MICRO-MOLECULAR-WEIGHT DETERMINATION BY VAPORDENSITY METHODS
This paper presents three simple methods for micro-moleculer-weight determination based on vapordensity and pressure relations. Results of t?rpical determinations are tabulated.
. . . . . .
1. Micro-Molecular-Weight -
Determination by the Victor-Meyer Method
It is a well-known fact that the grammolecular weight of a substance in the xaseous state occupies a volume of 22.4 liters a t 0°C. and under a pressure of 76 cm. Hg (I). I t follows that the determination of the molecular weight of a substance may be effected by determining the volume of a known mass under definite conditions of temperature and pressure. This is the well-known method of VictorMeyer ( Z ) , (3). Apparently the method has not been adapted to the determination of the molecular weight of extremely small quantities of material. One of the authors ( 4 ) has described a modified Meyer apparatus for use with small quantities of material that is readily constructed of regular laboratory stock and combines ease of manipulation with a saving in space. In this paper an apparatus is described that admits of molecularweight determinations of extremely small samples, i . e., true micro quantities.
1I
PRINCIPLE OF THE METHOD
A small tube whose volume is accurately known is filled with dry, pure mercury and inverted in a small porcelain crucible also filled with mercurv. A small bulb containing the samplc whose molecular weight is under investigation is allowcd to float to the top of the mercury-filled tube. The tube and crucible are now immersed in a 1819
F I G U R E1 . - G K N F R A L A R A P P A R ~ T UYO,, S
HASCL~HIISI. OP
~~~~P,"P~,"";~~~C~ MEYER Mgmo~
1820
JOURNAL OF CHEMICAL EDUCATION
OCTOBER, 1932
liquid bath that can be gradually heated. The temperature of the bath is slowly raised until the sample is vaporized and pushes the mercury out of the tuhe to a calibrated mark. After the mercurv level inside and outside of the tube bas been equalized the temperature of the bath is taken. The distance from the surface of the mercury to the surface of the bath liquid must he taken as added pressure on the gasified material. The barometric pressure is read. From these data, the known weight of the sample weighed on the microbalance and the known volume of the tube previously calibrated with mercury or water, the molecular weight of the material can be calculated. In arriving a t the final pressure exerted on the gasified substance the vapor pressure of the mercury may be neglected a t temperatures under 200°C. as less than the experimental error. At a temperature of 200°C. the pressure is of an order of 18.3 mm.; a t 300°C. it has become 246 mm. (5). DESCRIPTION OF APPARATUS
A threeliter beaker of water serves as a FIGURES 2 AND 3.-CONSTXUC- convenient bath if the substance boils under TIONAL DETAILS OX. GAS-MEASURthe conditions of the experiment at apING TUBES 90°C. or less. An air stirrer ~roximatelv r is useful in attaining a constant temperature in all portions of the bath but a hand stirrer may be substituted. The general arrangement of the apparatus is shown in Figure 1. The confming tube is of approximately 12 cm. length and 0.6 cm. internal diameter. The lip is slightly flared to offer convenience in introducing the sample bottle. For details see Figure 2. The tuhe is calibrated to a mark near the lip so that the mercury levels can be equalized within the limits imposed by the small porcelain crucible. For precision the tuhe may be drawn out in the manner shown in Figure 3. ObFIGURE 4.-CONSTRUCTIONAL DETAILSOF viously in the use of the latter SAMPLING BULBGREATLY ENLARGED tuhe the volume of the sample bottle need not he considered. A sampling bulb is shown in Figure 4. This bulb is made as small as possible commensurate with the volume of liquid to be weighed.
VOL. 9, NO.10 MICRO-MOLECULAR-WEIGHT DETERMINATION
1821
As high-temperature bath liquids one may use glycerin or various salt baths. In any case the liquid employed must be capable of being heated to a temperature considerably above that of the substance under test without ebullition or undue fuming. PROCEDURE
The confining tube is calibrated with mercury a t room temperature in the usual manner. For very precise work, using the tube shown in Figure 3, the calibration should be made over the range of temperature encountered with the highest boiling bath. For convenience in handling, the calibrated tubes are fastened to a glass rod. In the case of a non-corrosive bath liquid this is readily accomplished by means of rubber tubing. For work with corrosive bath liquids the tube must be fused to the rod. If this is carefully done witli a sharp flame there will be no distortion of the calibrated tube. After calibration the tube is filled with dry mercury and inverted in a small crucible of the same material. The tube should be free of air bubbles after this operation. The sampling bulb is weighed on the microbalance, grasped by means of a forceps, and slightly warmed in a micro-burner flame. Its tip is immediately placed beneath the surface of the liquid under investigation. As the bulb cools liquid is drawn in. The operation is repeated as the bulb usually does not completely fill on the first trial. A small volume of air should be left in the tip of the bulb. Reweigh the bulb. The increase in weight is the weight of the sample taken for the$etermiuatiou. Grasp the bulb with a forceps, hold it mouth downward over the mercury, and touch it with a glass rod heated in the Bunsen flame. The liquid expands and when it has completely filled the bulb quickly insert under the mercury-filled tube, still mouth downward, and release. The sample rises immediately to the top of the tube. Place the tube and crucible in the bath and gradually raise the temperature of the latter until the sample has vaporized and lowered the mercury to the calibration mark. Equalize the mercury levels inside and outside the tube. Record the temperature of the bath, the barometric pressure, and the height of the liquid TABLE I Determination of the Molecular Weight of Several Purified Materials Mdniol
Benzene Acetone Carbon tetrachloride Toluene Ether
WEighi,
r.
0.004160 0.003312 0.008820 0.004850 0.004786
TcmorrPrcrVolume oturc Boromdric sure Consclad of Tubr. Bolh, Pmrurr, Bolh, Pressure. Molecular WEigh1 cc. 'C. cm. T i p cm.Hg. rm. Hg 0 Colc.
1.65 86.0" 1.65 65.3" 1.65 82.2' 1.65 126.0' 1.65 51.3'
73.0 73.0 73.0 73.6 73.6
1.0 1.0 0.9 1.2 1.6
74.0 74.0 73.9 74.8 75.2
78.0 76.2 58.0 57.3 160.1 153.8 97.7 92.1 74.1 78.0
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O~OBER 1932 ,
above the mercury surface. By raising and lowering the temperature of the bath several times the mercury may be made to pass and repass the calibration mark. The average of the temperatures corresponding to the point of exact coincidence with the calibrated mark may be recorded. The molecular weight can now be calculated. Several typical determinations are summarized in Table I, p. 1821. Microsco~icDetermination of Molecular Weights by the VictorMeyer Method The prinaple of this method is simple. A drop of the substance whose molecular weight is under determination is confined in a 1.5 to 3 mm. bore capillary between t w o drops of mercury. The capillary is heated in a vapor bath to a temperature sufficient to volatilize the sample. One of the mercury drops, the other being sealed, takes up a new position corresponding to the increase in volume of the substance in the gaseous state a t the temperature of the experiment. Knowing the temperature of the vapor thermostat, the diameter of the capillae, the distance between the drops of mercury, and the mass of the substance, one can readily determine its molecular weight. The construction of the FIGURE~.-~KANGEMENT OI. APPARATUS POR +ne M ~ c n o s c o ~ rDETERMINATION c OR MOLECULAR a~varatus is shown in .. WEIGHTS Figure 5. The capillary is heated in the vapor of a liquid boiling in the flask. A square of wood or cork protects the apparatus from the heat source. Choice of a liquid depends upon the boiling point of the substance whose molecular weight is being determined.
2.
VOL. 9, NO. 10 MICRO-MOLECULAR-WEIGHT DETERMINATION
1823
PROCEDURE
The boilmg point of the substance whose molecular weight is to be determined is ascertained by the method of Smith and Menzies (6). The density of the material is determined by the method outlined by Wartenberg (7). The sample is conlined in a short section of thick-walled 0.5 mm. internal diameter glass tubing that fits snugly in the capillary. For a 0.0011 g. sample of benzene this capillary was 6.2 mm. long. A 0.0005 g. sample of the same material occupied a 3.0 mm. length of tubing. The diameter of the capillary tube having been previously determined under the microscope, the length of the drop is measured by means of a low-power objective and micrometer ocular. A drop of dry mercury is placed in the capillary, and the tube containing the sample transferred from the microscope stage to the capillary. The tube is slightly inclined and as the mercury drop moves down the tube the sample container is pushed in. When the container is flush with the juuction of the bell and capillary proper, it is followed by a small quantity of mercury and a plug whittled from a soft wood, e. g., white pine. The capillary is mounted in the vapor thermostat in a horizontal position. An organic liquid, whose boiling point is 10 to 15 degrees higher than that of the sample, is placed in the flask. The diameter of the capillary having been previously determined the liquid in the flask is boiled and when the mercury drop has come to rest the volume occupied by the substance in &e gaseous state is measured. Too large a sample must be avoided. The volmne occupied by the substance in the liquid state and the volume of the container is subtracted from the gaseous volume in calculating the results. Typical results obtained using this method are shown in Table 11. TABLE I1 Typical Determination of the Molecular Weight of Benzene (Wa) TVeighl Sample, P-
3.
Volume Vapor, CC.
Temp. of Bolh Sac. Butyl Alcohol, 'C.
Boromclric Pmsura, cm. Hn.
Molmulor Waighl Obr. Calr.
Micro-Molecular-Weight Determination by a Differential VaporPressure Method
Of the existent methods for determination of molecular weights by vapor pressure mention must be made of those advanced by W'il and Bredig (8), Barger (9), and Perman (10). Blackman ( l l ) , (12) offers a method unique in simplicity of apparatus and freedom from technical complica-
JOURNAL OF CHEMICAL EDUCATION
Ocroa~n,1932
tion. A modification of his apparatus and method to meet the exigencies of work on a micro-scale is presented. THEORETICAL
When equimolecular quantities of different substances are dissolved in equal weights of the same solvent, the elevation of the boiling point is constant. It follows that the volumes of two solutions with their vapors in communication, a t equilibrium in regard to vapor pressure, will be inversely proportional to the number of dissolved molecules. This may be expressed by the relation,
W 1 and W z are the respective weights of solutes of molecular weight MI and MZdissolved in solvents of volumes Vl and Vz. Simplification of the formula is obtained by taking Wl equal to Wz, in which case i t reduces to the expression,
If equal volumes of solutions containing the same weight of dierent solutes be allowed to evaporate to the state of equilibrium represented by equation (2) the two solutions will FIG6.-ARRANGEMENT OF APPARATUS thenceforth evaporate equally. EOR MICRO-MOLEC~AE-WEIGHT DETERMINA-The first few readings must be TION BY A DIFFERENTIAL VAPOR-PRESSURE rejected as representing apMETHOD
VOL.9, No. 10 MICRO-MOLECULAR-WEIGHT DETERMINATION
1825
proximations to a state of equilibrium. After a state of equilibrium has been reached the ratio of Vzto V,is a constant for the determination in progress. CONSTRUCTION OF APPARATUS
Figure 6 shows details of construction of the apparatus used in the determination. The bulbs containing the samples of known and unknown are made by cutting IO-cc. pipets a t the junction of the bulb and stem and sealing. The bulbs are mounted in a heating bath consisting of a threeliter beaker of water. PROCEDURE
Prepare a solution of a substance of known molecular weight in a solvent which readily dissolves both the known and unknown material. The standard material should have a molecular weight of approximately 100. If the molecular weight of the unknown material is found b y experiment to be very close to 100 the determination should be checked by using as standard a substance whose molecular weight is considerably greater than that of the unknown. A suitable concentration for both known and unknown is effected by using 0.0100 g. material per cc. Two to five cc. of each solution is used for a determination. The solutions are added to the bulbs by means of a drawn-out pipet. By carefully regulating the temperature of the bath the lower boiling solution will boil off before the other to the state of equilibrium represented by equation (2). After equilibrium has been attained the solutions vaporize at equal rates. When the solutions have boiled down to the $ate of equilibrium (20 to 60 minutes), the pipets are withdrawn from the bath, cooled to room temperature, and weighed separately on the balance. Since the weight of the solvent is proportional to its volume we may use the weights so obtained in the equation after subtracting the weight of the dissolved solute. Results of several typical determinations are shown in Table 111. TABLE m Typical Determinations of Molecular Weight Using the Diflerential Vapor-Pressure Method Solute
Solvan1
Ether
1 1 Bvlb (dandord)
Resorcinol
2nd Bulb (Unknown1
~ e m p weight sol. or Both, BpilLrium, a. T. 1 x 1 Bulb 2ndBulb
Naphthalene 36.0'
Chloroform Naphthalene Azobenzene
2.31 1.43 1.11
1.89 1.15 0.92
64.2" 26:3 1.79 1.51
1.82 1.25 1.02
Molecular Weight Obs. Colr.
134.2 128.06 136.3 132.8 134.5 Av. 18.5 1 182.10 184.0 189.9 186.3 Av.
1826
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Ocrossn, 1932
Summary The results obtained on various substances are shown in Tables I to 111. In general the various methods yield accurate values. The results obtained with the differential vapor-pressure apparatus are good provided no solute deposits on the walls of the bulb and neither substance is volatile in the vapor of the solvent. Bumping of the solutions while boiling can be prevented by adding several previously weighed platinum tetrahedra to each bulb. Literature Cited (1) (8')
(3) (4)
(5)
(6) (7) (8) (9)
(10) (11)
(I$)
GETMAN, ''Outlines of Theoretical Chemistry,"4th ed., John Wiley & Sons. Inc., New York City. 1927, p. 8. BIGELOW,"Theoretical and Physical Chemistry," The Century Co., New York City, 1912, p. 165. WALKER,"Introduction to Physical Chemistry," 6th ed., The Macmillan Co.. New York City, 1910, p. 198. BLANK,J. CBEM.EDUC.,8,546 (1931). HODGMAN AND LANGE,"Handbook of Chemistry and Physics." 13th ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1928. S m x AND MBNZIES,J. Am. Chem. Soc., 30, 897 (1910). WARTENBERG. Ber., 42, 1126 (1909). W ~ AND L BRZDIO,
