A Reusable Apparatus for the Convenient Determination of the Molecular Weight of Air- or Moisture-Sensitive Compounds Robert W. Zoellner Northern Arizona University, Flagstaff, AZ 86011 The determination of the apparent molecular weight of a substance is an essential step in the identification and characterization process. Such molecular weight determinations are often carried out using osmometric, cryoscopic, or ehullioscopic methods. However, few commercial laboratories are equipped to handle air- or moisture-sensitive compounds routinely,' and such services are often limited to determinations by osmometric or cryoscopic methods with a limited choice of solvents. Some in-house methods have been reported that overcome the problem of the lack of commercial sources for the determination of the apparent molecular weight of an air- or
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moisture-sensitive compound. Such methods include the ebullioscopic methods of Walker and Ashhy2 and of Bulkin and T e r g i ~ These .~ methods do have some disadvantages, however. includine being constructed of fragile or complicated components (sich asboiling chambers, which need to he made by skilled glassblowers, and Beckman thermometers for the precise determination of temperature), requiring that the compound be stable a t the boiling point of a chosen solvent, using relatively large quantities ifsolvent (60 to 75 mL) and sample, and necessitating the accurate determination of the difference in the boiling point of the pure solvent and the solution under fairly constant conditions. The Siener method of molecular weieht determination4 " involves the prinriple of isothermal distillation of two solutions solelv in vaoor ohase rontact. The distillation proceeds until the apparent concentrations of the solutions h e equal. An apparatus that employs the Signer method has been described by C l a ~ k However, .~ Clark's design is not easily applied to air-sensitive compounds and is not reusable. De&bed herein is an easily ionstructed, reusable apparatus for the determination of the apparent molecular weight of air- or moisture-sensitive compounds employing Signer's method of isothermal distillation. Although the apparatus needs- to ~he constructed bv a ~comoetent elassblower. the ~ ~ ~ design is inherently less fragile thanthose iescribed fo; the ehullioscopic methods. The components from which the apparatus is constructed are commercially available, and other ~ o t e n t i adisadvantaees l of the ehulliosco~icmethods generally do not apply to &is apparatus.
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Apparatus The accompanyingfigure illustrates the modified Signer molecular weight apparatus. One 5-mL Mohr pipet, A, sealed at its constricted (outlet)end, is glassblown at the opposite (tap) end to each of the two spherical sample bulbs, C. The dead volumes of the Mohr pipets, B, formed when the ends of the pipets were sealed, are calibrated by filling each dead volume individually with distilled water (another liouid of known densitv .~ . mieht .. also be used) and weighing the apparatus. In practice, thrsr dead volumea are sppnximacrly 0.4 to0.5 mL. Each of the sampk bulha, (.', is appruximal~ly 10 mL. The remainder of the apparatus is constructed as illustrated. The sample fill tubes, G, have standard taper ( S m ) 14/35 inner joints, D, attached to them, which are capped during the experiment with S/T 14/20 outer jointed caps, E. (The combination of an S/T 14/35 inner
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Work done in part at Kansas State University, Manhattan. KS 86606~
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und Kolbe Mikroanalytisches Laboratorium IPostfach Mhenweg 17. 4330 Mulhein a. d. Ruhr 1. Fed. Rep. of Germany)offersthe routine determinat~onof the molecular we ght of alr-sensitive compounds using cryoscoplc methods. Signer, R. Liebigs Ann. Chem. 1930,478, 246. Clark, E. P. Ind. Eng. Chem., Anal Ed. 1941, 13,820-821. Walker, F. W.; Ashby, E . C. J. Chem. Educ. 1968,45,654-656. Bulkin, B. J.; Tergis, P. J. Chem. Educ. 1979, 56, 280. 01 04 08
Modified Signer molecular welgM apparatus. me imered arrows refer to lhe follnwino: bulbs; B, =~A., Mohr ~- olmts sealed at one end and attached to sam~le calibrated dead volume; C, sample bulbs; D. SIT 14/35 Inner ground giass joint; E. SIT 14/22 outer ground giass jointed cap; F, greaseless high-vacuum valve; G, sample fill tube. ~~~
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Journal of Chemical Education
joint with an SrT 14/20 outer jointed cap helps prevent grease fmm contaminating the solutions.) The central greaseless valve, F,6 and its joint are chosen so as to mate directly to a vacuum srjtem for evacuation of the apparatus and cryogenic distillation of solvent into the apparatus. Procedure
The operations concerned with filling the apparatus are performed inside a dry nitrogen-filled glove box or glove bag. Accurately weighed samples of a known molecular weight compound (ferrocene, MW 186.04, freshly sublimed after recrystallization, is a convenient standard) and of the unknown are placed in the apparatus through the fill tubes, G , such that one sample bulh receives the known and the other receives the unknown. (When a theoretical molecular weight can be estimated for the unknown, the relative amounts of the standard and unknown may be adjusted so that the amounts of each compound present are approximately equal. If this can be accomplished, equilibrium within the apparatus will be achieved more rapidly.) After the apparatus has been filled, the fill tubes, G , are capped and the central greaseless valve closed. The apparatus is removed from the glove box or glove bag and connected to a vacuum svstem (such as a Schlenk line with a moderate vacuum of 1t o 10 mtorr) via the joint on the central greaseless valve. The apparatus is evacuated, and approximately 7 to 10 mLof adry, degassedsolvent are cryogenically distilled into the apparatus. (The only limitation on the choice of solvent is -
