formed with a wide variety of electrolytes, the only limitation being that the p H must not exceed 8, if the gel is to be stable. A salt-bridge can be converted from one supporting electrolyte to another within minutes and no harm is done to the frit if the silica gel is allowed to dry. The leak rate also is quite acceptable. The addition of fumed silica to a supporting electrolyte does not significantly affect its conductivity. The resistance of 1.00M KCI in our conductivity cell was 4.14 ohms. The resistance increased insignificantly (4.16 ohms) after the KCI was gelled with 773 fumed silica. The resistance between the mercury pool and the analyte salt-bridge in the coulometry cell, commonly used in this laboratory, was 100 ohms when a 1.00M KCI solution and gel were employed. Currents as
high as 100 mA have been passed through the cell without deleterious heating effects. ACKNOWLEDGMENT The authors thank William A. Jeunger of the Cabot Corp., Burlingame, Calif., for providing the samples of Cab-0-Si1 fumed silica, and Warren Harden for making the technical illustration.
RECEIVEDfor review August 14, 1970. Accepted September 20, 1970. Work performed under the auspices of the US. Atomic Energy Commission.
Design and Evaluation of a Precise, Continuous Photoelectric Spectropolarimeter P a u l E. Reinbold Department of Chemistry, Bethany Nazarene College, Bethany, Okla. 73008
Karl H. Pearson' Department of Chemistry, Cleveland State Uniuersity, Cleveland, Ohio 44115
THIS PAPER DESCRIBES the modifications done in our laboratory to a Perkin-Elmer Model 141 polarimeter to obtain continuous optical rotatory dispersion data over the entire spectral region which can he obtained with the original calcite polarizing and analyzing prisms and the 1P 28 photomultiplier detection systems. The modifications are not extremely difficult and require only a minimum of technical services. The modifications consisted essentially of adding an optical bench which contained a double grating monochromator and a continuous high intensity light source. The accuracy and precision of our modified spectropolarimeter will be discussed. EXPERIMENTAL Instrumental Modifications. The light source containing the sodium and mercury lamps along with the associated filters and optics were removed from the Model 141 polarimeter. An adjustable optical stand was easily fabricated to support and allow optical alignment of the new continuous light source and monochromator with the polarimeter. This optical stand was fabricated from 3-inch angle iron, a/h. inch thick, with welded sides and back of 3 inches X 3/1s inch flat steel. The overall dimensions of the stand are 13l/$ inches long by 13 inches wide. Four holes are drilled into the angle iron, two to match the original bottom holes for holding the sodium and mercury housing, and two to match the original alignment pin holes. These holes were drilled slightly larger than those in the polarimeter to allow final optical alignment; the platform was attached to the polarimeter with two of the original bolts in the bottom holes and two s/16-inch x ll/&inch bolts in the pin alignment holes. Three S/ls-inch holes were tapped into the flat platform in a triangle (two near the point of attachment to the polarimeter and one at the hack of the platform) into which s/,$ X 31/2-in~hbolts were threaded for height adjustments. The four metal legs of the Bausch & Lomh monochromator were removed and four holes were drilled into the platform to match those of the monochromator; the monochromator was attached to the platform with '/Anch X 1-inch bolts. A B&L 250-mm To whom all correspondence should be addressed.
Figure 1. Photograph of the modified Perkin-Elmer Model 141 polarimeter with the Bausch & Lomh monochromator and xenon light source
............................ L S L
A
5
............................ PBE
M O D E L 141
Figure 2. Block diagram of the continuous spectropolarimeter L: A: P: S: C: PM: LS:
Lens
Analyzer prism Polarizer prism Collimating slits Cell Photomultiplier, RCA 1P 28 Light source, B&L 150-watt xenon lamp (catalog no. 33-86-
20-01) M:
Monochromator, B&L double grating (catalog no. 33-86-66).
double grating monochromator, catalog no. 33-86-66, with a B&L 150-watt xenon lamp, catalog no. 33-86-20-01, was chosenfor the light source. Figure 1 is a photograph of the completed modification described above, showing in detail the attachment of the B & L monochromator and the xenon light source to the PerkinElmer Model 141 polarimeter. Optical alignment adjustments were made so that the center of the light beam passed through the center of the collimating apertures of the polarimeter. Figure 2 shows the block diagram of the spectro-
ANALYTICAL CHEMISTRY, VOL. 43, NO, 2, FEBRUARY 1971
-
293
+!
000 9 OG
800
*. 0
i
1
I
,703
z
o
I
600
L
I-
i
5 00
i
P
P
[SUCROSE],
g 11 00 m i
Figure 3. Plot of a o b s DS. [sucrose] at five different wavelengths 0650nm; 0589nm; A546nm; 0365nm; 240 nm.
560
580
WAVELENGTH,
600
z
nm
Figure 4. Optical rotatory dispersion spectra of the neodymium-D-(-)-PDTA complex :
metal perchlorate solution with standard disodium dihydrogen D-( -)-1,2-propylenediaminetetraacetate, buffered at pH 5.0 with acetic acid-sodium acetate buffer, and diluted to volume.
- Cary 60 recording spectropolarimeter; 0 Modified Perkin-Elmer Model 141
RESULTS
spectropolarimeter
polarimeter. Because of the excellent basic design of the polarimeter section of the Model 141,optical alignment was easily obtained. Because of the positions and apertures of the collimating slits and lens of the polarimeter, the slight divergence of the light beam from the B&L monochromator was found to be not critical in the optical alignment. Solutions. STANDARDSUCROSESOLUTIONwas prepared by dissolving 3.0100grams of Baker analyzed grade sucrose in deionized water and diluting to 1 liter. STOCK SUCROSESOLUTIONwas prepared by dissolving 6.0200grams of Baker analyzed grade sucrose in deionized water and diluting to 1 liter. This solution was diluted volumetrically to give various concentrations. 0.01M NEODYMIUM-D( -) PDTA SOLUTION was prepared by mixing 1 :1 stoichiometric amounts of the standardized 294
Table I. Comparison of Observed and Calculated ORD Values for Sucrose in Water at 20 "C, c = 3.01 grambiter, I = 1 decimeter WaveError, Error, length, Calculated," Observed,b a nm CY deg 650 +O. 162 +o. 160 -0.002 -1.23 0.166 0.168 -0.002 640 -1.19 0.172 630 -1.15 0.174 -0.002 0.179 620 0.180 -0.001 -0.56 0.185 610 0.186 -0.001 -0.54 0.192 600 0 0.192 0 0.200 0.200 0 590 0 0.207 0.207 580 0 0 0.216 0.214 570 +0.002 +0.93 0.223 0.223 560 0 0 0.232 0.232 550 0 0 0.242 540 0.241 +0.001 +0.41 0.252 0.251 +0.001 +0.40 530 0.262 0.263 +O. 38 +0.001 520 0.273 0.274 510 +o, 001 +o. 37 0.285 0.287 +0.71 500 +0.002 0.298 490 0.300 +O. 67 $0.002 0.314 0.311 +0.003 480 +0.96 0.327 0.329 410 +O. 61 +0.002 0.342 460 0.344 +o. 002 $0.58 0.360 0.362 +0.002 +O. 56 450 0.378 0.380 +0.002 440 +O. 53 0.398 0.400 +0.50 +0.002 430 0.421 +0.001 +0.24 0.420 420 $0.23 0.444 0.445 +0.001 410 0 0.470 0.470 0 400 0.498 -0.20 0.497 390 -0.001 -0.38 0.527 380 0.529 -0.002 -0.53 0.561 0.564 -0.003 370 0.605 360 0.602 +O. 50 +0.003 0,644 0.644 350 0 0 -0.43 0.688 0.691 -0.003 340 -1.08 0.744 0.736 330 -0.008 -1.49 -0.012 0.792 320 0.804 -1.61 0.857 310 0.871 -0.014 -1.79 0,949 -0.017 0.932 300 -1.73 1.038 -0.018 290 1.020 -1.49 1.124 -0.017 280 1.141 -1.35 1.246 -0.017 270 1.263 -0.21 1.404 -0.003 260 1.407 1,580 -0.002 250 1.582 -0.13 -0.22 1.791 -0.004 240 1.795 Calculated by computer program. Values are the average of 3 runs on the modified instrument described here in.
Table I shows the evaluation of the results of the standard sucrose solution (3.0100grams/l.) from 650-240 nm obtained on the modified spectropolarimeter. From 650-340 nm the error is within the specifications of the absolute reading of the instrument. The error from 330-240 nm is larger in absolute magnitude than the specified +0.002",but is within the specifications of the instrument for optical rotations of these magnitudes (-1.0'). The calculated values of the optical rotations of the standard solution were obtained from the equation 21.648 = X* - 0.0213 which assumes single-termed Drude equation behavior, using a computer program to give the calculated optical rotations (A is in microns).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
Figure 3 shows a Beer’s-type plot with &bs us. concentration of sucrose a t five wavelengths. The wavelengths 650 and 240 nm are the practical limits for the modification as reported herein. These wavelength limitations are due to the response of the 1P 28 photomultiplier, the absorption of the calcite polarizer and analyzer prisms, and the necessity t o purge with nitrogen a t lower wavelengths. The three wavelengths 589, 546, and 365 nm are very common wavelengths for which optical rotations are quoted in the literature. The linearity of the plots is excellent and within the instrumental limitations described above. Figure 4 shows the excellent resolution and accuracy of the spectrum obtained o n the modified spectropolarimeter taken at 1-nm increments, compared t o the spectrum obtained with the same solution on a Cary 60 recording spectropolarimeter. The extremely sharp f-f transitions of the neodymium-D( -)PDTA solution are dramatically resolved with this modified Perkin-Elmer spectropolarimeter. The accuracy of this complex spectrum is excellent, as is shown by the small scatter of the data points for the observed rotations. Thus, the data show the four peaks and four troughs which occur over the 25 nmregion of this spectrum. IIISCUSSION
The modification described above has many attractive features. The Bausch & Lomb monochromator, containing
a double grating modified Czerny-Turner mounting, does not require additional filters to suppress the higher orders of the grating at wavelengths longer than 550 nm that are required with single grating monochromators. The extremely low stray light of the double grating B&L monochromator allows the entire spectrum t o be obtained without the necessity of additional operations o n the optical system and subsequent changes in sensitivity. The second major advantage is the high intensity xenon lamp source which can be used over the entire wavelength range (650-240 nm), thus eliminating the necessity of changing lamps to cover the spectral region. Thus, effectively, the time required to run a n O R D spectrum from 650-240 nm is considerably shortened. Because of the high intensity of the xenon lamp, our modification has the capability of handling samples of high absorptions throughout the entire spectrum. Finally, our modifications to obtain ORD spectra can be easily made o n the Model 141 polarimeter for an approximate cost of $2000. RECEIVED for review July 30, 1970. Accepted October 20, 1970. This research was supported by the Robert A. Welch Foundation Fellowship Grant A-262 and the Research Council of Texas A & M University. Appreciation is expressed to the Research Council of Texas A & M University for a postdoctoral fellowship t o P. E. R.
Weighing Procedure for Nonvolatile, Air and Water Sensitive Solids Sidney G. Gibbins Departinent of’ Chemistry, Unicersity of Victoria, Victoria, B. C . , Canada THEOBJECTIVE of the technique described here is t o obtain for analytical purposes a weighed quantity of a nonvolatile, air and water sensitive solid. Previous t o the described operation, the substance is purified by suitable methods. This is the first of a series of notes describing methods of analysis and purification of such solids. The apparatus used employs allglass high vacuum systems. Stopcocks which inevitably leak or freeze on long term exposure to solvents and contribute contaminating grease are either eliminated or minimized t o a n extent that these shortcomings are not significant. These procedures are particularly suitable when dry box methods provide insufficient protection. The techniques were adequately tested in the characterization of Mg4Br3.SC10.SFeHB (I). The borosilicate weighing-transfer apparatus, Figure 1, comprises of three sections: ( A ) original sample vessel; ( B ) solvent-sample intermediate transfer vessel ; (C) weighed fragile bulb sample receiver. The solid is transferred from ( A ) to ( B ) by solution. The solvent is removed by distillation and the sample is then dry transferred to ( C ) . Sintered disks 1 (coarse) prevent glass chip contamination. The apparatus is operated in a vertical plane and rotated about a horizontal axis (perpendicular t o the apparatus plane) to effect various operations. Section (C) is a weighed 10/30 outer standard taper joint equipped with a fragile bulb of approximately 1-ml volume. The bulb is connected t o the joint by a 4-mm 0.d. glass tube
/ 7
2 2 I
8
C
7
A
(1) S. G. Gibbins, Abstract 94, 25th Annual Regional Meeting of the American Chemical Society, Seattle, Wash., June 1970.
Figure 1. Weighing transfer apparatus
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
295
