Undergraduate Polymer Experiment (the author replies) - Journal of

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To the Editoc

Literature Cited 1. Molyneur, P,JChom Educ, 1891,68.467-469. 2. Mills. I.; Cvitas, T.; Homann, K KsUay. N.; Kuehitsu, Eds. Quontirks, Units and Symbols inPhysicol Chamisfry; IUPACBlackwell: Oxford, 1988. 3. &lwsrd, G.H.;Rndlay, T. J. V. S I Ch~micolDoto,2nd ed.; W~ley:Sydneg 1974. 4. Wesst, R. C., Ed. Handbook ofChemisfry mnd Physies, 67th ed.: CRC Ress: Bas

Rston, FL, 198M87.

Philip Molyneux Macrophile Associates 53 Crestway, Roehampton London SW15 5DB, U.K Reactions of Rare Earth Metals To the Editor: The recent article about the rare earth elements bv Solomon and Lee ( I ) perpetuates the widespread miscouception that the rare earth metals dissolve in acids vieldina only typical, trivalent rare earth salts. When t h i acid is hot, dilute acetic acid, the reaction yields a precipitate. This compound appears to be a mixed oxide-hydride: two hydride (or acetate) ions, bearing a total charge of 2-, replace one oxide ion, also bearing a charge of 2-, in the incompletely filled fluorite crystal lattice (2).This pheuomenon would make a n excellent research topic.

I was oleased to hear of Professor Dust's interest in our PEG moiecular weight experiment. I am very familiar with Professor dust's work in this area. As a matter of fact, in our graduate-level Polymar Chemistry Laboratory we have students determine M, bv both the titrimetric and proton NMRmethods. The e&&re to NMR is extended to include measurement of carhon-13 TIrelaxation times of PEG'S and their dimethoxy derivatives. In PEG intermolecular bydrogen-bonding restricts motion of the carbons near the hydroxyl group and decreases corresponding TI values. This effect is not observed in ethers. Although we have performed the NMR experiments for several vears. Mr. Bernier and I wanted to em~hasizethe exposure of lower level students to polymer concepts. Therefore. we chose not to mention the ex~erimentsfor advanced &dents Kathryn R. Wllllams University of Florida P.O. Box 117200 Gainesville, FL 32611-7200 Other Publications on C6Ck and CzCls To the Editor:

Literature Cited

reference8 therein.

G. L. Silver EG&GMound Applied Technologies P.O. BOX 3000 Miamisburg, OH 45343-3000

Arecent article [J. Chem. Educ. 1994,71,7041 described the formation of C6C16 and CzCl6 from the high-temperature reaction of CCL and Cr203.Readers may be interested in knowing that the formation of C6C16and C2C16from this reaction and from the decomposition of CC14was described ~reviouslvin this Journal [J. Chem. Educ. 1978.55. 52; J. Richard L. Keiter Eastern Illinois University Charleston, IL61920

Undergraduate Polymer Experiment To the Editoc I n a very thorough article K. R. Williams and U. R. Bemier described a n undergraduate polymer experiment in which students determined the number-average molecular weight, M,, of a variety of polyethylene glycol (PEG) samples [J. Chem. Educ. 1994, 71, 2651. The method emoloved was a titration-based e u d - m o u ~analv&a&ydridh sis of the b[s (i,2,4,5-henzenetetracarb~x~lic derivatives. Unknowns were DreDared . . bv mixing known masses of two commercial PEGS of nominal molecular weiehts of 200 and 400 elmol.. res~ectivelv.These molecular weights happen to fail nicely into the region of M,, that can be determined rapidly and with reasonable accuracy (8%)by proton NMRin dimethyl sulfoxide solvent (1). Consequently, a possible extension of the experiment for use in upper-level courses c a n be suggested; students could initiallv estimate the M" of their s a m ~ l e sbv 'H NMR. followed bv the derivatization and titration analysis outlined hy fllllmns and Bern~er.Such an extenswn &~ld permlt students to iudae the "fitness to purpose" ofeach analytical method. A

Literature Cited 1. Dust, J. m.; Fang,2. H.;Hsrris, J.M.Mmmmoloeulea 1990.23, 3742.

Julian M. Dust Sir Wilfred GrenfeliCollege Comer Brook, NF, Canada, A2H 6P9

956

Journal of Chemical Education

Organic Hourglass Inclusions To the Editor:

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As an analvtical chemist in the oimnent industrv. .. I found the article "Organic Hourglass Inclusions," [J. Chem. Educ. 1994.71. 5841 verv interestine. Since BASF is one of the major s;p&iers-of aikali blue p&ment for which fuchsin is a n intermediate, I just had to try the experiment. I t worked nicely and I am looking forward to future articles on this subject (which the authors allude to on p 585"other candidates . . .are in preparation"). I found that the demo can be "s~eeded-UD" mi. bv" eoinrr cro. I placed 10 drops of sulfate on a micro slide and then added 1 d r o ~of 0.1% acid fuchsin which was mixed-in with a plastic disposable pipet. I n 30-60 min enough water had evaporated from the prep to start the crystallization process to produce the hourglass crystals. The dried prep contained a number of perfectly formed crystals that could he saved "as-is" on the microscope slide and viewed under low power (50x1 for observation. This process may work as a quick screening test for future hourglass crystal systems. James Benkn Research Associate BASF Corporation 491 Columbia Avenue Holland MI 49423 u

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