask. The diameter of the inside of the cury at 0" and t o C., respectively, and t i s the temperature of the collecting ring must be approximately t,he same as that of the inbarometer. Correct t)he distillation temperature t80 760-mm. side of the neck of the flask. pressure as follows: A Precision Thermometer and Instrument Co. thermometer (Catalog Ro. 72-B, range - 10" t o +50° C. ih 0.1' C. intervals) is T ~ B=OTabs. d T / d P (760 - PO) used. It should be calibrated t o the nearest 0.01" C. a t the ice where 7'760 and Tobe.are the corrected and observed distillation point by immersion in crushed ice. The thermometer is suptemperaturcs, respectively. val'ue of 0.035, calculated from t'he ported by means of a 24-inch length of '/*-inch Fiberglas sleeving data of Egan and Kemp (I), is used for d T l d P for normal variathreaded through a hole in a not,ched cork that fits the top of the t,ions in atmospheric pressure. Add the thermomet'er ice-point condenser. A small cork is fitted into the hole in the larger cork correction, if any. Calculate the per cent of distillate collected a t and is used to grip t,he sleeving and support the thermometer at each oint at which the temperature is read by dividing t'his volthe proper level. ume gy the total volume of distillate collected and multiplying A thermometer reader (Precision Scientific Co., Cat'alog S o . this ratio by 100. I t is convenient to compare lots of methyl 8968) is used for accurate temperature readings. I t is held in bromide by calculating the kmperature range over which t'he place on the neck of the distilling flask by several small springs, first 9570 of the distillate is collected. The temperature of the obtainable from Ace Glass, Inc., Vineland, N . J. 95% point is obtained by plotting the per cent dist'illate against A 100-ml. Pyrex cylinder is used for measuring the sample. the temperature and reading the 9 5 5 point from the curve. An accessory dry ice box is required for cooling the graduate and samples. -411ground-glass joints and stopcocks are lubricated with DowCorning silicone stopcock lubricant. Table I. Comparison of Flasks
+
PRECAUTIONS
Methyl bromide vapors are poisonous, and great care should be exercised to avoid breathing them. All boiling ranges should be carried out in a well-ventilated hood, and the analyst should be provided with a canister-type respirator which should be used whenever methyl bromide is transferred or there is any possibility of high concentrations of mcthyl bromide in the atmosphere.
Commercial CHaBr Simplified flask 6 C / Ci flasks 3.60 Start 3.70 3.62 3.71 0 3.66 3.70 50 3.74 90 3.75 3.84 3.81 95 3.90 97 3.92 4.88 Dry 4.61 a hverage of 2 boiling range';. b .\rerage of 4 boiling ranges. .Iverage of 3 boiling ranges. Distillate,
QTF
Purified CHgBr Simplified QTF flaska flask 3.66 3.61 3.66 3.61 3.65 3.62 3.63 3.64 3.65 3.63 3.66 3.64 3.84 3.83
PROCEDURE
Place the sample and the 100-ml. graduate, closed with a cork stopper, in a n accessory dry ice box t o cool. Assemble the apparatus as shown in Figure 1, but do not, insert thermometer, and close the top of t h e condenser tube with a cork stopper to prevent condensation of mi,isture from the atmosphere. Fill the dry ice compartment with dry ice. Fill the condenser reservoir wit,hsmall pieces of dry ice, then cautiously pour in trichloroethylene t o a point about 2.5 cm. (1 inch) from the top. The trichloroethylene evaporates and must be replenished from time to time. Add dry ice as required during the cmrse of the distillation.
EXPERIMENTAL
To adapt the Quiggle-Tongberg-Fenske flask for routine boiling range determinations the inner tubes that served to pump the liquid-vapor mixture over the temperature-measuring element weie omitted. The heating finger was shortened as much as possible and the stopcock at the bottom of the heating finger was omitted. It was considered desirable to measure the temperature of the vapor, but low and erratic temperature measure-
V O L U M E 20, NO. 8, A U G U S T 1 9 4 8 Table IT.
Effect of Thermometer Position
Distillate,
% St?rt 25 50 80 90 95 96 97 98 100
135
Position 1 4.39 4.39 4 31 4.26 4.04 4.09 4.09 4.09 4.19 4.3 8.7
Temperature, a C. Position 2 Position 3 3.69 3.72 3 71 3.71 3.74 3.72 3 74 3.78 3.83 3 80 3.86 3.89 3.93 3 92 3.94 3.94 3.96 3 97 4.1 4.1 6.1 4.7
Position 4 3.64 3.66 3.72 3.74 3.78 3.84 3.92 3.95 4.02 4.2 5.3
ments vere obtained during the initial tests due to cold condensate running down the thermometer. This was eliminated by means of the thermometer guide, which centers the thermometer. in the neck of the flask, and by removing the bulge from the outer member of the standard-taper joint of the top of the flask. A comparison of the performance of the Quiggle-TongbergFenske flask with that of the simplified flask when both commercial and rectified methyl bromide are used is shown in Table I. In general, slightly lower temperatures are obtained with the simplified flask. This flask would be expected to have a tendency toward fractionation of a sample and this may account for part of the temperature difference, especially a t the beginning of the distillation, when commercial methyl bromide is used. The data for purified methyl bromide in the Quiggle-TongbergFenske flask indicate that no superheating occurs during 97V0 of the distillation. As a matter of fact the temperature falls appreciably near the end of the distillation. This effect is real, as it occurred consistently, but no explanation is known. The dry point temperatures are, of course, of only qualitative value because of lack of visibility. The data for purified methyl bromide in the simplified flask indicate only a slight tendency of 0.02" C. superheating during the first 95% of the distillation. Because the Quiggle-Tongberg-Fenske flask is a boiling point apparatus measuring the temperature of the liquid and vapor in equilibrium, it should give reliable boiling point data. The data in Table I show t h a t the boiling point of methyl bromide, when this flask is used, is 3.65' C. if the 50% distilled temperature is taken. When the simplified flask is used, the boiling point is 3.62' C. Table I11 indicates that this value is precise to 10.04 "C. The boiling point of methyl bromide is variously reported in the literature. The most reliable value is probably that of Egan and Kemp (I), 3.56" =t0.05" C. Although the authors' value is slightly higher than theirs, the t n o ranges overlap, indicating satisfactory agreement. The effect of using the thermometer a t the following positions was determined: 1. Thermometer bulb inside heating finger. 2. Thermometer bulb in liquid just above heating finger. 3. Thermometer bulb just above the surface of liquid a t start of distillation (6 cm. below normal position). 4. Thermometer bulb just below take-off tube (normal position). The first position gave definite superheating throughout the distillation with the minimum superheating after about 90% of the sample had distilled. While the other three positions gave somewhat different temperature indications, the differences between them are small and it appears that the temperature could be measured a t any one of these positions. The highest position was adopted because it gave the greatest temperature spread over the significant portion of the range (0 to 95%), which seemed advantageous from the standpoint of estimating purity (see Table 11). Distillation rates between 3.8 and 8.4 ml. per minute gave distillation temperatures that agreed well within the precision of the method. However, it is considered desirable to maintain a uniform rate even though no effect of variation in rate could be observed. Because of the low lag of the heater it is possible to
control the rate precisely by setting the Variac a t any predetermined point. A rate of 5 to 6 ml. per minute was chosen arbitrarily as being sufficiently rapid to complete the distillation in a reasonable length of time. The precision was determined by distilling ten portions of the same lot of methyl bromide in succession. The limit of uncertainty (LU,) of several points between the start and the 91% point was =t0.04' C. or better, but increased sharplv a t 97%. The poorest precision (*0.57" C.) was obtained a t 99%. The rapid rise in the temperature during the last 4% of the distillation, due to superheating as well as to the presence of small amounts of high boiling impurities and the effect of slight errors in the measurement of the volume, are sufficient to account for poor precision in this region. Representative precision data are shown in Table 111. One of the impurities to be expected in methyl bromide is dimethyl ether. A plot of distillation temperatures against the composition of methyl bromide- dimethyl ether mixtures was shown to be linear up to a concentration of 0.84% by weight of dimethyl ether. The starting temperature !vas loivered 1.09' C. per weight per cent of dimethyl ether and the 5-ml. temperature was lowered 0.81 'C. per per cent. Judged by the precision of the temperature measurements, the precision of measuring the dimethyl ether content at either the first drop or the 5-mL point is 10.037%. The following equation applies to the first drop temperature. C = 0.02 (3.62 - t ) where C is the concentration of dimethyl ether in weight per cent and t is the starting temperature in degrees Centigrade. ~
~~
Table 111. Precision of Boiling Range Test NO
E 0 10 Average L U I o f method
Start 3.64 3.66 3.63 3.65 3.65 3.65 3.64 3.63 3 62 3.62 3.64 *0.04
91% 3.85 3,85 3.83 3.86 3.87 3.86 3.88 3.87 3.86 3.85 3.86
96% 3.93 3.94 3.94 3.97 3 ,95 3.95 3.97 3.98 3.95 3.95 3.95
10.04 &0.05
97% 4 00 3 99 3 97 4 02 3 97 4 00 4 06 4 09 3 99 4 02 4.01 10.12
99% 4.23 4.07 4.23 4.55 4 27 4.70 4.53 4.41 4.33 4.42 4.37 10.57
100% 5.63 5.33 5 33 5.33 5.32 5.53 5.41 5.31 5.32 5.42
5.39 &0.33
Methanol might also be expected in methyl bromide. The addition of 0.55% by weight of methanol lowered the entire boiling range by approximately 0.1 ' C. Larger amounts, 0.73 and 0.97y0, tended to raise the distillation temperature, although the starting temperature of even these mixtures was below that of the original. These data show that methanol forms a constant boiling mixture with methyl bromide. The exact composition of the constant boiling mixture cannot be determined from these data but it must be near 0.55% by n-eight of methanol, as this mixture gave uniform Ionwing of the distillation temperatures throughout the entire range. These data indicate that the boiling range is of little value in detecting the presence of small amounts of methanol in methyl bromide. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of George S. Haines, who made many helpful suggestions during the course of the work, and of R. A. Remke and members of the Control Laboratory staff for their contributions toward the design of the apparatus. LITERATURE C I T E D
(1) Egan, C. T., and Kemp, J. D., J . Am. Chem. SOC., 60,2097 (1935). (2) Quiggle, D.,Tongberg, C. O., and Fenske, M. R., IND. EICI. CHEM.,ANAL.ED., 6,466-5 (1934). RECEIVED February 11, 1946.
