Diffusion Cell Scanning Attachment for Beckman Model DU Spectrophotometer
, ....__.F. Felicetta, and Joseph 1. McCarthy, Pulp Mills Research and Departments of Chemistry and Chemical Engineering, University of Washington, Seattle, Wash. Fvnrt Rnrkl Vinrn.it
VOR measuring the absorbance of
r mati?rialsthat have diffused into a gel lFelic'etta, V. F., Markham, A. E., Peniston, Q. P., Mecarthy, J. L., J . Am. Chen1. Soc. 71, 2879 (1949); Felicetta, V. IF., Ahola, A,, McCarthy, J. L., Ibid., 78, 1899 (1956)l a suitable device was nee&,d to position a diffusion cell accuratelji in the path of the light beam. Such an attachment for the Beckman Model DU spectrophotometer has been designed and built. It consists of a cell holder, directly connected to a finely threaded horizontal carrier screw, which is mounted in a screw housine which also serves as a cover for thz light-proof cell compartment of the spectrophotometer equipped with a photomultiplier tube attachment. DESIGN REQUIREMENTS
The absorbance-distance measurements need to be of high precision, as they are used not only to evaluate diffusion coefficients of pure substances or average diffusion coefficients of mixtures, but also to estimate the frequency distribution of diffusion coefficients of components in polymolecular systems. To calculate the distribution, the ahsorbance-distance relationship obtained in a single diffusion experiment is used to calculate the first four moments of the distribution function. With the Beckman DU spectrophotometer, ahsorbance measurements in the range of 0.2 to 1.0 have an average reproducibility within 0.2 to O.3Yob. This degree of accuracy was required for the position setting of the diffusion cell, which, in the way the measurements were performed, corresponded to a reproducibility in the range of 0.02 to 0.03 nun. This made necessary a stable bolder Present address, Swedish Forest Products Research Laboratory, Stockholm, Sweden.
for the cell, rigidly mounted and attached to a precision screw. A further requirement was speed in securing data. The spectrophotometer could he adjusted for drift and variations in light source intensity most conveniently only at the beginning and end of a set of measurements, which usually required 10 to 12 minutes. The gel slowly shrinks at the boundary, because of evaporation of solvent, and the measurements have to be recorded as fast as possible. A simple method of inserting the cell firmly into the cell bolder helped to reduce the over-all time requirement. Figure 1 shows the device with the screw housing, S, and the cell holder, H , in which the cell is to be inserted through slot C. A 8/8-inch steel screw having 40 threads per inch is the cell holder carrier. It is mounted to the screw housing with combination thrust and hall bearings, B1 and BI. A lathe dial, D, of 1-inch over-all diameter and 3/rinch bore, divided into 100 divisions per turn, and a turn wheel, W , are used for reading and adjusting the movement of the cell bolder. The vernier scale. Ti. made from a vernier depth gage IL. 'S. Starret Co., Athol, Mass., Catalog No. 448-15M), was included for the calibration and close control of the carrier screw, which had an average of 40.13 turns per inch. Absorbance measurements usually were made at every full screw turn, and the vernier scale served only for indicating the number of turns. The bar, A', which holds the sliding scale of the vernier, is fastened to the screw carriage, serving as an extra alignment. Two horizontal X 3/16 inch brass guide bars for the cell holder carrier are screwed to the wall of the screw housing. The corresponding guide tracks, T, in the cell bolder carrier appear in Figure 2. The lower part of the screw housing has a flange which serves as a stop- and ligbtproof cover for the cell compartment. The screw housing also extends inch into the cell compartment with a very
close fit, so that the housing IS rigidly held by the cell compartment. Figure 2 is a detailed drawing of the front and side views of the cell bolder carrier. Dimensions not given are dependent upon the height of the screw housing and the position of the carrier screw, which are not critical. The cell holder (Figure 3) is comprised of two side plates, P, end spacer, R,, and bottom spacer, Rz, to make up the slot for insertion of the diffusion cell. The cell bolder is fastened to the cell holder carX Z1/4 rier hy two screws. Three inch flat brass springs, two on one of the side plates above and below the slit and one on the underside of the cell bolder carrier, hold the diffusion cell firmly in proper position. The diffusion cells were designed and made in these laboratories. They are carefully squared with respect to one side and the open end. The diffusion cell is inserted into the cell holder with the open end-the diffusion houndaryagainst the end spacer, R1, which has a depression to form a small air space, so that the houndary is not disturbed. After a cell is inserted, it protrudes a few millimeters from the cell holder to facilitate removal. On a small book, K , at the end of the bottom spacer a rubber
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Figure 2. Front and side views of cell holder carrier
VOL. 29, NO. 12, DECEMBER 1957
1903
band is attached to hold the cell flush with the bottom spacer. The screw housing and the cell holder are made of brass, with the lower part of the cell holder blackened by a surface treatment. The standard cell compartment with the light-proofing assembly (Beckman Instruments, Instruction LIanual 305, page 35, Figure 23) was used, modifled only by displacing the two extra upper screws, outward and downward a few millimeters into the corner below the shoulder of the standard (Beckman 2390) sample compartment. There still was room to use the cell compartment with the ordinary cells of 1-em. path length. MOUNTING
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circular saw to height less than that of the cell holder. Thus a uniform height of the light beam is obtained for all measurements taken on a diffusion cell. To minimize reflections, the diaphragm and slit holder were blackened by a surface treatment. For evaluation of the light intensity over the light beam after passage through the two exit slits, a standard “slit block” was used. Two wellsquared pieces of brass with beveled ends were mounted on a brass bar, so that the beveled ends were 0.115 mm. apart, forming a narrow slit. With this block in the scanning device, the intensity of the incident light beam can be measured, for example, at every 0.05 turn (0.03 mm.) over all the beam.
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SECTION 1
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A SECOND LIGHT SLIT
The Beckman DU spectrophotometer has an adjustable exit slit which can be used at small openings when the instrument is equipped with a photomultiplier tube receiver. This exit light beam, however, is divergent to the extent of about 2’ and its width does not decrease proportionately with a decrease in the exit slit width. A light beam in the visible range using exit slit widths of about 0.1 mm. had a width of 2 to 3 mm. in the middle of the cell compartment, with a light intensity varying horizontally over the beam. For determination of absorbance at different positions in the diffusion cell, the width and intensity distribution over the light beam must be established to permit observations to be properly corrected. The exit light beam was made narrower by the installation of a second slit in the aluminum plate of the cell compartment. The cell holder is set in from the
Figure 4.
L,’”-l
Slit holder and slit
lower part of the carrier, leaving space at the incident light side for another slit which can protrude slightly into the cell compartment. Figure 4 illustrates the second slit holder, F , and the type of diaphragm, E. The aluminum holder is inserted into the entrance hole of the aluminum plate of the cell compartment and aligned with the slit vertically by two pegs, G. The collar, L , of the slit holder is set in flush with the aluminum plate on the side of the incident light by lathing out the plate to a depth of inch centrally around the 1-inch opening. Holes to accommodate the two pegs in the collar of the slit holder are also provided. The slit was cut into diaphragm E by a thin
Such measurements were made after each refocusing of the hydrogen lamp light source, as well as with each set of diffusion measurements. Besides showing the light intensity of the incident beam over the slit opening, they gave accurate estimation of the width of the light beam passing through the diffusion cell. By comparing this estimation with the width of the second slit and the standard slit block slit as measured with a micrometer slide, and considering the slight divergence of the beam, the parallel alignment of the standard slit block and the diaphragm slit can be checked. Such a control is important t o assure that the light beam is perpendicular to the diffusion direction. Absorbance values obtained as described can be corrected for finite width of the light beam and for linear variation in intensity of the light across the beam according to procedures described elsewhere. John Sundling constructed the scanning device now in use.
Induction-Heated Ebulliometer Vernon A. Zeitler’ and Charles A. Brown,2 Department of Chemistry, Western Reserve University, Cleveland 6, Ohio
of a series of new inP organic compounds of moderately high molecular weights necessitated REPARATION
construction of a suitable apparatus for determination of molecular weights. The first Fompound prepared, tetrakistriphenylsiloxytitanium, [(CsHs)sSiOl4Ti,had such low solubility in the usual solvents at the freening point that cryoscopic methods failed. The Rast method could not be used because this compound reacted with camphor. As the compound demonstrated some solubility in boiling ben-
Present address, Central Laboratory,
T~~~~~ i v i ~The i ~ D~~ ~ , Chemical co.,
Free ort, Tex. * #resent address, Chemical Products Plant, General Electric CO.,Cleveland, Ohio.
1904
ANALYTICAL CHEMISTRY
zene and toluene, ebullioscopic measurements seemed promising. A sensitive vacuum-jacketed ebulliometer using Menzie-Wright differential vapor pressure thermometers has been described ( 3 ) . The internal heater of platinum wire requires several metalmetal and metal-glass seals. Replacement of the heating element \\-auld be a major repair job. The use of a ground-glass plug containing the heating element has been suggested (1). As this plug fits into the bottom of the solution well, two disadvantages are apparent: A tight seal must be secured without the aid of soluble lubricants; the plug provides an additional heat leak through the vacuum jacket. The vacuum-jacketed ebulliometer described (Figure 1) is similar to that of
Kitson, but modified to provide for inductive heating. An iron ring under the Cottrell pump serves as the secondary for a radio-frequency generator and provides heat for the solution in the ebulliometer. Induction heating eliminates the need for seals in the electrical heater circuit, It does not introduce a leak for either solution or heat. It permits thorough cleaning of the boiling chamber without possible damage to the heating element. The radio-frequency generator (Figure 2) is an adaptation of a Tesla coil (4). The primary coil of the generator is wound on a form constructed from 1/8-inch Lucite and has an inner diameter of 70 mm. This coil form consists of four coaxial Lucite rings supported by six vertical strips notched to hold 33 turns of No. 12 wire. These strips are made from three pieces of Lucite (1 x 6 inches), clamped
