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 o n 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 o r freeze o n 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
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(1) S. G. Gibbins, Abstract 94, 25th Annual Regional Meeting of the American Chemical Society, Seattle, Wash., June 1970.
Figure 1. Weighing transfer apparatus
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5 cm long. The volume of the bulb and about 2.5 cni of the stem is calibrated with mercury t o later account for air buoyancy. The standard taper joint is black waxed to 8. Section ( B ) is essentially planar with the three principal arms a t 120' to each other. Bulbs 2 and 3, each 100 ml, provide additional volume for the solvent. A high vacuum, solvent resistant, grease is used in stopcock 4 which is the vacuum cup type. Thickened portions 5, permit sealing off 4 and 6. While dimensions are not critical, 12- to 14-mm tubing provides adequate strength without being bulky. Sections that are sealed off and fragile bulb break seal tubes generally d o not exceed 12 mm 0.d. After sealing the sample tube A onto B a t 7 by the usual glass blowing methods, the apparatus is pumped down to 10-j Torr and flamed out. A measured quantity of dry purified solvent stored in the vacuum line is then distilled into 3. If mercury is present in the vacuum line, its condensation in 3 is avoided by placing a n intermediate trap in series between the weighing apparatus and the high vacuum line. The solvent, initially in the vacuum line, is transferred to the intermediate trap by condensation a t a temperature as close to 25 "C as convenient, e.g., 0 "C. The solvent, using similar temperature conditions, is then transferred from the intermediate solvent trap to bulb 3. If any small droplets of mercury collected in the intermediate trap, their condensation in bulb 3 can be avoided by incomplete distillation of the solvent into 3. The stopcock is then either sealed off or closed. An essential precaution during all subsequent operations involving the solvent is that regions 4 and 8 be kept above room temperature to avoid solvent condensation and attack. This is readily accomplished by a heat gun. It is good practice to keep the solvent, except for brief periods, below room temperature. The sample tube ( A ) fragile bulb is broken by a Teflon (Du Pont) encased bar magnet 9. The solvent is then poured onto the sample and the resulting solution is then poured back into bulb 3. The pouring operation is accelerated by cooling the receiver bulb. If the sample is incompletely dissolved, the solvent is condensed by dry ice, for example, back in bulb 2, and the pouring process repeated. When all the sample has been transferred t o bulb 3, the apparatus is attached t o the vacuum line via either the stopcock or the fragile bulb break seal 6. The solvent is then distilled out. The solid which now adheres t o the walls of bulb 3 is scraped off by means of a n internal iron nail 10 manipulated by a n external magnet. The fine powder drops into (C). Plugs may be dislodged by the nail, If charing in the fragile bulb neck is likely due to sample adhesion, solvent is condensed on the region to be sealed by dry ice. The neck is washed internally, the solvent is distilled out, and the fragile bulb sealed off. The two parts of Section (C) are cleaned and reweighed in the conventional manner, taking into account air buoyancy. Glass weight loss as a result of the sealing process is less than
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When not in use, the iron bars 9 and 10 are arrested by external bar magnets taped to the vessel. Such an apparatus may also be used for approximate solubility measurements. The qualitative dry transfer step can be modified in the following manner to accomplish quantitative transfer. After the bulk of the dry solid has fallen into the fragile bulb, 2 to 3 ml of solvent are transferred from the vacuum line to bulb 3 by condensation. The solid adhering to the wall is dissolved and the solution is poured into the fragile bulb section. Care must be taken to avoid wetting the black wax seal. This possibility is reduced if the standard taper joint 8 is of the drip tip type with the end projecting 10 m m or more beyond the ground surface. At a rate commensurate with the absence of bumping, the solvent in the fragile bulb is now condensed back in 3 by liquid nitrogen or dry ice. After the walls of 3 are washed, the solution is again poured back into the fragile bulb. Walls elsewhere in the vessel are washed free of solute by orienting the apparatus so that the fragile bulb section is almost vertical and then condensing the solvent on the walls by placing a piece of dry ice in the appropriate position. These operations should be so carried out as to avoid solution transfer to the stopcock region. Prior to sealing off the fragile bulb, the solvent is removed by distillation into the vacuum line. The intermediate transfer of the solid t o bulb 3 rather than directly by solution to the fragile bulb receiver is necessary when a large quantity (25 t o 50 ml) of solvent is used. Such a quantity of solvent in the fragile bulb region would wet the black wax seal and result in sample contamination by black wax and probable exposure to the atmosphere. If only a few milliliters of solvent are used, the intermediate transfer step is omitted. Choice of the procedure used depends on sample size and solubility, and whether the transfer is to be quantitative. The described method for solvent transfer from the vacuum line to the weighing transfer apparatus guarantees both dryness and freedom from mercury. There does not appear to be a simpler alternative method t o accomplish this purpose. Experience with the very hydrolytically sensitive iron hydride (I) and tetrahydrofuran demonstrated the extreme necessity of double drying the solvent. "Anhydrous," spectroscopically pure solvent was dried initially by sodium benzophenone in a closed vessel equipped with a stopcock and standard taper joint. The drying capacity, as indicated by the blue color, of the sodium benzophenone was not exhausted. Without exposure t o the atmosphere, the solvent was then distilled into the vacuum line and condensed in a vessel containing additional sodium benzophenone. RECEIVED for review August 28, 1970. Accepted November 3,1970.
NO. 2, FEBRUARY 1971
