cell, N o . 6, was used (Table I). This suggests that microdiffusion is affected by the capacity (and geometry) of the cell. Although three sodium hydroxide pellets were used in the center ~ l ltwo , are adequate for acids having less than four carbons. When two are used with isobutyric and isovaleric acids, aggregates of crystals of their sodium salts develop over the pellet surface. Although this seemed an interesting modification of a technique ( 5 ) for the collection of crystals of the salts of these acids, it indicates the possibility that the surface of the pellet had become entirely coated. For this reason, three pellets were used in most of the experiments reported. As in distillation, the fraction of acid transferred in microdiffusion wries slightly, if a t all, with concentrationa t 2 hours and 20°, 0.201, 0.212, and 0.213 mg. of 0.27, butyric acid left, respectively, 93.1, 92.8, and 89.57, acid remaining in the outer well of the Conway apparatus. These data for the tenfold diluted sample are similar to those for 2yo butyric acid (92.9, 92.1, and 91.1) appearing in Table 111. The lower limit of the quantity of acid detectable, therefore, should be decided only by the accuracy of the titration. When two-component acid mixtures were examined, the relative behavior in microdiffusion was consistent with expectations based on the Duclaux constants. Figure 2 shows a chromatogram from the residue of the outer !yell
of a Conway cell which contained butyric and acetic acids. Comparison of the effluent from the original solution with that of the Conway cell indicates losses, in 2 hours a t 20°, of 20% of the butyric acid and 11% of the acetic. A similar experiment with hexanoic and formic acids showed losses of 30% of hexanoic and 5.6% of formic acid. The data in Table IV describe the microdiffusion of organic acids from nonaqueous solutions. The order of escape from benzene, chloroform, cyclohexane, heptane, and their mixtures reflects the relative vapor pressures of the pure solutes. As expected, each acid escapes less readily from the mixed solvents as the butanol content increases. Effluent 1 has no measurable butanol content, while effluent 3 has 8.63% volume. The per cent acid remaining in the outer vessel of the Conway cell is different for the three acids in any of the effluent solvents shown. This difference is sufficient to permit characterization of these effluent acids in butanolchloroform solvents which release the acids from silica gel. As expected, Values for 4.06 and 8.63% butanol-chloroform mixtures are similar to effluents 2 and 3, which have corresponding compositions. Cyclohexane, heptane, and benzene allow the most rapid release of formic and acetic acids. The relative rate of escape, however, of these two acids is not constant in the three solvents. Such loss permits unequivocal
identification of these three acids by the use of two or more nonaqueous solvents. One cannot deduce the rate of escape of a n organic acid from a mixed solvent by using the data for the separate solvent components. For example, 2.15and 55.29% of formic acid remain in benzene and ether, respectively, after 2 hours. The mixture of equal volumes of benzene and ether permits 65.247, of the acid t o remain in the outer cell vessel in 2 hours, a value greater than that for either benzene or ether separately. An obvious advantage of nonaqueous microdiffusion is its direct applicability to formic, acetic, and propionic acids in solvents employed with silica gel columns. Thereby, at the R1 under scrutiny, either Characterization of the acid is permitted or lack of homogeneity of a chromatographic zone can be detected. LITERATURE CITED
(1) Buyske, D. A,, TVider, Pelham, Jr., Hobbs, 11. E., ANAL. CHEX. 29,
105 (1957).
(2) Corcoran, G. B., Ibid., 28, 168 (1956). (3) Donaldson, E(. O., Tulane, V. J.,
Marshall, L. M., Ibid., 24,773 (1952). (4) Duclaux, E., Ann. chim. et p h y s . 2 , 289 (1874); Trait4 de Microbiologic, Paris, 3, 384 (1900). (5) Schwyzer, R., Acta. Chem. Scand.
6 , 219 (1952). (6) Yohe, G. R., Lansford, M. W.,
Trans. Illinois State Acad. sei.
59 (1952). RECEIVED for review April 25, 1957. Accepted August 17, 1957. L. h1. Marshall is on leave from Howard University, Washington, D. C. 45,
Ultramicromethod for Molecular Weight Determination G. 0. GUERRANT Exploration and Production Research Division, Shell Development Co., Houston 25, rex.
b Geochemical studies have created a demand for a method for determining molecular weights of organic mixtures using only a fraction o f 1 mg. of sample. The test material i s suspended from a calibrated helical tungsten spring enclosed within a vessel containing a standard reference solution. Solvent i s distilled isothermally from the reference solution into the test material until the vapor pressures are equal. The molecular weight of the test material i s calculated from the sample size, the amount o f solvent distilled, and the molality of the standa r d reference solution. Molecular weights can be determined accurate to 3%.
A
of the molecular weight of hydrocarbon fractions obtained from extracts of oil source sediments is KNOWLEDGE
necessary in the general characterization of these sediments. Because of the difficulty of isolating these substances, the sample is often limited to less than 1 nig. hIost of the classical methods have been ruled out, as it appears unlikely that they could be modified sufficiently to apply to these samples, A cryoscopic method, used ljuccessfully on the micro scale by Rast ( I I ) , is considered unreliable for the compounds of interest because they are insoluble in the available solvents. Wash (7’) has applied a vapor-pressure comparison method, which is accurate to 27, when 2 to 8 mg. of sample are employed, to the determination of molecular weights below 700. Puddington ( I O ) devised a similar method depending upon the depression of vapor pressure as measured by means of a sensitive differential U-tube manometer, in which 1 mg. or less of com-
pound was required and accuracy was within 2Yc, These methods are complicated by the required purity of the solvents employed. Differences in vapor pressure have been measured by a thermoelectric method ( I S ) , which is limited by the accuracy of measurement of the temperature difference a t two thermocouple junctions and by changing temperature gradients within the system. Barger ( 1 ) described a n isothermal distillation method, employing drops of solution in a capillary. This has been improved by Niederl and associates ( 8 ) , but requires careful techniques and is time-consuming. A method based on the same principle has been developed by Signer (12) and modified by Clark (3). The apparatus congists of two graduated tubes joined by an inverted U-shaped section. The standard and the unknown are placed in VOL. 30, NO. 1, JANUARY 1958
143
opposing graduated tubes; the system is evacuated and allowed to equilibrate a t constant temperature. This technique was modified by Childs (2) by including a reservoir of pure solvent in the system. Solvent distills into both solutions in proportion to their respective molar concentrations. Parrette (9) applied the isothermal distillation technique to the determination of the molecular weight of polymers with molecular weights between 1000 and 30,000. A less fragile isothermal distillation apparatus has been described by Hoyer (4), but in insufficient detail, Molecular weights were determined with samples of only 0.4 rng. with an accuracy within 3% and with a distillation time of about 30 minutes. This method shows promise for determining the molecular weight of the type of samples of interest. This paper describes a modification of the isothermal distillation method for determining molecular weights, in which the amount of solvent required to equalize the vapor pressure of a solution of the unknown and a standard solution is determined eravimetricallv on a helia1 spring bzance of the McBain3akr type (5). An accuracy within 1% can be obtained with less than 1 ng. of material. The opaator time per letermination is 2 hours. Highly puriled solvents are not required.
~
Figure 1. Helix assembly and equilibration ye '
-
of Nichrome wire spaced a t 1'/4-iuch intervals and eoverine the nnt,ire interior snhace of the box. This is laced through air-cell asbestos paper and held in place on the sides, top, and bottom of the bath by aluminum sheets. In the door, the heater wire is placed between the Lncite sheets. Air is agitated by a 10-inch circnlm fan located in the ceiling of the bath. The temperature-sensing element is located in front of the fan, where there is the greatest air movement within the bath and where the greatest amount of temperature fluctuation within the bath is likely to occur. A Gaertner Scientific Corp. (Catalog No. M930-342) micrometer slide cathetometer with a 100-mm. scale, reading to 0.001 mm., is mounted upon a specially constructed optical bench so that it can be moved and exactly repositioned. A Microchemical Specialties Co. quartz torsion fiber microgram balance (Catalog No. ZOO) of 10-mg. sample capacity was used to weigh the samples. A square piece of glass-fiber filter paper (H. Reeve Angel and Co., Inc., 52 Duane St., New York 7, N. Y.), 3 to 4 mm. in size, holds the sample for weigh~
~~
~~
~~~
~
~~~~
~~
~~~
~
~
APPARATUS
The helix assembly and equilibration ,esse1 is shown in Figure 1. The helix Consists of a coil of about 2 meters of 01.076-mm. (3-mil) tungsten wire wound 0m n a 13-mm. mandrel, so that adjacent turus touch. The coil is wound with a t ension of approximately 90% of the 'cireaking strength of the wire and an,-",.., ".PI I".".uvulu I "" "" : ..--:..-L - L nraLGu a/uru~npr;uuLvt; plate which has a thermostatic control set to give a surface temperature of approximately 350' C. Pans are made from circles of aluminurn foil 13 mm. in diameter and 13-microns (0.5 mils) thick. They are shaped hemispherically by pressing them into a piece of sponge rubber with the bottom of a test tube. A short length of wire of No. 28 B. & S. gage supports the pan a t the end of the helix. A quartz fiber, placed between the helix and pan, positions the pan a t the proper distance from the solution. A quartz reference rod is placed concentric to the helix. The equilihration vessel is constructed from a 45/50 standard glass joint with a reservoir to contain 200 ml. and leaving an undistorted area for viewing the helix. These vessel4 are mounted within a thermostated air bath (Figure 3
-\
.
Z)
The thermostated air bath and cathetometer assembly are shown in Figure 3. The bath was constructed of Transite '/%inch thick, covered with Masonite s/18 inch thick, and the door was constructed of two sheets of Lueite '/s inch thick, with a 6/rinch spacing. 144
ANALYTICAL CHEMISTRY
Figure 2. Thermostoted air bath with door oDen to show interior assembly The door was counterbalanced for ease of opening, with a combination of slidmg panels on the front and plywood hetween the Lucite to make the bath light-tight. The bath was made 21/s feet high, 4 feet wide, and l'/%feet deep, so that 12 equilibration vessels could be accommodated. The temperature within the bath is controlled to a +0.003" C. by a Model 1053 Thermotrol, which is a time-cycle temperature controller with reset. It was designed by Shell Development Co., Emeryville, Calif., and built by Hallikainen Instruments, Berkeley, Calif. Two heaters are attached to the thermoregulator. One, mounted around the periphery of the blower, is a bare Chrome1 wire of open-coil construction controlled by means of a Powerstat to give approximately 150 watts. The other heater, of approximately 1000watt capacity, consists of a network
Figure 3. Thermostoted air bath and cathetometer assembly
ing and during the isothermal distillation.
value of a reference position on the bottom of the helix and on the quartz reference rod. Add approximately 100 ml. of 0.1 molal standard reference solution to the reservoir portion of the equilibration vessel by means of a funnel. Carefully close the equilibration vessel. If fingerprints have been removed from the equilibration vessel by wiping, remove the electrostatic charges by proper grounding before the vessel is closed; otherwise, the helix assembly will be drawn against the interior of the vessel by the charge. Repeat this procedure with each equilibration vessel used. Close the constant temperature air bath and start it. After the system has equilibrated for 3 days, record the cathetometer readings of the reference positions on the bottom of each helix and on the corresponding reference fiber. Open the equilibration vessel and substitute a clean flask with 0.05 molal standard reference solution. Repeat the equilibration procedure. Determine a blank for each standard reference solution and apply it to the molecular weight calculation. If the apparent molecular weight obtained with 0.05 molal standard reference solution is significantly different from that obtained with 0.10 molal reference solution, repeat the determination with 0.15 molal reference solution. Plot the molecular weight values against the corresponding concentrations of standard reference solution. Extrapolate to zero concentration and read from the curve the average molecular weight of the sample to the nearest whole number
PROCEDURE
Prepare standard reference solutions of 0.05, 0.10, and 0.15 molalities with an accuracy to 0.5y0. These solutions should consist of a reference compound of accurately known molecular weight dissolved in a solvent free of mater and boiling below 100" C. Pure octadecane in benzene is satisfactory for the determinations of molecular weights of petroleum fractions. Carefully clean and dry the equilibration vessel and the helix assembly. Rinse the helix assembly with the pure solvent that is employed, avoiding contamination during handling. Assemble the equilibration vessel and the helix assembly in position within the constant temperature bath. Calibrate the helix by hanging known loads in the range 0 to 40 mg. on it and determining the elongation. Carefully lubricate the upper portion of the large ground-glass joint with a hydrocarbon-insoluble lubricant consisting of a mixture of sucrose, dextrin, and water. The hydrocarboninsoluble lubricant consisting of starch, mannitol, and glycerol, frequently used with organic solvents, is unsatisfactory, as glycerol is soluble in benzene. Using a microgram balance, weigh a sample of test material of a size expected to require 20 mg. of solvent for equilibration. Weigh the sample on a piece of glass-fiber filter paper and with an accuracy to 0.5%. Wash the lower helix assembly with pure solvent to remove any contamination. Then transfer the sample on the paper to an aluminum pan and suspend it from the helix. Record, to the nearest 0.05 mm., the cathetometer millimeter scale
GENERAL DISCUSSION
When a sample is placed in a closed system in the presence of a solution
I
0 0
I
0
n; E 1.5-
under isothermal conditions, vapor will distill from the solution and condense in the sample until the vapor pressures of the two solutions are equal, At equilibrium the mole fractions will be equal, if the solutions adhere to Raoult's law, or both exhibit the same deviation from it. The volume of reference solution must be sufficiently large that the amount of solvent distilled makes an insignificant change in its concentration. The molecular weight can be calculated by the relationship 1000 w hI.17'. = ms
where
11.W.= molecular weight w = sample weight, nig. m = molality of the solution, moles per 1000 grams of solvent s = weight of solvent in the sample solution, mg. The primary difficulties in application of the isothermal distillation method are the slowness with which equilibrium is reached and the precise degree of temperature control required, The time required t o reach equilibrium can be shortened by minimizing the amount of solvent to be distilledminimizing the sample size and adding solvent to the sample to approximate the molality of the reference solution. Fluctuations in temperature will produce a corresponding variation in the vapor pressure of the solvent. If the temperatures of the reference solution and the unknolvn are not the same, the partial molal volumes will not be equal a t the isopiestic point, Therefore, a molecular weight based on this assumption will be in error, This error is given by the expression
HELIX No 4
E ' r
-
0
0
0
0
>*
I-
5 - 1.3-
where
HELIX N o 3
m
p'
vapor pressure of the pure solvent in the test sample po' = vapor pressure of the pure solvent in the reference solution nl = moles of standard reference compound n2 = moles of solvent in reference solution
I-
0
z w
HELIX
m
-
0 0
0
0 0
No 2
1.2-
1.1
c 0
0
HELIX
No I
I
I
I
I
I
I
I
5
IO
15
20
25
30
3!
Figure 4.
LOAD, mg.
Calibration of helices
=
When the concentrations of the solutions are 0.1 molal with benzene as solvent a t 35.00" C., a difference of 0.01' C. between the test sample and the standard reference solution results in an error of 6% in the molecular weight. A helical spring balance was used for weighing the solvent gained during the isothermal distillation process, because VOL. 30, NO. 1, JANUARY 1958
145
'""h
DISTANCE FROM SOLUTION, mm.
80
6ol\ = -
0 "40
i -I I
I-
START
FINISH
36 53
0 20
3~
2 2ot a
E
I
"'
0.4% AT 68 h f .
I
Effect of molality on equilibration rate
of its simplicity of operation and ease with which it could be enclosed within the isothermal system. Tungsten helical springs (6) are preferred to quartz, because they are less fragile and are easy to make from wire; quartz helices require special skill in their construction. In the load range in which the helices are used, the sensitivity is independent of load; Figure 4 shows plots of helix sensitivity us. load. The maximum deviation of calibration values from the mean for helices 1, 2, and 3 appears to be less than =tl% relative. Helix 4 shows somewhat greater deviation; however, it is more sensitive and was not well constructed. Better reproducibility in weighing was not obtained because of hysteresis and vibration effects. An air bath was considered more desirable than a liquid bath because the helix pan could be made more accessible without disturbing the helix assembly. The operating temperature of the bath mas selected to be somewhat above maximum room temperature, 37' C., so that cooling would not be required to maintain temperature control. The major portion of the heat is applied uniformly along the walls of the bath where the heat loss occurs, considerably reducing the gradients within the bath and making the temperature more uniform. Wall heating was necessary to obtain adequate temperature control. Before wall heating was used, all heat for the bath was provided by bare Chrome1 mire of open-coil construction 146
ANALYTICAL CHEMISTRY
I
I
\ 1
TI M E , hr. Effect of distance of reference solution from
T I M E , hr, Figure 5.
I
\
Figure 6. sample on equilibration rate
with an unshrouded centrifugal blower of high capacity for stirring. Such an arrangement resulted in unsatisfactory temperature control. EXPERIMENTAL RESULTS
For the initial phase of the esperimental investigation, benzil was eniployed both as the test material and as the solute in a benzene reference solution. The time required for equilibration and the effect of molality of the reference solution upon the equilibration were studied, Samples of benzil of 0.30 mg. each were weighed accurately on a quartz torsion fiber microgram balance and equilibrated with 0.0418, 0.0884, and 0.1664 molal solutions. The logarithm of the per cent of solvent to be distilled plotted against the time in hours is shown in Figure 5 for the molalities used. If points are taken 5% before equilibrium, just beyond the straight-line portion of the plots, the elapsed times are approximately 13, 31, and 58 hours for the 0.1664, 0.0884, and 0.0418 molal reference solutions. Conditions used with the 0.0418 molal reference solution were duplicated, except that the sample was placed approximately 20 mm. nearer the solvent. Figure 6 shows that the elapsed time was approximately 43 and 58 hours, when 5% of the solvent remained to be distilled. The distillation time was shortened approximately 25%; however, the path mas made too
short and the test material entered the reference solution when 370 of the solvent remained to be transferred. Another test sample of adobenzene was equilibrated with the 0.0418 molal reference solution. The theoretical amount of benzene that it should contain a t equilibrium was 22.8 mg. The sample was compared under identical conditions to a benzil test sample previously investigated which had a theoretical weight gain of 31.8 mg. (Figure 7). The amount of solvent to be distilled is directly proportional to the time required for its distillation. A series of blank determinations mas made in which the test procedure was followed, omitting the test sample (Table I). In some instances, droplets of solvent could be observed on the coils of the helices. These helices had been washed with benzene prior t o calibration to remove any material soluble in benzene. If the weight gain during a molecular weight determination was small, the blank could contribute as much as 5y0 t o the total weight. This blank appeared to be inversely proportional to the molality of the solution, and an average value of the molality of reference solution multiplied by the blank in milligrams was determined. For subsequent determinations, the blank was determined for a solution of a particular molality from this average value. -4small pad of glass-fiber filter paper supported each sample during weighing and was transferred to the helix pan to
Figure 7. Effect of quantity of solvent to be distilled upon equilibration rate
vapor pressure. Sample sizes of 0.10 to 0.20 mg. were employed and the operating temperature was 36" C. The molecular weights of fractions obtained by distillation of a crude petroleum residue in a Distillation Products Industries Model ChIS-5 molecular still were determined by the isothermal distillation method and also by the twin thermistor ebullioscopic method (Table 111). Sample sizes of 0.5 to 1.5 mg. were used in the isothermal distillation method, and the standard reference solution was octadecane in benzene. The mean molecular weight values obtained by the tmo procedures agreed vithin 2% relative. DISCUSSION OF RESULTS
I
0
IO
20
30
40
50
1
60
TIME, hr.
absorb the solution formed and to prevent objectionable creeping from the pan. Blank determinations were made 1%-ithoutthe pad and sample; no detectable difference was found in the values obtained with and without the glass filter pad. Blank determinations were made with approximately 30 times more glass-fiber filter paper than in the original determinations. A detectable amount of solvent nas adsorbed on the glass fiber and amounted to 0.02 mg. for a piece of glass fiber of the size employed in a molecular weight determination, This mas a contribution of about 7% to the blank obtained with 0.1 molal reference solutions. The remainder of the blank appears to be due to adsorption of benzene on the helix assembly. To determine the reproducibility of the procedure, nine molecular weight determinations were made on test samples of benzil that varied in size from 0.20 to 0.40 mg., and with benzil as solute in the reference solution (Table 11). Both benzene and carbon tetrachloride were employed as solvents. The average relative error was =t2.3% and the coefficient of variation was 2.6l%. The molecular weight of azobenzene was determined with benzil in benzene as the reference solution, The molecular weight was found to increase with molality (Figure 8). Extrapolation of the results to zero concentration gives a molecular weight value nearer the theoretical value of 182.2. The molecular weight was determined on a sample of trilaurin (Eastman white label grade) having a theoretical molecular weight of 639.0 (Figure 9). The
value obtained was 594, which is lower than the theoretical value, suggesting that the sample had undergone partial hydrolysis; this was confirmed by determination of the acidity. The molecular weight was then determined by the twin thermistor ebullioscopic method, employing successive additions of sample and calculation of the molecular weight from the average slope of a plot of boiling point depression against the cumulative a-eight of sample additions. Cyclohexane was used as solvent and a molecular weight of 584 was obtained. The mean molecular weight values obtained by the two methods agreed n-ithin 2% relative. =Ittempts were made to determine the molecular weight of benzoic acid, but sample loss occurred because of high
Table 1.
Helix
Blank LIolality Solute Benzil
2
Benzene
.4zobenzene Benzil
3
Benzene
Benzil
1
a
Determination of a Mean Value of the Blank
Solvent Benzene
KO.
The most satisfactory concentration of standard reference solution was 0.10 molal. The equilibration time when 20 mg. of solvent was distilled from 0.10 molal solutions was 2 days. More dilute reference solutions gave somewhat more erratic results, because of contamination effects and higher blank values; the equilibration time was also longer. Use of more concentrated solutions resulted in shortening of the equilibration time; however, the amount of solvent distilled was less and greater deviations from ideality were observed. The variation of equilibration rate with molality, (Figure 5 ) is apparently due to the difference in the amount of solvent that must be distilled. Further proof of this is given by Figure 7, where the time is directly proportional to the amount of solvent distilled. The straight-line portion of the plots in Figures 5, 6, and 7 indicates that the distillation rate is essentially first order with respect to the difference in vapor pressures of the standard reference and test solutions. The initial phase of the distillation corresponding to zero time and 100% of solvent to be distilled does
4
Benzene
Benzil
4
Carbon tetrachloride
Benzil
Molality 0.0411 0.0881 0.1008
0.0411 0.0881 0.0884 0.0411 0.0881
0.1644 0.0411 0.0881 0.0930
hIg.
X mg.
0.647 0.145 0.245 0.627 0.286 0.307 0.698 0.579 0.179 0.732 0.389 0.476
0.0266
0.01425
0.0247
0.0257 0,0252 0.0271 0,0287
0 .O51Oa
0,0327 0.0300 0.0343 0 .0442a
Mean 0.0283
Results omitted from mean. VOL. 30, NO. I , JANUARY 1958
147
L
=
L
I
/
t
l7?L
I 0.01
'0
MOLALITY
I 0.10
I 0.15
I
470
OF BENZIL I N BENZENE
Figure 8. Variation of apparent molecular weight of azobenzene with molality of benzil reference solution
not fall on the straight-line portion of the plot, because the distillation was started a t room temperature and some time elapsed before the path was a t operating temperature. The distance through which the solvent must travel has an effect upon the distillation rate (Figure 6). As the distillation progresses, the distance the solvent must travel is reduced, tending to speed up the distillation process. This does not
Table II.
Benzene
Carbon tetrachloride
Molality
become significant until the amount of solvent remaining to be distilled is less than lo%, and then it results in deviation from a straight-line behavior (Figures 5, 6, and 7 ) . The distance from the reference solution to the sample support pan must be sufficiently great that the sample does not enter the reference solution. Unknowns cannot be plotted as in Figures 5, 6, and 7 until after equilibration, a8
0.0418 0.0884 0.0884 0.0884 0.1664 0.1664 0.0930 0.0930 0.0930
Mean Coefficient of variation, % Theoretical molecular weight 210.2.
Molecular Weight" 215.7 203.4 203.8 206.9 213.8 203.6 209.6 215.5 204.6 208.5 2.6
Error, +2.6 -3.2 -3.0 -1.6 +1.7 -3.1 -0.3 +2.5 -2.7 f2.3
Q
Table Ill. Molecular Weight of Petroleum Residue Fractions Molality of Reference Solution Fractions Octadecane in Benzene No. 8 No. 10 No. 12
0.0500 0.1000 0.1505
Mean Coefficient of variation, yo Ebullioscopic values, mean Difference in mean values, yo 148
ANALYTICAL CHEMISTRY
440 443 419 424 432 3 437 1
0.10
0.15
OF BENZIL IN BENZENE
Figure 9. Determination of molecular weight of trilaurin (ebullioscopic molecular weight 584)
Determination of Molecular Weight of Benzil Standard with Benzil as Reference Material
Solvent
0.05
MOLALITY
486
558
489 493 482 508 492 2 503 2
537 598 547 580 569 4 558 2
.SR5 ---
the amount of solvent to be distilled is unknown. If sufficient time is allowed for the equilibration, it is unnecessary to make intermediate measurements of weight gain. It has been found convenient to allow 3 days for equilibration, which permits two series of determinations each week. Equilibration time can be shortened through use of smaller samples, the limiting factors being helix sensitivity and size of the blank. Solvent can be added to the sample to speed up the equilibration, but it is difficult to add to the suspended sample, and evaporation is great because of the large surface area. The distillation rate can be increased by raising the temperature; however, lubrication of the ground-glass joint may be a problem. The air bath is not properly constructed for operation a t temperatures much above room temperature. The normal illumination present in the room caused a loss of solvent from the helix pan. In one instance in which 0.2027 mg. of azobenzene was equilibrated in the dark with 0.0418 molal benzil in benzene, the pan became lighter by 0.208 mg. in 15 minutes' exposure. This represented a change of 1% in the total elongation. Apparently, the pan is not in direct thermal contact with the bath and its temperature is raised by the absorption of light, resulting in evaporation of solvent. Carbon tetrachloride as the solvent gave accuracy comparable to that attained with benzene; however, solutions of benzil in it decomposed upon exposure to light. If there is no deviation from Raoult's law or if both solutions exhibit the same deviation, there mill be no varia-
tion of apparent molecular weight with molality. This appears t o be the case with the petroleum distillate fractions analyzed using octadecane in benzene reference solution (Table 111). When deviations from ideality occur, more reliable determinations can be made by determining the apparent molecular weights a t several finite concentrations and extrapolating to zero concentration, as was done for azobenzene and trilaurin (Figures 8 and 9). As more than one concentration of standard reference solution can be used with the same test material, a new sample does not have to be weighed and transferred. The test sample must dissolve completely in the solvent employed. In general, solvents in which the sample shows the greatest solubility should exhibit the least deviation from ideality. The solvent employed must have a vapor pressure low enough to make evaporation losses insignificant during handling. High vapor pressures are desirable for a more rapid equilibration. Molecular weight standards above molecular weight 400 were unavailable; therefore, the method n-as evaluated by comparing molecular weight values obtained by isothermal distillation and twin thermistor ebullioscopic methods. The mean molecular weights obtained by the two procedures agreed within 2% relative (Table 111). Under the conditions employed, the method is applicable to materials that boil above 275’ C.; otherwise, the vapor pressure is sufficient to cause sample loss. Loss of benzoic acid, boiling point 249’ C., was observed by a progressive decrease in weight on the helix pan after the system was equilibrated. KO loss of azobenzene, boiling point 293’ C., occurred even after it was left in the system for pro-
longed periods. Unknowns can be checked for volatility in the same manner by observing whether or not the equilibrated unknown and solvent gets progressively lighter over a few days a t equilibrium. Because straight-line plots were obtained in Figures 5, 6, and 7, molecular weights were calculated from data taken before equilibrium was reached, using the expression
where a = weight gain a t equilibrium, mg.
m = slope, hours-’ t = time, hours w = weight gain a t time t, mg.
Both a and m are unknown and are obtained graphically from the intersection of a plot of a us. m values and a second set of values upon substitution of t3 and w 3 for tz and w2. This method of calculation of molecular weight from data taken prior to equilibration of the system has been applied to the data shown in Figures 5, 6, and 7. The average relative error of seven determinations was 8.5%. With average values of w and t taken from a mean curve drawn through a plot of w us. t values, this error is expected to be less than 5%. This calculation method has the advantage that values can be obtained in one working day instead of 3 days; however it is less accurate and requires more operator time. CONCLUSIONS
The results obtained in this investigation indicate that molecular weights can be determined by the proposed procedure with a fraction of a milligram of test material. An accuracy to 3% is attainable. The temperature of the
equilibrium system must be controlled for prolonged periods to =k0.005° C. The method is inapplicable to materials boiling below 275’ C. Three days of elapsed time are required for a determination. The operator time per single determination, however, is only 2 hours. ACKNOWLEDGMENT
The author is grateful to Max Blunier for proposing use of the helical spring balance in the isothermal distillation method and for conducting initial experiments which indicated the feasibility of the method. LITERATURE CITED
Barger, G., J . Chem. SOC.85, 286 (1904). Childs, C. E., ANAL. CHEM.26, 1963-4 (1954). Clark, E. P., IND.ENG. CHEM., ANAL.ED.13,820-1 (1941). Hover. Herbert. Mikrochemie 36/37, i169-73 (1951). McBain, J. W., Bakr, A. &I.,J . Am. Chem. SOC.48,690-5 (1926). Madorsky, L., Rev. Sei. Instr. 21, 393-4 (1950). Nash. L. K., ANAL. CHEM. 19, 799-802 (1947). Niederl, J. B., Kasanof, D. R., Risch, G. K., Rao, D. S., Mikrochemie 34, 132-41 (1948). I Parrette, R. L., J . Polymer Sei. 15,447-58 (1955). (10) Puddington, I. E., Can. J . Research 27B, 151-7 (1949). (11) Rast, Karl, Ber. 3727-8 (1922). (12) Signer, R., Ann. Chem., 478, 246-66 (1930). (13) Taylor, G. B., Hall, M. B., ANAL. CHEM.23, 947-9 (1951). RECEIVEDfor review May 27, 1957. Accepted September 5, 1957. Division of Analytical Chemistry, Symposium on Analytical Contributions to Research in Petroleum Geochemistry, 131st Meeting, ACS, Miami, Fla., Bpril 1957. Publication 111, Shell Development Co., Exploration and Production Research Division. I
Multipurpose Standard for Microchemical Analysis W. H. SMITH National Bureau of Standards, Washingfon 25;
b As a standard for use in ultimate analysis of organic compounds, 5chloro 4 hydroxy 3 methoxybenzylisothiourea phosphate is suggested.
- -
I
- -
1946 Ogg and Willets (3) proposed benzylisothiourea hydrochloride as a “new standard for use in ultimate analysis of organic compounds especially suited for microprocedures.” It has N
D. C.
been extensively used by microchemists. Benzylisothiourea hydrochloride contains carbon, hydrogen, nitrogen, chlorine, and sulfur. A compound, 5 - chloro - 4 hydroxy - 3 - methoxybenzylisothiourea phosphate, which is related in structure and contains also phosphorus, oxygen, and the methoxyl group, is proposed as an alternative standard. This compound contains all the elements in the seven microchemi-
-
cal standards now issued by the National Bureau of Standards, except iodine. “/2
CH2. S . C
0
C1
“€I. H3P0d
OCHB
OH C9HI4C1S2OeSP,molecular weight -344.73 VOL. 30, NO. 1, JANUARY 1958
149
