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
LTHOUGH 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. 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
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
A. B.
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
spring is compressed by a manually operated screw, L. The whole unit is designed to be mounted in a commercial infrared beam condensing unit and the functional parts of the cell are only 1 inch thick to fit into the restricted focal area of the lens system.
I
w
CELL MATERIALS
