Thermistor Micromethod for Molecular Weight ABRAHAM WILSON, LOUIS BINI, and ROBERT HOFSTADER American Cyanamid Co., Bound Brook, N. 1.
b A highly sensitive thermistor method for molecular weight determination has been used with water, benzene, 1,4dioxane, carbon tetrachloride, ethyl acetate, and chloroform as solvents. The apparatus is essentially a twin thermistor bridge, by means of which may b e detected a temperature difference as low as 1.5 X 1 0-40C. arising from the difference in solvent activity between drops hanging on the thermistors a t 30" C. A useful technique measures the unknown solution against a series of standards and thereby avoids a separate calibration.
T
first use of thermistors in the application of the "electro-osiiiotic" technique of Hill (4)and Baldes and Johnson ( 1 ) for determining t h e activities of solutions of nonvolatile solutes was by Brady, Huff, and JIcI3ain ( 2 ) . Other variations of this method (3) as applied to molecular weight determinations have been reported (3, 6, 6). This paper describes two microtechniques. One is an internal calibration technique (IC) in n-hich the solution of unknown is used as a referenee for comparison with a scries of standards. It has advantages over a standard calibration technique (SC), which is a two-stage technique for measuring tenipcrature HE
T I
I
3 55/50 EFLON
OADING DROPPEfiS
THERMISTORS 1
24 CM. ~
I I I
i
I
i i ~
2 L
Figure 1.
Thermistor cell
differences of a series of standards vs. solvent, and then a series of unknown us. solvent. A simple apparatus for routine molecular weight determinations is also described. APPARATUS
The thermistor cell, which is kept submerged in a bath controlled a t 30' 0.01" C., is shown in Figure 1. The measuring chamber, lined with filter paper and aluminum foil, is soaked with solvent to saturate the apparatus with solvent vapor. One-foot glass thermistor probes (Gulton Industries Yo. L 419) having nominal resistances of 10,000 ohms a t 25" C., and matched to within 1%, form two arms of the Wheatstone bridge (Figure 2). In series with the measuring thermistor is R-14, a 50-ohm, lo-turn potentiometer, each dial division of which represents 0.05 ohm. About 0.75 volt is usually applied to the bridge. When the bridge is unbalanced as a result of a temperature difference between the probes, rebalance is achieved by adjustment of the series potentiometer. A temperature difference of 1.5 X 10-4' C., corresponding to one dial unit, is easily detectable. The chassis and all leads are carefully shielded a t ground potential to avoid excessive alternating current pickup.
*
PROCEDURES AND CALCULATIONS
Both methods have been used for molecular weight determination. At present, the IC method is used for rapid molecular weight determinations and the SC technique for activity-concentration measurements. Internal Calibration. T h e zeroadjust potentiometer, R-11, is set so t h a t bridge balance is achieved with zero temperature difference between the thermistors when about one fourth to one third of the series resistor, R-14, is in the circuit. The molecular weight of the unknonm is estimated and a fen- milliliters of solution are made up, which would give a response of about 300 dial units if measured against the solvent (based on the estimated molecular weight and the solvent constant). A series of three or four standard solutions of a known reference compound with molalities above and below that estimated for the unknown is prepared. A drop of unknown is placed on the reference thermistor, while a droD of the most dilute standard is placed*on the measuring thermistor. The thermistors are lowered into the measuring chamber
and the series recistance is adjusted for null balance. The time required for reaching a steady state is inversely proportional to the volatility and heat of condensation of the solvent. Optimum times are established by trial and error. At a fixed time-5 minutes for chloroform and carbon tetrachloride and 10 for water, benzene, 1,4-dioxane, and ethyl acetate-the dial reading a t null balance is recorded and the standard solution drop is renewed for duplicate determination. Usually three readings are taken a t each concentration of the standard solution. Finally, the unknown solution iy measured against itself. Figure 3 shows that the data fall on a straight line, the equation for which is obtained by a least squares procedure. The readings for unknown us. unknown are then substituted in the equation to calculate the molality, from which the molecular weight is obtained Standard Calibration. All measurements are made with a drop of solvent on the reference thermistor. The calibration of a pair of thermistors for a particular solvent consists of the response measurements of a series of solutions of a suitable standard solute, and determination of the equation of the curve of recponse us. molality, the slope of which is characteristic of the solvent. Unknowns are then determined in the same way, and usually require the preparation of three or more concentrations. The equation representiiig the response us. n-(light of unknon-n per 1000 grams of qolvent is determined, and the molecular n eight is obtained from the ratio of the calibration dope to the unknown slope. Xlter-
I
I
Figure 2.
Thermistor bridge
R-1-10. 5 6 0 ohms each R-1 1. 0 to 100 ohms, 1 0-turn R-12, 1 3.. 7000 ohms R-14. 0 to 5 0 ohms, 1 0-turn R-15. 5 6 0 0 ohms T-1. Measuring thermistor T-2. Reference thermistor Output to Kintel electronic galvanometer, 240 A.
VOL. 33, NO. 1, JANUARY 1961
135
natively, single point calculations may be made by using the readings for the unknown and the calibration slope. The equations are :
R - RQ= Km (1) dial reading for solution us. solvent RQ = dial reading for solvent us. solvent K = solvent constant, dial units molal-' m = molality R - RQ = kw (2; where k = unknown constant, units/ gram/1000 grams of solvent w = grams of unknown per 1000 grams of solvent where R
=
M. W. = K/lc (3) There was no evidence of curvature in our plots of R vs. concentration; hence the slopes used are the same as at zero concentration. I n more concentrated solutions, Equations 1 and 2 should be written with activities rather than concentrations, and curve fitting and extrapolative techniques may be used to evaluate the molecular weight. RESULTS AND DISCUSSION
Examples of molecular weight determination on known compounds are given in Table I. The average deviation from theory is slightly less than 1%. To calculate a standard deviation, each deviation from the mean was divided by the mean molecular weight and converted to percentage. The estimated standard deviation of these normalized deviations, calculated in 3.79%. Since, the usual way, was with the exception of sucrose in water, these results are for solutions from 0.01
*
Table 1.
Solute Anthracene Sucrose N'-(3,4Dimethyl-5-isoxazolyl sulfani1amide)b o-Dinitrobenzene a
MOLALITY
Figure 3. Response curve for o-dinitrobenzene, unknown vs. naphthalene in benzene standards
- R = 94.2 + 7133 m
--Response for unknown VP. itself. grams benzene
to 0.03 molal, this precision should be considered satisfactory. Table I1 lists the solvents studied in order of increasing sensitivity, which, as expected, is also the order of decreasing latent heat of vaporization. Following the example of Brady (2) and of Higuchi (3)) the thermodynamic efficiency of the process is calculated as the ratio of the observed molal temperature rise to the calculated temperature rise, where T,
where T ,
k
=
K X 0.05 320
= observed rise, "C:/molal
0.05
= =
observed slope (Figure 3) ohm per division
Molecular Weight Determinations
Solvent Carbon tetrachloride \JTater Ethyl acetate
Actual Molality 0.0108 0.1040 0.0136
Benzene 0.0292 Ethyl acetate 0.0095 Chloroform 0.0166 Mean of three determinations. Sample 59-6, Association of Official Agricultural Chemists.
Table II.
1,)
Solvent Water. 1,4-Dioxane Benzene Ethyl acetate Chloroform Carbon tetrachloride
136
30" C., Cal./G. 579.5 107.1 103.7 91.8 61.9 50.1
ANALYTICAL CHEMISTRY
(4)
~
K,
Exptl. Slope, Units/Molal 1,470 4,630 7,250 7,710 11,000 14,610
Molecular ICa 175 341 273 168 169 167
Weight Theory 178.2 342.3 267 168.1 168.1 168.1
Concentration 4.91 5 grams per 1000
320
thermistor response, ohms per "C. RTao T', = (5) 1000 I , where T ' , = theoretical rise, "C./molal To = 303.2'K = latent heat of vaporization I, at 303.2" K., calories Der gram =
As Higuchi has shown, the highest efficiency is obtained with water, although this is the least desirable solvent from the standpoint of sensitivity. The low value for 1,4-dioxane is probably attributable to traces of water. The IC technique has obviated some of the difficulties encountered with the SC method. Because each measurement represents a calibration of a standard vs. unknown, long-term changes in thermistor resistances have no effect, as they do when one depends on solvent calibrations made some time in the past. Daily changes in line voltage, ambient temperature, etc., are not important over the period required to make a determination. As far as we can see, the most important consequence of Higuchi's evacuation technique is that i t avoids the effect of a varying relative humidity, the increased sensitivity being of minor importance. However, repeat measurements using the I C technique carried out on differ-
Solvent Parameters
Tm,
Ex tl., Deg.,!Molal 0.230 0,726 1.13 1.21 1.72 2.28
T'm,
Calcd., DegJMolal 0.315 1.71 1.76 1.99 2.95 3.64
Efficiency, % T,/T', 73 43 64 61 58 63
Relative Sensitivity Tm/T,(H20) T',/T',(H20)
1.0 3.2
4.9
5.3 7.5
10
1.0
5.4 5.6 6.3 9.4 12
ent daj-s have been within experimental error and it is felt that this method is preferable, especially when routine measurements are required. The amount of sample needed for a molecular weight determination varies with molecular weight and solvent sensitivity. If a molecular weight of 200 is assumed, preparation of a solution containing 1 gram of solvent, and a required temperature rise which will give a A R of 100 units, the sample requirements are as follows: HzO (least sensitive). m = 100/1470 = 0.068 molal Sample %-eight = 0.068 X 200 X 1 13.6 mg.
=
CC1, (most sensitive). m = 100/14609 = 0.00685 molal Samale reauired = 0.00685 X 200 X 1 1.37 h g . =A
No measurements on “pure” samples above M.W. 400 have been made. However, the question of extension to higher molecular weights is a matter of the sensitivity of a method, the precision depending only upon the replication of response a t a given molality or activity. Because this technique is from five to ten times as sensitive as an ordinary cyroscopic measurement with a Pt thermometer, the accessible molecular weight range is accordingly extended.
LITERATURE CITED
(1) Baldee, E. J., Johnson, F., Biodynamica 46, 1 (1939); 47, 1 (1939). (2) Brady, A. P., Huff, H., McBain, J. W., J. Phys. & Colloid Chem. 55, 304
(1951). (3) Higuchi, W. I., et al., J . Phys. Chem. 6 3 , 996 (1959). (4) Hill, A. V., Proc. Roy. SOC.(London) A127. 9 11930). (5) Muher; R. H., Stolten, H. J., ANAL. CHEM.2 5 , 1103 (1953). (6) Keumayer, J. J., Anal. Chim. Acta 20, 519 (1959). RECEIVED for review February 9, 1960. Accepted September 1, 1960: Presented in part at the Meeting-in-Miniature, North Jersey Section, ACS, January 26, 1959.
Microtechnique for the Infrared Study of Solids Diamonds and Sapphires as
Cell Materials
E. R. LIPPINCOTT and F. E. WELSH’ Department o f Chemistry, University of Maryland, College Park,
C.
Md.
E. WEIR
National Bureau of Standards, Washington, D. C.
b A microtechnique for obtaining the infrared spectra of solids and corrosive liquids, utilizing sample weights as low as 4 pg., is described. A cell in which diamonds or sapphires are used as window material i s employed to obtain spectra in the 2- to 35micron region. The visible and ultraviolet regions can also b e studied. Spectra are obtained routinely, easily, and rapidly without many of the limitations inherent in other procedures. So far as is known, the method is applicable to all solids.
A
the infrared spectra of liquids and gases may be obtained by simple methods, recording the spectra of solids is sometimes difficult. Solids may be studied by several techniques, with the nature of the specimen generally suggesting the applicable method. However, each particular method has various disadvantages, either with respect to sample handling techniques or to the quality of the spectrum obtained. Accordingly, it is desirable t o supplement the available techniques for solids with new ones that may increase the number and type of solid samples capable of being studied in the infrared region. LTHOUGH
Present address, Midwest Research Institute, Kansas City, Mo.
A cell utilizing diamond or sapphire windows has been used to obtain the spectra of a Ride variety of solids in the 2- to 35-micron region. As far as is
u -FRONT VIEW
SIDE VIEW,
known, the cell may be used in a routine manner to study infrared spectra of all solids. The method is essentially a microtechnique giving spectra on specimens weighing as little as 4 pg. (4 X 10-8 gram). The same cell can be used throughout the visible and ultraviolet regions on solids and extremely corrosive liquids in a routine manner. The cell was initially designed to study the effect of high pressures on solids ( G I ) , but the ease of obtaining spectra on solid specimens led to the investigation of its use as a routine method for the study of solids. The disadvantages and advantages of this technique are compared with those of other methods. DESCRIPTION OF CELL
Figure 1. mond cell A. B.
Schematic diagram of dia-
Diamonds or sapphires Brass disk C. Washer D. Steel piston E. Bearing F. Pivot G. Thrust bearing H. Pivoted pressure plate 1. lever arm J. Calibrated spring K . Thrust plate 1. Screw
The apparatus used is shown in Figure 1. Two gem-cut Type I1 diamonds, A, comprise the infrared cell proper. The culets of each diamond are ground and polished t o form small irregular octagonal flat surfaces parallel to the tables. The specimen is squeezed into a thin film between these small surfaces, which have areas varying between and 10+ sq. inch. To eliminate axial alignment problems two diamonds having markedly different surface areas are used. Each diamond is seated on its tabular face over a small hole in a close-fitting recess in a brass disk, B. The disks in turn are seated on rubber washers, C, cemented into a VOL. 33, NO. 1, JANUARY 1961
137
