Modified Menzies-Wright Ebulliometer for Molecular Weight Determinations R. E. KITSON, A. N. OEMLER, AND JOHN MITCHELL, J R . Ammonia Department, E. I . du Pont de Nemours & Company, Inc., Wilmington, Del. The basic ebulliometric technique of Menzies and Wright for the determination of molecular weights has been modified to permit more rapid, precise determinations on a wide variety of materials. The use of differential thermometers filled with liquids other than water at temperatures above their normal boiling points permits greater precision over a wider range of solvents than is possible with the water thermometer. A modified ebulliometer is described which incorporates the improvements of previous workers in the field, together with a newly designed boiler and pump and which permits determination, precise to about l%, in about 1 hour per sample.
T
cal reaction are usually of such magnitude as to be apparent to the operator or, in case of doubt, a rerun with radical changes in solvent and standard material quickly indicates whether reaction has taken place.
HE method of Menzies and Wright (8) for the determination of molecular weight by the increase in the boiling point of a liquid is a well established technique and has been used, with some modifications, in this laboratory for a number of years. In the course of this work, and particularly after the development of the organic-filled differential thermometers described in the preceding paper ( 6 ) ,the ebulliometer has been redesigned in order to increase the speed, precision, and accuracy of the determination. This paper describes the ebulliometer as finally evolved, and presents typical data obtained with it. The ebulliometric technique is thoroughly established, but the method has involved a rather large number of variables, the exact control or measurement of which was necessary to provide accurate results. By measuring the instantaneous difference in the boiling point of the pure solvent and the boiling solution, the differential thermometer of Menzies (7) removed, among other things, the effect of barometric pressure changes in the course of a determination. Subsequent investigators (1, 2, Q, 9) have not attempted to change the procedure of Menzies and Wright (8) greatly. All (except Barr and Anhorn, 1)used the water thermometer, converted the millimeter reading thus obtained to degrees Centigrade by Menzies' chart ( 7 ) , measured the volume of solution a t the boiling temperature, and employed carefully purified solvents. illthough these investigators claimed a general precision of 1% for the procedure, the authors rarely have been able to achieve such reproducibility using the various published modifications. Examination of the details of the method suggested that the addition of a known standard immediately before and after the addition of each portion of unknown might permit substantial improvements in the procedure; the effects of changes in the purity of the solvent, volume of the boiling liquid, boiling rate, and other variables would be minimized, because the unknown would be measured in terms of a known standard. (This general technique has been proposed by Swietoslawski, 10). The molecular weight mav be calculated by the relatively simple equation
'
EXPERIMENTAL WORK
Apparatus and Chemicals. The ebulliometer in which most of the data given below were obtained is indicated in Figure 1. Two instruments were actually used. Although minor constructional differences existed between them, the accuracy and precision of the results were substantially identical.
Y -Y'
@ x-x
7
where m, and m uare the molecular weights of unknown and standards, respectively, At, and Atu are the observed temperature change of known and unknown, and w e and uu are the weight of standard and unknown added. The main sources of error remaining in such a procedure, in addition to the four variables in the above equation, are those due to the change in the boiling rate and volume during a given series of determinations, and to interreactions among solvent, standard, and unknown. The boiling rate and volume are readily controlled. Errors due to chemi-
0
3
6
9
12 15 cm,
w
O i2345cm. LLLuJ scale
Scale
Figure 1. Modified Menzies-Wright Ebulliometer 404
I
V O L U M E 21. NO. 3, M A R C H 1 9 4 9
405
The molecular weight standard most frequently employed was b e n d (Eastman Kodilk Co. No. 35, moleoular weight 210.22).
(Sake care obviously must be used a t this point; new batches of standard should be cheeked against old before use.) The material wm pelleted into 100-mg. pills with a small press. Other sstisfactory standards include hexamethylbenzene, hexaethylbenzeno, naphthalene, and anthracene. Solid samples were pilled before addition to the ebulliometer. Liquids were added from a weight buret.
Figure 2.
'
Ebulliometer and Auxiliary Components Assembled
The ebulliometer consists of a 30-ml. boilerin which an electrical heater, made of 225 cm. (7.5 feet, 50 ohms) of No. 30 Nichrome wire, is suspended wound on a glass helix, and spotwelded t o tungsten wire, which is sealed through the walls of the boiler. A second air of tungsten leads, connected to the first by flexible nickel l e d , provides the external contacts for the heater. At the top of the boiler, four integral pumps are so installed that all the vapor produced must pass through them, forcing a steady mixture of vapor and solution over the lower bulb of the thermometer. The pumps are constructed of glass tubing 5 mm. in outside diameter, each with uncanstrieted tip about 3 mm. long extending into the thermometer chamber. Condensed solvent returns to the boiler through a glass tube running through and under the heater. A rapid and efficient circulatory system is thus provided, whioh permits the instrument to reach equilibrium quiokly after additions of solute. The differential thermometer is suspended in the main chamber of the cbulliometer, so that the lower bulb is just below the level of the pump.jets. The thermometer is surrounded by an inner mantle, which serves to roteet it from the returning condensate, and from portions of sokte which otherwise might lodge on the stem. This mantle is slotted to drop over the pump jets, and may be removed through the main condenser to facilitate cleaning of the thermometer chamber. Just above t.he upper bulb of the thermometer, a side arm enters the lharmometer chamber and is used for sample additions. The top of the sample arm is closed with a T 24/25 joint and stopper. The thermometer is mounted on a T 35/28 joint a t the tap af the main arm of the ebulliometer. An 8-mm. tube near bhe top of each arm is connected to a T stopcock, which is fastened to a drying tube that dries all the air "breathed" during a normal run. Cooling water for the condensers is circulated by a small pump from a simple thermostat, constant in temperature to * 1"C. The heater is connected, through an ammeter, to a variablevoltage rtutotrsnsformer. With a 50-ohm heater, between 1 and 2 amperes normally are used, The transformer, ammeter, and pilot light are all mounted in a small metal box, together with an "Off-On" double-pole single-throw switch which supplies current to both the transformer and the circulating pump motor. Thermometer readings (liquid levels) are made most satisfsctorily with a esthetometer. A small Lumiline or fluorescent lamp is mounted vertically in back of the cbulliometer to aid in reading the thermometers. Two sets of differential thermometers were used, each set containing, respectively, benzene, toluene, p-xylene, n-pentane, nhexane, and n-heptane; they were oonstrueted and filled by the methods described in the previous paper (6). The ebulliometric solvents were C.P. materials, used as p x chased, except that the aromatic solvents were dried over sodium metal.
Procedure. Before analyzing a sample, the operator must decide what solvent and what differentid thermometer are t o be used. The nature of the solvent is dependent entirely upon the sample, and no general rules are laid down here for its selection except that the solvent should boil a t least 150" C. below the solute. Having chosen the solvent, the choice of the thermometer is relatively simple. For average sensitivity, a thermometer filled with a liquid having a boiling point equal to or slightly less than that of the solvent should be used. Far the maximum sensitivity, as when materials of high molecular weight are being analyzed, a thermometer filled with liquid boiling as much as 20' or 30" C. below the ebulliometrio solvent may be chosen. Thus when beneene or carbon tetrachloride is used as solvent, a benzene thermometer should be used for average sensitivity, while a thermometer filled with n-hexane or acetone will provide greater sensitivity. I n making a molecular weight determination, the ebulliometer and its auxiliary components are assembled asshown in Figure 2. Pipet 30 ml. of the ebulliometrie solvent through the sample side arm. insert the desired thermometer in the main arm. and D U t a
Geed with the next Series. If B dinerent Solvent IS to be used, sev-
eral rinses with the new solvent will be nepded before making the run.
calculate a molecular weight: the weight and subsequent change of temperature (expressed conveniently as millimeters rise in the
ANALYTICAL CHEMISTRY
406
capillary arm of the thermometer) for a pellet of standard and the corresponding data for a pellet of unknown. Obviously, weight and temperature differences must be expressed in the same respective units, but the character of these units is unimportant. Substitution of these values, together with the molecular weight of the standard, in Equation 1, will permit the calculation of the unknown molecular weight. It is usually desirable to add a pellet of standard both before and after the addition of the sample: in this case, the standard data can be averaged before substitution in the formula.
If a number of standard and sample additions are made, it is convenient to calculate a K value for the standard, and then use this value in the subsequent calculations.
culated according to familiar statistical procedures, and these values converted to per cent of the average. Table I11 shows a similar set of data, obtained for a series of unknown samples on which six or more values were obtained.
Table 11. Reproducibility with Known Material Solvent Benzene Carbon tetrachloride
Toluene Xylene
Thermometer Liquid Benzene Hexane Benzene Heptane Toluene Heptane Toluene Xylene
Probable Error, % Single value Mean 0.6 0.3 1.7 1 .o 4.1 1.6
0.7
Av.
These K values are calculated for a single run. If they show good random agreement, they are averaged, and the molecular weight of the unknown is computed for each addition of unknown, as follows (3)
If a trend is obvious between K values, two successive values are averaged and used to calculate the sample run between them. The o erator must seek the cause of the trend, and recognize a possibfe decrease in accuracy and precision in this case. When a large number of similar samples are to be run using the same solvent, the same ebulliometer, and the same thermometer, and when the best accuracy is not required, the value for K can be established by a suitable number of calibration runs, and then assumed to remain constant. Its value can be checked a t intervals. If this procedure is used, the controllable variables, such as condenser water temperature, boiling rate, solution volume, etc., must be maintained as constant as possible.
3.4 1.9 3.3 3.8 2.4
0.2 1.4 1 .o 1.7 1.1 1.0
The samples analyzed to date have generally been run in triplicate, and hence do not lend themselves readily t o statistical study. The following treatment was made of data covering about 150 samples, run consecutively. The deviation from the mean was calculated for each of the three values in the sample, and from this the average deviation was obtained. This value was converted to per cent of the average. For 120 consecutive samples this average deviation ranged from 4.7 to 0.1%; the average was 1.5%. In the course of running these samples, the value of K (Equation 2) was recorded, to give some indication of the precision obtainable by establishing this value, and then omitting the standard addition during each individual run. Table IV summarizes these data.
Table 111. Reproducibility with Unknown Materials EXPERIMENTAL R E S U L T S
Because of the established nature of the ebulliometric procedure, no special work has been done with the modified apparatus to check the molecular weights of known compounds. The data in Table I are typical of the results obtainable. These materials were analyzed in the same manner as routine samples, generally with triplicate additions during scparatc runs on the individual substances.
Table I. Typical Results with Knowns Known Benzil (c.P. as received) Benzil (machine pelleted) Azohenzene
Molecular Weight Known Found 210.2 210.1
Reference Standard Solvent B e n d (recrys- Benzene talliaed) Benzil (hand Benzene pelleted)
Thermometer Benzene Benzene
210.2
210.0
Benzil
Carbon tetrachloride Benzene
Benzene
182.2
183.1
Benzene
280.3
282.1
Carbon tetrachloride Benzene
Benzene
266
254
Benzene
226.4
224.1
Cyclopentyl3.5-dinitrobenzoate Heptadecanol
Benzil Bend
n-Cetane
Benzil
The data in Table 11, which indicate the precision of thc method, were obtained as follows. The ebulliometer was operated in the manner described above. The solvents and thermometers listed were inserted into the apparatus, and then successive weighed portions of a known standard, usually benzil, were added, and the resulting rises in temperature were noted. From these data the value, milliequivalents millimeter of thermometer liquid rise, was calculated. At east six portions of standard were added. The probable error of a single value, and the probable error of the mean, were then cal-
per
Sample No. 1 2 3 4 5 6 7
Molecular Weight Found 315 432 416 471 450 296 254
Probable Error, % Single value Mean 3.2 1.4 3.1 1.2 2.4 1.1 2.6 1.1 2.5 1 .o 1.4 0.6 1.7 0.7
Experience with the ebulliometer has shown that one operator can analyze seven or eight routine samples in triplicate per 8-hour day. This allows time for sample pilling (if necessary), calculating and reporting results; and the necessary cleanup. Completely unknown samples require longer times, depending somewhat upon the operator's skill in selecting suitable solvents, thermometers, etc., without undue experimentation. The instrument has been used on samples boiling as low as 200' C., and on polymeric materials with apparent molecular weights as high as 2500. The accuracy of the instrument for these materials of high molecular weight has not been established definitely, but precision between results on such samples, while lower than those obtained on lower molecular weight samples, does not appear to be inconsistent with the lack of homogeneity and low solubilities of such samples, and the probable errors in measuring very small temperature differences. DISCUSSION OF R E S U L T S
Ebulliometer Design. Before designing the present apparatus, considerable development work was undertaken on the various components. The experience of others was reviewed, and certain features, such as the vacuum jacket and the immersion electrical heater, were adopted directly. The development of thermometers filled with nonaqueous liquids has been presented in a previous paper (6). These new thermometers, and the techniques developed to handle them, permitted liberties with
407
V O L U M E 21, NO. 3, M A R C H 1 9 4 9 Table IV.
Variation of Value of K for a Benzene Thermometer
Solvent Ebulliometer Carbon Tetrachloride NO. n Av. deviation, 7* n 2 176 43 1.3 27 3 3.1 92 n number of runs.
-
Benzene Av. deviation, 1.4 1.4
the ebulliometer design which were not available to previous workers in the field. The principal changes in the ebulliometer involve the boiler, heater, and pumping arrangement. After testing numerous designs, the removable pumps used previously were abandoned in favor of the sealed-in design. Two-, three-, and four-jet arrangements were tried, the last giving the best combination of pumping speed and ease of glass blowing. One ebulliometer was’constructed in which the 3-mm. tips on the jet end of the pumps were omitted. Lack of these tips did not seem to affect the results obtained with the instrument, but the solvent running down the sides of the thermometer chamber obviously interfered with the pumping action. Except for their diameter, the pump arm dimensions did not appear to be critical. ,4number of different styles of heaters were tried. The boiling rate appeared to be a simpler function of energy input, if the heater was completely surrounded by the boiling liquid, or was mounted on a small, glass helix. (The latter design is the more rugged, and is preferred.) Heaters mounted on a relatively large support showed a tendency towards lower boiling rates for a given energy input. KO decomposition of the solvents mentioned in this paper or a wide variety of solutes has been observed on the Nichrome heaters over 18 months of operation. Enclosing the top of the apparatus, to prevent the introduction of moisture into the ebulliometer during a run, was necessary primarily in order to permit determinations in humid weather. The side arm permitted the addition of sample without disturbing the thermometer. The chief disadvantages are the inconvenience and expense involved in replacing the heater wire, in the event of a burnout, and the difficulty of cleaning the boiler. The latter could be reduced by the use of platinum instead of Sichrome wire for the heater, but this in turn makes the heater more susceptible to burn out. Ketchum’s (5) method of mounting the heater on a glass joint, FTas published after the development of the ebulliometer here described. Although its adoption in the present ebulliometer was seriously considered, no models containing it have yet been made, because a comparatively large grind would be required for the authors’ device with the attendant increased possibility of leakage. However, they plan to try Ketchum’s arrangement in a further development of their apparatus. The present procedure calls for reading the change in temperature, upon the addition of a pellet ef solute, only as the change in the height of the liquid in the capillary arm of the thermometer. However, the level of liquid in the lower bulb also changes as the level in the capillary changes (because the total volume is constant). In a well constructed thermometer this change is linear, and a small, constant correction should be added to the change of liquid level in the capillary arm to give the true change in height. Actually, this is unnecessary in this procedure, since the error term is essentially equal for successive thermometer readings and cancels out in the Ats and A t , terms in Equation 1. As dt is small (ca. 0.01 a C.), dp/dt is assumed to be linear. One feature of all previous equipment, which is completely omitted in this design, is the small metal clip, originally recommended by Nenzies and Wright (8),for conducting condensed solvent from the condenser wall to the upper stem of the thermometer. The authors have found that this arrangement fails to
achieve its objective of stabilizing the thermometer reading. Among the substitutes tried, the most successful was a small coldfinger condenser; to the bottom of which the thermometer was attached; this device has also been reported by Colson (3). The cold finger was used for some time on all thermometers and seemed to increase the thermometer stability. After several months of routine operation, however, circulation of liquid through this condenser was discontinued without apparent effect on the stability of the thermometer. The authors have no explanation for this apparent anomaly. Analytical Results. The chief concern in the development of the present ebulliometric method was the need for a rapid, relatively precise procedure for the determination of the molecular weights of a wide varity of samples. In general, it was desired to establish this value to 1%. Greater accuracy is of little value in establishing the nature of pure compounds or polymeric materials of low molecular weight. Consequently, no great effort was expended to refine the method to give more precise results, although this probably could be done. On one occasion it was desired t o establish very definitely the molecular weight of a compound, and three independent sets of triplicate values were obtained. The difference between the average value for the nine results and for the average of any set of three determinations was considerably less than 1%. Of the solvents and thermometers used, the benzene combination has been the most generally useful. Benzene is a highly versatile solvent, easily obtained in a’ high degree of purity; it boils smoothly, and has good ebullioscopic constants. The benzenefilled thermometer is generally used with benzene, although the n-hexane thermometer will, give greater sensitivity. As a companion solvent, for materials not readily measured in benzene, the authors most frequently use carbon tetrachloride with the same pair of thermometers. Materials of very high molecular weight are run in xylene with either the p-xylene or toluene thermometer. Other solvents, such as ethanol, 1-propanol, ethyl ether, and toluene, have been used successfully. The authors have not been able to obtain known materials of very high molecular weight, and hence are unable to verify the accuracy of the present procedure for such materials. Extensive data on samples, ranging in molecular weight from 1000 to 2500, indicate a fairly good precision, although not so good as a t lower molecular weights; this, however, is not inconsistent with the known inhomogeneity of such samples. ACKNOWLEDGMENTS
The authors wish to acknowledge with thanks the direct aid and criticism given by Donald M. Smith and the assistance of C. W. Hammond for his design and construction of the auxiliary equipment used with the ebulliometer. Thanks also are due E. J. Mulderick and Lydia B. Wishowsky, who obtained virtually all the experimental data presented in this paper, and R. G. Kester and T. M. Brown, who built the various ebulliometers used and described. LITERATURE CITED
(1) Barr, W. E., and Anhorn, V. J., with Hanson, W. E., Instruments, 20, 342 (1947). (2) Colson, A. F.,Analyst, 57, 757 (1932). (3) Zbid., 71,322 (1946). (4) Hanson, W. E., and Bowman, J. R., IND.EXG.CHEM.,ANAL. ED., 11,440(1939). (5) Ketchum, Donald, AXAL.CHEM.,19,504 (1947). (6) Kitson, R.E., and Mitchell, J., Jr., Ibid., 21,401 (1949). (7) Menzies, A.W. C., J. A m . Chem. Soc., 43,2312(1921). (8) Mensies, A.W. C., and Wright, S. L., Zbid., 43,2314 (1921). (9) Smith, J. H. C., and Milner, H. W., M i k r o c h m k , 9, 117 (1931). (10) Swietoslawski, W., “Ebulliometric Measurements,” p. 172, New York, Reinhold Publishing Corp., 1945.
RECEIVED May 18, 1948. Presented before the Division of Bnalytical and Micro Chemistry at the 113th Meeting of the . ~ X E R I C A N CHE\IICAL SoCIETY, Chicago, 111.
